bp code gs146-1

GS 146-1

COLUMN INTERNALS

September 1995

Copyright © The British Petroleum Company p.l.c.

rezaei

tadvin-arm

Copyright © The British Petroleum Company p.l.c.All rights reserved. The information contained in this document is subjectto the terms and conditions of the agreement or contract under which thedocument was supplied to the recipient's organisation. None of theinformation contained in this document shall be disclosed outside therecipient's own organisation without the prior written permission ofManager, Standards, BP International Limited, unless the terms of suchagreement or contract expressly allow.

BP GROUP RECOMMENDED PRACTICES AND SPECIFICATIONS FOR ENGINEERING

Issue Date September 1995Doc. No. GS 146-1 Latest Amendment Date

Document Title

COLUMN INTERNALS

(Replaces BP Engineering Standard 168)

APPLICABILITY

Regional Applicability: International

SCOPE AND PURPOSE

This document covers general requirements for the design, fabrication, supply, testing andinstallation of trays, packings and other column internals.

AMENDMENTSAmd Date Page(s) Description___________________________________________________________________

CUSTODIAN (See Quarterly Status List for Contact)

Fluid SeparationIssued by:-

Engineering Practices Group, BP International Limited, Research & Engineering CentreChertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LN, UNITED KINGDOM

Tel: +44 1932 76 4067 Fax: +44 1932 76 4077 Telex: 296041

GS 146-1Column Internals PAGE i

CONTENTS

Section Page

FOREWORD ............................................................................................................... iii

1. INTRODUCTION................................................................................................... 1

1.1 Scope 11.2 Application of this Specification........................................................................... 11.3 Quality Assurance................................................................................................ 1

2. COLUMN TRAYS.................................................................................................. 1

2.1 General Arrangement and Size Requirements....................................................... 12.2 Fractionator Tray Design Details ......................................................................... 22.3 Materials and Thickness....................................................................................... 42.4 Structural Details................................................................................................. 52.5 Clamping and Bolting Arrangements.................................................................... 72.6 Seal Welding ....................................................................................................... 82.7 Manways and Access........................................................................................... 82.8 Gasketting ........................................................................................................... 82.9 Drainage.............................................................................................................. 92.10 Tolerances for Fabrication and Assembly .......................................................... 92.11 Vortex Breakers ............................................................................................. 102.12 Blanking on Sieve or Valve Trays ................................................................... 112.13 Tray Installation.............................................................................................. 112.14 Reflux Inlets ................................................................................................... 112.15 Feed Inlets ...................................................................................................... 12

3. PACKED COLUMNS........................................................................................... 13

3.1 General Arrangement......................................................................................... 133.2 Materials and Thickness..................................................................................... 133.3 Structural Detail ................................................................................................ 143.4 Packing.............................................................................................................. 143.5 Distributors - Gravity......................................................................................... 143.6 Distributors - Pressure ....................................................................................... 163.7 Ancillary Equipment .......................................................................................... 163.8 Tolerances ......................................................................................................... 17

4. ACCUMULATOR TRAYS .................................................................................. 17

4.1 General Arrangement......................................................................................... 174.2 Manways and Access......................................................................................... 174.3 Structural Requirements .................................................................................... 184.4 Seal Welding ..................................................................................................... 19

GS 146-1Column Internals PAGE ii

5. VESSEL FEATURES ........................................................................................... 19

5.1 Internal Pipes..................................................................................................... 19

6. DESIGN, DELIVERY AND INSTALLATION................................................... 20

6.1 Design calculations ............................................................................................ 206.2 Drawings ........................................................................................................... 206.3 Finishing, Handling and Storage......................................................................... 216.4 Testing and Inspection before Installation .......................................................... 226.5 Testing and Inspection after Installation. ............................................................ 23

7. GUARANTEES..................................................................................................... 23

7.1 Mechanical ........................................................................................................ 237.2 Process .............................................................................................................. 24

8. REGIONAL ANNEX............................................................................................ 24

8.1 USA .............................................................................................................. 24

APPENDIX A.............................................................................................................. 25

DEFINITIONS AND ABBREVIATIONS .............................................................. 25

APPENDIX B.............................................................................................................. 26

LIST OF REFERENCED DOCUMENTS............................................................... 26

GS 146-1Column Internals PAGE iii

FOREWORD

Introduction to BP Group Recommended Practices and Specifications for Engineering

The Introductory Volume contains a series of documents that provide an introduction to theBP Group Recommended Practices and Specifications for Engineering (RPSEs). Inparticular, the 'General Foreword' sets out the philosophy of the RPSEs. Other documents inthe Introductory Volume provide general guidance on using the RPSEs and backgroundinformation to Engineering Standards in BP. There are also recommendations for specificdefinitions and requirements.

Value of this Guidance for Specification

This document covers the major requirements of column internals appropriate to all BPBusinesses. Adherence to the procedures will ensure that the equipment is fit for purposewhilst being cost effective

Application

This Guidance for Specification is intended to guide the purchaser in the use or creation of afit-for-purpose specification for enquiry or purchasing activity.

Text in italics is Commentary. Commentary provides background information which supportsthe requirements of the Specification, and may discuss alternative options.

This document may refer to certain local, national or international regulations but theresponsibility to ensure compliance with legislation and any other statutory requirements lieswith the user. The user should adapt or supplement this document to ensure compliance forthe specific application.

Principal Changes from Previous Edition

This is a new BP Guidance for Specification which incorporates the substance of threeprevious standards listed below:

BP (America) Refining Practice RP 7-2-2 (Fractionating Trays and Column Packing) - April1992.

BP Chemicals Engineering Standard ES/ENG/520/01 (Column Internals) - April 1987

BP Engineering Standard No 168 (Distillation, Absorption and Extraction Column Internals) -April 1987.

GS 146-1Column Internals PAGE iv

Specification Ready for Application

A Specification (BP Spec 146-1) is available which may be suitable for enquiry or purchasingwithout modification. It is derived from this BP Group Guidance for Specification byretaining the technical body unaltered but omitting all commentary, omitting the data page andinserting a modified Foreword.

