pressure relief system design

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P

ressure relief systems are vitalin the chemical process indus-tries (CPI) for handling a wide

variety of situations. They areused to prevent pressurization abovea systems design pressure; for vent-ing during an unusual or emergencysituation; and for normal depressur-ization during a shutdown, as exam-ples. In some cases, such as when non-combustible gases including steam,air and nitrogen are used, venting intothe atmosphere may be an option. Inother cases, such as those typically en-countered in the hydrocarbon sector,elaborate systems for the disposal of

vented gases may be required.This article describes some of the

causes of overpressurization, thetypes of valves and rupture disksthat are available and some of thecomponents needed for a pressurerelief system. Example calculationsare given, as well as a list of instal-lation considerations.

Causes of overpressurizationAn overpressurization may resultfrom a single cause or a combination

of events. Typically, not all causeswill occur simultaneously. In case ofan external fire in vessels that pre-dominantly handle vapors, such asknockout drums, there may be a rapidtemperature rise in the metal accom-panied with a rise in pressure dueto expansion. In case of external firein vessels that contain liquids, therewill be a rise in pressure as the liq-uid vaporizes. Pressure may alsorise abruptly due to thermal expan-sion when a blocked-in pipeline orother equipment containing a liquidis heated. Relieving pressure underthese situations is essential to prevent

failure. It is also required in systemswhere a continuous flow of vapor orliquid is suddenly stopped by a down-stream blockage.

While a full description of the vari-ous causes of overpressurization is be-

yond the scope of this article, detailsare provided by API [1]. The followingis a partial list: Blocked outlet Failure of control valve Cooling water failure Power failure Instrument air failure Heat-exchanger-tube failure External fire

Safety relief valvesThere are several types of safety re-

lief valves available on the market,including the following.Conventional: Conventional pres-sure-relief valves are susceptible toback pressure. Such valves are notrecommended when the total backpressure exceeds 10% of the set pres-sure. For systems operating at pres-sures close to atmospheric or at lowpressures, the limit of 10% is rarelyachieved. Therefore, these valves findapplication mainly in high-pressuresystems, or in systems that relieve to

the atmosphere (for example, steamand air).Balanced bellows: Balanced pres-sure-relief valves are used when con-

ventional pressure-relief valves can-not be used because of the reasonsmentioned above. Such valves arenot susceptible to back pressures ashigh as 50% of set pressure. The valveopening is independent of the backpressure. At higher back pressures,these valves will still relieve at the setpoint, but with a reduction in capac-ity. Therefore, it is recommended thatif balanced pressure-relief valves areto perform as rated, the back pressure

should be limited to about 50% of theset pressure.Pilot operated: In such valves, themain pressure-relief valve opensthrough a pilot valve. Pilot operated

valves are used in the following cir-

cumstances: 1) The pressure-relief-valve set pressure is lower than 110%of the operating pressure and 2) whenhigh back pressures are applicable.The opening of the valve is indepen-dent of back pressure.

Rupture disksA rupture disk is a pressure reliev-ing device that is used for the samepurpose as a relief valve, but a disk isa non-reclosing device or in otherwords, once it is open it will not close.

This means that whatever is in thesystem will continue to vent untilstopped by some form of intervention.The following are the applications ofrupture disks [3]:For quick action: Rupture disksare very fast acting. Therefore, theyare used in cases where relief valvesmay not be fast enough to prevent acatastrophic failure. Some engineersprefer to use rupture disks to preventheat-exchanger-tube ruptures becausethey are concerned that pressure relief

valves would be too slow to preventpressure build-up scenarios.To prevent plugging of relief

valves: In certain processes, the pro-cess fluid contains solid particles thatmay cause blockage within a relief

valve, rendering it useless. In suchcases, a rupture disk is normally usedupstream of the relief valve. Purifiedterephthallic acid plants (PTA) is atypical application example.Handling highly viscous liquids:In systems handling highly viscousliquids, such as polymers, depressur-ization through a relief valve may betoo slow for a given situation. A rup-

Feature Report

40 CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2008

Feature Report

Siddhartha MukherjeeLurgi India Company Ltd.

