gp 04-10 22 feb 2006

Guidance on Practice for Drainage Systems

GP 04-10

BP GROUP ENGINEERING TECHNICAL PRACTICES

Document No. GP 04-10

Applicability Group

Date 22 February 2006

22 February 2006 GP 04-10 Guidance on Practice for Drainage Systems

Downloaded Date: 6/17/2008 10:04:33 PM The latest update of this document is located in the BP ETP and Projects Library

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Foreword

This is the first issue of Engineering Technical Practice (ETP) BP GP 04-10. This Guidance on Practice (GP) is based on parts of heritage documents from the merged BP companies as follows:

British Petroleum RP 4-1 Drainage Systems.

Amoco A CV-PLT-DISP-G Civil—Plant—Disposal Systems—Guide. A CV-PLT-DISP-E Civil—Plant—Disposal Systems—Engineering Specification. A CV-PLT-DISP-S Civil—Plant—Disposal Systems—Concrete Protection SS Lined

Trench—Specification. A CV-PLT-BGDS-C Civil—Plant—Below-Grade Gravity Drainage & Sewer Systems—

Construction Specification.

Arco ES 801 Civil Design and Construction (Section 5).

Copyright 2006, BP Group. All rights reserved. The information contained in this document is subject to the terms and conditions of the agreement or contract under which the document was supplied to the recipient’s organization. None of the information contained in this document shall be disclosed outside the recipient’s own organization without the prior written permission of the Director of Engineering, BP Group, unless the terms of such agreement or contract expressly allow.



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Table of Contents

Page

Foreword .............................................................................................................................................. 2

1. Scope.......................................................................................................................................... 6

2. Normative references ................................................................................................................. 6

3. Symbols and abbreviations ........................................................................................................ 7

4. Basic requirements..................................................................................................................... 8

5. Legislation and standards .......................................................................................................... 8

6. Design......................................................................................................................................... 8 6.1. General............................................................................................................................ 8 6.2. Integration with processes .............................................................................................. 8 6.3. Waste minimisation ......................................................................................................... 9 6.4. Fugitive emissions of hydrocarbon gases ...................................................................... 9 6.5. Future developments ...................................................................................................... 9 6.6. Design factors ................................................................................................................. 9 6.7. Effluents........................................................................................................................... 9 6.8. Effluent types.................................................................................................................10 6.9. Effluent segregation ......................................................................................................11 6.10. Types of system ............................................................................................................12

7. Effluent volumes .......................................................................................................................14 7.1. Systems draining paved and/or unpaved areas ...........................................................14 7.2. Rainfall intensities .........................................................................................................14 7.3. Firewater volumes.........................................................................................................15 7.4. Groundwater infiltration.................................................................................................17 7.5. Bunded (diked) tank area flow capacity........................................................................17 7.6. Water discharge ............................................................................................................17

8. Layout and configuration ..........................................................................................................17 8.1. Layout of drainage systems..........................................................................................17 8.2. Process areas ...............................................................................................................17 8.3. Offsites areas ................................................................................................................20 8.4. Treatment ......................................................................................................................22 8.5. Measurement of effluent discharge rate .......................................................................23

9. Hydraulic design .......................................................................................................................23 9.1. General..........................................................................................................................23 9.2. Gravity based drainage systems ..................................................................................23 9.3. Closed drainage systems..............................................................................................26

10. Structural design of buried pipework........................................................................................29 10.1. Backfill ...........................................................................................................................29 10.2. Road and rail crossings.................................................................................................30



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10.3. Loads during testing......................................................................................................30 10.4. Thermal expansion........................................................................................................30 10.5. Submerged pipes ..........................................................................................................30 10.6. Settlement .....................................................................................................................31

11. Secondary containment............................................................................................................31 11.1. General..........................................................................................................................31 11.2. Exfiltration......................................................................................................................31 11.3. Infiltration.......................................................................................................................31

12. Ancillary structures ...................................................................................................................31 12.1. Manholes.......................................................................................................................31 12.2. Gully traps .....................................................................................................................35 12.3. Open ditches and channels ..........................................................................................36 12.4. Effluent collection and treatment (neutralisation) pits ..................................................36 12.5. Pumping sumps.............................................................................................................37 12.6. Soakaways and land drains ..........................................................................................37 12.7. Cesspools and septic tanks ..........................................................................................37

13. Control of fugitive gas emissions and venting of drainage systems........................................37 13.1. Control of fugitive gas emissions ..................................................................................37 13.2. Design of vents for open gravity drainage systems......................................................38 13.3. Extraction and treatment of vented gases ....................................................................39

14. Materials ...................................................................................................................................39 14.1. General..........................................................................................................................39 14.2. Resistance to effluents..................................................................................................40 14.3. Strength .........................................................................................................................40 14.4. Joints .............................................................................................................................40 14.5. Other..............................................................................................................................40

15. Construction and workmanship................................................................................................41 15.1. General..........................................................................................................................41 15.2. Construction ..................................................................................................................41 15.3. Connections to existing sewers ....................................................................................41 15.4. Testing...........................................................................................................................41 15.5. Back filling .....................................................................................................................42 15.6. Cleaning ........................................................................................................................42

16. Operation and maintenance.....................................................................................................42 16.1. General..........................................................................................................................42 16.2. Cleaning ........................................................................................................................42 16.3. Inspection ......................................................................................................................42 16.4. Rehabilitation.................................................................................................................43 16.5. Operational procedures (closed system only) ..............................................................44

Bibliography .......................................................................................................................................58

List of Tables

Table 1 - Advantages and disadvantages of alternative drainage systems .....................................46



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Table 2 – Material Selection ..............................................................................................................47

List of Figures

Figure 1 - Pressurised drainage systems - typical arrangements ....................................................49

Figure 2 - Pressurised drainage system - typical connection arrangement .....................................50

Figure 3 - Pressurised drainage system - typical line diagram of collection.....................................51

Figure 4 - Pumped drainage system - typical arrangements ............................................................52

Figure 5 - Manhole gully detail...........................................................................................................53

Figure 6 - Typical sealed manhole covers.........................................................................................54

Figure 7 - Typical standard 150 mm gully trap..................................................................................55

Figure 8 - Trapping of drain inlets to manholes.................................................................................56

Figure 9 - Typical offsites storage tank oily and clean water drainage layout ..................................57



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1. Scope

This GP provides guidance for selection, design, and installation of open and closed drainage systems in refineries, terminals, pipeline associated installations, chemical plants, drill sites, and jetties.

This GP does not include requirements for drainage systems on offshore platforms.

This GP does not include a comprehensive listing of legislation, regulations, codes of practice, and standards applicable to the detailed design of drainage systems. The designer is responsible for ensuring that the most recent version of the appropriate codes of practice and standards relevant to the proposed location are used for the design, construction, and testing of the systems.

2. Normative references

The following normative documents contain requirements that, through reference in this text, constitute requirements of this technical practice. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this technical practice are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies.

BP GP 04-30 Guidance on Practice for Civil Engineering. GP 24-10 Guidance on Practice for Fire Protection - Onshore. GP 42-10 Guidance on Practice for Design of Piping Systems (ASME B31.3).

American Concrete Institute (ACI) ACI 372R Design and Construction of Circular Wire- and Strand-Wrapped

Prestressed Concrete Structures. ACI 373R Design and Construction of Circular Prestressed Concrete Structures with

Circumferential Tendons.

American Petroleum Institute (API) API Spec 5L Specification for Line Pipe. API 15LR Specification for Low Pressure Fiberglass Line Pipe and Fittings.

British Standards (BS) BS 65 Vitrified clay pipes, fittings and ducts, also flexible mechanical joints for

use solely with surface water pipes and fittings. BS 534 Steel pipes, joints and specials for water and sewage. BS 1452 Piping systems for water supply. BS 1600 Dimensions of steel pipe for the petroleum industry. BS 2633 Class I arc welding of ferritic steel pipe work for carrying fluids. BS 2971 Class II arc welding of carbon steel pipework for carrying fluids. BS 3505 Unplasticised polyvinyl chloride pressure pipes for cold potable water. BS 3506 Unplasticised PVC pipes for industrial purposes. BS 3602 Steel pipes and tubes for pressure purposes: Carbon and carbon steel with

specified elevated temperature properties.



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BS 3604 Steel pipes and tubes for pressure purposes: Ferritic alloy steel with elevated temperature properties.

BS 3605 Austenitic stainless steel pipes and tubes for pressure purposes. BS 4515 Process of welding of steel pipelines. BS 4622 Grey iron pipes and fittings. BS 4660 Unplasticised PVC pipe and plastics fittings nominal sizes 110 and 160

for below ground gravity drainage and sewerage. BS 4772 Ductile iron pipes and fittings. BS 4870 Approval testing of welding procedures. BS 4871 Approval testing welders working to approved welding procedures. BS 4991 Propylene copolymer pressure pipe. BS 5178 Prestressed concrete pipes and fittings for drainage and sewerage. BS 5391 Acrylonitrile butadiene styrene pressure pipe: Pipe for industrial uses. BS 5392 ABS - Fittings for use with ABS pressure pipe Part 1. BS 5480 Glass reinforced plastics, joints and fittings for use for water supply or

sewerage. BS 5481 Unplasticised PVC pipe and fittings for gravity sewers. BS 5556 General requirements for dimensions and pressure rating of pipe of

thermoplastics materials. BS 5911 Precast concrete pipes and fittings for drainage and sewerage. BS 6437 Polyethylene pipes (type 50) in metric diameters for general purposes. BS 6464 Reinforced plastics pipes, fittings and joints for process plant. BS 6572 Blue polyethylene pipes up to nominal size 63 for below ground use for

potable water. BS 6730 Black polyethylene pipes up to nominal size 63 for above ground use for

potable water. BS 7336 Polyethylene fusion fittings with integral heating elements for use with

polyethylene pipes for the conveyance of gaseous fuels. BS 8010 Code of practice for pipelines.

International Standards Organisation (ISO) ISO 2531 Ductile iron pipes, fittings, and accessories for pressure.

3. Symbols and abbreviations

For the purpose of this GP, the following symbols and abbreviations apply:

ABS Acrylonitrile butadiene styrene.

d Internal pipe diameter.

EDPM Ethylene - propylene terpolymer.

GRE Glass reinforced epoxy.

GRP Glass reinforced plastics.

ks Surface roughness.

TEL Tetraethyl lead.



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TML Tetramethyl lead.

UPVC Unplasticised polyvinyl chloride.

VOC Volatile organic compound.

4. Basic requirements

a. Detailed specification of material for closed systems, if appropriate, shall be performed in accordance with GP 42-10.

b. Drainage system shall provide safe, reliable, and economic system for collection and transport of effluents and surface water to treatment areas and discharge points.

c. Effect of effluent beyond point of discharge with respect to quantities and quality of effluent should be a major consideration.

d. Overall system should be kept as simple as possible in terms of construction, operation, and maintenance. To meet this objective, open, gravity based drainage systems should be used for wastewater effluent drainage, if legislation permits.

e. Design of plant drainage and sewer systems shall be subject to BP approval.

f. Conceptual design should/shall be agreed with BP before detailed design.

5. Legislation and standards

a. Handling and disposal of effluents and surface water drainage shall be subject to approval of local authorities and subject to legislation within that country or state.

b. Standards relating to gaseous emissions, contaminants, and waste sludges shall be considered together with quality and quantity of effluent discharged.

c. If constructing or upgrading drainage system, consideration should be given to compliance with new and potential future standards.

d. If standards set by local authority, BP standards, and current legislation are different, most stringent standard shall be adopted.

6. Design

6.1. General

a. Drainage system should be considered in very early stages of design of plant as part of initial infrastructure development layout.

b. Drainage system should be designed in accordance with locally recognised and accepted standards except as otherwise described below.

1. Standards shall be subject to review by BP.

2. In the UK, BS 8005 and BS 6297 are acceptable, if applicable.

6.2. Integration with processes

Consideration should be given to integration of drainage into process facilities.

If process conditions permit, this may provide financial savings.

It may be useful to incorporate the drainage system into the P&I diagram.



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6.3. Waste minimisation

Waste should be minimised.

a. Unnecessary mixing of water, oils, and chemicals before entering the drainage system should be reduced.