Feedback and Further Information

Users are invited to feed back any comments and to detail experiences in the application of BPRPSE's, to assist in the process of their continuous improvement.

For feedback and further information, please contact Standards Group, BP International orthe Custodian. See Quarterly Status List for contacts.

GS 146-1Column Internals PAGE 1

1. INTRODUCTION

1.1 Scope

This Specification covers general requirements for the design,fabrication supply, testing and installation of trays, packings and othercolumn internals.

1.2 Application of this Specification

This Specification shall be applied as the sole means of specifyingcolumn internals to a manufacturer, and forms a working documentwhich shall be transmitted directly to manufacturers' without being re-written as a project or contractor document.

1.3 Quality Assurance

Verification of the vendor's quality system is normally part of the pre-qualificationprocedure, and is therefore not specified in the core text of this specification. Ifthis is not the case, clauses should be inserted to require the vendor to operate andbe prepared to demonstrate the quality system to the purchaser. The quality systemshould ensure that the technical and QA requirements specified in the enquiry andpurchase documents are applied to all materials, equipment and services providedby sub-contractors and to any free issue materials.

Further suggestions may be found in the BP Group RPSEs Introductory Volume.

2. COLUMN TRAYS

2.1 General Arrangement and Size Requirements

2.1.1 For vessel of 840 mm (33") diameter and larger, tray panels,downcomer sections and other tray components shall be of sectionalconstruction with sections removable through the vessel manways.

2.1.2 When a lifting beam is not available within the column, formed decksections shall have a maximum component weight of 50 kg and amaximum length of 3000 mm (118").

A minimum number of plate sections shall be used in the floors ofdowncomers and accumulator trays.

2.1.3 When a lifting beam facility can be provided within a column, the lengthand weight restrictions shall be governed by the ability to installsectional components with the vessel in both the horizontal andvertical.

The factors which govern the maximum weight/length are those necessary to ensuresafe working in a confined space.


2.1.4 All tray components shall be designed to permit installation andremoval from the top side.

To design the internals to be installed from both sides is an expensive addition thatshould be avoided unless absolutely necessary.

2.1.5 For vessels smaller than 840 mm diameter, cartridge tray assembliesshall be provided and the vessel shall be flanged to permit installationand withdrawal of the cartridge.

Cartridge trays are invariably assembled in the fabrication shop.

2.2 Fractionator Tray Design Details

2.2.1 Sieve tray deck hole sizes should normally be selected between 4 mmand 12 mm (5/32 - 1/2") diameter. Larger hole sizes shall be usedwhen the specification sheet specifies a fouling or corrosive service.The ratio of the hole pitch to hole diameter shall be a minimum of 2.0.

Hole sizes larger than 25 mm (1") are more likely to weep, will restrict capacityand cause a lower efficiency. They are not normally used.

2.2.2 Holes in sieve trays shall be formed with the punch exit on theupperside of the tray. The manufacturer shall ensure throughfabrication and installation that this requirement is met for all decksections. Burrs on the sieve tray holes which would cause injury topersonnel, should be removed.

2.2.3 On sieve and valve trays the hole area shall be uniformly distributed inthe contacting area approximating equilateral triangular pitch. Holearea per tray shall be within ±1% of the area specified.

2.2.4 In the closed position the valve shall not make full peripheral contactwith the tray deck.

Valve caps should be provided with an indentation which prevents such peripheralcontact and thus helps to prevent the valve from sticking closed.

2.2.5 Trays having more than two passes should be designed for equalbubbling area per pass rather than equal flow path length per passsubject to a minimum flowpath length of 450 mm (18").

In some multi pass designs this may cause problems with meeting minimum manwaydimensions. In such cases this should be discussed with the owner..

2.2.6 Trays with three or more passes shall be provided with vapour andliquid balance ducts.


These are tubes or ducts that connect the different vapour flow areas such thatpressures over the columns are equalised. The design should be discussed with BP.

2.2.7 Trays shall be designed to handle the vapour and liquid loads given onthe tray data sheets. Standard sieve tray performance data shall bespecified on an FRI basis.

It is recognised that proprietary trays using sieve decks will be designed accordingto the manufacturer's own procedures and in this case the basis used forcalculating jet flood, entrainment, maximum sustainable loading and downcomerbackup shall be provided to the purchaser.

2.2.8 Where the column is to be used offshore it may be subject to sway. Insuch cases the trays should be designed such that they operatesatisfactorily within the sway tolerances approved by BP.

In the absence of stated tolerances, a sway of ±0.5 degree may be assumed inaddition to the conventional fabrication tolerances used although it will benecessary to consider motion in all six axes and, to some extent, the accelerationsand periods of motion. It is the responsibility of the manufacturer to ensure thatthe hydraulic design meets these criteria.

2.2.9 No sieve tray shall be designed for more than 80% (FRI basis) of jetflood. Where sieve trays are used for vacuum service a maximum of75% (FRI basis) of jet flood shall be used. No valve tray shall bedesigned for more than 85% of jet flood. Where valve trays are usedfor vacuum service a maximum of 77% of jet flood shall be used.

2.2.10 Downcomer clear liquid back-up shall be limited to a maximum percentof tray spacing plus weir height as listed below:-

Down comer Backup CriteriaSystem Pressure Maximum %

Liquid Backupbarg psig< 2.0 < 30 60

2.0 to < 5.5 30 to < 80 555.5 to < 10 80 to < 150 50

10 to 14 150 to 200 45> 14 barg > 200 40

2.2.11 Where the design liquid loading on a straight segmental overflow weirwould be less than 0.9 litre/min. cm (0.6 USGPM/inch), 'picket fence'type overflow weirs shall be provided. The minimum horizontaldistance between pickets shall be 50 mm (2"). The minimum verticalheight of picket above the weir shall be 150 mm (6"). The effectiveweir rate at normal design flows shall be no less than 0.9 litre/min. cm(0.6 USGPM/inch).


2.2.12 If the method of weir attachment is such that gaps could be leftbetween the weir plate and the shell wall and/or between the weir plateand the tray weir edge, slide plates shall be provided to seal these gaps.

2.2.13 For sloped or stepped downcomers, the bottom downcomer area mustnot be less than 50% of the top downcomer area.

In many cases the difference will be 20-40%.

