psv calculations flare

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L T- CHIYODALIMITEDDate: Feb.15, 2007 LTC-PB-P0-004

Rev: 1

PROCEDURE FOR PRESSURE

SAFETY VALVE CALCULATIONS

AND FLARE SYSTEM DESIGNPage 1 of 116

L&T -CHIYODA PERSONNEL ONLY L&T- CHIYODA PERSONNEL ONLY L&T- CHIYODA PERSONNEL ONLY

PROCEDURE FOR

PRESSURE SAFETY VALVE CALCULATIONS

&

FLARE SYSTEM DESIGN

1General

RevisionNUT/MPR NPK/KNK/RHD SS Feb., 15, 2007

0 First Issue SJR SS MH March, 12, 1996

Revision.No.

Description Prepared By Reviewed By ApprovedBy

ApprovedDate


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CONTENTS

1 SCOPE ..................................................................................................................................... 5

2 CODES AND PRACTICES................................................................................................... 5

3 DEFINITION OF TERMS..................................................................................................... 6

3.1 Pressure Relief Device ............................................................................................................ 6

3.2 System pressures ..................................................................................................................... 6

3.3 Device Pressures...................................................................................................................... 7

3.4 Relieving conditions ................................................................................................................ 7

4 PRESSURE RELIEF VALVES............................................................................................. 7

4.1 Types of Pressure Relief Valves............................................................................................. 8

4.2 Back Pressure.......................................................................................................................... 9

5 SET PRESSURE, ACCUMULATION LIMITS AND RELIEVING PRESSURE........ 11

6 OVERPRESSURE................................................................................................................ 14

6.1 Over Pressure Criteria ......................................................................................................... 14

6.2 Principal Causes.................................................................................................................... 15

7 PSV RELIEF LOAD CALCULATIONS AND PHILOSOPHY...................................... 15

7.1 External Fire.......................................................................................................................... 15

7.2 Blocked / Closed Outlets (Exit block).................................................................................. 21

7.3 Cooling or Column Reflux or Pump around failure.......................................................... 21

7.4 Tube Rupture / Plate & Frame Heat Exchanger Failure.................................................. 22

7.5 Control Valve failure............................................................................................................ 25

7.6 Hydraulic / Thermal Expansion .......................................................................................... 28

7.7 Power Failure (Steam or Electric)....................................................................................... 29


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7.8 Instrument Air Failure......................................................................................................... 30

7.9 Air Cooled Exchanger failure.............................................................................................. 30

7.10 Cooling Water failure ........................................................................................................... 31

7.11 Abnormal Heat Input ........................................................................................................... 31

7.12 Check Valve Mal-operation ................................................................................................. 31

7.13 Loss of Heat in Series fractionation system........................................................................ 32

7.14 Liquid Overfill....................................................................................................................... 32

8 SIZING FOR PRESSURE RELIEF VALVE .................................................................... 35

8.1 Sizing for Vapor or gas relief............................................................................................... 35

8.2 Sizing for Steam Relief ......................................................................................................... 37

8.3 Sizing for Liquid Relief ........................................................................................................ 37

9 DESIGN OF PIPING UPSTREAM OF RELIEF DEVICE ............................................. 39

10 DETERMINATION OF FLARE DESIGN CAPACITY.................................................. 40

11 SIZING OF FLARE HEADER ........................................................................................... 42

12 DESIGN OF PIPING DOWNSTREAM OF RELIEF DEVICE...................................... 44

13 FLARE STACK SIZING ..................................................................................................... 45

13.1 Flare Stack Diameter ............................................................................................................ 45

13.2 Flare Stack Height ................................................................................................................ 45

14 DESIGN OF FLARE KNOCKOUT DRUM...................................................................... 47

14.1 Horizontal Knockout Drum................................................................................................. 47

14.2 Vertical Knockout Drum...................................................................................................... 48

15 DESIGN OF SEALS IN FLARE SYSTEM........................................................................ 49

15.1 Sealing of the Flare Stack..................................................................................................... 49

15.2 Sealing of Piping Headers .................................................................................................... 49

16 PURGING OF FLARE HEADER AND FLARE TIP....................................................... 52


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16.1 Procedure for Calculating Flare Header Purge................................................................. 52

16.2 Procedure for Calculating Flare Tip Purge........................................................................ 52

17 P&I DIAGRAM FOR FLARE SYSTEM........................................................................... 52

18 ANNEXURES........................................................................................................................ 53

18.1 Annexure-1 [Tables, Figures (as per API-520/521)].......................................................... 53

18.2 Annexure-2 (Environment factor data) .............................................................................. 68

18.3 Annexure-3 (Vapor pressure and Heat of vaporization of pure single component

paraffin hydrocarbon liquids) ................................................................................. 70

18.4 Annexure-4 (Sizing for Two-phase Liquid/Vapor Relief)................................................. 71

18.5 Annexure-5 (Examples for Calculation of Relief load) ..................................................... 83

18.6 Annexure-6 (Typical Flare Load Summary sheet) .......................................................... 109

18.7 Annexure-7 (Flare Header / PSV outlet line sizing) ........................................................ 110

18.8 Annexure-8 (Flare stack, Figure-A, B) ............................................................................. 112

18.9 Annexure-9 (Flare knock out drum, Figure-C) ............................................................... 114

18.10 Annexure-10 (Seal drum, Figure-D) ................................................................................. 114

18.11 Annexure-11 (Typical flare system P&I Diagram).......................................................... 115

18.12 Format for Relief load calculation sheets ......................................................................... 116

19 OTHER REFERENCES .................................................................................................... 116

19.1 Handbook by Crosby.......................................................................................................... 116

19.2 Questions and Answers for API-520 / 521 ........................................................................ 116


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1.0 SCOPE

This document covers the standard design procedure to perform PSV sizing calculations. The

safety of personnel and the protection of equipment due to overpressure are the basis for the

design, sizing, and selection of pressure relieving systems. All systems and pressure relief

devices shall meet the applicable codes, industry standards and practices as well as related

owner/PMC job instructions.

The objective is to apply a systematic examination to all modes of operations and engineering

intentions to the mechanical integrity of the equipment and piping systems based on all

credible incidents. Provisions shall be made to contain or safely relieve any excessive pressures

in the system. These provisions shall include utilization of the applicable standards as listed in

further sections.The equipment and piping systems shall be designed, fabricated, tested, and assembled in

accordance with project specifications and shall be subject to the vendors quality assurance

and control procedures, including third party inspection.

The practices outlined in this document shall be followed, for all Process unit areas including

related Utilities, Offsite, licensor and non-licensor packages. Also this manual presents the

standard design procedure of a flare system.

2.0 CODES AND PRACTICES

API RP 520 Part I and II : Recommended Practice for the Sizing, Selection andInstallation of Pressure-Relieving Devices in Refineries.

API RP 521: Guide for Pressure-Relieving and Depressuring systems. API STD 526: Flanged Steel Pressure-Relief valves. API STD 527: Commercial Seat Tightness of Safety Relief Valves with Metal to Metal

Seats

API STD 2000: Venting Atmospheric and Low-pressure Storage Tanks (Nonrefrigerated and refrigerated)

ASME Boiler and Pressure Vessel Code, Sec I, Power Boiler ASME Boiler and Pressure Vessel Code, Sec VI, Recommended Rules for Care and

Operation of Heating Boilers ASME Boiler and Pressure Vessel, Sec VIII, Pressure Vessels, including Appendix ANSI/ASME B31.3, Chemical Plant and Petroleum Refinery Piping ANSI/ASME Power Piping B31.