E.g., oil slops can be collected in drums and not poured into the drains, solid wastes can be screened, and local separators can be used.

b. Procedures should be revised and equipment should be modified so that waste is reduced.

6.4. Fugitive emissions of hydrocarbon gases

Fugitive emissions of hydrocarbon gases from conventional gravity drainage systems should be reduced by changing work practices and methods of operation (see �13).

If required, new drain systems may be installed that can almost eradicate fugitive emissions (see �6.10.3). These new drains are, however, expensive to install and more complex to operate. Thus, major cost savings can be derived by avoiding discharge of oily materials into the drainage system.

6.5. Future developments

Future use of technology to automatically monitor flow rates and effluent composition in drainage systems may allow operations to be better regulated.

6.6. Design factors

The following technical and financial considerations should be assessed by designer in selection and planning of drainage systems:

a. Safety of system with respect to site for which it is selected.

b. Nature and quantity of effluent to be conveyed.

c. Effluent segregation requirements.

d. Legislative/environmental/social considerations.

e. Cost:

1. Construction.

2. Operation and maintenance.

f. Design life.

g. Location of existing buildings and services (to be connected or negotiated).

h. Topography of site.

i. Method of construction and associated disruption of operations.

j. Material, jointing method, size, length, and depth of pipework.

k. Condition of existing service.

l. Secondary containment requirements.

m. Site ground conditions that may affect method and materials of construction and, consequently, cost.

n. Ground contamination.

6.7. Effluents

a. At early stage of design, every source of effluent should be identified.



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The range of possible effluents may vary significantly.

b. Characteristics of materials present in system shall be assessed, including:

1. Estimated maximum and minimum rates of flow.

2. Concentration.

3. Maximum temperature (and temperature fluctuations) of effluent.

4. Potential chemical reactions.

5. Effluent pressure upon entry.

6. Details of potential future additional materials in system.

c. Every effort should be made to segregate clean and contaminated water.

If connecting to existing plant or drainage systems, effluent details may be provided by BP. Otherwise, the process design contractor should provide this information.

6.8. Effluent types

6.8.1. General

a. Potential effluents are described in �6.8.2 through �6.8.10.

b. Descriptions are a guide, and there are no distinct boundaries between categories.

c. Exact definitions shall depend on legislation and treatment facilities available.

6.8.2. Clean water

Clean water is water that is not liable to be contaminated under normal operating conditions and should normally be discharged from site without further treatment.

This usually originates as rainwater or in some cases as emergency fire cooling water.

6.8.3. Contaminated water

Contaminated water is water from areas liable to be contaminated, e.g.:

a. Runoff from contaminated paved areas.

b. Use of hoses for washdown and firefighting in contaminated areas.

c. Laboratory wastes.

Some areas may be contaminated indirectly, e.g., by particle fallout from stacks - this may even come from outside the site boundary.

6.8.4. Oily water

Oily water is water contaminated by oil to varying degrees and may originate at the following sources:

a. Drainage from pipe trenches.

b. Spillages and leaks from process equipment.

c. Cooling water from water cooled glands and bearings.

d. Drainage from sample points, level gages, drain cocks, etc.

e. Water from transformer bays.

f. Pump stations, meter proving stations, manifolds, roof drains from floating storage tanks.

g. Drainage from circulating cooling water systems that may be contaminated with oil.



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6.8.5. Acids, chemicals, solvents, and other process fluids

a. Discharges of these effluents should generally be regulated as part of production process.

b. Reference should be made to P&I diagrams for details.

c. These effluents should be intercepted and reprocessed rather than discharged after treatment.

6.8.6. Liquefied gas LPG/LNG

Discharges result from spills and routine maintenance.

6.8.7. Lead-alkyl compounds

Lead-alkyl compounds are found at the following sources:

a. Leaded motor spirit tankage.

b. TEL/TML blending plants.

c. Leaded motor spirit pumps.

6.8.8. Detergents

Detergents may be used in washing down plant or vehicles.

6.8.9. Solids

a. Solid waste may be either particulate matter, such as clay particles, or product of industrial process, such as pellets or granules.

b. Solid effluents should be avoided if possible (with exception of �6.8.10).

6.8.10. Domestic sewage

Domestic sewage includes waste from toilets, washrooms, kitchens, and cleaners sinks (but not from laboratories).

Domestic sewage systems are also known as “foul” or “sanitary” drainage systems.

6.9. Effluent segregation

6.9.1. Number and types of systems

Number and types of systems should be optimised because of limits on cost and available space taken by systems.

Ideally, each effluent has its own segregated drainage system, with each system specified in terms of capacity, ancillary structures, fittings, and materials in accordance with the particular requirements of that effluent. The waste treatment facility would also be effluent specific, hence, more efficient.

6.9.2. Degree of effluent segregation

Degree of effluent segregation depends on various factors that are covered in �6.9.2.1 through �6.9.2.4.

6.9.2.1. System specification

a. Specification and features of system containing mixture of effluents shall be those of effluent that requires highest level of integrity and treatment. If concentration of the effluent requiring most treatment is below legislative requirements, the requirement for the highest level of integrity may be varied subject to acceptance by BP.



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b. Branches and feeders joining system shall also meet this specification, unless special precautions are taken to isolate them, e.g., suitable water seals.

6.9.2.2. Reactions

Effluents shall not be mixed if physical, chemical, or biological reactions occur.

For example, detergents in oily wastes harm the treatment process and solvent mixing with water (at temperature above solvent boiling point) causes boil outs - a release of vapour.

6.9.2.3. Treatment

Concentrated effluent should not be diluted by large volumes of water if the effect of the larger volume of less concentrated effluent is more severe.

6.9.2.4. Varying flow rates

a. To prevent siltation and blockages in pipes, minimum velocity shall be achieved.

b. Velocity may be compromised if shared system is sized to cater for intermittent large flows while continuous flows are smaller (see �9.2.3 through �9.2.6).

6.10. Types of system

6.10.1. General

a. Vented piped gravity drain systems are inexpensive and should be used in most situations.

b. Higher integrity drainage systems should be considered if increase in costs can be justified for reasons, such as environmental, safety, or legislative requirements.

c. The following sections provide guidance on types of drainage systems that should be used. Table 1 lists advantages and disadvantages of main drainage system types.

Table 1 is intended to provide initial guidance. The final type(s) of system and the features required depends on the types and quantities of effluent to be drained and the legislative requirements.

6.10.2. Open gravity systems

6.10.2.1. Vented piped gravity drain system

Effluent in vented piped gravity drain systems in pipes laid to falls between manholes should be subject to the following:

a. Manhole and gully inlets shall be trapped to prevent spread of fire if effluent system contains flammable gases or liquids.

b. Manholes shall also be vented to maintain atmospheric pressure in each section of pipe to help avoid pressure locks developing in system.

c. Vapours expelled from system should be kept to a minimum.

d. Traps in manholes are achieved by installing dip pipes. An example of a typical dip pipe is shown in Figure 8.

In the case of third party or acquired installations, existing traps may consist of a totally submerged system controlled by a weir in the downstream manhole or by use of a system with conventional “straight through” manholes but with totally submerged pipes laid as inverted siphons.



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6.10.2.2. Ditch gravity drain system

Channels/ditches carrying effluent flow in open channels or drains shall be trapped to prevent spread of fire.

6.10.3. Closed systems

6.10.3.1. General

a. Systems closed to atmosphere shall be provided if there is legislation or safety reasons that restrict gaseous emissions to atmosphere from oily water or chemical drainage systems.

b. Process information should also be used to identify which effluent streams should also be closed.

c. Three types of closed system may be considered:

1. Closed gravity systems.

2. Pumped systems.

3. Pressurised systems (or vapour recovery/purged systems).

The United States is leading the way with rigorous legislation on air quality and ground contamination related to drainage systems on industrial sites. There are restrictions on the amount of volatile organic compounds (VOCs) that can be released to the atmosphere. Several European countries are also developing legislation that will have an impact on the types of drainage system commonly used by BP, such as vented systems with traditional uncontained spigot and socket pipes.

In all cases, the maximum process pressure must be established and used to design the pipework.

6.10.3.2. Closed gravity systems

a. System should be configured as conventional gravity system with sealed access manholes and gas vent collecting system.

b. Materials and construction techniques used should be of higher integrity than with open systems.

c. Generally, joints should be welded and tested to higher standards than with open systems.

d. Connections to drainage system shall be by airtight connections at tanks, bunds (dikes), process units, etc.

e. At changes of gradient and direction, bends should be used instead of manholes.

f. Changes of pipe size should be made using flat backed tapers.

g. Connections from laterals to main sewer line should be by flat tees or branches.

h. To ease cleaning of system, rodding points should be provided. These points should take form of a “Y” branch on sewer pipe, with rodding branch pointing upwards and extended to suitable access point.

6.10.3.3. Pumped systems

a. Pumped system may be run below or above ground.

b. Lift stations or fully pumped systems should be considered in drainage design.

Effluent lift stations lift flow from one gravity system and deliver to a second similar system. Such stations contain only short lengths of pressurised pumping main local to the lift station.



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Pumping stations with associated pumping mains lift and deliver to a distant treatment facility. Such systems contain long lengths of pressurised pumping main. These are normally located aboveground but can be buried as necessary.

The main components of pumped systems are:

• Localised gravity systems feeding to the lift/pumping station sump. • The effluent lift/pumping station. • A pressurised pumping main.

6.10.3.4. Pressurised systems

a. Pressurised drainage systems should flow under gravity but without access manholes and without conventional venting.

b. Vapour space above liquid flow shall be large enough to allow displacement of gases above liquid level. This air space shall then be filled with inert gas at low positive pressure.

c. Inert gas should be injected at discrete points in system to prevent accumulation of hazardous vapours. Venting of inert gas/vapour mix should be provided at controlled vent facility where gases are removed for treatment.

d. Connections to drainage system shall be by airtight connections that shall have pressure reducing and isolating valve. These connections should be aboveground for ease of access.

e. Bends, pipe size changes, connections to laterals, and rodding points should be as for closed gravity systems (see �6.10.3.2).

This system is difficult to operate and maintain and is expensive to install and operate.

7. Effluent volumes

7.1. Systems draining paved and/or unpaved areas

a. Systems draining paved and/or unpaved areas should be designed for greater of the following:

1. Firewater = firewater plus effluent.

2. Rainwater = rainwater plus effluent.

b. Effluent shall include all dry weather flow. Drainage systems carrying these effluents shall not be allowed to flood.

Usually, process and domestic effluents flow continuously in comparatively limited volumes. These volumes can be obtained from process information or standard tables.

It is more difficult to determine water quantities, whether the source is rain or firewater.

7.2. Rainfall intensities

a. Range of storm return periods between 1 and 10 yr should be used for different areas on site, depending on how acceptable risk of flooding is in each catchment area and balance of risk and cost.

For example, in tank farms, where risk is low and some flooding could be tolerated, a return period of average once in 1 yr would be acceptable. If design capacity cannot be exceeded so frequently, e.g., sections of treatment plant, an acceptable return period would be higher, e.g., average once in 10 yr.



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A storm with an average return period of once in 10 yr is of the order of 25% to 50% greater than a 1 yr return period storm, although this depends on the rainfall history of the area. This affects the size of the drainage system required.

b. Contributing area for rainfall drainage to sewers shall be assumed to be 100% of paved area.

c. Contribution of runoff from unpaved areas should be considered on case by case basis and may constitute significant proportion of flow.

d. If time of concentration is less than 10 min, maximum hourly rate of rainfall should be applied as “flat” rate.

If rainfall rates are not specified, design should be based on formulae derived from local records or, in the UK, BS 8005, Table 3.

7.3. Firewater volumes

7.3.1. Area - fire exposed envelope

a. Area in which fire should be contained shall be determined.

b. This could be bunded (diked) area or part of process unit.

c. Area shall be determined by considering consequence of fire incident spreading from one area to another (fire risk analysis - see GP 24-10).

7.3.2. Volumes

7.3.2.1. General

a. Volumes of firewater should be contained or controlled in predetermined area (fire exposed envelope) such that firewater does not cause spread of fire by flowing into adjacent areas.

b. Water should be directed into drainage system and/or areas where water can do no harm.