2.2.14 Side downcomers shall be no less than 150 mm (6") in horizontaldistances from the column shell. This distance to be measured at thecentre line of the downcomer perpendicular to the inside face of thedowncomer.

Capsulated downcomers are exempted from this specification. The spacingbetween interior downcomers on proprietary multi-pass trays is often small, butthese are acceptable.

2.3 Materials and Thickness

2.3.1 The minimum design thickness of tray components shall be as follows:-

Thickness ThicknessMaterial mm nearest

gaugeMaterial mm nearest

gaugeCarbon steel 3 mm 10 Aluminium 2 mm 14Stainless steel 2 mm 14 Nickel Alloys 2 mm 14Monel 2 mm 14 Hastelloy 1.5 mm 16Copper 2 mm 14 Zirconium 1.5 mm 16

2.3.2 The thickness of pressed alloy steel bubble caps and the riser thicknessshall not be less than 1.5 mm (0.059") and 2.0 mm (0.079")respectively.

2.3.3 The thickness of proprietary valve units shall be set by the manufacturerbased on process requirements for the tray.

2.3.4 More than one weight of proprietary valves shall normally be achievedby using thicker gauge material. The spot welding of a single weighton a standard valve is an alternative but in cases where severe corrosionis anticipated any weights shall be fully seal welded.

2.3.5 In all cases proprietary valve units shall be manufactured fromcorrosion resistant material for the intended duty and stainless steel is aminimum requirement.

Carbon steel valve units should never be used.


2.3.6 Tray rings and downcomer bolting bars shall be in the same material asthe vessel. Tray rings and downcomer bolting bars for clad or linedvessels shall be in the same material as the vessel cladding or lining,subject to thermal stress limitations.

2.3.7 The dimensions of the tray support rings and downcomer bolting barsshall be provided by the manufacturer.

The following minimum and maximum dimensions shall be used:-

Column IDRing

Width

Minimumthickness

allow

excluding corrosionance

MaximumThickness

mm mm Corrosionresistantmaterial

Othermaterial

mm ins. mm ins. mm ins.less than 900 40 6 0.24 6 0.24 15 0.59901-1800 50 6 0.24 6 0.24 15 0.591801 - 3500 65 6 0.24 6 0.24 15 0.593501 - 6000 75 6 0.24 10 0.39 20 0.79above 6001 100 6 0.24 10 0.39 20 0.79

2.3.8 Vessel Corrosion allowance shall apply to all surfaces of welded-onitems in contact with the process fluids.

2.3.9 Vessel Corrosion allowance shall apply to one side of all removablesupport beams. Vessel Corrosion allowance does not have to beapplied to replaceable/removable tray parts.

2.3.10 For trays of corrosion resistant materials all bolting shall be of the samematerial as the removable internals with which it is connected .

2.3.11 Monel valve trays are prohibited.

2.4 Structural Details

2.4.1 Loose beams unsupported by shell brackets are not acceptable.

In the context of deck support beams, a loose beam is defined as a completelyseparate member whose position is maintained only by tray to support ring clampsand tray to beam clamps.

2.4.2 Certain larger diameter columns will require support beams that aredistinct members separate from the stiffened tray sections. These


separate beams, whether rolled joists or channels, shall be securelyrestrained against vertical and horizontal movement.

2.4.3 At least 360 mm (14") clearance shall be maintained between thebottom of the beams and the tray deck below when the main beams arelocated at right angles to the liquid flow direction on the tray below.This applies to both integral stiffeners and separate beams. Beamsrunning parallel to the liquid flow on the tray below are not subject tothe clearance requirement.

2.4.4 The design load shall include the weight of the internal element (with itscorrosion allowance) plus the following uniform downward loads atdesign temperatures.

(a) Fractionation trays. The greater of 1470 N/m2 (150 kg/m2, 30psi) or the weight of water 50 mm (2") over the highest weirsetting.

(b) Areas under downcomers. The greater of 2940 N/m2 (300kg/m2, 61 psi) or a head of water one half the height of thedowncomer.

(c) Draw-off pans. The greater of 6865 N/m2 (700 kg/m2, 143 psi)or a head of water to the top of the pan.

(d) Total draw-off trays. The greater of 6865 N/m2 (700 kg/m2,143 psi) or a head of water to the top of the vapour riser or theoverflow weir.

(e) Baffle tray with a weir. The greater of 6865 N/m2 (700 kg/m2,143 psi) on the projected area or the weight of water 50 mm(2") over the weir height.

(f) Vacuum residue stripping trays where steam stripping is used.An upward force of 49 N/m2 (5 kg/m3, 1.0 psi) shall be used.

Calculations shall exclude the corrosion allowance.

2.4.5 Stiffeners or a retaining device shall be provided on downcomers tolimit the horizontal movement of the bottom edge of the downcomerwhen the bottom section of the downcomer exceeds both 250 mm(10") in depth and 600 mm (24") in length.

2.4.6 Structural shapes or clips shall be provided along the bottom edge ofthe downcomer to ensure proper clearance between the bottom of thedowncomer and the tray deck. The design and installation of theseshapes or clips shall be such as to not interfere with the liquid flow


exiting from under the downcomer. These clips may also be designedto satisfy 2.4.5.

2.4.7 Adjustable weirs are not acceptable unless specifically required by theproject.

The specification of adjustable weirs is now an uncommon practice due to the easewith which these can be incorrectly installed or otherwise maladjusted duringoperations. The levelness of the tray therefore depends on the levelness of the traysupport ring and this requires an appropriate check.

2.4.8 Tray support rings shall be welded to the wall of the vessel to provide aflat support for the assembled tray sections. The upper surface of thetray support ring shall conform to the stated tray elevation. The ringshall be level to within the tolerances given in 2.10.4.

2.4.9 When the data sheet specifies a corrosive or vibrating service nonotched corners shall be provided. All corners shall be formed using a5 mm (0.2") radius minimum; minimum bend radii shall be three platethickness.