Wherever the code differs and/or conflicts, the more appropriate practice shall apply in

agreement with Client/PMC/Owner.


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3.0 DEFINITION OF TERMS

3.1 Pressure Relief Device

Actuated by inlet static pressure to prevent a rise of internal fluid pressure in excess of

specified design value. The device may be a pressure relief valve, a non-reclosing pressure

relief device or a vacuum relief valve.

Pressure Relief Valve:A pressure relief device

designed to open and relieve

excess pressure and to recloseafter normal conditions have

been restored.

a). Relief valve: Valve opens

normally in proportion to the

pressure increase over the

opening pressure. Used

primarily with incompressible

fluids.

b). Safety valve: Characterized

by rapid opening or pop action.

Normally used withcompressible fluids.

c). Safety Relief valve: May be

used as either a safety or relief

valve depending on the

application.

Non-reclosing pressure

relief device:A pressure relief device

which remains open afteroperation.

a). Rupture disk device:

Actuated by static

differential pressure

between the inlet &

outlet of the device and

designed to function by

bursting of a rupture

disk.

a). Pin-actuated device:

Actuated by staticpressure and designed to

function by buckling or

breaking a pin, which

holds a piston or plug in

place.

Vacuum relief

Device:

3.2 System pressures

(Refer Annexure-1, Figure-1)

Maximum operating pressureis the maximum pressure expected during normal system

operation. Maximum allowable working pressure (MAWP) is the maximum permissible gaugepressure at the designated coincident temperature. This pressure is determined by the

vessel design rules for each element of vessel using actual nominal thickness, exclusive

of any other allowances such as corrosion etc. The MAWP is normally greater than the

design pressure but must be equal to design pressure when design rules are used only to

calculate the minimum thickness for each element and calculations are not made to

determine the value of MAWP. The MAWP is the basis for the pressure setting of the

pressure relief devices.

Design pressure of the vessel along with design temperature is used to determine theminimum permissible thickness of each vessel element. This pressure may be used in

place of MAWP where MAWP has not been established. Design pressure is equal to orless than MAWP.


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Accumulation is the pressure increase over the MAWP of the vessel allowed duringdischarge through pressure relief device, expressed in pressure units or % of MAWP or

design pressure.

Overpressure is the pressure increase over the set pressure of the relieving deviceallowed to achieve rated flow, expressed in pressure units or % of set pressure. It is sameas accumulation when the relieving device is set to open at MAWP of the vessel.

3.3 Device Pressures

Set pressure is the inlet gauge pressure at which the device is set to open under serviceconditions. In general, the set pressure of single installed PSV is equal to the MAWP of

the protective equipment. If the MAWP is not defined, the design pressure would be

applicable for the set pressure.

Backpressure is the pressure that exists at the outlet of pressure relief device as a result ofthe pressure in the discharge system. It is the sum of the superimposed and built-up

backpressures.

Built-up Backpressure is the increase in pressure at the outlet of pressure relief devicethat develops as a result of flow after the pressure relief device or devices open.

Superimposed backpressure is the static pressure that exists at the outlet of pressure reliefdevice at the time the device is required to operate. It is the result of pressure in the

discharge system coming from other source and may be constant or variable.

3.4 Relieving conditions

The term relieving conditions is used to indicate the inlet pressure and temperature on a

pressure relief device during an overpressure condition.

4.0 PRESSURE RELIEF VALVES

Pressure relief devices are required for all equipment subject to overpressure that results

from outside pressure sources, external heat input or exothermic reactions. This section

summarizes the design approach to the sizing and selection of pressure relief devices to

protect equipment against overpressure from operating and fire contingencies.

All pressure relief devices shall be stamped with the ASME Code Symbol for Section I or

for Section VIII application as required.

All pressure relief valves shall be bench tested to verify the set pressure prior to final

installations, except those requiring in situ testing for ASME Section I applications.Acceptable types of pressure relief devices include spring-loaded pressure relief valves,

pilot-operated pressure relief valves, rupture disks and rupture pins.

Pressure relief valves shall be designed and constructed in accordance with API STD 526

and API STD 527 and sized in accordance with API RP 520 PT I and API RP 521.

For pressure relief valves in water and steam services, appropriate sections of the ASME

Code shall apply. The ASME Code shall be the minimum acceptable where local codes do

not cover relief valves or are less stringent.

Weight-loaded pressure relief valves shall not be used without OWNER / PMC approval.

Venting and breathing equipment for low-pressure, aboveground storage tanks at less than

1.03 bar gauge (15 psig) shall be sized as specified by API STD 2000, Sections 1-3 or APISTD 620, Section 6.


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4.1 Types of Pressure Relief Valves

4.1.1 Conventional pressure relief valve

It is a spring loaded pressure relief valve whose operational characteristics are directly affectedby changes in the backpressure. (Refer Annexure-1, Figure-2)

The operation of a conventional spring loaded pressure relief valve is based on a force balance

(Refer Annexure-1, Figure-19). The spring load is preset to equal the force exerted on the

closed disc by the inlet fluid when the system pressure is at the set pressure of the valve. When

the inlet pressure is below the set pressure, the disc remains seated on the nozzle in the closed

position. When the inlet pressure exceeds set pressure, the pressure force on the disc

overcomes the spring force and the valve opens. When inlet pressure is reduced to a level

below the set pressure, the valve re-closes. The pressure at which the valve re-seats is the

closing pressure. The difference between the set pressure and the closing pressure is blow

down.

4.1.2 Balanced pressure relief valve

It is a spring-loaded pressure relief valve that incorporates a bellows or other means for

minimizing the effect of backpressure on the operational characteristics of the valve. (Refer

Annexure-1, Figure-3)

When a superimposed backpressure is applied to the outlet of a spring-loaded pressure relief

valve, a pressure force is applied to the valve disc which is additive to the spring force. This

added force increases the pressure at which an unbalanced pressure relief valve will open. If

the superimposed backpressure is variable then the pressure at which the valve will open will

vary (Refer Annexure-1, Figure-22).In a balanced-bellows pressure relief valve, a bellows is

attached to the disc holder with a pressure area AB, approximately equal to the seating area of

the disc, AN, (Refer Annexure-1, Figure-23). This isolates an area on the disc, approximately

equal to the disc seat area, from the backpressure. With the addition of a bellows, therefore, the

set pressure of the pressure relief valve will remain constant in spite of variations in back

pressure. It is important to remember that the bonnet of a balanced pressure relief valve must

be vented to the atmosphere at all times for the bellows to perform properly.

When the superimposed backpressure is constant, the spring load can be reduced to

compensate for the effect of backpressure on set pressure and a balanced valve is not required.Balanced pressure relief valves should be considered where the built up backpressure is too

high for conventional pressure relief valve. Balanced pressure relief valves may also be used as

a means to isolate the guide, spring, bonnet and other top works parts within the valve from the

relieving fluid.