Pressure-ReliefSystem Design

Unexpected high-

pressure situations canbe relieved with a proper

relief-system design


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ture disk is the preferred choice.Loss prevention: In systems han-dling low-molecular-weight hydrocar-bons, there is always a chance thatsome material will pass through thepressure relief valve into the flaresystem leading to material loss. Insuch cases, a rupture disk is normallyused upstream of the relief valve.For economic reasons: Many pro-cess industries use exotic materials,such as titanium and Hastelloy C.In these cases, rather than having

a pressure relief valve made of Has-telloy C, it may be cheaper to have arupture disk made of Hastelloy C fol-lowed by a stainless-steel pressure-relief valve.

PRESSURE RELIEF SYSTEMSOpen and closed systemsIn cases where non-hazardous fluidsare used, such as steam, water and air,a typical pressure-relief system con-sists of several pressure relief valvesthat discharge through short tail pipesto the atmosphere. These systems aretermed open disposal systems.When hazardous fluids, such as hy-

drocarbons are in use, the tail pipesare connected to a common flareheader, which is ultimately routed toa flare stack where the hydrocarbonsare burned. In many cases, the fluidrelieved is toxic or flammable. In suchcases, it is mandatory to dischargethe gases through a closed disposalsystem such as the flare. The flaresystem converts the flammable va-pors to less objectionable compoundsby combustion.

Components of closed systemsPressure-relief valve-outlet pip-

ing:The flare system starts with out-let pipes from the various pressure re-lief valves of a unit. These valves arepiped together to a common unit-flareheader, which is routed to the unit-flare knockout (K.O.) drum.Unit flare header:Discharge pipesfrom the pressure relief valves in in-dividual units are connected to therespective unit-flare headers. Theseheaders are either connected directlyto the main flare header, or are routedto the flare K.O. drums, which in turnare connected to the main flare header

(Figure 1) with a recommended mini-

mum slope of 1:500. All unit flareheaders are continuously purged fromthe upstream end towards the respec-tive K.O. drums to avoid ingress ofair into the system. Fuel gas, or inertgases, such as nitrogen are typicallyused as purge gas.Unit-flare knockout drum:In caseswhere the discharge from a unit is ex-pected to contain appreciable quanti-ties of liquids, especially corrosive,fouling and congealing liquids, a unit-flare K.O. drum of a suitable size is

mandatory. Another reason for requir-ing such a drum may be that it is notfeasible to have all headers continu-ously slope toward the main-flare K.O.drum. In this case, the unit flare head-ers are sloped toward the unit-flareK.O. drums. The vapors from thesedrums are then routed to the mainflare header [2].

Unit-flare K.O. drums are sizedto separate particles in the rangeof 300600 microns, and hold liquiddischarge for 510 min from a single

source. The liquid collected in thesedrums should preferably be drainedby gravity to the blowdown drum. Ifa congealing type of liquid is likely tobe discharged, the drums should beheat traced or provided with steamcoils [2].Main flare header:The main flareheader receives discharge through in-dividual unit-flare headers, or throughunit-flare K.O. drums. The flare headershould not have pockets and should befree draining toward the nearest K.O.drum, typically with a slope of 1:500.

Although flare headers are nor-mally sized based on pressure drop,

CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2008 41

01-PSV-01

01-PSV-02

01-PSV-03

01-PSV-04

01-PSV-05

Unit 01

02-PSV-01

02-PSV-02

02-PSV-03

02-PSV-04

02-PSV-05

Unit 02

03-PSV-01

03-PSV-02

03-PSV-03

03-PSV-04

03-PSV-05

Unit 03

Unit flareK.O. drum

Unit flareK.O. drum

Unit flareK.O. drum

Main flareK.O. drum

Main flare header

Flare stack

Battery limit(unit 01)



FIGURE 1. A network of flare head-ers may be used in a complex process

DEFINITION OF TERMSSet pressure:The inlet gauge pressure at which the pressure relief valve is set to openunder service conditionOverpressure:The pressure increase over the set pressure of the relieving device ex-pressed as a percentageRelieving pressure:The sum of the valve set pressure and the overpressureSuperimposed back pressure:The static pressure that exists at the outlet of a pressurerelief device at the time the device is required to operate. It is the result of the pressure inthe discharge system coming from other sources and may be constant or variableBuilt-up back pressure:The increase in pressure in the discharge header that developsas a result of flow after the pressure relief device opensBack pressure:The pressure that develops at the outlet of a pressure relief device afterthe pressure relief device opens. It is the sum of the superimposed and the built-up backpressures