2.5 Clamping and Bolting Arrangements

2.5.1 Downcomers shall be designed to ensure that no vapour by-pass willoccur. Where the downcomer is assembled in sections the top sectionshall be bolted through at least two matching holes or horizontal slotsin the bolting bar and the plate. One clamp or bolt hole shall occur asclose to the end of each section as possible. Bolts or clamp pitch shallbe a maximum of 100 mm (4"). All bolt holes shall be sealed bywashers.

2.5.2 Where carbon steel decks are specified, the non-threaded componentsof clamping assemblies may be fabricated from carbon steel. Threadedcomponents shall be compatible with the process requirements and thebolting material shall be AISI 304 stainless steel as a minimum.

2.5.3 Where the decks are specified in stainless steel, the clamp assembliesshall be fabricated from the same material.

2.5.4 The maximum pitch between clamps shall be 180 mm (7") in the trayactive area and 100 mm (4") on the downcomer bolting bars and,where used, on the downcomer floor. Each separate tray anddowncomer section shall have a clamp located as close to each corneras possible.

2.5.5 The bolting for attaching major beams shall be provided with doublenuts or equivalent device.


2.5.6 Fasteners must not be welded to tray components.

2.6 Seal Welding

Where seal welding is required, for various tray sections please refer toSection 4.4.

2.7 Manways and Access

2.7.1 Tray manways shall provide a minimum access opening size of 380 mmx 600 mm (15" x 24").

2.7.2 Tray manways shall be removable from both above and below the tray,unless access is only from one side. The bolting shall be designed forpositive closure orientation.

2.7.3 Tray manways shall be vertically aligned to allow maximum workingaccess.

This clause is given in order to ease maintenance. The growing legalisation tostagger manways on alternate trays so that injury due to a fall is minimised maybecome an important issue in some projects. Advice must be sought in cases ofdispute.

2.7.4 A tray manway shall be provided on each liquid pass.

2.7.5 Where practicable, access through an accumulator tray shall be via oneor more risers. The cap(s) covering the riser(s) concerned shall bedetachable.

Where a riser is used as a manway, its width shall not be less than 450mm (18") in the longer direction and 380 mm (15") in the shorterdirection. The top of such a riser shall have a welded brim bar toprevent injury to personnel. The maximum width shall be governed bythe need to provide an adequate number of risers to ensure goodvapour distribution.

2.7.6 Minimum inter-tray work space shall be 300 mm (12").

2.8 Gasketting

2.8.1 Gaskets shall not be used to seal the tray active area.

2.8.2 Gaskets shall not be used to seal horizontal downcomer plates andbolting bar junctions and any baffling that may be used in the base ofthe column.

2.8.3 Gasketted joints for total draw-off trays are permitted.


Where a stringent (Cat 1) leak test is required, gaskets may not be used as it isimpossible to achieve a suitably leak tight tray.

2.8.4 Gasket material shall not contain any asbestos and shall be an asbestossubstitute suitable for both the process fluid and steam, up to the vesseldesign temperature.

2.8.5 The use of packing, cements, caulking, etc. to achieve liquid or vapourtightness or for any other purpose is prohibited.

2.8.6 Tray spacing requirements shall be as follows:-

Tray location SpacingAbove top tray A minimum of 1.5 times the normal tray

spacingFeed tray Normal tray spacing plus 150 mm (6 in)

whichever is the greater.Shell manhole Manhole internal diameter plus 150 mm (6

in) or the design tray spacing, whichever isgreater.

The feed tray spacing should not be less than 600 mm (24").

2.9 Drainage

2.9.1 Trays shall have drain holes only where items, e.g. bubble caps, giving apositive seal are used.

2.9.2 All seal pans and sumps shall have drain holes. Holes, whereappropriate, shall be located in the pan floor closest to the columncentre where appropriate to avoid vapour ingress into the downcomerliquid.

2.9.3 For weir loads exceeding 1.2 m3/h.m3 (.5 USGPM/ft2) the drain holearea should be based on 0.1% of the area to be drained. For lower weirloads the basis should be 0.01%. Hole diameter should be between 5-13 mm (0.2" - 0.5"), limited to a maximum of 2 holes per section.

2.9.4 Drain holes shall not be provided on trays or pans having a nozzlepositioned so as to allow complete drainage.

2.10 Tolerances for Fabrication and Assembly

2.10.1 The mean outlet weir height shall be set within ±2.5 mm (0.1") of thespecified value. The difference between the maximum and minimumheight dimensions shall not exceed 3.0 mm (0.12").


2.10.2 When inlet weirs are used, the height tolerance shall be ± 3.0 mm(0.12").

2.10.3 The under-downcomer clearance shall be set within + 3.0 mm (0.12")or - 0.0 mm of the specified value.

2.10.4 Tray levelness and tray support levelness between high and low pointson the vessel circumference shall be:-

Column diameter Maximum out of levelmm ins. mm ins.

< 1000 41 3 0.121000 to < 1800 71 4 0.161800 to < 3500 138 5 0.20

+3500 138 6 0.24

2.10.5 The location of tray support rings shall be set within the followingspecified distances:-

(a) ± 6 mm (0.24") from the reference plane with the maximumhigh to low difference not exceeding 6 mm (0.24").

(b) ± 3 mm (0.12") top of support to top of adjacent support.

2.10.6 The maximum uncovered joint opening between tray sections shall be 5mm (0.2") in the shorter direction and 25 mm (1") in the longerdirection. The sum total of the areas of uncovered joint openings onany one tray shall not exceed 1% of the active hole area on that tray.

2.10.7 The minimum overlap between the support ring and the outsidediameter of trays, pans and similar items shall be 20 mm (0.8").

2.11 Vortex Breakers

2.11.1 Vortex breakers shall be fitted in all vessels at nozzles leading to pumpsuctions. They are also required at all other pipe outlets where swirlmay interfere with the separating of phases, e.g. in the separation ofwater in oil-water separators.

Vortex breakers may be of the cross type or grating type. The choice is usuallymade according to the space available.

2.11.2 Vortex breakers shall be fitted to all internal pipes discharging liquidfrom accumulator trays.


2.12 Blanking on Sieve or Valve Trays

2.12.1 Blanking of active holes shall be achieved using flat plate blankingstrips bolted to the tray. Blanking strips shall be distributed uniformlyin the active area and should be between 40 mm (1.6") to 100 mm (4")wide, so as to completely cover holes.