4.1.3 Pilot operated pressure relief valve

It is a pressure relief valve in which the major relieving device or main valve is combined with

and controlled by a self-actuated auxiliary pressure relief valve (pilot). (Refer Annexure-1,

Figure-6)

A pilot operated relief valve consists of the main valve, which normally encloses a floating

unbalanced piston assembly, and an external pilot. The piston is designed to have a larger area


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on the top than on the bottom. Up to the set pressure, the top and bottom areas are exposed to

the same inlet operating pressure. Because of the larger area on the top of the piston, the net

force holds the piston tightly against the main valve nozzle. As the operating pressure

increases, the net seating force increases and tends to make the valve tighter. This featureallows most pilot operated valves to be used where the maximum expected operating pressure

is higher than the percentage shown in Annexure-1, Figure-1. At the set pressure, the pilot

vents the pressure from the top of the piston; the resulting net force is now upward causing the

piston to lift, and process flow is established through the main valve. After the overpressure

incident, the pilot will close the vent from the top of the piston; thereby re-establishing

pressure, and the net force will cause the piston to reseat.

The lift of the main valve piston or diaphragm, unlike a conventional or balanced spring-

loaded valve, is not affected by built-up backpressure. This allows for even higher pressures in

the relief discharge manifolds. The pilot vent can be either directly exhausted to atmosphere or

to the main valve outlet depending upon the pilots design and users requirement. Only abalanced type of pilot, where set pressure is unaffected by backpressure, should be installed

with its exhaust connected to a location with varying pressure (such as to main valve outlet).

Slight variations in back pressure may be acceptable for unbalanced pilots.

4.2 Back Pressure

Pressure existing at the outlet of a pressure relief valve is defined as backpressure. Regardless

of whether the valve is vented directly to atmosphere or the discharge is piped to a collection

system, the backpressure may affect the operation of the pressure relief valve. Effects due to

backpressure may include variations in opening pressure, reduction in flow capacity, instability

or a combination of all three.

Backpressure, which is present at the outlet of pressure relief valve when it is required to

operate, is defined as superimposed backpressure. This backpressure can be constant if the

valve outlet is connected to a process vessel or system, which is held at a constant pressure. In

most cases, however the superimposed backpressure will be variable as a result of changing

conditions existing in the discharge system.

Backpressure, which develops in the discharge system after the pressure relief valve opens, is

defined as built-up backpressure. Built-up backpressure occurs due to pressure drop in the

discharge system as a result of flow from the pressure relief valve.

The magnitude of the backpressure, which exists at the outlet of a pressure relief valve, after ithas opened, is the total of the superimposed and built-up backpressure.

4.2.1 Effects of superimposed back pressure on pressure relief valve opening

Superimposed backpressure at the outlet of a conventional spring loaded pressure relief valve

acts to hold the valve disc closed with a force additive to the spring force. The actual spring

setting can be reduced by an amount equal to the superimposed backpressure to compensate for

this.

Balanced pressure relief valves utilize a bellow or piston to minimize or eliminate the effect of

superimposed backpressure on set pressure. Many pilot operated pressure relief valves havepilots which are vented to atmosphere or are balanced to maintain set pressure in the presence

of variable superimposed back pressure. Balanced spring loaded or pilot operated pressure


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relief valves should be considered if the superimposed backpressure is variable. However, if

amount of variable superimposed backpressure is small, a conventional valve could be used

provided:

The set pressure has been compensated for any superimposed back pressure normallypresent and

The maximum pressure during relief does not exceed the code-allowed limits foraccumulation in the equipment being protected.

4.2.2 Effects of back pressure on pressure relief valve operation and flow capacity

Conventional Pressure Relief Valves:

Conventional pressure relief valves show unsatisfactory performance when excessive

backpressure develops during a relief incident, due to the flow through the valve and outletpiping. The backpressure tends to reduce the lifting force, which is holding the valve open.

Excessive built-up backpressure can cause the valve to operate in an unstable manner. This

instability may occur as flutter or chatter. Chatter refers to the abnormally rapid reciprocating

motion of the pressure relief valve disc where the disc contacts the pressure relief valve seat

during cycling. This type of operation may cause damage to the valve and interconnecting

piping. Flutter is similar to chatter except that the disc does not come in to contact with the seat

during cycling.

In a conventional pressure relief valve application, built-up back pressure should not exceed

10% of the set pressure at 10% allowable overpressure. When the back pressure is expected toexceed these specified limits, a balanced or pilot operated pressure relief valve should be

specified.

Balanced Pressure Relief Valves:

A balanced pressure relief valve should be used where the built-up backpressure is too high for

conventional pressure relief valves or where the superimposed back pressure varies widely

compared to the set pressure. Balanced valves can typically be applied where the total back

pressure (superimposed + built-up) does not exceed approx. 50% of the set pressure. The

specific manufacturer should be consulted concerning the backpressure limitation of a

particular valve design.

With a balanced valve, high backpressure will tend to produce a closing force on the

unbalanced portion of the disc. This force may result in a reduction in lift and an associated

reduction in flow capacity. Capacity correction factors, called back pressure correction factors,

are provided by manufacturer to account for reduction in this flow. Typical backpressure

correction factors may be found for compressible fluid service in figure-30 and for

incompressible fluid (liquid) service in figure-31.

Pilot-Operated Pressure Relief Valves:

For pilot-operated pressure relief valves, the valve lift is not affected by back pressure. Forcompressible fluids at critical flow conditions, a back pressure correction factor of 1.0 should

be used.


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4.2.3 Effects of back pressure and header design on pressure relief valve sizing and

selection

The pressure relief valve discharge line and flare header must be designed so that thebuilt-up backpressure does not exceed the allowable limits.

In addition, the flare header system must be designed in order to ensure that thesuperimposed backpressure caused by venting or relief from another source will not

prevent relief valve from opening at a pressure adequate to protect equipment as per

applicable code.

For a balanced pressure relief valve, superimposed backpressure will not affect the setpressure of the relief valve. However total backpressure may affect the capacity of the

relief valve. Sizing a balanced relief valve is a two step process:- The relief valve is sized using a preliminary backpressure correction factor, Kb.

- Once a preliminary valve size and capacity is determined, the discharge line and

header size can be determined based on pressure drop calculations.

- The final size, capacity, backpressure and backpressure correction factor can then

be calculated.

For a pilot operated pressure relief valve, neither the set pressure nor the capacity istypically affected by backpressure for compressible fluids at critical flow conditions.

Tail pipe and flare header sizing are typically based on other considerations.

5.0 SET PRESSURE, ACCUMULATION LIMITS AND RELIEVING PRESSURE

Contingency Single Valve Installations Multiple Valve Installations

Maximum

Set pressure%

Maximum

Accumulatedpressure %

Maximum Set

pressure %

Maximum

Accumulatedpressure %

Nonfire Cases

First Valve 100 110 100 116

Additional

valve(s)

- - 105 116

Fire Cases

First Valve 100 121 100 121

Additional

valve(s)

- - 105 121

Supplemental

valve

- - 110 121


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All values are % of MAWP. The maximum accumulated pressure equals to the relieving

pressure of PSV.