Abbreviations

P1 Pressure at pipe inlet, kg/cm2aP2 Pressure at pipe outlet, kg/cm2aW Gas flowrate, kg/hZ Gas compressibility factorT Gas temperature, K

Mw Gas molecular weight, kg/kmolf Moodys friction factork Ratio of specific heats, Cp/Cva Absolute pressure, as in kg/cm2ag Gauge pressure, as in kg/cm2g


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velocity cannot be ignored. A Machnumber in the range of 0.20.5 isrecommended. The third criterionthat should be checked is the changein density of flare gases along thelength of the flare header. In manycases where flare discharges are athigh temperatures, the flare gases

cool down due to the length of theflare header. This leads to an increasein density and, correspondingly, a de-crease in flowrates. Therefore, whileestimating pressure drops in suchflare lines, it is a good idea to dividethe header into sections and estimatepressure drops separately.Main-flare knockout drum: In ad-dition to the unit-flare K.O. drum, amain-flare K.O. drum close to the flarestack should be provided. This takescare of condensation in the header

that results from atmospheric cooling.Similar to the unit-flare K.O. drums,these drums are also sized to separateout liquid droplets of 300600 micronsin size, and for holdup of 2030 min [1]of liquid release. Pumps are installedwith the K.O. drum to transport anycollected liquid to a safe location. Thepumping capacity should allow theliquid holdup to be emptied in 1520min. When congealing liquids are inuse, the drums should be providedwith steam coils [2].Seal drum: Seal drums are locatedclose to the flare stack or are sometimesintegral with the flare stack. These

drums protect against flame flash-back from the flare tip. The seal drumshould have a diameter of at least twotimes the flare pipe diameter [1].Flare stack: Flare stacks are usuallyelevated structures designed to burnflammable vapors.

Relief system pipingInlet piping: The inlet piping fromthe protected equipment to the pres-sure relief valve should be sized toprevent excessive pressure loss thatcan cause chattering with consequentreduction in flow and damage to seat-ing surfaces. The recommended prac-tice is to limit the total pressure dropin the inlet piping to 3% of the safety-

valve set pressure [1]. The piping isdesigned to drain towards the pro-tected vessel.Discharge piping: In case of over-pressurization in a vessel, the rel-evant pressure-relief valve will startto open at the set pressure. At thismoment, the downstream pressure atthe valve is the superimposed backpressure of the system. The valvekeeps opening as the pressure buildsup. The resultant flow creates a built-up back pressure on the dischargepipe. As long as the built-up backpressure is less than the overpres-sure after the valve opens, the valvewill remain open and perform satis-factorily. If however, the built-up backpressure develops at a rate greater

than the overpressure, it will tend toclose the valve. Therefore, proper siz-ing of discharge pipes is very impor-tant in such systems.

Discharge piping and manifoldsare sized for the contingency thatproduces the largest relief load. Pipesizing is carried out by working back-

ward from the battery limit of theunit flare header up to the outlet ofindividual, pressure safety valves.The superimposed back pressure ofthe flare header at the battery limitis defined. Thereafter, based on therelief loads, pressure drop calcula-tions are carried out to arrive at theback pressure of the individual, pres-sure relief valves. In the course ofthe calculations, two parameters arechecked for compliance: The Mach number at each pipe sec-

tion should not exceed 0.5 The back pressure at each safety

valve should not exceed 3050% ofthe set pressure

The isothermal flow equation basedon the outlet Mach number is given by

API [1]. This method calculates pres-sure buildup backward up to the out-let of relief valves, thus avoiding theneed for trial and error methods:

(1a)or,

Feature Report


95,648 kg/h69,356 kg/h28,012 kg/h

Unit flareheader

Mainflare

header

To main flare

K.O. drum

12,8

70kg/h

13,4

22kg/h

10,5

36kg/h

14,0

76kg/h

12,9

60kg/h

15,0

52kg/h

16,7

32kg/h

ABCD

PSV-06

PSV-05

PSV-04

PSV-03

PSV-02

PSV-01

PSV-07

PSV-08

PSV-09

PSV-10

PSV-11

PSV-12

FIGURE 2. This sketch of a typical flare system is used as the basis for the samplecalculations explained in the given example

TABLE 1. RELIEF LOAD SUMMARY

Relief ValveTag No.