2.12.2 Blanking strips shall be installed at right-angles to liquid flow, coveringcomplete rows if practicable.

2.12.3 For trays using circular valves, valves shall be removed prior toblanking.

2.12.4 Blanking shall not be used to adjust installed hole area to the specifieddesign value.

2.13 Tray Installation

2.13.1 The manufacturer shall supply an installation procedure for each type oftray to be supplied to include tolerances for installation and crossreferenced with drawings.

2.13.2 When tray installation takes place with the vessel in the horizontalposition, the manufacturer shall supply an installation procedure whichtakes account of the vessel diameter and the out of roundness of thevessel in the horizontal.

All sections of the trays including tray manways shall be fitted whentraying takes place in the horizontal.

2.13.3 The manufacturer shall supply the following extra material to allow forfield losses during installation:-

(a) Bolt, nuts and washers 10%

(b) Tray clamps 3%

(c) Gasketting material 20%

Operational spares are not required.

2.14 Reflux Inlets

2.14.1 Refluxes shall enter an inactive tray area behind an inlet weir or falsedowncomer acting as a seal dam. The inlet pipe shall be either an openended 'T' or a slotted branch distributor.

The choice will depend on the need to maintain a certain minimum exit velocity.


2.14.2 The seal dam height shall be 50 mm (2") greater than the inlet pipenominal diameter up to a maximum dam height of 200 mm (8").

2.14.3 Branch pipe velocities shall be a maximum of 1 m/s (3.3 ft/sec) and slotvelocities a maximum of 2 m/s (6.6 ft/sec).

2.15 Feed Inlets

2.15.1 Liquid feeds shall enter through an open ended branch to dischargevertically downward into the downcomer of the tray above the desiredfeed location or via a slotted branch distributor discharging onto theactive side of the downcomer at the desired feed tray level.

It is recognised that a number of variations in design may be used. These should becarefully reviewed.

A distributor shall be used when flashing of liquid in the downcomercould result in premature flooding.

2.15.2 Two phase, vapour-liquid, feeds shall enter the tray zone via a slottedbranch distributor which discharges against a wear plate attached to thedowncomer bolting bars.

2.15.3 Wear plate thickness shall be a minimum 5 mm (0.2"); spacers shall beprovided to hold the wear plate off the downcomer.

2.15.4 Vapour inlets shall normally be a plain open inlet nozzle. The inletnozzle shall be oriented approximately at right angles to the flow ofliquid on the tray above.

2.15.5 For liquid feed pipes which cross the tray active area the minimumclearance from the bottom of the pipe to the tray below shall be350 mm (14").

2.15.6 High velocity (i.e. approaching sonic velocity) vapour inlet lines shallbe designed to dissipate the energy. The inlet pipe shall be sized tolimit the fluid velocity to a maximum of 70% sonic velocity. Aminimum straight run of 2 m shall be provided to the column entry andthe feed shall enter the tower tangentially. A hood shall be providedextending at least to one quarter of the column circumference from thecolumn inlet point.

2.15.7 For high velocity inlets the minimum distance from the top of the inletpipe to the seal pan of the tray above shall be the greater of one pipediameter or 500 mm (20"). The minimum distance from the bottom ofthe inlet pipe to the surface of any liquid below shall be the greater oftwo pipe diameters or 600 mm (24").


3. PACKED COLUMNS

3.1 General Arrangement

3.1.1 All non random packing elements, support grids or structures, anddistributor systems and hold down grids shall be of sectionalconstruction with sections removable through the column manways.Column manways shall be provided at the distributor system.

3.1.2 When a lifting beam is not available within the column any componentsection referred to in 3.1.1 shall have a maximum weight of 50 kg (110lb.) and a maximum length of 3000 mm (118"). All other components,excluding structural beams, shall have a maximum weight of 120 kg(265 lb.) and a maximum length of 4000 mm (157").

When a lifting beam facility can be provided within a column the weightand length restrictions shall be governed by the ability to installsectional components with the vessel in the vertical.

The factors which govern the maximum weight/length are those necessary to ensuresafe working in a confined space.

3.1.3 Gravity distributors shall be provided as standard. Justification shall beprovided for any alternative offered.

The quality of the liquid distribution is the key factor to good packing performance.More even liquid distribution can be achieved with gravity distributors. Particularemphasis is necessary in distillation duties where efficiency is important.

3.2 Materials and Thickness

3.2.1 The standard material for all items referred to in 3.1.1 and randompacking elements shall be stainless steel. The specific grade of stainlesssteel or alternative material shall be suitable for the process fluid.

Carbon steel may be suitable for the process fluids and carbon steel will not rustless than stainless steel. Carbon steel is not recommended because of themaintenance requirements. It is very difficult to prevent surface rust by excludingair/water in a maintenance condition when column man entry is necessary.

3.2.2 The thickness of packing shall be specified so that the maximumdistortion of packing elements at the bottom of the tallest packedheight shall be no more than 2% of the corresponding standarddimension.

Distortion under compression will effect both the efficiency and the hydrauliccapacity of a packing.

The thickness of packings shall be determined by the manufacturer and shallconform to his current practice. This will depend on the grade, and therefore


rigidity, of the packing grade used. It is frequently 0.15 mm (0.006") but may withsome packing be as low as 0.1 mm (0.004") thick.

3.3 Structural Detail

The design load shall include the weight of the packing elements plusthe weight of the process fluid when the section is operating in the fullyflooded regime. The minimum process fluid at flood shall be taken asequivalent to 10% of the packing volume.

3.4 Packing

3.4.1 The supply of random packing shall be made only on the basis of fillinga given column diameter to a given depth. The manufacturer shall beresponsible for the supply of sufficient packing to take account of anysettlement allowance or dimensional variations.

Column sections are often non-circular and the packing elements per unit volumequoted by manufacturers are averages. The supply of additional packing at theoutset is more cost effective than the remedial work necessary to meet the specifiedheight. Packing performance is almost always expressed in terms of packed height.