There are several methods of calculating firewater volumes. The first, in �7.3.2.2, is recommended for initial sizing of the firewater system. For a more accurate calculation of volumes, the methods in �7.3.2.3 through �7.3.2.6 should be followed, referring to GP 24-10.

7.3.2.2. Preliminary design

a. Total firewater demand for installations with fire risk/hazard should be between 800 m3/hr (59 gal/s) and 2 000 m3/hr (147 gal/s). Average rate of 1 360 m3/hr (100 gal/s) should be sufficient, unless plant is particularly congested, for which higher figure should be used.

b. For preliminary design purposes, it should be assumed that water will be applied as follows:

1. 70% evenly distributed over area of 1 000 m2 (1 200 yd2) located anywhere within process area.

This is intended to cater for large plant areas, where the firewater would realistically be concentrated over a section only (i.e., 1 000 m2 (1 200 yd2) - and not dispersed over the whole area resulting in an inadequately designed drain capacity).

2. 100% evenly distributed over whole of process area.

c. For process areas of smaller than 1 430 m2 (1 710 yd2), maximum design intensity should not exceed that given by a., unless BP specifies otherwise.

d. In assessing total firewater demand for any site, area of plant with largest firewater demand should be used as governing factor. If this demand exceeds 2 000 m3/hr (2 600 yd3/hr,



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consideration should be given to separation of part of area or plant in question by passive means (e.g., physical separation by fire walls to reduce demand to more reasonable level).

7.3.2.3. Minimum flow

Minimum flow is that required to contain fire and is not normally used for design of drainage systems.

Guidance is given in GP 24-10 on the volume of water that may be applied in different types of process or tank storage areas and is based on the surface area of plant and volumes of cooling water applied per unit area.

7.3.2.4. Design flow

a. Design flow should ideally be used as basis of drainage design and is the figure contained in pre fire plans.

b. Design flow is based on minimum flow and is adjusted in fire risk analysis and firewater losses deducted.

Design Flow = Min. Flow + V1(actual output) - V2(firewater losses)

Typically, the increase from minimum flow to design flow is 25% to 30%, depending on the equipment used, as the equipment can exceed its rated output. Mobile monitors may need to deliver twice the volume of water to achieve the same cover as a fixed system.

7.3.2.5. Firewater losses

It is recognised that some losses occur between firewater being applied and water entering drainage system due to overspray, evaporation, infiltration into surrounding ground, etc.

These losses are influenced by factors, including climatic conditions, ground infiltration (see �7.4), duration of water application, type of application, and structural types, and should be assessed on case by case basis.

7.3.2.6. Maximum flow

a. Maximum flow shall be total flow rate from firefighting equipment (both fixed and mobile) that could conceivably be directed to a fire in fire exposed envelope.

b. Consequences of over application of water by flooding drainage system shall be evaluated with respect to escalating fire.

Maximum flow rates should only be used if it is likely that design flow rates would be exceeded by the firefighters.

7.3.3. Combined flow rates

a. Consequences of applying design flow rates (or parts of these) to adjacent areas shall be considered.

If, for example, a tank on fire threatens three adjacent bunded (diked) tanks, the total amount of water that must be drained is the design flow for each of the tanks, plus any allowance for foam for the tank on fire.

b. Design of downstream pipework should allow for combination of flow rates from adjacent areas.

c. Design of effluent treatment and discharge facilities should be checked against combined flow rates.



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7.4. Groundwater infiltration

a. No allowance for groundwater infiltration should be made in design of new drainage systems.

b. In analysis of existing systems, allowance may be made, based on site observations.

7.5. Bunded (diked) tank area flow capacity

a. Flow capacity of bunded (diked) tank area drainage system shall allow for greater of:

1. Drainage of accumulated rainwater within bund (dike) in less than 4 hr.

2. Continuous drainage of firewater used for cooling purposes (see �7.3.1).

b. Tank base shall not become submerged.

7.6. Water discharge

Water discharge proposals for new facilities shall be subject to BP approval (see �8.4.2).

Many countries require all drainage, including that used for fire protection, to be rendered harmless before being discharged into local authority drains, rivers, or sea. The exception to this is where water is being used for the protection of life.

8. Layout and configuration

8.1. Layout of drainage systems

a. Layout of drainage systems should be decided at same time as plant layout.

b. Impact on drainage systems of future developments in plant, waste treatment facilities, and improved practices, such as waste treatment and vent gas extraction, should be considered.

c. If possible, main drain lines should run along edge of plant areas and roads to minimise impact of future drainage work on operational areas.

8.2. Process areas

8.2.1. Paved areas

a. Process units shall be paved and divided into catchment areas to contain water using paving gradients and kerbing/bunds (dikes).

b. Catchment areas shall be arranged and drained in such a way as to prevent, in fire incident, spread of firewater and/or flammable liquids to unaffected areas (see �7.3.1).

c. Paved areas beneath pipework or plant carrying LNG shall not be subject to ponding. To prevent Rapid Phase Transition, the paved area shall have sufficient fall to remove water from the area at risk.

8.2.2. Area layout

a. Shape and size of catchment area draining to each manhole/gully should be related to process equipment which it surrounds such that leakage of liquids from equipment is not directed under other equipment before reaching drainage system.

b. Layout should be determined at early stage of design, in conjunction with plant layout and using risk assessment techniques.

c. Size of each catchment area should be minimised, while taking account of most efficient drainage layout.

d. Within process areas, paving should be sloped at gradient no flatter than 1 in 80 in large open areas or 1 in 60 in restricted areas.



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e. Vertical fall across paving should not exceed 250 mm (10 in).

The use of 100 mm (4 in) kerbing/bunding (diking) around the perimeter of the catchment area and around sensitive process units aids containment of firewater and the separation of effluents.

In determining the shape and features of each catchment area, it is important to maintain safe and convenient access routes for people and vehicles. Kerbs in certain areas may create trip hazards and limit vehicle access - ramps may be necessary.

8.2.3. Entry points for effluent into drain system

8.2.3.1. General

a. There are four main types of collection point for effluent entering drain system:

1. Manhole gullies.

2. Gully traps.

3. Tundishes.

4. Channels.

b. Manhole gullies, gully traps, and tundishes shall have rodding points.

8.2.3.2. Manhole gullies

If rain/firewater is to be drained from paved areas, centre of each catchment area should have combined manhole gully.

Figure 5 shows typical details for a manhole gully. Details of manhole gullies are given in �12.1.3.

8.2.3.3. Gully trap connections

a. Individual trapped gullies should be used if lower volumes of effluent have to be collected.

b. Figure 7 shows typical details which meet the requirements of this GP.

c. Within process plot limits, gullies should generally be connected by individual lines from each to manhole, which shall be trapped on entry to manhole. An example which complies with this GP is shown in Figure 8.

If the physical obstruction of foundations makes it difficult for gullies to be connected to manholes in their section of the process area, connections may be made to adjacent areas, provided that they are suitably trapped.

Each end of the pipe connecting a gully to a manhole is to be trapped using a water seal. This can lead to the following “double trapping” effects.

• Depression of water seals and escape of vapour through the weakest. • Increase in flow resistance, requiring additional hydraulic head on the upstream

side of the gully to maintain hydraulic capacity. This can lead to flooding.

Hydraulic analysis of this situation has shown that the “BP Standard 100 mm (4 in) gully” fails in bullet one above by allowing vapours to be released back into the process area. Larger sized gullies (150 mm (6 in) [see Figure 7] and 200 mm (8 in)) designed in accordance with �12.2.2 permit sufficient head to prevent escape of vapours upstream and overcome the flow resistance in bullet two. If possible, it is recommended that combined manhole gullies (see �8.3.1) are used to overcome the double trapping effect.



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8.2.3.4. Process drain connections

a. Process drain connections should be via tundishes.

b. If several process drains connect to underground drain system at same or closely adjacent locations, collector drains and branches should be used. In such cases, collector drain shall be connected direct to manhole and trapped on entry.

8.2.3.5. Drainage channels

Drainage channels should only be used to collect flows (especially large volumes of water), if there is no risk of fire spread from flammable liquids and vapours and combined drainage system is appropriate.

The disadvantage of open channels is that they could contain burning hydrocarbon for the unrestricted length of the channel.

The use of concrete channels can simplify the drainage layout by reducing the number of manholes and associated underground pipework. The grading of the paving can also be simplified with a single fall to the channel. Several proprietary drainage channel systems are available. However, some of these contain plastic components or polymer/concrete mixes that may be susceptible to chemical or hydrocarbon damage and should be checked for suitability before use.

8.2.3.6. Valves

a. Valves should be incorporated into open and closed systems to isolate sections of pipe.

b. Valves located underground should be mounted in suitable concrete valve pits.

8.2.4. Manhole location

a. Manhole location should be determined at early stage in layout design to allow vents to discharge in safe areas with minimum length of underground vent pipe.

b. Manholes should generally not be located in accessways within process units or where crane outriggers may be placed.

c. Manholes located outside or on edge of process units should generally be at least 5 m (16 ft) from edge of road.

8.2.5. Drains crossing foundations

a. Drains should not be laid below or through structural foundations.

b. Drainage system and foundations should be designed such that drains can be laid above upper surface of foundation that they cross.

Precautions should be taken to allow for differential settlement where drains laid in unsupported ground (outside the limits of a piled foundation) join those laid within or above a foundation.

Drainage positions and foundation types should be examined at an early stage with respect to groundwater conditions to ensure compatibility.

8.2.6. Other features

Placing roofs over items of process equipment or tanks reduces amount of rainwater that becomes contaminated. This is especially useful if there is a low fire risk or if the firewater drain could be valved.



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8.3. Offsites areas

8.3.1. Oily water

8.3.1.1. Oily water from storage tanks

a. Water drawoff from oil storage tanks and roof drains (floating roofs only) should be drained to oily water drainage system (see Figure 9).

The most efficient way to carry out drawoff is to use an automatic valve that shuts if the amount of oil reaches a predetermined level. If this is not possible, some form of segregation at source, usually consisting of a sump connected to the oily water system, may be necessary if control of the drawoff cannot be guaranteed. Excess oil released into the sump can be recovered.

b. Connection to oily water system should be valved outside bund (dike). Valve should remain closed except if draining oily water, under control, from area.

c. Areas within bunded (diked) area that are heavily contaminated with oil (e.g., under valves/manifolds) should ideally be paved and bunded (diked) and connected to oily water system (instead of clean water system).

d. Storage areas should be sized to contain products that are spilled, until such products can either be passed into drainage system or returned to tank.

The tops of the walls of any sumps should be sufficiently high to prevent rainwater from within the bund (dike) flooding into the sumps during periods of intense and prolonged rain.

8.3.1.2. Oily water from storage tanks with leaded product

a. Effluent from leaded motor spirit tanks shall pass to combined separator within bunded (diked) area.

b. Water shall be drained off to oily water drainage system and motor spirit pumped back into storage tanks.

c. Outlet to pit shall connect to valve outside bund (dike). Valve shall normally be kept closed such that drainage is only let into system if known to be lead free. Figure 9 shows a typical example which complies with this GP.

The tops of the walls of any sumps should be at the elevation indicated in �8.3.1.1.

8.3.1.3. Oily water from other leaded product areas

a. Effluent from leaded motor spirit pumps shall pass to combined separator within bunded (diked) area.

b. Water shall be drained off to oily water drainage system and motor spirit recovered.

c. Drainage from TEL/TML blending plants shall be intercepted by holding pit.

d. Outlet to pit shall connect to valve that is normally kept closed.

8.3.1.4. Oily water from transformer bays

a. Generally, drainage shall be to oily water system via a trapped gully.

b. If transformer bays are remote from oily water sewer, discharge may be to clean sewer, although valved collecting sump should be used.

The requirement for oily water drainage can be removed in areas of high rainfall if a roof is constructed over the transformer bay and a sump is used to collect intermittent spills that can then be pumped out.



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8.3.2. Clean effluent

8.3.2.1. Clean water from storage tank areas

a. Rain and firewater from storage tank areas should be drained by clean water drainage system (An example of this is shown in Figure 9).

b. If surface of storage area is not naturally impermeable, lining material (rigid or flexible) shall be provided for both surface and features, such as ditches and sumps. Lining material should be integrated properly with under tank lining system.

c. Open channel

1. Surface of ground within tank bunds (dikes) should be graded to shallow open channel around inside of bund (dike).

2. Channel should discharge into silt chamber and via pipe through bund (dike).

3. Drain should be valved outside bund (dike) in convenient position to enable discharge to be controlled without operator having to enter bunded (diked) area.