Relief load, kg/h

Coolingwater failure

Externalfire

PSV-01 12,870 13,453

PSV-02 6,750

PSV-03 10,536 9,872

PSV-04 14,076 8,970

PSV-05 5,783

PSV-06 12,960 11,423

PSV-07 15,052 6,432

PSV-08 5,764

PSV-09 16,732 8,976

PSV-10 7,432

PSV-11 13,422 5,133

PSV-12 9,984

Total load 95,648


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(1b)

The Mach number at the outlet ofeach pipe section is given as follows

[1]:

(2)In the next section, a solved exampleillustrates the procedure.

EXAMPLEAn extractive distillation plant hastwelve safety valves. There are twomajor relief scenarios: cooling water

failure and external fire. Table 1 sum-marizes the relief rates under thesetwo conditions. The governing case isthe cooling water failure because itoccurs plant wide. External fire oc-curs only at localized areas and therelief loads come from only a coupleof safety valves. Hence, we will con-sider the cooling water failure casehere. For simplicity, we will assumethat set pressures of all safety valvesare 5.0 kg/cm2g.

The flare network is divided intosegments as shown in Figure 2. Seg-ment 1 is a section of the flare headerbetween the battery limit A and point

B. Likewise, segment 2 is a section ofthe flare header between points B andC. Let us assume a superimposed backpressure at point A of 1.5 kg/cm2a.

Piping between points A and B

DataFlowrate: 95,648 kg/h (Table 1)Pressure at point A,P2: 1.5 kg/cm2aMolecular weight of vapor,Mw: 86Temperature of vapor: 100CStraight length of pipe : 15 mNumber of elbows: 0Number of valves: 1Number of tees: 0Compressibility factor: 0.95Ratio of specific heats, Cp/Cv: 1.4

Viscosity: 0.000009 kg/m s

Roughness factor, :0.0001m (0.1 mm)

Pipe diameter,D(assumed):0.30 m (12 in.)

Calculations

Density of vapor: PMw/ZRT = 4.2899 kg/m3

Volumetric flowrate:95,648/(4.2899 x 3,600) = 6.193 m3/s

Velocity in pipe: 6.193 x 4/( x (0.30)2)

= 87.61 m/sReynolds number,NRe:

0.30 x 87.61 x 4.2899/0.000009 = 12,528,371Outlet Mach Number,M2:

3.293 x 107(95,648/(1.5 x 0.302)) x(0.95 x 373/1.4 x 86)0.5= 0.4003

Fannings friction factor (to be solvedby iteration):

1/f = 4log[/(3.7D) + 1.256/(NRef)]

= 0.0038Moodys friction factor: 4 x 0.0038

= 0.0152

Equivalent length: 17.8 mfL/D: 0.0152 x 17.8/0.3Using Equation (1b),P1/P2 is calcu-lated to be: 1.08226

P1: 1.08226 x 1.5 = 1.6234 kg/cm2aHence, the pressure at point B is1.6234 kg/cm2a

Piping between point B andPSV-01, 1st trialData

Set pressure of PSV-01: 5.0 kg/cm2gFlowrate: 12,870 kg/h (Table 1)Pressure at point B,P2:

1.6234 kg/cm2aMolecular weight of vaporMw: 86Temperature of vapor: 100CStraight length of pipe: 30 mNumber of elbows: 3Number of valves: 1Number of tees: 0Compressibility factor: 0.95Ratio of specific heats, Cp/Cv: 1.4

Viscosity: 0.000009 kg/m s

CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2008 43

TABLE 2. FLARE DISCHARGE PIPING CALCULATIONS

Segment

Setpres-sure

Linesize

Linesize

Flow-rate

Pres-sureP2

Den-sity

Rough-nessfactor

Pipelength

No. oftees

No. ofvalves

No. ofelbows

Pres-sureP1

Pres-sureP1

% ofsetpres-sure

Machno.

kg/cm2g

m in. kg/h kg/cm2a

kg/m3 mm m kg/cm2a

kg/cm2g

A to B 0.30 12 95,648 1.5000 4.2899 0.1 15 0 1 0 1.6234 0.5904 0.4004

B to PSV-01 5.0 0.10 4 12,870 1.6234 4.6428 0.1 30 0 1 3 2.7780 1.745 34.9 0.4480