Example: Determination of Relieving Pressure for a Single-Valve Installation (OperatingContingencies)

Characteristic Value

Valve Set Pressure Less than MAWP

Protected vessel MAWP, psig 100.0

Maximum accumulated pressure, psig 110.0

Valve set pressure, psig 90.0

Allowable overpressure, psi 20.0

Relieving pressure, P1, psia 124.7

Valve Set Pressure Equal to MAWPProtected vessel MAWP, psig 100.0





Example: Determination of Relieving Pressure for a Multiple-Valve Installation

(Operating Contingencies)


First Valve (Set Pressure Equal to MAWP)Protected vessel MAWP, psig 100.0





Additional Valve (Set Pressure Equal to 105% of MAWP)




Allowable overpressure, psi 11.0Relieving pressure, P1, psia 130.7

Example: Determination of Relieving Pressure for a Single-Valve Installation (Fire

Contingencies)


Valve Set Pressure Less than MAWP



Valve set pressure, psig 90.0Allowable overpressure, psi 31.0



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Valve Set Pressure Equal to MAWP


Maximum accumulated pressure, psig 121.0Valve set pressure, psig 100.0



Example: Determination of Relieving Pressure for a Multiple-Valve Installation (Fire

Contingencies)


First Valve (Set Pressure Equal to MAWP)


Maximum accumulated pressure, psig 121.0Valve set pressure, psig 100.0



Additional Valve (Set Pressure Equal to 105% MAWP)






For steam Boilers:

As per ASME Boiler and Pressure Vessel Code, Section-I,

Set pressure and Accumulation limits

Single Valve Installations Multiple Valve Installations

Maximum

Set

pressure %

Maximum

Accumulated

pressure %(As per ASME PG-

72 & PG-67.5)

Maximum

Set pressure

%

Maximum

Accumulated

pressure %(As per ASME PG-

72 & PG-67.5)

First Valve 100 103 ** 100 103 **

Additional

valve

- - 103 103 **

** Maximum up to 106% of MAWP (as per ASME PG-67.2). However, normally safety

valves shall be designed to attain full lift at a pressure no greater than 3% above their set

pressure (As per ASME PG-72).

All values are % of MAWP. The maximum accumulated pressure equals to the relieving

pressure of PSV.


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Number of PSVs

Each boiler shall have at least one safety valve / safety relief valve and if it has more than

500 ft2(47 m2) of bare tube water heating surface, or if an electric boiler has a power input

more than 1100 kW, it shall have two or more safety valve / safety relief valves. For aboiler with combined bare tube and extended water-heating surface exceeding 500 ft2(47

m2), two or more safety valve / safety relief valves are required only if the design steam

generating capacity of the boiler exceeds 4000 lb/hr (1800 kg/hr).

6.0 OVERPRESSURE

6.1 Over Pressure Criteria

All equipment and piping systems must be protected when the internal or external pressure can

exceed the design condition of the system due to an emergency, upset condition, operationalerror, instrument malfunction or fire. Pressure relieving devices are installed to ensure that a

system or any of its components are not subjected to pressures that exceed the code-allowable

pressure accumulation. Any circumstance that reasonably constitutes an overpressure type

hazard under the prevailing conditions shall be analyzed and evaluated.

Assumptions

- It is assumed that trained operators will staff the plant.

- In evaluating a given emergency condition, certain assumptions must be made

concerning equipment not affected by the emergency in order that relief rate may be

determined.

- The simultaneous occurrence of two or more conditions which could result in

overpressure will not be considered if the causes are unrelated, i.e., if no process,

mechanical, or electrical commonality exists among the causes.

- The opening and closing action of control valves and the automatic start-up of

equipment will not be considered as a substitute for pressure relieving devices for

equipment protection because power supply to these items in an emergency is not

considered reliable. As a general rule, final overpressure protection is to be provided

by means of a mechanical pressure-relieving device.

- Equipment not affected by a utility failure being evaluated will be considered to remainin operation while control functions and other systems will be assumed to operate as

designed.

- Flow rates through the equipment and other conditions during the emergency will be

assumed to be at the normal rates except where the particular primary emergency case

under consideration would alter the flow.

- In case of fire, the flow is assumed to have stopped and been contained within a defined

system.

- The possibility of an operator inadvertently opening or closing any one valve or taking

any incorrect action in the wrong sequence or at the wrong time will be considered.

(However, block valves, electric switches, and other equipment items that are locked or

car sealed in the correct position will not be considered involved in any cases of

operator error).


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6.2 Principal Causes

The following lists some common principal causes of overpressure, which shall be analyzed to

determine the individual relieving flow rates for pressure relieving devices. Also, clarification

of the failure and overpressure protection device is provided where applicable.

The list is not intended to be all-inclusive but is intended to serve as a guide.

1. External Fire

2. Exit Block Or Blocked Outlet

3. Cooling Or Column Reflux Failure Or Pump around failure

4. Tube Rupture

5. Control Valve Failure

6. Hydraulic / Thermal Expansion7. Power Failure

8. Instrument Air Failure

9. Loss of fan in air cooled exchangers

10. Cooling water failure

11. Abnormal heat input to reboiler.

12. Check Valve mal-operation

13. Loss of Heat in series fractionation system

14. Liquid Overfill

7.0 PSV RELIEF LOAD CALCULATIONS AND PHILOSOPHY

7.1 External Fire

Assume that all fluid flow to the equipment has stopped, and that the liquid level inside the

equipment is at the top of its normal working range.

In calculating fire loads from individual vessels, assume that vapor is generated by fire

exposure and heat transfer to contained liquids at operating conditions. The calculation

procedure is as mentioned below.

For determining pressure relief device capacity for several interconnected vessels, each vesselshould be calculated separately, rather than determining the heat input on the basis of the

summation of the total wetted surfaces of all vessels. Vapors generated by normal process heat

input are not considered. No credit is taken for any escape path for fire load vapors other than

through the pressure relief device (which may be a common relief valve for more than one

connected vessel), nor is credit allowed for reduction in the fire load by the continued

functioning of condensers or coolers.

Equipment, which normally operates dry, must be evaluated for the expansion of vapor or

supercritical fluid due to fire. A procedure is as mentioned under section for unwetted area

calculations.

The insulation system for an equipment item shall be considered individually. Credit may betaken for equipment insulation in reducing the required relief load if project specifications

concerning fireproofing insulation are met.


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See calculation procedure for details.

For vessels filled with both a liquid and a solid (such as molecular sieves or catalysts), the

behavior of the vessel contents normally precludes the cooling effect of liquid boiling. Hence

fireproofing and depressurizing should be considered as alternatives to protection by pressurerelief devices, unless provision of pressure relief is required by local regulations.

Piping and piping components are generally not considered to require protection against

overpressure due to fire exposure, consistent with requirements of ASME B31.3.

To determine the total vapor capacity to be relieved when several vessels are exposed to a

single fire, a processing area may be divided into a number of smaller single fire risk areas by

increased spacing. A single fire risk area is defined as a group of equipment items that is

surrounded on all sides by clear access ways that are at least 6 metre wide. The space under

pipe racks is considered an access way if it is at least 6 metre wide. For the estimation of the

vapor relief load, it is assumed that all (and only) the equipment contained within a single fire

risk area is exposed to the same fire. The largest of the vapor relief loads calculated from eachof the individual fire risk areas into which the plant is subdivided is used as the basis for the

analysis of the vapor collection system (if any) based on fire exposure.

Overpressure protection from fire exposure for heat exchangers: In general, heat exchangers do

not need a separate pressure relief device for protection against fire exposure since they are

usually protected by pressure relief devices in interconnected equipment or have an open

escape path to atmosphere through cooling water return lines. This is true even if the heat

exchanger has a manual block valve between it and the pressure relief device since it is not

expected that operators will close this valve during a fire incident. However, in situations

where a fail-close control valve or an automatically actuated emergency isolation valve could

isolate the heat exchanger from the pressure relief device providing protection against fireexposure, a separate pressure relief device to protect the exchanger may be required.