4. Valve should be normally closed.

d. If necessary to meet local authority or statutory requirements, interceptor pits shall be provided. Pits should generally be sited outside bund (dike) but before final discharge from site.

8.3.2.2. Clean water from storage tank areas with leaded product

Requirements for clean water drainage should be same as in �8.3.2.1.

8.3.2.3. Clean water from LPG storage areas

Drainage of surface water from area around and under liquefied gas storage vessels should be discussed and agreed with BP.

LPG storage areas in the UK are designed in accordance with HSG-34.

Excessive amounts of LPG depress the water seals in the system and allow LPG to pass through the traps. This negates both the integrity of the trap and the hydraulic performance of the system.

If there is a discharge of LPG, there is a possibility that the gully seal may freeze. This phenomenon has previously been taken advantage of by designing LPG tank bund (dike) outlets such that the water seal freezes quickly to prevent loss of containment. Further advice is necessary if this is being considered as a solution.

8.3.2.4. Clean water from area land drainage

a. Rainwater falling on unpaved uncontaminated ground within process area should normally be disposed of by natural percolation into subsoil and evaporation.

b. If land is not sufficiently permeable for natural removal to be effective without undue ponding, surface should be graded to suitably located trapped gullies discharging to buried pipe system.

c. If this is not practical, land drains should be provided.

Pipe drains, if required, should be arranged to discharge into the clean water drainage system or such other system as may be specified by BP. It may be necessary to remove additional sludge and grit from this system, and so larger sumps should be provided.



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8.3.3. Drainage from buildings

8.3.3.1. Industrial buildings/workshops

a. Floor drains in pump or compressor houses and workshops shall be connected to fully trapped and vented manholes if they form part of system draining oily or chemical contaminants.

b. Hazardous gases shall not be allowed to enter building from drainage system.

8.3.3.2. Control rooms

a. Drains shall not be located within control rooms.

b. Other areas of control buildings, electric substations, and switchrooms should have appropriate type of drainage systems (usually sewage wastewater).

8.3.3.3. Laboratory drainage

a. To maintain control over waste disposal, laboratory collection points should be used.

b. Uncontaminated waste liquids should be drained to sewage wastewater system.

It is not good practice to dispose of laboratory waste via the sink system, as this can involve the need for costly glass drainage systems within the laboratory.

8.3.4. Other buildings

a. Other buildings should be connected to sewage wastewater system, if “domestic type” waste is being drained.

b. Canteens shall be connected via grease trap.

8.4. Treatment

8.4.1. Effluent treatment

Treatment of oily and chemical waste and subsequent discharge shall comply with requirements of local and national authorities, in addition to BP requirements relating to health, safety, and environment.

It is anticipated that an effluent treatment plant would feature low shear pumps, storage capacity, primary separation, filters, and biological treatment.

8.4.2. Holding basins

Discharge of system will be specified by BP.

Although some local authorities may permit direct discharge into a river or the sea, it should generally be assumed that future, more stringent requirements may require some form of holding basin and/or treatment plant to be installed. Specific provisions should be made for example to accommodate future requirements on layout and the direction of flow.

A suitable holding basin would be designed to capture and retain the first 10 mm (3/8 in) of rainfall (or equivalent firewater) from paved areas.

8.4.3. Sewage treatment

Sewage handling and treatment shall comply with requirements of local authority and BP.

Failing any such requirements, the design and construction should comply with the British Standard Codes of Practice or approved alternatives.



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The drainage should preferably be discharged to the nearest local authority sewer. If this is not economical or practical, biological treatment may be necessary. In the case of isolated buildings, there are three alternatives to the above options: cesspools, septic tanks, and prefabricated sewage treatment plants. Cesspools are the most basic and cheapest, but septic tanks are the preferred choice if there is no possibility of polluting underground potable water supplies.

8.4.4. Removal of solids

a. Process solids, such as pellets and granules, should be removed at source before entry into drainage system using screens that can be regularly cleaned.

b. Alternatively, decanting system further downstream may be appropriate if solids are coming from a number of sources.

8.5. Measurement of effluent discharge rate

Simple weir or monitoring device should be installed at or near final effluent discharge point.

9. Hydraulic design

9.1. General

a. Drainage systems should be gravity based open drainage systems.

b. On occasion, particularly for process drainage, closed drains may be appropriate because of technical, safety, or legislative requirements.

The design methods and criteria for both open and closed systems are closely related.

Gases or vapours that may be carried forward with effluent or evolved during course of treatment or due to contact with other effluent may affect flow regime of system.

c. Hydraulic design involves consideration of the following:

1. Maximum and minimum flows.

2. Sediment transport capacity.

3. Degree of surcharge or controlled flooding that can be tolerated.

4. Hydraulic capacity required that necessitates determination of pipe size, gradient and condition, nature of liquid to be carried, and vapour pressure likely to arise in system.

9.2. Gravity based drainage systems

9.2.1. General

a. Guidelines in �9.2.2 through �9.2.12 may be applied to open gravity systems in which controlled surcharging is allowed and closed gravity systems with partially full (0,7d) pipes (i.e., no surcharging) operating at or near to atmospheric pressure.

b. For equipment requirements of closed gravity systems, see �9.3.1.

9.2.2. Design methods

Colebrook-White formula may be used for design of gravity drainage systems.

For most drainage designs, hand methods of calculation are adequate for the small areas involved. If firewater (including dry weather effluent flow) is the critical design case, these constant flows shall directly provide the design flows throughout the system. If the rainfall case (including dry weather effluent flow) is to be checked,



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flows should be calculated throughout the system by the Modified Rational Method or the procedures of BS 8005 or similar document.

Design may be performed by computer if appropriate software is available. Steady state design can be produced on a spreadsheet. Rational method design can be done using proprietary software, such as Hydraulics Research Ltd's WALLRUS and SPIDA.

Some software allows direct production of sewer long sections for import into propriety CAD software. More complex computer programs are available for the design and analysis of large and complex sewer networks. Examples are WALLRUS-SIM and WALLRUS-HYD in the UK and the U.S. Stormwater management model (SWMM). Such programs should only be used for drainage if:

• Rainwater flows are critical. • The design involves major modifications to an existing drainage system that may

result in under capacity of existing sewers.

Calculation of open channel flows should be done using the Manning formula.

9.2.3. Velocities

a. Velocities shall be kept within range that prevents damage to pipes and fittings and allows self cleansing.

b. Pipe runs should be designed to accommodate maximum expected flow if running just full.

c. For some lengths of drain, flow from emergency use of fire hoses may greatly exceed normal process and rainwater flows.

9.2.4. Design velocity

a. Design velocity (from combined process and rainwater flows) should be approximately 1 m/s (3,3 ft/s).

b. Velocities for firewater or emergency flows may exceed 1 m/s (3,3 ft/s).

9.2.5. Minimum velocity

a. If possible, piped drains shall be designed to attain minimum velocity of 0,75 m/s (2,46 ft/s) )either from process flows alone or from combined process and rainwater flows (return period of 1 in 2 mo).

b. Velocity shall be attained minimum of six times per year to achieve periodic cleansing of drains.

9.2.6. Maximum velocity

a. High velocities present problems due to high friction losses and, hence, head losses.

b. If design velocities exceed 3 m/s (9,8 ft/s) at any point in system, pipe manufacturer should be consulted to ensure no erosion will take place.

c. In oily water systems, velocities should not exceed 1,2 m/s (3,93 ft/s) to avoid emulsification.

9.2.7. Open ditches

a. Ditches in fine sands or silts should be lined.

b. In unlined open ditches, velocity should be kept sufficiently low to prevent scouring.

c. Velocity shall be selected in accordance with local soil conditions and construction but may typically be in the range 0,5 to 0,8 m/s (1,6 to 2,6 ft/s).



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d. If velocity is likely to be high (e.g., greater than 0,8 m/s (2,6 ft/s)), such that scouring of the bed or sides would result, ditches in cohesive soils or coarse sands shall be suitably revetted.

e. Higher velocities shall be necessary if oily water is being drained.

If considered necessary, ditches may be bottomed in concrete to facilitate cleaning.

9.2.8. Siltation

a. If possible, introduction of solids into drainage system should be avoided.

b. Drainage system should be capable of carrying solids in system with minimum maintenance effort.

c. If minimum velocities for periodic cleansing of system cannot be obtained through process or rainwater flows, flushing facilities should be installed to provide flow of 0,75 m/s (2,46 ft/s) in each pipe run of system.

Flushing facilities should be actively considered at arid sites with occasional or unreliable rainfall or if large quantities of sediment are likely to enter the system.

Fine solid particulate matter, such as clay particles from storm water runoff on unpaved areas, should, if possible, be excluded from oily water drains. The solids adhere to oil droplets forming neutrally buoyant particles that are difficult to separate in a gravity oily water separator.

d. If lengths of sewer are designed to be permanently flooded either for hydraulic reasons or for reasons of safety (e.g., ditch firetraps), design flows should be increased by 10% as allowance for siltation.

e. Further analysis should performed if large quantities of solid material are able to enter system.

9.2.9. Surcharging and flooding

a. In open gravity systems only, surcharging of drains should be taken into account in emergency conditions (or extreme rainfall) to provide sufficient hydraulic capacity, provided that surface flooding is not increased.

b. Under maximum flow conditions described in a., hydraulic gradient within drainage system should extend no higher than 300 mm (12 in) below any point of entry into system.

c. If there is risk of flooding, each drainage catchment area should be assessed using recognised risk assessment technique.

If some flooding can be accepted, sensitive areas should be bunded (diked) or kerbed and the water directed away using the paving falls. Allowing flooding in a controlled manner in safe areas can provide additional storage for the drainage system until the peak flow conditions have eased.

9.2.10. Pipe roughness

In assessing frictional head loss of effluents flowing in drains, pipe roughness factors (ks) shall be chosen to take account of future mature condition of pipe, considering materials of construction and nature of effluents drained.

Use of conservative values increases hydraulic gradient and depth of system and consequently cost of excavation required to meet design flow.

If effluent is likely to produce large quantities of chemical precipitate, the roughness values are greatly increased. Special consideration of roughness values is necessary in such conditions. An increase in ks from 0,3 mm to 6,0 mm (0,012 in to 0,250 in)



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reflecting a 10% loss in cross section due to precipitate build-up reduces the design flow by nearly 40%.

9.2.11. Head losses at manholes and fittings

Hydraulic designs of sewers should take due account of likely head losses at manholes and fittings.

These losses are dependent upon the detailed design, and it is not therefore possible to give exact figures - most design manuals provide approximations. By choosing certain standard details, head losses can be reduced. For example, the introduction of a socket or bell mouth outlet instead of a plain straight end pipe reduces the exit head losses by between 40% and 80%. Alternative sections increase cost, though. Bell mouths are approximately 3 times more expensive than plain ends, although sockets are only 1,1 times more expensive (based on average UK costs).

9.2.12. Gradients

a. Trench excavation should be limited to 6 m (19 ft 8 in) depth because of construction difficulties and associated high cost - especially if water tables are high.

b. Physical difficulties with accurate laying of pipe at very flat gradients dictates minimum gradient.

c. Minimum gradients for pipe size ranges should be as follows:

Pipe diameter Gradient Less than DN 150 (NPS 6) 1 in 80 DN 150 (NPS 6) to DN 450 (NPS 18) 1 in 250 Greater than DN 450 (NPS 18) 1 in 500

In the UK, reference should be made to BS 8301 for foul drainage to ensure that pipes are self cleansing.

9.3. Closed drainage systems

9.3.1. Closed gravity systems

9.3.1.1. General

Backflow from high pressure to low pressure systems across common drain systems in event of mal operation should be considered during design.

9.3.1.2. Drainage lines and headers

Drainage lines should fall towards closed drain drum.

9.3.1.3. Closed drain drum

a. Sizing of closed drain drum should be based on largest item of equipment likely to be drained to closed drain drum and contents of inlet drains.