B to PSV-11 5.0 0.15 6 13,422 1.6234 4.6428 0.1 30 0 1 3 1.8282 0.7952 15.9 0.2077

B to C 0.25 10 69,356 1.6234 4.6428 0.1 15 0 0 0 1.7528 0.7198 0.3863

C to PSV-03 5.0 0.15 6 10,536 1.7528 5.0129 0.1 30 0 1 3 1.8712 0.8382 16.8 0.1510

C to PSV-04 5.0 0.15 6 14,076 1.7528 5.0129 0.1 30 0 1 3 1.9616 0.9286 18.6 0.2017

C to PSV-09 5.0 0.15 6 16,732 1.7528 5.0129 0.1 30 0 1 3 2.0449 1.0119 20.2 0.2398

C to D 0.20 8 28,012 1.7528 5.0129 0.1 15 0 0 0 1.8111 0.7781 0.2258

D to PSV-06 5.0 0.15 6 12,960 1.8111 5.1796 0.1 30 0 1 3 1.9832 0.9502 19.0 0.1797

D to PSV-07 5.0 0.15 6 15,052 1.8111 5.1796 0.1 30 0 1 3 2.0417 1.0087 20.2 0.2088


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Feature Report


Roughness factor, :0.0001m (0.1 mm)

Pipe diameter,D(assumed):0.08 m (3 in.)

Calculations

(detailed steps not repeated)Density of vapor: 4.6428 kg/m3

Volumetric flowrate: 0.7700 m/sVelocity in pipe: 153.186 m/sReynolds number,NRe: 6,321,901Outlet Mach number: 0.6998Fannings friction factor: 0.0052Moodys friction factor: 0.0208Equivalent length: 40.65 m

P1/P2: 2.67214P1: 4.3380 kg/cm2a

Hence, the back pressure at PSV-01 is4.3380 kg/cm2a, or 3.3050 kg/cm2g.

The ratio of the back pressure tothe set pressure for PSV-01 works outto 66.10% which is more than thatrecommend for balanced bellow-type

valves. In addition, the Mach numberis 0.6998, which is greater than therecommended limit of 0.5. Therefore,we need to increase the line size in thestretch between B and PSV-01. Let usnow select a line size of 4 in. for thesecond trial.

Piping between point B andPSV-01, 2nd trialData

Set pressure of PSV-01: 5.0 kg/cm2gFlowrate: 12,870 kg/hPressure at point B,P2:

1.6234 kg/cm2aMolecular weight of vapor,Mw: 86Temperature of vapor: 100CStraight length of pipe: 30 mNumber of elbows: 3Number of valves: 1

Number of tees: 0Compressibility factor: 0.95Ratio of specific heats, Cp/Cv: 1.4

Viscosity: 0.000009 kg/m sRoughness factor, : 0.0001m (0.1 mm)Pipe diameter,D(assumed):

0.10 m (4 in.)Calculations

(detailed steps not repeated)Density of vapor: 4.6428 kg/m3

Volumetric flowrate: 0.7700 m/sVelocity in pipe: 98.03 m/sReynolds number,NRe: 5,057,041Outlet Mach Number: 0.4480Fannings friction factor: 0.0049

Moodys friction factor: 0.0196Equivalent length: 43.31 m

P1/P2: 1.71121P1: 2.7780 kg/cm2a

Hence, the back pressure at PSV-01 is2.7780 kg/cm2a, or 1.745 kg/cm2g.The ratio of the back pressure to

the set pressure for PSV-01 is 34.90%,which is acceptable. The Mach num-ber is 0.4480, which is also within ac-ceptable limits.

A summary of parameters and cal-culation results is given in Table 2.

INSTALLATION FEATURESCorrect installation of relief valvesand the associated relief system is

very important for their proper opera-tion during upsets to ensure the safetyof personnel and the plant. Some use-ful guidelines follow: Pressure relief valves should be

connected to the vapor space of theprotected equipment

The inlet line should be self drain-ing back to the process vessel. Thisis to prevent accumulation of liquidthat can corrode or block the sys-tem. Likewise, the outlet line fromthe pressure relief valve should be

self-draining to the flare header. Tomeet this requirement, it is recom-mended that relief valves are in-stalled at a high point in the sys-tem (Figure 3)