Fire exposure protection for heat exchangers that are provided with blocks and bypasses to

permit cleaning while the rest of the unit is operating, present a special situation. Again,

interconnected equipment usually provides the required overpressure protection but these

exchangers are expected to be occasionally isolated from the system. In this case, one of two

options is available to provide protection: installing a pressure relief device or relying on

operating procedures. If the operating procedure option is used, this operating procedure must

direct the operators to drain all liquid from the exchanger immediately upon isolating it from

the system, and maintaining the exchanger dry" and unpressurized during the period of time it

is isolated from the pressure relief device that would normally provide protection. To increase

the probability that this operating procedure is followed, a caution sign to that effect shall be

permanently placed at the block valves of all exchangers equipped with a bypass.

Fire exposure overpressure protection for air-cooled exchangers is discussed in below

mentioned calculation procedure.


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CALCULATION PROCEDURE FOR EXTERNAL FIRE SCENARIO:

Refer ANNEXURE-5, Section-18.5.1 (Examples for Calculation of Relief).

1. For Wetted Surface:

The following formula should be applied. The process flows from / to the system would

be stopped and the protective equipment is assumed to be contained within defined

system.

L

QW = ...(Eq.01)

Where adequate drainage and firefighting equipment exist;82.021000 AFQ = ; For British unit..(Eq.02)

82.027140 AFQ = ; For Metric unit..(Eq.03)

Where adequate drainage and firefighting equipment do not exist;

82.034500 AFQ = ; For British unit..(Eq.04)

82.061000 AFQ = ; For Metric unit(Eq.05)

Where;

British unit Metric unit

W : Relieving Capacity lb/h kg/h

Q : Total heat absorption (input) to the

wetted surface

Btu/h kcal/h

F : Environmental Factor (#1) - -

A : Total wetted surface (#2) ft2 m2

L : Latent heat (#3) Btu/lb kcal/kg

In calculating the total wetted surface of the equipment, the expanded volume of the liquid inthe vessel should be used. The expanded volume includes the thermal expansion of the liquid

as it is heated from its initial temperature to its boiling point at the accumulated vessel

pressure.

These equations apply to process vessels and pressurized storage. For storage vessels with

design pressure of 15 psig (100 kPa) or lower see API 2000 for recommended heat absorption

due to fire

(#1) Environmental Factor

Refer to Annexure-2

(#2) Wetted Surface Exposed to Fire

The wetted surface area used to calculate heat absorption for a practical fire situation is

normally taken to be the total wetted surface within 25 ft (7.62 m) above grade. Grade"


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usually refers to ground level, but any other level at which a major fire could be sustained,

such as a solid platform, should also be considered. In the case of vessels containing a variable

level of liquid, the high level is considered. Specific interpretations of A to be used for various

vessels are as follows:1. Horizontal Drums

The wetted vessel surface within 25 ft (7.62 m) above grade, based on high liquid level, is

used.

2. Vertical Drums - The wetted vessel surface within 25 ft (7.62 m) above grade, based on

high liquid level, is used.

3. Fractionators and Other Towers - An equivalent tower dumped" level is calculated by

adding the liquid holdup on the trays to the liquid at high liquid level hold up at the tower

bottom. The surface that is wetted by this equivalent level and which is within 25 ft (7.62

m) above grade is used. Level in the reboiler is to be included, if reboiler is an integral

part of the column

4. Storage Spheres - The total surface exposed within 25 ft (7.62 m) above grade, or up to

the elevation of the centerline whichever is greater, is used.

5. Shell and Tube Heat Exchangers and Piping - The surface area of a tower reboiler and its

interconnecting piping should be included in the wetted surface of exposed vessels in a fire

risk area. The surface area of piping, other than that for reboiler, is not normally included

in the wetted surface area.

6. Storage tanks - Maximum inventory level up to the height of 25 ft (7.62 m) (portions of

the wetted area in contact with foundation or ground are normally excluded). For tanks of

15-psig operating pressure or less; see API STD 2000.

7. Air Cooled Exchangers:

Refer to API RP 521 sect. 3.15.7

Or

Only that portion of the bare surface on air-cooled exchangers located within the fire zone area

being evaluated needs to be considered in the calculation of fire loads. Air fins located directly

above pipe racks are also normally excluded since they are shielded from radiation by the

piping. The bare area is used instead of the finned area because most types of fins would be

destroyed within the first few minutes of fire exposure.

The following types of air-cooled exchangers need not be considered in the calculation of relief

loads due to fire:

Gas cooling services. There will be no vapor generation due to fire and the tubes are likely to

fail due to overheating.

Air-cooled partial or total condensers that meet the following criteria:

a. The tubes are sloped so that they are self-draining.

b. There is no control valve or pump connected directly to the condenser liquid outlet.

For these services, condensation will stop in the event of a fire, and any residual condensate

will drain freely to the downstream receiver. However, in this case, the normal condensingload for the air-cooled condenser must be added to the calculated fire load from other sources,

unless it can be established that the source of condensing vapors would stop in the event of a


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fire.

For air-cooled condensers that do not meet the above criteria, and for liquid coolers, the wetted

area used to calculate the relief load should be the bare area of the tubes located within the fire

zone area and within 25 feet (7.5m) above grade (or any other surface at which a major firecould be sustained, such as a solid platform). For tubes located higher than 25 feet (7.5m)

above grade (or other surface at which a major fire could be sustained), the wetted area shall be

taken as zero for forced draft units (the tubes would be shielded from radiant heat exposure by

the fan hood) and as the projected area (length times width) of the tube bundle for induced

draft units.

8. Piping:

It may be appropriate to add a percentage of the vessel area to account for vapor generation in

piping associated with the vessel under consideration.

(#3) Latent Heat calculationsIf relieving pressure is beyond critical pressure, use 50 Btu/lb as latent heat.

Single Component Systems:

Refer to Annexure-3 (Vapor pressure and Heat of vaporization for pure single component

paraffin hydrocarbon liquids)

Or

For single component systems, the term equals the latent heat of vaporization at relievingconditions. It may be determined from a flash calculation as the difference in the specific

enthalpies o f the vapor and liquid phases in equilibrium with each other, or it may be obtained

from API RP 521, Appendix A, Figure A-1 or other literature sources. For such systems, thelatent heat, the vaporization temperature, and the physical properties of the liquid and vapor

phases in equilibrium remain constant as the vaporization proceeds. The peak relief load will

always occur at the start of the fire, when the wetted surface, A, and consequently, the heat

input, Q, are both at a maximum.

Multi-component Systems:Refer to Annexure-3 (Vapor pressure and Heat of vaporization for pure single component

paraffin hydrocarbon liquids)

Or

For multi-component systems, the vaporization of the liquid initially in the vessel at the start of

the fire proceeds as a batch distillation in which the temperature, vapor flow rate andphysical properties of the vapor and liquid in equilibrium with each other change continuously

as the vaporization proceeds. The peak relief load may or may not coincide with the start of

the fire. In general, such systems require a time-dependent analysis to determine the required

relief area and the corresponding relief rate. The following approach is suggested: Assume

that all vapor and liquid inflows into and outflows from the vessel (other than the fire relief

load) have stopped.

Using the composition of the residual liquid inventory in the vessel, perform a bubble point

flash at the accumulated pressure. In doing this flash, the flow rate of the feed stream to the

flash can be set at any arbitrary value. For convenience, it is suggested that the mass flow rate

be set numerically equal to the mass inventory of liquid initially in the vessel or 1000 units of

mass flow rate (lb/h or kg/s).