For very large vessels, there should be provision to reduce the inventory to a minimum using normal process outlets. The size of the drain drum can then be based on the lowest practical inventory of the vessel and piping. Consider supply operations and the likely overflow from storage tanks.

b. Requirement for electric heaters to maintain temperature of liquid in drum should be considered.

c. Centre section of the drum shall have extra high liquid level switch.

d. Operation of extra high liquid level switch should open emergency dump valve.



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Operation of the extra high liquid level switch shall cause an emergency dump valve to open, allowing the drum contents to discharge. It should be noted that the high level control should be designed to avoid the risk of liquid carryover to the flare system.

9.3.2. Pumped systems

a. Flows from process units, tanks, drains, etc., should be fed by short gravity sections designed by conventional methods to pumping station wet well. Figure 4 provides typical details of arrangement of pumped drainage system.

Pumping station design should comply with GP 04-30 or other suitable codes, such as BS 8005 and WAA Sewers for Adoption.

b. Wet wells and pumping mains shall not be oversized.

This is to avoid excessive retention times of the effluent.

c. Pumps should be located in dry wells or aboveground to facilitate maintenance and removal.

d. Wet wells shall be of air tight construction with adequate venting to provide hydraulic stability.

If wet well pumps are used (with lifting guide rails for maintenance), the airtight construction requirement remains and must not be compromised.

e. Pump heads and capacities

1. Pump heads and capacities should be chosen to accommodate both normal process flows and emergency fire water flows or rainwater flows.

2. Separate pumps may be needed for each of these duties.

3. No less than 33% standby capacity shall be provided at pumping stations.

f. Pump and pumping pipework design should be performed by normal hand methods of calculation.

g. Pump and pipe capacities shall be defined as peak flows in system.

h. Pumps for liquids containing oils

1. Emulsification of oil globules makes treatment and separation difficult.

2. For liquids containing oils, pumping stations shall be designed using low speed screw impeller pumps to minimise emulsification.

3. Pumping plant should generally be low speed centrifugal pumps with operating speeds below 970 rpm.

4. Consideration should be given to use of Archimedean screw pumps, particular care being taken in design to achieve adequate venting of screw.

i. Computer aided analysis should be used to determine transient pressures due to pump operation or failure.

Proprietary programs are available for this analysis.

j. In order to limit both silting and emulsion, pumping mains shall be designed for velocity of between 0,8 and 1, m/s (2,6 ft/s and 3,9 ft/s) for maximum process flows alone or from combined process and rainwater flows.

In general, high velocities present more problems due to high friction losses and, hence, head losses.



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k. In assessing frictional head loss of effluents flowing in pumping mains, pipe roughness factors should be chosen that take account of future mature condition of pipe, with strong consideration given to materials of construction and nature of the effluents drained.

l. Colebrook-White formula should be used for relating friction of pipe walls, gradient, flow, and pipe diameter for aqueous liquids. The following values shall be used for assessing friction loss in rising mains for all pipework:

Operating velocity k

Less than < 1,0 m/s (3,3 ft/s) 6,0 mm (236 mil) 1,0 - 1,2 m/s (3,3-3,9 ft/s) 3,0 mm (118 mil) 1,2 - 1,4 m/s (3,9-4,6 ft/s) 1,5 mm (59 mil) Greater than 1,4 m/s (4,6 ft/s) 0,6 mm (24 mil)

m. If effluent is likely to produce large quantities of chemical precipitate, roughness value shown in l. may be increased and hydraulic capacity substantially reduced. Individual assessment of roughness shall be performed for such systems and may require:

1. Research of parameters used in existing drainage designs for similar processes.

2. Site measurement of friction losses in similar lines.

n. If hazardous vapours are generated, venting should be provided at entry points to pumped system and to pumping station wet wells.

o. Hazardous vapours should be removed and transferred by closed piping system to treatment plant.

p. Mechanical and electrical equipment should be designed for required hazardous area classification.

9.3.3. Pressurised systems

a. Main design requirement of pressurised system is that liquids and gases entering and/or generated within system are contained and released from system through controlled outlets.

b. Pressurised system is designed in a similar manner to open gravity drainage system, except that pipes shall always run part full to maintain continuous gas phase above liquid flow. Design of system shall not, therefore, permit full pipe flow or surcharge conditions to occur.

Design of pressurised systems normally forms part of the design scope for the process design package. Figures 2, 3, and 4 provide typical details and layout arrangements for a pressurised drainage system.

c. Design criteria to be adopted for pressurised system should be similar to those detailed previously for gravity drainage systems with the following additional criteria:

1. Since maintenance of continuous gas phase is required throughout system, design flows into system should be given careful consideration (see �16.5).

2. Design shall be performed for total flows from connections or for combination of flows, if it is determined that flows can be adequately regulated.

3. To avoid surcharge and hydraulic instability, system should be designed for maximum operating depth of flow in sewer of 0,7d.

4. In order to limit siltation within sewers and maintain self cleansing, minimum flow velocities shall be 0,5 m/s (1,6 ft/s).

d. Detailed design of pipe bends and junctions is required to avoid high local head losses and consequent surcharge of system. The following measures may be adopted to avoid excessive head losses:



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Change in pipe diameter Flat backed tapers to be used with level soffits

Bends

Limit velocities to less than 1,2 m/s (3,9 ft/s). Use Pipe bends of radius > 3d. Limit bends to angles of 45 degrees or less.

Junctions

Limit losses to 0,1d. Branches to be used with level soffits. Use 45 degree junctions.

e. Detailed design and management of gas phase is critical to operation of system. Main design features for gas systems should be as follows:

1. Tapers for changes of pipe size and tees of branches should be designed with level soffits such that continuous gas phase is maintained above liquid surface.

2. Connections to drainage system should be valved in order to give controlled discharge into system. Each connection point shall have pressure reducing arrangements (valves or liquid seals) to prevent overpressurising drainage system.

3. Inert gas injection points shall be provided at connection points to maintain inert gas levels.

4. Inert gas/liquid vapour removal shall be required at pumping station/treatment works located at downstream end of system.

5. Rodding/cleaning points should have isolating valve and inert gas purging system to prevent release of vapours during use.

6. Rodding points should be located:

a) Upstream of connections to system.

b) At major changes in direction of main sewers (greater than 45 degrees).

c) At appropriate points along straight length of sewers (adjacent to branches and at maximum spacing of 50 m (163 ft)).

7. Gas pressure system should be designed to operate at low pressures, between 6,9 and 34,4 kPa (1 psi and 5 psi).

8. Effluent sampling points should be provided at each connection point.

10. Structural design of buried pipework

10.1. Backfill

10.1.1. General

a. Backfill configurations for rigid and flexible pipes should be as given in national standards and pipe manufacturer information.

b. Installation of pipes in ground should result in construction of composite soil/pipe structure.

c. To ensure integrity of pipe system, pipe material, its strength class or wall thickness, and bedding shall be designed with due regard for surface loads that may be applied, type and quality of soil and backfill, and quality of workmanship that may reasonably be expected at that particular site.

d. If drains are in short runs of small diameter (e.g., within process areas), bedding and backfill should be of nominal design with respect to pipe strength, except if vehicular or other exceptional top loads may be applied.



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e. For vehicular and other exceptional top loads and for long runs of large diameter pipework (e.g., in tankage and other extensive areas), pipe/soil structure should be designed to make maximum use of soils’ properties.

f. In all cases, if backfill is required to provide support to surface construction, e.g., paving or pipe supports, backfill should be of similar material and compaction to surrounding soil.

10.1.2. Additional requirements for flexible pipes

a. Maximum deflection should be limited to 50% of maximum value stated by manufacturer.

b. Imported bedding and backfill

1. If flexible pipes are laid underground in poor soil conditions, it may be necessary to use imported bedding and backfill to achieve desired soil strength for type and thickness of pipe being used.

2. In such cases, ability of natural soil of trench sides to support compacted embedment should be checked and trench and/or backfill and/or pipe designs modified as necessary.

10.2. Road and rail crossings

a. If pipe runs laid in open cut trenches cross under roads and railways, pipes and their bedding should be designed to support maximum expected applied load with adequate factor of safety, e.g., greater than 1,5.

b. If it is not possible to provide pipe and/or backfill of sufficient strength, whole pipe should be installed in adequate load bearing sleeve or protected by reinforced concrete slab.

c. Concrete protection

1. For rigid pipes of 300 mm (12 in) diameter or less, structural concrete haunching, surround, or arched capping is an acceptable alternative to sleeving.

2. If such concrete protection is used, movement joints consisting of minimum of 25 mm (1 in) of compressible packing should be provided in concrete at each flexible joint of pipe.

10.3. Loads during testing

a. Pressure pipelines, such as pumped effluent lines and deep inverted siphons, shall be tested to pressure equal to either 1,5 times working pressure or sum of working pressure and surge pressure, whichever is greater (in accordance with BS 8010).

b. Pipe structural design should therefore allow for hydrostatic pressure in combination with various potential external loads.

10.4. Thermal expansion

a. Pipe should have sufficient movement joints such that pipe may freely expand and contract.

b. If requirement in a. is not possible, manholes shall be designed to resist thrusts applied to walls by such thermal expansion or contraction (see �12.1.8).

10.5. Submerged pipes

a. If it is intended that sections of pipes remain full to act as traps, method of jointing should be examined to ensure integrity.

b. Otherwise, pipes should be at such levels and gradients that liquids are not retained in any part of system, except manholes.



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10.6. Settlement

Likely future ground settlements, particularly if constructing on reclaimed or filled land, should be strongly taken into account.

11. Secondary containment

11.1. General

a. Secondary containment of drainage system may be necessary to prevent leakage and/or damage to piping materials and to meet legislative requirements.

b. All components of system shall be contained to same standard.

11.2. Exfiltration

a. Effect on surrounding ground of leakage of effluents (both aggressive and non aggressive) from drain should be assessed and include chemical reactions, dissolution, or flocculation.

b. Contamination of groundwater and/or soil and damage to foundations shall be avoided.

c. Secondary containment may take several forms:

1. Advanced: e.g., double pipes and concrete trenches.

a) With these systems, provision can be made for leak detection.

b) Containment system can drain to collection sump(s) and be vented to prevent accumulation of hazardous vapours.

2. Basic: e.g., membrane pipe “jackets” and membrane trench lining.

Leak detection is more difficult, but effluents are contained.

11.3. Infiltration

a. Installing drainage systems in contaminated ground may adversely affect pipe materials and lead to accelerated corrosion and loss of integrity.

b. Composite pipe materials should be used to protect outside of pipe, e.g., plastic coated ductile iron pipe.

c. Material chosen shall depend on compatibility with aggressive chemical in ground and effluent being carried.

12. Ancillary structures

12.1. Manholes

12.1.1. General

a. Manholes in open drainage systems shall be located where there is pipe diameter, gradient or direction change, and at major junctions.

b. For sewers of less than 0,9 m (3 ft) diameter, manholes should not be more than 100 m (327 ft) apart.

c. For larger sewers, spacing may be up to 100 times pipe diameter up to maximum of 200 m (654 ft).

12.1.2. Trapping

a. Influent drains in manholes, other than manholes on sanitary drains and certain chemical drains (see exceptions in �b.), shall be effectively trapped.



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An example which meets these requirements is shown in Figure 8. If it is necessary to depart from this drawing, the ld provide for adequate seal of 230 mm (9,1 in) and facilities above the water level in the manhole for rodding, while at the same time permitting any separated oil to travel in the normal direction of flow and not be held up in the system.

b. There are two exceptions to �a. for which trapping may be omitted:

1. If manhole is in a safe area and is only necessary for change in pipe direction or gradient. No additional influent pipes are connected. If any are added at a future date, manhole shall be modified to include trap and vent. This modification may alter hydraulic flow and should be considered at initial design stage.

2. For certain chemical effluents carrying solids in suspension, it may not be desirable to trap drain lines at manholes. Such drains should be identified in applicable plant specification and should preferably have catchpit type manholes.

12.1.3. Combined manhole-gully

a. Specific guidelines for combined manhole-gullies are as follows:

1. Sealed area of gully shall be not less than open grated area of gully. This reduces seal depth required to prevent vapour escape and effects of evaporation in hot climates.

2. Grate over open areas of gully shall be arranged for easy removal.

3. Cover over sealed part of gully shall be secured to prevent removal during normal operations.

It is important to ensure that the size and design of the gully grating does not restrict the maximum capacity of the outlet pipe.

b. Details of typical combined manhole gully that complies with this GP are given in Figure 5.