3.4.2 The depth of packing for each bed shall be stated by the manufacturerto provide the number of mass transfer stages required and to preventphysical distortion of the packing in service.

3.4.3 The manufacturer of structured packing shall ensure that peripheralsealing is provided at each layer to minimise vapour by-pass due tocolumn ovality, manufacturing tolerances, etc.

Peripheral sealing should be provided at each layer around the completecircumference. The sealing medium should be in contact with both the column walland the packing. Visual inspection at installation will be required to check this.

3.5 Distributors - Gravity

3.5.1 The liquid distribution to the top of the packed bed shall be uniformover the vessel cross section for the whole design operating range.This uniformity of flow shall be maintained as the liquid impinges on tothe top of the packed bed.

3.5.2 The outermost liquid distribution points shall be placed as close aspracticable to the vessel wall and the following maximum dimensionsshall be used.

Randomnominal

packingsize

Structuredsurface

packingarea

Maximum distancevessel wall

from

mm ins. m2/m3 ft2/ft3 mm ins.


< or = 26 < or = 1.0 > 261 > 80 20 0.827 to 61 1.0 to 2.4 121 to 260 37 to 79 30 1.2

> 61 > 2.4 < or = 120 < or + 37 40 1.6Grid type Use manufacturers

limit

The area irrigated by the distributor shall be a minimum of 95% of thetower cross sectional area, as defined by a circle linking the outermostdistribution points.

Particular attention shall be given to ensure that flow from theoutermost orifices is not blocked by the presence of any other piece ofequipment in the column.

3.5.3 Distributors shall be provided with means of levelling.

3.5.4 The design of the gravity distributor and any associated parting boxesshall allow them to be levelled to within the tolerances specified in 3.8.

3.5.5 Liquid supply to the parting box shall be so arranged that aeration ofthe liquid in the box and liquid swirl shall be minimised.

3.5.6 Distributor design shall be subject to a minimum orifice diameter whichwill be a function of the process fluid characteristics. This minimumdiameter shall not be less than 2 mm (0.08").

Despite best efforts to filter all liquid streams entering packed columns, industrialfluids are not sufficiently clean to operate for prolonged periods with orifices below2 mm (0.08") diameter. In many cases this chosen orifice diameter will be greaterthan this minimum and must always be considered at the design stage, bearing inmind the nature of the fluid being distributed and the process environment.

3.5.7 All orifices shall be formed with the punch exit on the liquid outlet sideof the distributor.

Small irregularities during manufacture are to be minimised near orifices so as tominimise differences in exit flow rates. Where orifices are laser cut this clauseneed not apply. Drilled holes should not be used other than in tube distributors.

3.5.8 The manufacturer shall provide within the distribution system a meansto minimise particulate fouling of the distributor and define the methodof cleaning such systems.

Even in the cleanest process duties periodic cleaning will be a requirement.External filters on all liquid feed lines should always be provided and located asclose as is practicable to the feed point. The filters will extend the period betweencleaning.

3.5.9 Covers shall be provided on all final distributor channels.


3.5.10 A minimum liquid depth of 30 mm shall be used in parting box ordistributor channels.

A minimum depth is necessary to minimise variations in head along the length ofthe channels.

3.6 Distributors - Pressure

3.6.1 Spray nozzle distributors shall be designed so that the theoreticalcoverage ratio is a minimum of 3 where the coverage ratio is defined asthe ratio of the total geometrical spray footprint area to the columnarea. Nozzles shall be arranged so that there are no unirrigated areas.Circular full cone spray nozzles shall be used.

The exact locations of nozzles to achieve the most uniform coverage is a function ofthe characteristic of the particular nozzle selected. This is an area which should bediscussed with the owner.

3.6.2 Where ladder type distributors are selected, and where plain holes areused, all internal burrs shall be removed which would otherwise affectthe liquid flowrate. Any pipe system shall have a vent hole to preventaccumulation of vapour.

3.6.3 Feed pipework to the distributor shall be sized to limit the maximumvelocity to 1.5 m/s (4.9 ft/sec).

3.6.4 For fouling services the minimum free passage through the spray nozzleshall be equivalent to a diameter of 3 mm (0.12").

In many cases larger free passages will be required.

3.6.5 External filters shall be provided to prevent passage of material thatmight otherwise block the nozzle. The mesh size should be one third ofthe nozzle free passage diameter.

3.7 Ancillary Equipment

3.7.1 All packed beds shall be provided with a bed limiter, the design ofwhich will be a function of the particular packing and distributor typebeing used. In all cases the bed limiter design shall be such as to offerthe minimum possible coverage of the packing below. In this contextangle sections are not acceptable.

Where structured packing is used, any bars constituting the bed limitershall be orientated so that they do not lie along joins between adjacentblocks of packing.


3.7.2 Support grids shall be such as to impose the minimum resistance tovapour flow.

3.8 Tolerances

The elevation difference between low and high points in the gravityliquid distributor shall not exceed 2 mm (0.08").

4. ACCUMULATOR TRAYS

4.1 General Arrangement

4.1.1 Accumulator tray risers shall be uniformly spaced over the tray.

The total riser area shall be consistent with the requirements of the column duty.Thus in vacuum operation and operations where minimum pressure drop normallyis required, a maximum riser area is to be used consistent with the restraintsimposed by mechanical considerations.

4.1.2 Accumulator trays used for all vacuum columns and/or where all liquidleakage has to be avoided, shall incorporate the following features:-

(a) chimney caps vee shaped.

(b) vapour emergence area at the pouring end, or ends, of the capshall be fully blanked.

(c) risers oriented to be parallel to the general flow of liquid acrossthe tray.

These requirements are made to minimise the possibility of liquid re-entrainmentand to minimise liquid residence time, a factor particularly required in hightemperature vacuum service.

4.1.3 Accumulator tray caps shall be oriented so that liquid pouring from onecap shall not enter the vapour emergence area of an adjacent cap.

4.2 Manways and Access

4.2.1 Where practicable, access through an accumulator tray shall be via oneor more risers. The cap(s) covering the riser(s) concerned shall bedetachable.

Where a riser is used as a manway, its width shall not be less than450 mm (18") in the longer direction and 380 mm (15") in the shorterdirection. The top of such a riser shall have a welded brim bar toprevent injury to personnel.