Flash the liquid from the preceding flash at constant pressure and the weight percent vaporized


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equal to 1% to 5%. Divide the heat duty calculated for this flash by the mass flow rate of

vapor generated. The result is the heat absorbed per unit mass of vapor generated, . NOTETHAT, IN GENERAL, THIS VALUE WILL NOT EQUAL THE LATENT HEAT OF

VAPORIZATION, NOR WILL IT EQUAL THE DIFFERENCE IN VAPOR AND LIQUIDSPECIFIC ENTHALPIES. In fact, the value thus calculated will generally exceed the latent

heat of vaporization, especially in the case of wide boiling mixtures. The reason is that a

significant portion of the heat absorbed goes into raising the temperature of the system (most

of which is residual liquid at this point) to the equilibrium temperature of the flash (i.e. sensible

heat).

Using the value of calculated from Step 3; calculate the relief vapor rate, W

2. For Un-wetted Surface:Un-wetted wall vessels are those in which the internal walls are exposed to a gas, vapor or

super-critical fluid. The following formula should be applied:

( )

=

1506.1

1

25.1

11

'1406.0

T

TTAPMW W .(Eq.06)

Where;

W : Relieving Capacity lb/hr

M : Molecular Weight of Gas lb/lbmole

P1 : Relieving pressure (=set pr.+allow. Over press.+atm. Press.) psia (lb/in2A)

A : Exposed surface area ft2

TW : Vessel wall temperature

The recommended maximum vessel wall temp. for the usual carbon

steel plate material is 1100 F (593.33 C). Where vessels are

fabricated from alloy materials, the value for TW should be changed

to more appropriate recommended maximum.

R

T1 : Gas temperature, absolute, in R, at the upstream relieving pressure,

determined from the relationship,

n

n

T

P

PT

= 11

Where,

Pn: Normal operating gas pressure, psia (lb/in2A)

Tn: Normal operating gas temp. in R

R

Relieving temperature for wetted & un-wetted surface are often above the design temperature

of the equipment being protected. If, however, the elevated temperature is likely to cause

vessel rupture, additional protective measures should be considered such as:

Cooling the surface of a vessel with water Depressuring systems

Earth-covered storage


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7.2 Blocked / Closed Outlets (Exit block)

Refer ANNEXURE-5, Section-18.5.2(Examples for Calculation of Relief).

The capacity of the relief device must be at least as great as the capacity of the sources of

pressure. If all outlets are not blocked, the capacity of the unblocked outlets may properly be

considered.

The quantity of material to be relieved should be determined at conditions that

correspond to the set pressure plus overpressure instead of at normal operating

conditions.The effect of friction drop in the connecting line between the source of overpressure and the

system being protected should also be considered in determining the capacity requirement.

Base for relief capacity (blocked outlet):

Liquid relief Vapor relief

Maximum liquid pump-in rate Total incoming steam and vapor that

generated therein at relieving conditions

7.3 Cooling or Column Reflux or Pump around failure

Refer ANNEXURE-5, Section-18.5.3(Examples for Calculation of Relief).

Reflux Flow Failure - In some cases, failure of reflux (e.g., pump shutdown or valve closure)

will cause flooding of the condenser, which is equivalent to the pressure relief valve capacity

required for total loss of coolant. Compositional changes caused by loss of reflux may producedifferent vapor properties, which affect the relieving capacity. Usually, a pressure relief valve

sized for total tower overhead will be adequate for this condition, but each case must be

examined in relation to the particular components and system involved.

Pump around Flow Failure - The relief requirement is in the vapor condensed by the pump

around circuit evaluated at the relieving pressure and temperature. Pinch out" of steam heaters

may be considered, if appropriate. When pump around duty is high, or the feed to the

fractionators is highly superheated, loss of a pump around may cause a significant reduction in

tower cooling and result in dry-out of the tower. Therefore, the potential for dry-out should be

evaluated. The relief load due to fractionators dry-out is usually the sum of the entire vapor

feeds entering the fractionator plus any stripping steam or reboiler vapor (where applicable).Because of the difficulty in calculating detailed heat and material balances at relieving

pressure, the simplified bases described in following table have generally been accepted for

determining relieving rates.

1 Total condensing The relief requirement is the total incoming vapor rate to the

condenser, recalculated at temperature that corresponds to the new

vapor composition at relieving pressure and the heat input

prevailing at the time of relief.

The surge capacity of the overhead accumulator at the normal liquid

level is generally limited to less than 10 minutes. If cooling failure

exceeds this time, reflux is lost, and the overhead composition,temperature and vapor rate may change significantly.

2 Partial The relief requirement is the difference between the incoming and


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condensing outgoing vapor rate at relieving conditions. The incoming vapor rate

shall be calculated on the same basis as total condensing.

If the composition or rate of the reflux is changed, the incoming

vapor rate to the condenser should be determined for the newconditions.

3 Fan Failure (AFC

failure)

Because of natural convection effects, credit for a partial

condensing capacity of 20% to 30% of normal duty is often used

unless the effects at relieving conditions are determined to be

significantly different.

4 Louver closure Louver closure on air-cooled condensers is considered to be total

failure of the coolant with the resultant capacity established in point

1 & 2.

5 Top-tower reflux

failure

Total incoming steam and vapor plus that generated therein at

relieving conditions less vapor condensed by side stream reflux.

6 Pump aroundcircuit

The relief requirement is the vaporization rate caused by an amountof heat equal to that removed in the pump around circuit. The latent

heat of vaporization would correspond to the latent heat under

relieving conditions.

7 Side stream

reflux failure

Difference between vapor entering and leaving section at relieving

conditions.

7.4 Tube Rupture / Plate & Frame Heat Exchanger Failure

Refer ANNEXURE- 5, Section-18.5.4(Examples for Calculation of Relief).

Regarding the heat exchangers, there are some failure modes where the lower pressure side

could be exposed to fluid from the high-pressure side.

When design pressure of the low-pressure side is equal to or greater than ten-thirteenth the

design pressure of the high-pressure side, no need to calculate the relieving rate due to tube

rupture.

Tube failure shall be considered a potential source of overpressure for the low-pressure side of

heat exchangers except for the following heat exchanger types:

(a) Tubular reactors and waste heat boilers with tubes 1.5 in. (38 mm) and larger in diameter,

in which the tubes have wall thickness equivalent to process piping, and in which the

tubes are welded to the tube sheet.,

(b) Double-pipe exchangers except those with multiple tubes.

(c) Shell and tube exchangers that meet ALL of the following criteria:

(1) Tube vibration is not likely based on a rigorous tube vibration analysis.

(2) Tube wall thickness is at least one standard gauge thicker than the minimum required

for the specified material or a detailed equipment strategy has been developed, documented

and reviewed by experienced equipment specialists (both mechanical and metallurgical).

The equipment strategy must specifically recognize the application of the 6mm corrosion

hole concept (see below) and, consider all potential Equipment Degradation Modes. In

addition, inspection data with similar designs, process conditions and metallurgy should

confirm that no degradation has been found.


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(3) The tubes are not subject to erosion.

(4) The tubes will operate at temperatures warmer than -150F (-101C).

(5) The tubes are not subject to fatigue or creep.

(6) The process fluid will not cause aggressive corrosion or degradation of tubes and tube

sheets (for example pitting from salt deposits, corrosion from acidic condensates or stress

corrosion cracking).

(7) An appropriate tube inspection program will be developed for the exchanger bundle in

consultation with Materials Engineering specialists.

All these heat exchanger types shall be evaluated for potential overpressure in the event of

leakage through a 0.25in. (6mm) Hole due to corrosion.