12.1.4. Design to control leakage

a. For oily water and chemical sewer systems, manholes should be designed as water retaining structures.

b. Design shall take into account both external water pressure from ground water and internal water pressure from effluents in manhole.

c. Walls should be not less than 225 mm (9 in) thick.

d. Pipes through walls shall be sealed.

e. If possible, puddle flanges should be used for sealing.

As an alternative to puddle flanges, the use of self sealing sodium bentonite seals around the pipe can be considered. These expand upon contact with water.

Construction joints should also be sealed at the kickers and vertical joints with EDPM nitrile chemical resistant water bars or sodium bentonite seals.

Guidelines for the design of water retaining structures are provided in BS 8007, ACI 372R-03 and ACI 373R-97.

12.1.5. Internal protection

a. Chemical manholes should generally be protected internally by either:

1. Suitable continuous chemical resistant membrane

a) Suitable continuous chemical resistant membrane should be applied to walls and base slab.



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b) Walls and base slab should be lined with 105 mm (4 1/8 in) thick chemical resistant bricks, bedded and jointed in suitable chemical resistant mortar.

c) Joints should not exceed 3 mm (1/8 in) in thickness.

2. Sheet plastic linings

a) Sheet plastic linings to manholes, pits, etc., should be adequately anchored to base, walls, and roof of structure.

b) Such anchoring should take account of potential unbalanced hydrostatic pressure between lining and structure.

c) Joints to such linings should be tested by high voltage spark testing. If this is not possible, joints should be multipass welded type.

b. Suitable protective tanking for effluents being drained should be applied to internal surfaces of manhole in accordance with manufacturer instructions.

c. Underside of roof slab should be protected with suitable chemical resistant membrane.

Good surface preparation for linings is essential. For resins, this should consist of mechanical abrasion (or as a minimum nail or chemical etching), particularly in the splash zone.

12.1.6. External protection

a. Detailing and mix design for concrete structures shall include allowance for soil or ground water that contains chemicals which may have deleterious effect on concrete.

b. In severe conditions, structures should be tanked externally.

External tanking can comprise sheet liners, resins, or proprietary sodium bentonite sheets in geotextile/board form (except in saline conditions if the expansion properties of the clay are reduced).

12.1.7. Other types of manhole

12.1.7.1. General

For clean water and sanitary sewer systems, manholes should be precast concrete construction (shallow manholes - brickwork), provided that long term settlement or other movement of surrounding ground is not expected.

12.1.7.2. Catchpit manholes

a. Catchpits should have flat base slab, finished level of base slab being at least 150 mm (6 in) below invert level of outgoing drainpipe.

b. Retention time in manhole should be at least 1 min.

If there is a particular problem with solids, it may be better to provide a specific chamber for removal of the material rather than relying on maintenance procedures.

12.1.8. Design for movement

a. Flexible joints shall be provided at connections to structures, including thrust blocks, valve chambers, manholes, and rodding eyes.

b. Joints shall be as follows:

1. First joint less than or equal to one pipe diameter or 550 mm (21 in) from structure, whichever is greater.



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2. Second joint not less than two pipe diameters or 450 mm (18 in) from first joint, whichever is greater for pipe diameters up to 750 mm (30 in) or from 450 mm (18 in) to 750 mm (30 in) for pipe diameters up to 1 m (3,3 ft) (see �10.4).

12.1.9. Manhole roof slabs

Consideration should be given to constructing the roof of the manhole as a slab, with cast in lifting lugs. It can be removed for construction and major repairs, enabling work to be performed more safely and efficiently.�[1]

12.1.10. Manhole covers

a. Sealed manhole covers shall be used:

1. Within or adjacent to process area limits and other hazardous areas.

2. In offsite areas on manholes that are trapped or located near roadways.

b. A cover seal which complies with a. is shown in Figure 6. Sealant shall be selected with due regard to solvents with which it may come into contact.

If subject to high vehicular traffic flows or heavy mobile equipment, gas sealed manhole covers can be rebated into the manhole and covered by a normal heavy duty or structural cover. This reduces the risk of sealant being squeezed out and the consequent loss in integrity.

c. For clean water drains or land drains, minimum sizes should provide clear opening of 550 mm (22 in) diameter or 600 mm (24 in) by 450 mm (18 in).

Manhole covers should be located over the outlet side of the manhole to allow the insertion of jetting equipment, CCTV survey equipment, and flow monitors. This aids maintenance procedures.

d. For oily water, chemical, or acid drains, manhole covers should provide minimum clear opening of 750 mm (30 in) x 600 mm (24 in), as it may be expected that access is required for personnel wearing breathing apparatus.

12.1.11. Chemical manhole covers

a. If effluent is liable to give off poisonous or flammable vapours, chemical manhole covers should be double seal type protected with suitable chemical resistant paint on underside.

b. Covers should bear clear warning of hazardous nature of manhole contents.

Consideration can be given to colour coding manhole covers (by painting) to indicate the type of effluent being drained. This aids identification of the correct system in the event of having to pump effluent directly into a manhole in an emergency or during inspections and repair.

12.1.12. Rodding points

Rodding points should not require access to manholes.

12.1.13. Sumps

Sump should be incorporated into base of manhole if volumes of sludge and grit are large enough to necessitate regular cleaning of manholes.

12.1.14. Access

a. Access to manholes should be by portable wooden ladders.

b. Step irons shall also be provided in manholes (except chemical manholes) to allow escape in emergencies.



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c. Step irons should be of galvanised steel and set into wall of manhole at 300 mm (12 in) centres, vertically and staggered.

The use of step irons for regular access should be discouraged, as the integrity of the steps cannot be guaranteed, especially if contaminated effluents are being drained.

12.1.15. Design for maintenance

a. Design should make allowance within manholes for additional future connections.

b. Access space in manhole for suction cleaning should be considered at design stage.

c. Corners within manhole should not be less than 300 mm (12 in) square.

12.1.16. Manhole identification system

New drainage systems shall have manhole identification system.

Manholes should have a code that identifies the type of effluent, a reference to the area it services, and an identification number. Code should be cast into the top of the manhole lid or be a corrosion proof plate. A further refinement of this that could save much investigation work at a later date is the simple addition of arrows showing connections, flow direction, and diameters.

Identification can also be applied retroactively and may be used in conjunction with colour coded covers (see �12.1.10).

12.2. Gully traps

12.2.1. General

a. Gullies shall be capable of draining maximum water volume without occurrence of extensive flooding of paved areas.

b. Within hazardous process area limits, other hazardous areas, and such nonhazardous areas as may be connected into oily water drainage system, drainage gullies shall be trapped with depth of seal not less than 150 mm (6 in) and should have adequate provision for rodding.

c. In other nonhazardous areas, such as administration areas, gullies may be concrete or stoneware, which should be concrete encased.

The use of plastic gullies is not recommended unless it can be shown that the ground does not (or will not) contain deleterious contaminants, such as hydrocarbons, that would damage the gully. Plastic gullies should not be used in any process area or areas where oily or chemical effluents are drained.

12.2.2. Gully design

Gullies should be designed in accordance with the following:

a. Depth from top of gully grating to top of gully outlet pipe shall be greater than 230 mm (9 in) to allow full flow to pass through outlet pipe and prevent escape of vapours.

b. Hydraulic capacity of each gully shall be defined as that flow which passes through gully grating without surcharge.

c. Pipe downstream of gully shall act as major hydraulic control of flow.

d. If appropriate, standard 150 mm (6 in) gully trap shown on Figure 7 should be used.

Use of the BP “standard 100 mm (4 in) gully” should be discontinued (see �8.2.3.3).



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12.3. Open ditches and channels

12.3.1. Genera

a. If velocity is likely to be high (e.g., greater than 0,8 m/s (2,6 ft/s)) (see �9.2.7), such that scouring of bed or sides would result, ditches in cohesive soils or coarse sands shall be suitably revetted.

b. Ditches in fine sands, silts, or material susceptible to scour should be lined.

c. If regular cleaning is considered necessary, ditches should be bottomed in concrete.

12.3.2. Firetraps

a. Trapping facilities should be provided to prevent fire spreading from one area to another via ditches.

b. Such firetraps should consist of minimum 9 m (29,5 ft) length of permanently flooded pipe with access sumps at each end.

Access to the sumps should be provided for tanker suction hoses.

c. Location and spacing of firetraps should reflect different usage of areas through which drain runs, e.g., process areas, black oils tankage, white oils tankage, and LPG storage areas.

d. Spacing of firetraps should not be greater than 200 m (654 ft).

12.3.3. Open drainage channels in process areas

a. Open drainage channels in process areas should be of reinforced concrete construction.

b. Base slab should be laid to falls of not less than 1 in 60 and, if possible, of 1 in 40.

c. Recessed channel grating covers should be provided to suit conditions specified in particular plant specification.

d. Mechanical strength of gratings shall be as described in �12.3.5.

12.3.4. Open drainage channels for chemical effluents

Open drainage channels for carrying corrosive effluent should be constructed in accordance with �12.3.3 and should be lined throughout internally as for chemical manholes (see �12.1.5).

12.3.5. Channel grating covers

Channel grating covers shall be of sufficient mechanical strength to support loads defined in GP 04-30.

12.4. Effluent collection and treatment (neutralisation) pits

a. Requirements for treatment pits should be as for manholes.

b. Acidic effluent should be preferably be neutralised at or near source.

c. Retained batch operation is preferred for neutralisation systems.

d. Neutralised effluent should be discharged to appropriate drainage system as specified by BP.

e. Effluent collection and treatment pits shall be designed and constructed to same criteria as chemical manholes (see �12.1.5).

Special consideration shall be given to thermal movement and the general stability of linings.

f. Provision should be made to allow removal of precipitates left by neutralisation process.



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12.5. Pumping sumps

Pumping sumps required in oily water drainage systems should be designed such that separated oil can be readily collected and pumped away to recovered oil system or other suitable place of disposal.

12.6. Soakaways and land drains

Soakaways and land drains should be provided for clean water use only.

12.7. Cesspools and septic tanks

Cesspools and septic tanks should, unless agreed otherwise with designer, be prefabricated, glass reinforced plastic construction installed strictly in accordance with manufacturer instructions.

13. Control of fugitive gas emissions and venting of drainage systems

13.1. Control of fugitive gas emissions

13.1.1. General

Fugitive emissions of hydrocarbon gases from conventional gravity drainage systems should be reduced by changing work practices and methods of operation.

New drain systems may be installed, which can almost eradicate fugitive emissions. These new drains are, however, expensive to install and more complex to operate. Major cost savings can be derived by avoiding discharge of oily materials into the drainage system.

13.1.2. Location of gaseous emissions from drains

13.1.2.1. General

Refinery oil, water/gravity drain systems produce fugitive gas emissions from three main sources, and action should be taken to minimize emissions at each location:

a. At point of entry to drainage system.

b. At vents and manholes along drainage system.

c. At point of exit, which could be treatment plant, pond, or final discharge during maintenance.

13.1.2.2. Emission at point of entry

a. Surface area of hydrocarbon exposed to atmosphere at entry point should be kept to minimum.

b. Freefall of effluent and oversize tundishes and catch basins should be avoided.

It is normal for entry points to be open to the atmosphere, as are tundishes and catch basins. It is also common for entry points to be trapped. This is a safety feature in reducing risk of spread of fire/explosion, and it also minimises gas escape from the piped system.

Emission at points of entry may be avoided by providing a continuous connection into the drainage system. This, however, is inappropriate in many cases due to the need for pressure interface control between any plant and drainage systems.

13.1.2.3. Emission at vents

a. To minimise emissions from vents, flow conditions should be kept as steady as possible.



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b. Large sudden discharges of process water or ballast water should be avoided.

The majority of emissions from vents in gravity drainage systems are caused by changes in the volume of liquid in the piped system. In a steady state flow condition, very little gas leaves the system through the vents. Large increases in volumes of effluent contained in the piped drainage system will cause correspondingly large volumes of gas to be emitted. Rainstorm and emergency fire water are the single biggest contributions to large changes in effluent volumes. Rain and fire water diverted to a segregated “clean water system” would significantly reduce gaseous emission from the source.