The maximum riser width should be governed by the need to provide an adequatenumber of risers to ensure good vapour distribution.

4.2.2 Where the height of a riser used as a manway exceeds 450 mm (18") aninternal rung ladder shall be provided as necessary, with strengtheningof the riser.

4.3 Structural Requirements

4.3.1 Major beams shall be attached by bolting to vertical brackets or footstep brackets, welded to the vessel shell. Bolt holes in brackets orbeams shall be slotted to allow for thermal expansion of the beam.Methods of restraint which rely on indirect fastenings are notacceptable.

Other methods of restraint may be used providing that both horizontal and verticalmovement of the beam is limited to that strictly required to contain the effects ofthermal expansion.

4.3.2 The deflection of assembled deck sections at operating temperatureshall not exceed a value of 1/900 of the column diameter. Where acorrosion allowance is used, the calculations for deflection shall bemade on the corroded thickness.

4.3.3 All elements shall be designed for a concentrated maintenance load of150 kg (330 lb.) at any point on the installed assembly, based on theallowable stress at 40°C (104°F). Calculations shall exclude thecorrosion allowance.

4.3.4 The design load shall include the weight of the internal element (with itscorrosion allowance) plus the following uniform downward loads atdesign temperatures.

The greater of 6865 N/m2 (700 kg/m2, 143 psi) or a head of water tothe top of the vapour riser or the overflow weir. Calculations shallexclude the corrosion allowance.

4.3.5 Allowable stresses for metals shall be as per the vessel code used forthe column design and construction, such as for example ASME Codeor BS 5500.

4.3.6 The distance between the upper edge of an accumulator tray riser capand the underside of the tray above or packing support above, shall notbe less than 300 mm (12"). When the riser is used as a manway theminimum distance shall be 450 mm (18").

4.3.7 The manufacturer shall ensure that the trusses above the riser caps ofaccumulator trays do not obstruct the flow of vapour from the risers.


Such structural members shall be oriented so that they are parallel to,and whenever possible immediately above, the riser caps.

4.4 Seal Welding

4.4.1 All joints in seal pans and discharge pans shall be fully seal welded.The minimum design thickness of seal pans and discharge pancomponents shall be 3 mm (0.12"). Trays designated as total draw-offtrays shall be designed to have all seams continuously welded, includingseams in risers and the peripheral seal with the tray support ring.

Accumulator trays with clamps and bolting have often failed in service. Thepresence of gasketting has often made failure more likely. Fully weldedconstruction has proved to be the solution to minimising failures. Long experiencehas shown that provided the tray is designed for full seal welding, damage due tothermal stresses will not occur even though there may be some minor localisedpanel buckling.

4.4.2 Seal welding shall normally be using a continuous fillet weld above theattachment and an intermittent weld below. Where the duty is specifiedas corrosive, continuous fillet welds should be used above and below.

4.4.3 Welding operators and procedures shall be qualified in accordance withthe vessel code used for the column construction, such as Section IX ofthe ASME Boiler and Pressure Vessel Code or BS 4870/4871.

4.4.4 All welding shall conform to BS 5500 or ASME Section VIII.

4.4.5 Accumulator trays shall be designed to be fully seal welded, includingthe periphery. The peripheral support shall normally utilise aconventional tray support ring welded to the vessel wall and not to anycladding which may be present.

5. VESSEL FEATURES

5.1 Internal Pipes

5.1.1 Internal distribution pipes shall have flanged and gasketted connectionsto facilitate installation and removal.

5.1.2 Internal piping shall be the same nominal alloy as the inside surface ofthe vessel. For ferric, austenitic or high alloys, such as Inconels, theminimum wall thickness shall be schedule 10 or 3/16th inch (4.7mm),whichever is less. For carbon steel or low chrome steels, the minimumwall thickness shall be schedule 80.


5.1.3 Internal flanges shall be ASME/ANSI Class 150 weld neck or slip-ontype unless otherwise approved. Internal flanges shall be of acompatible metallurgy with the material specified for the internal pipe.Internal flange bolting shall be sequentially tightened, and double nutsused to prevent loosening.

5.1.4 When an internal distributor or sparger is of the 'bayonet' type, theexternal flange shall be marked in such a manner that the position of theslots or holes will be known from the outside of the vessel.

6. DESIGN, DELIVERY AND INSTALLATION

6.1 Design calculations

The results of the rating calculations and related mechanical data shallbe summarised on the data sheets. The information shall be furnishedto the owner's engineer for their review prior to commencement offabrication.

6.2 Drawings

6.2.1 The following dimensions shall be referenced to the tray floor and beconsistent with the process and mechanical data sheets:-

(a) Weir heights

(b) Clearance under the downcomers

(c) Splash baffles

6.2.2 Plan and elevation drawings shall be provided by the manufacturer forapproval by the purchaser before fabrication commences, indicatingmaterials specifications and all design and assembly dimensions. Thesedrawings should contain the following minimum design information,where applicable, relating to every separate design section of the tower.

(a) clearance under the downcomer.

(b) inlet and outlet downcomer area.

(c) effective exit weir length.

(d) exit weir height.

(e) sieve hole diameter.

(f) valve type (reference number).


(g) valve thickness.

(h) actual number of sieve holes on valves.

(i) normal tray spacing.

(j) correct bolting torque requirements.

(k) level tolerances (distributors).

(l) fluid densities at operating conditions.

(m) maximum and minimum design liquid flowrates (distributors).

(n) elevation and orientation (in diagram) of the tray.

Note:- the above measurements shall apply to actual sizes as used forprocess design, and not to measurements that might be used forfabrication purposes.

6.2.3 One original transparency and two copies of each Final Fabricationdrawing shall be supplied to the Owner.

6.2.4 The orientation in degrees, and the elevation, shall be marked on alldrawings where appropriate.

6.3 Finishing, Handling and Storage

6.3.1 With the exception of packings, all items of equipment shall be markedwith a part number for identification and easy assembly. Like parts maybear the same number. Supporting documentation will cross referencebetween assembly procedures and drawings. Structured packingelements shall not be individually marked, but only the housingcontainer. Items shall normally be marked by metal-stamping. Formaterials subject to stress corrosion cracking these shall be markedusing an electric discharge pen (pyrograph) or equivalent.