If a pressure relief device is required to protect the low-pressure side, the relief rate is defined

by the maximum flow through the two open ends resulting from a guillotine cut of a single

tube at the tube sheet. In calculating this maximum flow rate, it is assumed that the normal

process flow into the low-pressure side has stopped and the pressure difference across the tube

opening is the difference between the maximum operating pressure of the high-pressure side

and the design (set) and/or relieving pressure of the low-pressure side.

Flow rate capacity from both side of a ruptured tube is defined as follows. It is based on a

single orifice equation with a discharge co-efficient of 0.7. For liquids that do not flash when

they pass through the opening or vapors, this formula shall be applied.

1. Liquid flow and conventional (conservative) equation for vapor flow:

( ) 12127.0 = PPAW ...(Eq.07)

2. Critical vapor flow:

+

+

=1

1

111

27.0

k

k

kkPAW

.(Eq.08)

In case k = 1.4 (conservative), then

11685.07.0 = PAW ..(Eq.09)

3. Non critical vapor flow 11685.07.0 = PAW

121 )(27.0 PPAYW = .(Eq.10)

P2 < 0.5 x P1


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=

r

r

k

krY

k

k

k

1

1

1

1

2

...(Eq.11)

In case k = 1.4 (conservative), then

=

r

rrY

1

15.3

286.043.1

.(Eq.12)

Where,

W : Mass flow rate kg/s

A : 1. For STHE: Cross sectional area of one side of ruptured tube x2

2. For PLHE: (**)

m2

P1 : Absolute upstream pressure based on maximum operating pressure pa a

P2 : Absolute downstream pressure (PSV set pressure) pa ar : P2/ P1 -

k : Ratio of specific heat, Cp/Cv -

: Density at upstream pressure kg/m3

(**) Plate and Frame Heat Exchanger failure case:

The following two types of failure modes are recommended based on experience(s) in past

projects

1) Failure mode of a 6 mm "pinhole" from one side to the other, which is referenced in

API RP 521.

2) Gasket Failure Mode (Rectangular opening)

The potential leak should be quantified as the flow through orifice in the same way we would

do it for a shell and tube exchanger (assuming flow from the high pressure side set pressure to

the low pressure side relief pressure). The size of the orifice should be calculated as the

hydraulic equivalent of a rectangular opening 0.0625 (1/16) inch wide, with a length equal to

the diameter of the relevant inlet or outlet (semi-cylindrical) flow header on the exchanger. The

Crane fluid flow handbook has equations for calculating the "hydraulic radius" for a circular

opening equivalent to a flow path of arbitrary cross-section. This method has the advantage of

being based on vendor input, and is consistent with the most industry practice.

For two phase flashing fluids, the flow models developed by DIERS and others shall be used in

determining the relieving rate through the failure.


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7.5 Control Valve failure


Automatic control devices are generally actuated directly from the process or indirectly from a

process variable (cascaded), e.g., pressure, flow, liquid level, or temperature. When the

transmission signal or operating medium fails, the control device will assume either a fully

open or fully closed position according to its basic design (the fail-safe position), although

some devices can be designed to remain stationary in the last controlled position.

When examining a process system for overpressure potential, it shall be assumed that any one

automatic control valve could be either open or closed, regardless of its specified fail-safe

action under loss of its transmission signal or operating medium.

When the control valve size (flow coefficient, Cv) is known it shall be assumed that this sizevalve is installed, and the maximum flow rate through the fully open control valve shall be

calculated based on the installed Cv. If the required relief area for any pressure relief device is

dependent on, or may be affected by, the maximum flow rate through a control valve, a

permanent sign shall be attached to the control valve stating that the installed Cv shall not be

increased without confirming the capacity of any pressure relief device that may be impacted

by the proposed change.

As a minimum, the following individual control valve failures shall be considered in the

analysis of control systems for determination of pressure relief requirements:

(a) Failure in the closed position of a control valve in an outlet stream from a vessel orsystem.

(b) Failure in the wide-open position of a control valve admitting fluid (liquid or vapor/gas)

from a high-pressure source into a lower pressure system.

(c) Failure in the wide open position of a control valve which normally passes liquid from a

high-pressure source into a lower pressure system, followed by loss of liquid level in the

upstream vessel and flow of high-pressure vapor. No credit is allowed for the response of

the level controller, which under normal conditions would close the control valve upon

loss of liquid level, since this scenario could be caused by the level controller failure. If

detailed analysis indicates that flow through the wide-open control valve is mixed phase,

then this should be considered when determining the maximum flow through the controlvalve. High pressure may also be generated in the piping system as a result of liquid slugs

being pushed by the vapor; hence the potential for excessive pressure from this event

should also be evaluated.

(d) Failure in the closed position of a control valve in a stream removing heat from a system.

(e) Failure in the open position of a control valve in a stream providing energy (heat) to a

system.

When a control valve is equipped with a bypass, the installed flow coefficient (Cv) of the

bypass valve shall not exceed that of the control valve. The following additional scenarios

shall be analyzed:

(f) The control valve fails wide open with its bypass valve partly open. To calculate the

relieving rate for this case, the flow rate through the partly open bypass valve is calculated


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using a Cv for the partially open bypass valve equal to 50% of the installed Cv of the

control valve in its wide-open position, regardless of the actual size of the bypass valve.

(g) The bypass valve is wide open with the control valve closed or blocked-in. The relieving

rate for this case is the flow rate through the wide-open bypass valve using the installedCv of the bypass valve in its fully open position.

For the control valve or its by pass valve that gives high differential pressure as described

below, the capacity of downstream PSV must be at least as great as the capacity passing

through the valve(s).

Where,

P1: Upstream pressure of control valve, kg/cm2A

P2: Downstream pressure of control valve, kg/cm2A

Flow rate through a Failure opened control valve is calculated as follows:

1. Liquid flow and conventional (conservative) equation for vapor or steam flow:

( )213.27 PPCW LVE = ..(Eq.13)

2. Critical vapor flow:

1

19.56T

MPCW VE =

...(Eq.14)

3. Non critical vapor flow:

( )22211

311 PPT

CW NVE =

.(Eq.15)

( )22211

7.65 PPT

MCW VE = ..(Eq. 16)

P1 P2 x1.5

P2 < 0.5 x P1


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4. Critical Steam Flow:

( )SHVE

T

PC

W +

= 00126.01

76.11 1.(Eq.17)

5. Non critical steam flow:

( )SH

VE

T

PPCW

+

=

00126.01

51.132

2

2

1

...(Eq.18)

Where,

W : Mass flow rate kg/hr

CVE : Control valve flow co-efficient, Or

Refer ANNEXURE-5, Ssection 18.5.5for CVEvalue table Or

Refer (***)

-

P1 : Pressure at control valve inlet based on the normal operating

pressure

kg/cm2A

P2 : Pressure at control valve outlet that is equal to PSV relieving

pressure

kg/cm2A

M : Molecular weight kg / kgmole

T1 : Temperature at control valve inlet K

: Upstream vapor density at normal conditions (= M/22.4141) kg/Nm3

L : Liquid density kg/m3

TSH : Steam degree of superheat (= Superheated temp. Saturated

temp.)

K

(***)

Alternate method for calculation of Cv (During initial stage before the control valve is

selected):

1. At first, please calculate process required CV value for corresponding control valve.2. Use 200 % of calculated required CV value for PSV calculation for no bypass

configuration across control valve.