13.1.2.4. Emission at point of exit

a. Effluent shall discharge into some form of pond, chamber, sump, or tank with increased surface area.

b. Such open ponds or chambers should have floating cover (zero air space).

c. Danger of hydrocarbon rich voids being trapped below cover should be considered.

Floating covers are restrictive if any form of mechanical treatment is required; the container then is covered, forming void spaces beneath the cover. The void is likely to contain high percentages of hydrocarbon gas that should be contained and/or treated.

13.1.2.5. Emission during inspection and maintenance

Cleaning work, if materials, such as sludges, have to be removed and disposed of (see �16.2), should be conducted in such a way as to minimise emissions to air.

Access to the drainage system is affected by lifting the manhole covers and exposing the gaseous void inside the manhole. It is no longer necessary to purge the air spaces in the drainage system before inspection. Explosion proof cameras and light systems are now available for remote inspection of the drains (see �16.3).

13.2. Design of vents for open gravity drainage systems

13.2.1. General (open gravity systems)

a. If necessary, manholes (and treatment pits) shall be fitted with 100 mm (4 in) minimum diameter vent pipes.

b. Vent pipes shall be required both for:

1. Hydraulic reasons.

2. If gaseous effluents (especially toxic or explosive) may be discharged into system or if gas may otherwise be released from effluent (for example by contact with hotter effluent stream).

c. Vent pipes shall be arranged with fall to drain condensation back to manhole.

d. Materials susceptible to corrosion shall be suitably protected.

13.2.2. Vent pipes discharging to atmosphere (open gravity systems)

a. If permissible to discharge to atmosphere, vent pipes shall discharge in open air, clear of areas accessible to personnel and at least 15 m (49 ft) away from permanent source of ignition.

b. Particular attention shall be given to ensure that vapours cannot be admitted to building via air intakes or windows. Vents shall be minimum of 5 m (16,5 ft) from such openings.



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c. Flame protection device

1. If venting gases may be ignited by intermittent ignition source (for example, lightning strike in exposed location), flame protection device shall be fitted.

2. Device should be type requiring minimum of maintenance consistent with being suitable for gases present in sewer.

3. Device shall be mounted such that it does not trap condensation and should be readily accessible for maintenance.

d. Vent pipes should be distinctively marked to warn personnel of hazardous nature of vapours that may be emitted.

13.3. Extraction and treatment of vented gases

a. If it is not permissible to discharge directly to atmosphere, drainage systems shall incorporate extraction of gases.

b. Treatment of these vented gases should be performed by proprietary treatment methods.

c. Treatment of vented gases shall require detailed treatability and design studies for types of gases extracted.

d. Methods that are commonly used for treatment are:

1. Hydrocarbon vapours:

a) Capture and direct to flare stack.

b) Capture and direct to activated carbon plant.

2. Water miscible vapours:

a) Capture and direct to water scrubbers.

b) Capture and return to effluent flow.

Due to increasing legislation, flaring off hydrocarbon vapour may not be acceptable.

In assessing the feasibility of activated carbon plant, account should be taken of disposal/replacement costs of spent carbon and use of carbon regeneration equipment.

14. Materials

14.1. General

a. Choice of material shall depend on type of effluents being drained.

b. Particular attention shall be paid to effects on materials of potential changes in physical, chemical, and biological properties.

c. Aggressive conditions in soil and potential movements of soil or of drainage structures shall be taken into account.

d. Degree of integrity of drainage system should be established.

e. Method of proving continuing integrity should be identified.

Pressure testing of pipework is the most direct method of establishing integrity, but this has cost implications regarding material choice.

f. Pipe material used under process areas should be such that potential for damage is minimised.



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If future access to pipes is not possible, the use of higher specification materials should be investigated (this is to include the types of joint system for each material).

A preinstallation survey is recommended such that methods of corrosion protection, the suitability of the natural soils for embedment, and design of structures, including anchorages, are given full consideration.

g. Materials for drainage pipework

1. Materials for drainage pipework should be chosen from those shown in Table 2.

2. Other materials may be used only after review by BP during design.

3. For chemical effluents, suitability of each material proposed should be confirmed in writing with manufacturer and shall be subject to BP review.

The specifications given in the material tables are predominantly of UK origin. If working elsewhere, local standards may be used providing the contractor can demonstrate they deliver performance equivalent to international best practice.

h. Installation of chemical resistant materials should be performed by specialists having experience with materials specified.

14.2. Resistance to effluents

Material and its jointing should be completely impervious and resistant to degradation by effluents being carried and contamination present in ground.

14.3. Strength

a. Material should have sufficient mechanical strength to support loads that it is required to carry in conjunction with designed backfill (see �10.1).

b. Material, if subject to long term loading, such as buried pipework, should not be subject to creep.

c. Material should, if possible, possess some flexible properties, either inherently or by virtue of its jointing system.

14.4. Joints

a. Joint integrity should be maintained for design life of drainage system or for as long as directed by BP.

b. Unless specified otherwise, underground applications shall have flexibly jointed pipework.

Joint rings for such pipework should comply with BS 2494 or equivalent.

c. Jointing materials should have resistance to specified chemical and physical conditions at least equal to that of materials being joined.

d. For oily water sewers, oil drains, and other drain systems to be buried in potentially hydrocarbon contaminated ground, joint rings and gaskets shall be acrylonitrile butadiene rubber (NBR). External corrosion protection coatings to pipes and fittings shall be hydrocarbon resistant.

14.5. Other

Membranes for secondary linings to manholes, effluent collection and treatment pits, and open drainage channels should be resistant to chemical conditions prevailing in drain system and ground in which system is being laid (see clauses �12.1.5 and �12.1.6).



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15. Construction and workmanship

15.1. General

Specifications for closed drainage systems that are regarded as process plant pipework should comply with GP 42-10.

If reference is made to spigot and socket pipes, the same guidelines generally apply to other types of pipes.

15.2. Construction

Unless agreed otherwise by designer, drainage works should be performed in accordance with specification based on this GP and local standards.

15.3. Connections to existing sewers

a. Connections to existing sewers should be made either by way of pipe saddle, new manhole, or new branch into existing manhole.

b. Before entering, breaking into, or connecting to existing sewer, drain or manhole, Vendor shall give notice of his intention to do so to authority responsible for pipe to which connection is to be made.

c. Proposed method of connection should be agreed by BP before commencing any work.

d. Construction should be performed under supervision of BP.

e. Connections using vitrified clay or concrete pipe on clean surface water sewers may use pipe saddle connection if:

1. Incoming sewer invert is equal to or higher than soffit level of existing sewer.

2. Incoming sewer pipe is of smaller diameter than existing pipe.

f. New manhole connection

1. New manhole connection should be made if existing sewer pipe has been bedded in concrete or if sewer flow cannot be stopped off during connection.

2. Connection may be used for any type of pipe.

3. Sanitary effluent sewers should be connected only by this method.

g. New branch connection into existing manhole should be used for sewer conditions other than those in �f.

15.4. Testing

a. Sewers and drains with water tight joints, regardless of pipe size, and manholes should be tested.

b. Test pressure should be appropriate to class of pipe, material used, and working pressure and comply with national standards.

In the UK, BS 8005 is the standard for testing and should be used for all testing, except oily and contaminated drainage systems.

c. Oily and contaminated drainage systems should show no detectable loss if tested.

BS 8005 is too lax for testing oily and contaminated drainage systems, in that it allows a certain amount of leakage to take place (1 l/hr/m) (0,86 gal/hr/ft).



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15.5. Back filling

Back filling should be performed as soon as possible after drain run has been satisfactorily tested.

15.6. Cleaning

a. Interiors of pipes, manholes, gullies, and pits should be left clean and free from rubbish and surplus material.

b. On completion, manholes, drains, and sewers, other than French drains, should be flushed from end to end with water and left clean and free from obstructions.

Consideration should be given to camera inspection prior to acceptance of the drainage system.

16. Operation and maintenance

16.1. General

Regular inspection and maintenance is essential to operation of drainage systems.

a. To preserve or improve hydraulic performance, debris shall be removed, leakage shall be prevented, and damage shall be repaired.

b. Each section of drainage system should be categorised according to its importance in system, which will be reflected in time between inspections. Importance depends on nature of plant or area being drained and where in system pipe is located.

c. To facilitate inspection and maintenance of drainage system, system maintenance layout should be produced, either as one of Vendor deliverables or as part of overall site drainage maintenance plan.

The plan would show main drain lines and manholes, with identification numbers (see �12.1.16). Also see �12.1.10 - coloured covers.

16.2. Cleaning

a. Drains shall be cleaned periodically to remove silts, sludges, and precipitates that impair performance of system.

The most common method uses pressurised water jets. If this action is performed, the seals on the system are temporarily broken. This increases the risk of any vapours spreading through the system and ignition occurring at a remote location.

b. If cleaning closed systems that are not permitted by legislation to release gas to atmosphere, system shall be purged prior to opening jetting points.

Sludge that collects in the drainage system and waste treatment plant is a contaminated material, impairs hydraulic flow, and can be expensive to dispose of.

16.3. Inspection

16.3.1. General

a. This clause generally covers open systems with conventional construction. Additional specific guidelines for alternative closed systems are provided in �16.5.1 and �16.5.2.

b. Inspection should be manual or remote, depending on three main factors: size of pipes, type of effluent carried, and whether system is open or closed. Principles of inspection are the same whichever method is used.



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In the UK, the WAA Manual of Sewer Condition Classification and Model Contract documents for manhole location surveys and non man entry sewer inspection provide guidance on the procedures involved in an inspection.

c. In addition to periodic surveys of drainage system, routine inspections should be made of the following features:

1. Manhole cover seals and sealant (especially after covers have been moved).

2. Vent pipes (for blockages or flame protection device damage).

3. Water trap seals (in manholes, tundishes, and gullies).

Routine inspections are essential to continual safe operation of the system.

16.3.2. Pressure testing

Pressure testing can prove the integrity of a system but not the condition. Pressure testing of concrete and clay pipes is not normally possible because of natural leakage.

16.3.3. Remote imaging systems

Other systems shall require some form of remote inspection.

Generally, closed circuit television cameras (CCTV) or SONAR are most appropriate for most drainage systems. The use of computer recognition systems enables the analysis of video tape records for pipe defects to be made more efficiently. The equipment used shall meet safety regulations for hazardous areas. It may need to be explosion proof, otherwise over pumping and purging may be necessary. This removes one advantage of remote inspection if access can be made without interrupting the flow. Components of an explosion proof CCTV system would require BASEEFA or similar approval.

16.3.4. Man access

a. Man access can be achieved in open systems if pipes are greater than 600 mm (2 ft) in diameter, but working is difficult in sizes below 900 mm (3 ft).

b. Safety rules determine whether entry can be permitted without draining or over pumping system.

c. Work shall comply with safety regulations for working in confined spaces.

Because of the hazardous nature of the drainage systems, man access into the drainage system should be performed only if no other method is available.

16.4. Rehabilitation

16.4.1. General

There are three options for rehabilitation: replacement, renewal, and renovation.

The choice of technique depends on the individual conditions of the site, the cost and nuisance involved with disruption, and the delay compared to the costs of each method.

16.4.2. Replacement

Replacement shall consist of complete reconstruction of system to take account of new flows and future development in addition to existing flow.



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16.4.3. Renewal

a. Reconstruction is to a design that caters only for original design flows.

b. Route of new system can be different to existing.

c. Alternative construction method can be used.

16.4.4. Renovation

Renovation is used to extend design life of drainage system, with same design flows continuing.

In operational plants, it is advantageous if repairs can be done from within the pipe, without the disruption of excavation. Such trenchless technology is becoming increasingly available. Repairs can be either structural or non structural. A variety of methods that can be used are available.

Structural repairs include inserting rigid pipe sections inside the existing pipe and/or grouting around the pipe perimeter with cementitious or resin based grouts.

Non structural repairs include patching with composite resin based materials or mortars, relining with flexible membranes cast in situ, and injecting resins into cracks. Robotic methods have been developed that can perform a range of patching and injection repairs without disrupting flows. Care needs to be taken with the joint between repair materials and the existing pipe section. This has been the location for failure in the past. Manufacturers should be consulted to ensure that repair materials and techniques are compatible with the effluents being carried.