6.3.2 A list with the part numbers of the items within the container shall beprovided both inside and outside all packing containers.

6.3.3 As far as possible the manufacturer shall pack containers so thatunpacking for installation shall be consistent with the order ofinstallation in the column.


6.3.4 Structured packings shall be packed in weather proof boxes. Consistentwith standard box sizes, elements which make up complete vessel crosssections shall be packed together for large diameter vessels.

6.3.5 Random dumped packings shall be packaged in bags, and shall beclearly marked with the part number of the item.

6.3.6 Carbon steel parts shall be protected in a dry storage area. Austeniticstainless steel components shall never be exposed to wetting by saltwater or salt spray. Protective packaging or a suitable removablecoating shall be used to prevent such exposure.

6.4 Testing and Inspection before Installation

6.4.1 One tray per type, including accumulator trays, shall be assembled inthe manufacturer's shop to the extent necessary to check dimensions.Examination shall be by the purchaser or his representative.

6.4.2 All accumulator trays shall be subject to a water leak test at site. Thepermitted leakage shall be as per the schedule below:-

Leakagecategory Typical service

Maximumrate of

acceptablelevel drop

mm/h inches/hour1 vacuum towers:-

lowest distillatedraw

10 0.4

2 vacuum towers:-except category 1

50 2.0

3 all other columns 200 8.0

The water leak tests will be made (with drain holes plugged) at the overflow weirheight or chimney height, as applicable.

6.4.3 One gravity distributor per type as used for fractionation service shallbe tested with water in the fabrication shop before dispatch. Liquidinlet means to the distributor parting box shall be the same as that to beused in the column. The tests shall be performed at design maximumand minimum flowrates as follows:-

(a) Measurement of the uniformity of flow from the parting box orpredistributor.

Samples should be taken from each part of the parting box thatdischarges into a separate channel.


Acceptance criteria: Liquid distribution at 50% of the maximum designflowrate, or a specified minimum design flowrate, to be within + or -4% of the mean.

(b) Samples taken at no less than 10 random locations over thecross-sectional area of the column according to the followingschedule:-

Column diameter Approximate number ofholes from which liquid is

mm ins collected from each location< 900 < 36 1

900 - 1800 36 - 70 5> 1800 >70 10

Acceptance criteria. Liquid distribution at 50% of the maximum designflowrate, or a specified minimum design flowrate, to be within ± 6% ofthe mean.

(c) Measurement of liquid depth at each end and at the centre ofeach channel.

Acceptance criteria: The distance between the liquid level and partingbox/channel/pan top must be a minimum of 10% of the partingbox/channel/pan depth. Under no conditions will an overflowingsystem be accepted.

6.4.4 When a spray distributor is used for a VDU wash zone, or similar zone,it shall be tested with water in the fabrication shop before dispatch.Tests shall be performed at design maximum and minimum flowrates.A nozzle inlet pressure versus flowrate relationship shall be derived.Nozzle dimensions shall be checked against manufacturers data.

Acceptance criteria: At any pressure the flow shall be within 20% ofthe stated catalogue value.

6.5 Testing and Inspection after Installation.

Weld spatter on downcomer bolting bars shall be removed to ensure aflush seal.

In the event that the downcomer does not form a continuous seal withthe bolting bars, the downcomer shall be reassembled using a gasket.


7. GUARANTEES

7.1 Mechanical

7.1.1 The manufacturer shall guarantee all items supplied by him againstdefective material, poor workmanship, improper design and failurewithin design loadings.

7.1.2 The manufacturer shall repair or replace free of charge any defectivematerial or workmanship found within the guarantee period.

7.2 Process

7.2.1 The manufacturer shall guarantee the ability to handle the vapour andliquid loads over the specified operating range whilst retainingefficiency, when hydraulic loadings are supplied by the purchaser.

7.2.2 The manufacturer shall guarantee the separation and the hydraulicthroughput when the process design and the hydraulic design is hisresponsibility.

7.2.3 The manufacturer shall replace free of charge any new items required tomeet the process guarantees.

7.2.4 The manufacturer shall guarantee that the design and fabrication ofliquid distributors for packed columns will meet the criteria foracceptance given in section 6.4.3 and 6.4.4.

7.2.5 Any modification necessary to meet the leak tightness qualificationgiven in section 6.4.2. shall be made free of charge by themanufacturer.

8. REGIONAL ANNEX

8.1 USA

Where tray spacing is 915 mm (36") or greater an access ladder shall beprovided at each tray manway.

This is a requirement of BP's refineries in the USA which at the moment (Sept '95)is not strictly required elsewhere.


APPENDIX A

DEFINITIONS AND ABBREVIATIONS

Definitions

Standardised definitions may be found in the BP Group RPSEs Introductory Volume.

gravity distributor: liquid distributor using only hydrostatic head

grid packings: ordered packing manufactured from cut out metal strips

manufacturer: manufacturer of column internals

owner: the operator of the column

pressure distributor: liquid distributor with pressurised head

random packing: packing manufactured as discrete elements distributed randomly

structured packing: ordered packing manufactured from pressed sheet metal sheets

Abbreviations

CS Carbon steelFRI Fractionation Research IncorporatedHF Hydrofluoric acid


APPENDIX B

LIST OF REFERENCED DOCUMENTS

A reference invokes the latest published issue or amendment unless stated otherwise.

Referenced standards may be replaced by equivalent standards that are internationally orotherwise recognised provided that it can be shown to the satisfaction of the purchaser'sprofessional engineer that they meet or exceed the requirements of the referenced standards.

British Standards

BS 4870 Approval Testing of Welding Procedures

BS 4871 Approval Testing of Welders Working to Approved Welding Procedures

BS 5500 Unfired Fusion Welding Pressure Vessels

BS 5750 Quality System for design, development, production installation and Part 1servicing.

American

ASME Section V111 Pressure vessels, Division 1 and Division 2Section 1X Welding and Brazing Qualifications

bp code gs146-1

Documents