3. Use 300% of calculated required CV value for PSV calculation with bypass valve

(same size as that of main control valve) configuration [take as 200% is max CV X

150% (50% is by bypass valve open)].

Note:

100% CV is process required CV value

200% CV is Max CV value

300% CV is Max CV + bypass valve open


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If the blocked-in liquid has a vapor pressure higher than the relief design pressure, then the

pressure relief device should be capable of handling the vapor generation rate.

7.7 Power Failure (Steam or Electric)

(1) Normal Individual and Process Unit Basis for Pressure Relief Sizing Considerations

The following contingencies shall be considered as the basis for evaluating overpressure

that can result from electric power failures:

(a) Individual failure of power supplies to any one item of consuming equipment, such as a

motor driver for a pump, fan or compressor.

(b) Total failure of power to all consuming equipment in a process unit supplied by a unit

substation.

(c) General failure of power to all equipment supplied from any one bus bar in a substationservicing one or more process units. Note that some substation designs include a

hierarchy of bus bars. With such an arrangement, a design contingency such as a ground

fault in a higher-level bus bar will result in loss of all power to the lower level bus bars.

In the case of the bus bar contingency, the basic assumption for this contingency is a ground

fault in the bus bar. Thus, the impact it will have on the equipment will be affected by the

design of the substation and the protective equipment provided. Some substations are designed

with normally closed circuit breakers isolating adjacent bus bars, when these are fed from the

same electrical feeder. When a ground fault occurs in a bus bar, these circuit breakers open,

thus isolating the fault and preventing the ground fault from extending to other bus bars andperhaps causing the complete substation to fail. The basic philosophy is to assume that

normally closed circuit breakers will function. For example, if the substation is designed such

that a single feeder provides power to two bus bars separated by a normally closed circuit

breaker, the design contingency for this design would be the loss of power to the equipment

connected to the bus bar having the ground fault. If in the example above, the substation were

designed without any circuit breaker, then the design contingency would be the loss of both

bus bars.

Other substation designs use normally open circuit breakers that are meant to close upon loss

of a power source to permit continued operation by obtaining power from a different source.

Since this type of protection implies action by a device/instrument in order to preventoverpressure in the equipment, no credit may be taken for the potential continuation of power

delivery. Hence, the contingency of loss of power to a bus and the normally open circuit

breaker failing to close and reestablish power needs is evaluated as a design contingency.

During design it may not be known from which bus bar a piece of equipment will be receiving

its power at the time of failure. Therefore, the combination of equipment losing power from

any single bus bar fault that results in the highest release rate shall be used as the design basis

for this contingency. Alternatively, the design specification may specify the arrangement of

equipment within the available bus bars.

For units in which spared equipment is supplied from different bus bars in the same substation,

loss of any one bus bar will, on average, result in loss of power to one-half of the equipment.Hence, for the design of a closed flare header system, a release equal to one-half of the release

for the worst combination of equipment loss can be assumed as a design contingency.


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(2) Consideration of Plant-wide Power Failure

The following general power failures on a plant-wide scale must be considered.

(a) Failure of purchased power supply to the plant.

(b) Failure of internally generated power supply to the plant.

(c) Total power failure in any one major substation

Total electrical power failure may result in loss of seawater, cooling water, steam and

instrument air if these utilities rely on electrically driven equipment for their availability.

In case of partial failure, equipment that is not affected by the failure of concern will be

considered to remain in operation and the controls will be assumed to operate as designed.

Reference to the electrical one-line diagrams and steam system P&IDs shall be made to

determine the extent of failure. For example, consider a cooling water circulating system

consisting of two parallel pumps in continuous operation, with drivers having different and

unrelated sources of power. If one of the two energy sources should fail, credit may be taken

for continued operation of the unaffected pump, provided that the operating pump would not

trip out due to overloading. Similarly, credit may be taken for partial continued operation of

parallel, normally operating instrument air compressors and electric power generators that have

two unrelated sources of energy to the drivers.

Backup systems which depend upon the action of automatic startup devices (e.g., a turbine-

driven standby spare for a motor-driven cooling water pump, with PLC control) shall not be

considered an acceptable means of preventing a utility failure for normal pressure relief design

purposes, even though their installation may be fully justified by improved reliability of plant

operations.

In cases of fan failure of the air-cooled exchangers, refer to section7.9

7.8 Instrument Air Failure

In case of total instrument air failure, the inventory in the instrument air receiver/header shall

be adequate to allow a safe shutdown without causing overpressure and subsequent release to

the flare header.

The failure position of control valves upon loss of instrument air shall be specified such that

potential hazards, including overpressure, are minimized. It shall be assumed that, upon partial

or total loss of instrument air, all control valves affected by the failure will assume their

specified failure position. Control valves that are specified to initially fail stationary shall beeither assumed to drift to their specified ultimate failure position or assumed to remain at their

last controlling position, whichever condition is more restrictive from an overpressure

protection standpoint.

7.9 Air Cooled Exchanger failure

Loss of air-cooled exchanger capacity may result from fan failure, inadvertent louver closure,

pitch control failure, or variable speed motor driver failure.

Refer Section-18.5.7, ANNEXURE- 5(Examples for Calculation of Relief).


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7.10 Cooling Water failure

(1) Normal Individual and Process Unit Basis for Pressure Relief Sizing Considerations

The following design contingencies shall be considered as the basis for evaluating overpressure

that can result from cooling water failures:

(a) Individual failure of water supply to any one cooler or condenser.

(b) Total failure of any one lateral supplying a process unit that can be isolated from the

offsite main.

(2) Consideration of Plant-wide Failure

The following general cooling water failures shall be considered:

(a) Failure of any section of the offsite cooling water main.

(b) Loss of all the cooling water pumps that would result from any design contingency in theutility systems supplying or controlling the pump drivers.

Relief load calculation can be done based on the following conditions:

Total Condenser : Total normal incoming vapor

Partial Condenser : Normal condensing rate


7.11 Abnormal Heat Input


The required capacity is the maximum rate of vapor generation at relieving conditions

(including any non-condensable produced from over-heating) less the rate of normal

condensation or vapor outflow.

In every case potential behavior of the system and each of its components shall be considered.

Some examples are:

Design value should be used for an item such as valve. Built-in overcapacity shall be used for burners, heater etc. Where limit stops are installed on valves, the wide-open capacity, rather than the

capacity at the stop setting, should normally be used. However, if mechanical stop is

installed and is adequately documented, use of the limited capacity may be appropriate.

In Shell & Tube heat exchange equipment, heat input should be calculated on the basisof clean rather than fouled conditions.

7.12 Check Valve Mal-operation


A check valve is not effective for preventing overpressure by reverse flow from a high-

pressure source. Experience indicates a substantial leakage through check valves.

The following guidelines apply to the evaluation of reverse flow through check valves as a


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potential source of overpressure.

(1) A pressure relief device is not required to protect piping against potential overpressure

caused by reverse flow if the pressure of the high-pressure source does not exceed the

short-term allowable overpressure for piping. The short term allowable overpressure forpiping is 133% of the maximum continuous pressure for the specified flange rating at the

flange operating temperature.

(2) A pressure relief device is not required to protect a pressure vessel against potential

overpressure caused by reverse flow if the pressure of the high-pressure source does not

exceed MAWP of the vessel. With the explicit approval of the OWNER / PMC, on a case-

by-case basis, a pressure relief device may not required if reverse flow from the high-

pressure source does not exceed the maximum allowable accumulated pressure of the

vessels.

(3) For

psv calculations flare

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