For reliable overpressure protectionit is best to install pressure relief

valves without any isolation valves.However, pressure relief valvessometimes do not reseat properlyand start to leak. Therefore, in manycases, two safety valves are installedto allow replacement of relief valves

that are leaking while the plant isin operation. This also facilitatestesting and servicing of relief valvesregularly without interrupting plantoperations

Whenever two pressure relief valvesare installed as mentioned above,isolation valves are provided for eachrelief valve (Figure 4). This is to fa-cilitate isolation of the A valve formaintenance while taking valve Bonline when required. In such cases, a-in. bleeder valve with an isolation

valve is recommended in between theisolation valve and the pressure relief

valve. This is needed because when

the A valve has been isolated, thesection between the isolation valveand the pressure relief valve is stillat the operating pressure of the col-

umn. The bleeder is used to depres-surize this section before the valveis removed from the line, otherwiseit could be an unsafe condition formaintenance personnel [4]

Whenever two pressure reliefvalves are provided, it is mandatorythat a mechanical interlock systemis installed between the respectiveisolation valves to ensure that oneof the isolation valves is open at alltimes. This is to eliminate opera-tor error, which might mistakenly

create a situation where both theisolation valves are in the closedposition, leading to unavailabilityof either of the relief valves for anyparticular equipment

Pressure relief valves in steam,water or air service are connectedto the atmosphere through a short

vertical pipe. To keep this pipe freefrom liquid accumulation, a smallweep hole is drilled at the lowestpoint of this pipe

Whenever a rupture disk is installedupstream of a pressure relief valve,it is important to have a pressureindicator in the section between the

Column

Safetyvalveoutlet line

Safety valve

Columnoutlet line

FIGURE 3. In order to make reliefvalves self-draining to the flare header,it is recommended that they be in-stalled at a high point in the system


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two (Figure 5). The reason for thisis that in case of a pin-hole leak inthe disk, vapors from the protected

equipment would pass through tothe section between the disk andthe safety valve. After some time,the pressures upstream and down-stream of the disk would be the sameand the disk would never burst

The unit and the main flare headershould slope towards the main,flare-header K.O. drum. This is toensure that condensed vapors, ifany, do not back up and accumulateimmediately downstream of thesafety valves

Edited by Dorothy Lozowski

References1. A Guide for Pressure-Relieving and Depres-

surizing Systems, API Recommended Prac-tice 521, American Petroleum Institute, 4thed., March 1997.

2. Process Design and Operating Philosophieson Pressure Relief and Disposal Systems,Oil Industry Safety Directorate, OISD-Standard-106, Government of India, August1999.

3. Leckner, Philip, Rupture Disks for ProcessEngineers Part-1, www.cheresources.com/asiseeit3.shtml

4. Sizing, Selection and Installation of Pres-sure - Relieving Devices in Refineries, Part

II Installation, API Recommended Practice520, American Petroleum Institute, 4th ed.,December 1994.

AuthorSiddhartha Mukherjeeis the

group leader process, at LurgiIndia Company Ltd. (A-30,Mohan Cooperative IndustrialEstate, Mathura Road, NewDelhi 110 044, India. Phone:+911142595365; Fax:+911142595051; E-mail:[email protected]). For the past eight years,he has been involved in a leadrole in the design, precom-

missioning and commissioning of chemical andpetrochemical plants in India and elsewhere. Hehas also been involved in inorganic and olech-emistry while at Lurgi. Prior to this, Mukher-jee worked as an environmental engineer withDevelopment Consultants Ltd. (Kolkata), doingvarious environmental assessment projects in-volving thermal power plants. Mukherjee earnedhis B.Tech. and Ph.D. Ch.E. degrees from the

Indian Institute of Technology, Kharagpur. Heholds lifetime memberships in Indias Instituteof Engineers and the Indian Institute of Chemi-cal Engineers.

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Inletpiping

Outlet piping

Pressurerelief valve

Isolationvalve

3/4"Bleeder valve Isolation

valves

Bypassline

Blind

Protected vessel

Rupture disk

Pressuregage

Relief valve

Protected

vessel

FIGURE 4. When two pressure reliefvalves are used, isolation and bleedervalves are needed to prevent an unsafecondition

FIGURE 5. When a rupture disk isinstalled upstream of a pressure reliefvalve, a pressure indicator is needed be-tween the two

pressure relief system design

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