16.5. Operational procedures (closed system only)

�16.5.1 and �16.5.2 describe the additional operation and maintenance procedures for alternative closed drainage systems.

16.5.1. Pressurised systems

Operation of the system

Operation of the system requires maintenance of the invert/vapour gas phase above the liquid flow throughout the system.

Prior to releasing effluent into the system, the system is purged with the inert gas. Following completion of this operation, the system is filled with inert gas to the system pressure.

Effluent can then be drained into the system at the controlled rates determined by the design, thus maintaining the continuous gas phase above the liquid level.

During operation, the gas pressure gradually increases as volatile gases are generated from the effluent flow. The design allows the gas pressure to increase until the upper system pressure is reached, at this point a gas relief mechanism located at the downstream end of the system opens and the mix of inert gas and vapour is extracted. The extracted gas is removed by a closed piping system to a treatment plant (see �13.3).

Extraction of gases continues until a low set pressure is reached, at which point further inert gas is injected into the system to return the pressure and to maintain the purging effect within the system.

Operation of the system is more difficult than a normal gravity system, since flows into the pressurised system have to be carefully monitored to maintain the gas phase above the liquid level.

Maintenance of the system



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Maintenance of a pressurised system is more complex than an open gravity system as access to the system is restricted. Ensuring correct operation of the effluent flows into the system and thus retaining the gas phase reduces the maintenance requirements.

Regular maintenance of the mechanical and electrical equipment associated with the gas injection and removal system is required for satisfactory operation.

Repair or removal of pipework within the system is more difficult than a conventional open gravity system since the pipelines require isolation and purging to remove any hazardous gases.

If installed below ground, secondary containment makes maintenance easier to perform, particularly with respect to location and isolation of leaks to the system.

16.5.2. Pumped systems

Operation and maintenance of the system

Operation of the system is controlled by the liquid levels in the pumping station wet well.

Effluent is pumped, either to a high level gravity pipeline, an underground pipeline, or direct to a remote treatment facility.

Repair or removal of pipework within an aboveground pumped system is easier than a gravity system since the pipes are laid above ground level and are generally of flanged joints.

If hazardous liquids or gases exist, maintenance within the wet wells is difficult. Since the sumps are not free draining, a separate drain down facility needs to be provided with a system of purging the air space.



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Table 1 - Advantages and disadvantages of alternative drainage systems

Drainage system Advantages Disadvantages

Open Gravity Open system - Most basic form of drainage. No environmental constraints on vapour discharge.

- Design, construction, materials, and operation/maintenance are conventional and well known/proven.

- Generally fail safe. - Design caters for variable flows. - Can operate under surcharge. - Vapours easily vented. - Relatively low operation and maintenance

cost. - Fire hazard can be controlled.

- Extensive systems expensive to construct due to depth.

- May require containment. - Requires oil interceptors and fire traps. - Requires regular venting (open) or gas

removal (at collection drum/pit for closed). - If laid in ground, possible exfiltration.

Closed Gravity Closed system - No emissions allowed to atmosphere.


- Generally fail safe. - Design caters for variable flows. - Relatively low operation cost. - Fire hazard can be controlled.

- Extensive systems expensive to construct due to depth.

- May require containment. - Requires regular gas removal (at collection

drum/pit for closed). - If laid in ground, possible exfiltration. - Higher maintenance cost than open gravity. - Difficult to inspect.

Pumped Closed system – No emissions allowed to atmosphere.


- Design caters for variable flows. - Pipework can be laid above ground or in

shallow excavation. - Design caters for segregation of effluents. - Fire hazard can be controlled. - Gaseous emissions to atmosphere are

minimal. - Higher integrity pipework. - Easily pressure tested to prove integrity.

- Dependent upon m/e plant. - Requires standby system for confidence of

operation. - Requires dedicated reception point. - Expensive to cater for large variable flows. - High operation and maintenance costs. - Possible septicity/chemical attack in

pumping mains. - Materials of construction for plant may be

expensive.

- Design, construction, and materials for piping system well known/proven.

- Generally fail safe for effluent flow. - Fire hazard can be controlled. - Possible to convert from existing gravity

system. - Pipework can be laid aboveground. - Gaseous emissions to atmosphere are

minimal. - Higher integrity pipework. - Easily pressure tested to prove integrity.

- Overall design, construction, and operation/maintenance not well known/proven.

- Air tight system required. - Design caters for fixed flows. - Design of gas system expensive and

specialist. - Extensive systems very expensive to

construct (if belowground). - If laid in ground, possible exfiltration. - High operation and maintenance cost. - Not suited to surface water runoff or

firewater.



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Table 2 – Material Selection

Service for which material is suitable/unsuitable

System type Jointing type Size

Range

Material

Example of typical

acceptable standard

Wat

er

Oily

Wat

er

Oil

Sol

vent

Cau

stic

Aci

d

Che

mic

al

Foul

Sew

age

Gra

vity

Pre

ssur

e*

Flan

ged

Wel

ded

Spi

got/

Soc

ket

mm (in) Note

Carbon steel and cement lined carbon steel

API 5L BS 534 BS 1600 BS 3602 BS 3604 BS 3605 BS 4515

Y1 Y Y Y Y3 Y1 Y1 Y Y Y Y2 Y2 BS 534

(butt weld) BS 2971

Y 60-220 (2 ½-8 ½)

1, 2, 3, a, b, c, d, e

Ductile iron BS 4772 ISO 2531

Y Y Y N N N N Y Y Y Y (BS 4772) (Table 45)

N Y 80-1 600 (3-64)

f g, g, h, i

Vitrified clay BS 65 Y N N Y Y Y Y Y Y N N N Y (BS 65)

100-1 000 (4-40)

j, k, l, m, o, p, q

Reinforced and prestressed concrete

BS 5911 (reinforced BS 5178 (prestressed)

Y Y N N N N N Y Y N N N Y4 150-2 400 (6-96)

4 r, s

UPVC BS 3505 BS 3506 BS 5481 BS 4660

Y Y N N N N Y5 Y Y Y N Y6 (BS 6464)

Y (BS 5481)

160-400

(6 ½-16)

5, 6, t, u, v

ABS BS 5391 BS 5392

Y Y N N N N Y Y Y Y N Y Y 12-225 (1/2-9)

Polyethylene BS 5556 BS 6437 BS 6572 BS 6730 BS 7336

Y Y N N Y Y Y Y Y Y N (BS 6464)

Y Y

Polypropylene BS 4991 Y Y N N Y Y Y Y Y Y N Y Y w

GRE and GRP (Glass reinforced epoxy and plastic)

BS 5480 BS 6464 API 15LR

Y Y Y8 Y8 Y Y Y Y Y7 N Y (BS 6464)

N Y 50-2 500

(2-100)

7, 8, x, z, aa

Austenitic stainless steel type 304

BS 3605 Y Y Y Y Y Y Y Y Y N Y Y (BS 4870) (BS 4871)

Y 60-1 000

bb cc

Glass N N N Y Y Y N Y n/a n/a n/a n/a n/a dd, bb

Notes: * Pressure refers to both pumped and pressurised systems. Y Suitable/applicable N Unsuitable/not applicable 1. Special precaution to be taken in use, e.g. internal linings. 2. Good for above ground and pressure mains. 3. Stainless steel should be used at high temperatures. 4. Flexible joints incorporating an O-ring gasket of a high quality rubber material. 5. Inorganic chemical wastes (not nitric acid).



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6. Chemicals can affect solvent welded joints. 7. Low pressure applications only. 8. Check long-term stability of material and consider likelihood of fire damage in pipe. a. Materials for jointing shall be checked in relation to the service. b. Provision for thermal expansion and differential movement required. c. Threaded and coupled joints also available. d. Cathodic protection may be necessary below ground. e. Special protection required externally if laid in ground. f. Normally used below ground. g. External protection should be provided in aggressive soil conditions. h. Maximum pressure rating 40 bar (580 psi). i. Provision for flexibility should be made. j. Care should be exercised during laying of pipes and the provision of bedding. k. Problem with in-line valves if required. l. Sleeved joints are also available. m. Jointing material can also be made from various materials – consult manufacturer. o. Specially suitable at normal temperatures and normally suitable at high temperature. p. Not suited to cyclic variations in temperature. q. Chemical resistant pipes available. r. Not recommended for effluent containing sulphide. s. Special precautions should be taken in aggressive ground conditions. t. Flexible pipeline design more complex than rigid pipe design. u. Good supervision required during installation. v. Choice of plastic material determined by effluent constituents. w. Polypropylene can be fusion welded. x. Choice of resin/epoxy and pipe details determined by effluent constituents. z. Good supervision required during installation. aa. Expensive if special resins required. bb. Very expensive. cc. Good for drainage effluent containing hydrocarbons, nitric acid, and caustic. dd. Laboratory drains only.



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Figure 1 - Pressurised drainage systems - typical arrangements

Clean Water

(2.8.1)

OpenNo Vents /traps (2.10.1)

ContaminatedWater2.8.2Holding BasinEffluent treamentUnit (ETU)Vents / Traps ? (2.10.1)

Detergent

2.8.7Oily Water(Low VOC) 2.8.3

Open

Vents & TrapsSealed ManholeETU (2.10.1

Sewerage2.8.9

NoVents/TrapsSewerage -Treatment

Solids 2.8.8

No Traps

High Volitate Organic Compounds (VOC) Oily water

All Closed Systems (2.10.2)

Gravity (2.10.2.1)

No ManholeSealed joints

Pumped (2.10.2)

Non Emulsifying pumpsHigher Spec’n MaterialsSealed tundishes

Pressurised

Vapour ExtractionSuction PumpsScrubberSealed Tundishes

BasicTreatment& DrainageRequirments

ComplexDrainage& expensiveTreatment

Clause Numbers refer to text? indicates options to be considered

Chemicals (2.8.4)

Non-Aggresive

Sealed manhole ?Vents / Traps ?Neutralisation Pit ?MaterialsHolding Basin (2.10.1)

Chemicals(2.10.2

Aggresive

ClosedMaterialsNeutralisation Pit

6..8.2

�6.10.2

�6.10.2

�6.8.3

�6.8.5

�6.10.2 �6.8.8 �6.8.4

�6.8.10 �6.8.9

�6.10.2

�6.10.3.2 �6.10.3

�6.10.3 �6.10.3

Exa

mpl

e of

ty

pica

l ac

cept

able

st

anda

rd



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Figure 2 - Pressurised drainage system - typical connection arrangement



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Figure 3 - Pressurised drainage system - typical line diagram of collection



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Figure 4 - Pumped drainage system - typical arrangements



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Figure 5 - Manhole gully detail



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Figure 6 - Typical sealed manhole covers

10mm min.

Sealing Material

Section of frame

Section of cover

SINGLE SEAL DOUBLE SEAL



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Figure 7 - Typical standard 150 mm gully trap

Notes: 1. Actual method of hinging airtight lid shall be decided by the manufacturer. Hinge pin shall be of mild steel galvanised. 2. Material cast iron shall be BS 1452. 3. The trap is shown to a manufacturers' design for a UK project. 4. Flanged sockets equal to BS 4622 shall be used in conjunction with trap if connection to flanged pipes is required. 5. Units shown are in mm. To convert to inches, divide by 25,4



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Figure 8 - Trapping of drain inlets to manholes

1. Dimensions are in mm. To convert to inches, divide by 25,4.

X



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Figure 9 - Typical offsites storage tank oily and clean water drainage layout

Notes: 1. If product spillage occurs and clean water could become contaminated, close valve and recover locally or allow to pass to final

separator. 2. Leaded or special products not suitable for mixing in oily water system shall be retained in local separator until product can be

recovered. Water remaining can then be allowed to drain to oily water system.



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Bibliography

British Standards (BS) [1] BS 2494 Elastomeric Seals for Joints in Pipework and Pipelines.

[2] BS 6297 Design and Installation of Small Sewage Treatment Works and Cesspools.

[3] BS 8005 Sewerage: Parts 0 to 5.

[4] BS 8007 Design of Concrete Structures for Retaining Aqueous Liquids.

[5] BS 8301 Building Drainage.

Health and Safety Guidance (HSG) [6] HSG 34 The Storage of LPG at Fixed Installations.

Water Authorities Association (WAA) [7] Sewers For Adoption.

gp 04-10 22 feb 2006

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