optimizing pressure relief systems

7/25/2019 Optimizing Pressure Relief Systems

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Environmental Manager

CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 201578

Pressure relief systems for the chemi-cal process industries (CPI) are es-sential to prevent a process system,or any of its components, from being

subjected to pressures that exceed the maxi-mum allowable accumulated pressure, byemergency venting to a closed relief system.

These relief systems are normally veryconservatively designed. For large, new

petroleum refineries with capacities around300,000 barrels/day (bbl/d), this can resultin costs of up to 1% of the total refinerycapital investments (Capex).

This article presents simple project alter-natives to traditional closed relief systems[1], based on American Petroleum Institute(API) standards, that can present significantinvestment-cost reductions.

BackgroundOverpressurization of process units canoccur due to several reasons as indicated in

API-521 [2]. Some of those reasons are thefollowing:

General power failureCooling water failureInstrument failureExternal fire

Normally, general power failure or utilityfailure results in the highest vapor load for aclosed pressure-relief system, and is there-fore used as the design case. Before sizing aclosed relief system, it is advisable to reducethese very high vapor loads by the following:

Use high-integrity protection systems1.(HIPS) as recommended in API-521,which can result in a significant reduc-tion of the vapor flowrates to the flare.

Realize dynamic-system load model-2.ing. This analysis for a complete petro-leum refinery is very complex and is notnormally used, but it can also result inflowrate reductions.

After defining the minimum possible vaporflowrates that correspond to the overpres-sure relieving rates defined by the designcase, the closed relief system may be sized.

Traditional closed systemsA closed pressure-relief system is designedto safely control overpressurization of pro-cess units during emergencies by relievingthe vapors to the flare, which destroys hydro-carbons in a high-temperature flame. Figure1 shows a typical closed relief system thatcollects vapors and liquids in process-unitheaders and separates the liquid in process-unit knockout (KO) drums before sending thevapor phase to the main flare header, and fi-nally to the flare unit for destruction.

In the traditional system, the unit KO drumsand the flare KO drums are projected for themaximum vapor and liquid flowrates as de-termined from the analysis of the overpres-sure causes and indicated in API-521 [2].

The KO drums, process units and flareunit, are sized to separate particles in therange of 300600 m in diameter, and tohold liquid discharge for 20 to 30 minutes asper API-521 item 7.3.2.1.2 for these maxi-mum flow conditions.

The unit flare headers and the main flareheader are also sized for these maximumflowrates. All the headers slope with a mini-mum inclination of 1:500 toward their respec-tive KO drums, and are continuously purged

Optimizing Pressure Relief Systems

Alternative designs for pressure relief systems may offer investment cost savings

Peter Cain

Process Consultant

IN BRIEF

BACKGROUND

TRADITIONAL CLOSED

SYSTEMS

OPTIMIZED CLOSED

SYSTEMS

COST SAVINGS

FIGURE 1. This sketch shows a simplified pressure-relief system for a petroleum refinery

1:500

1:500 Main flare header

Flarestack

Unit BL Unit BL Unit BL

1:500 1:500

Unit KO drum Unit KO drum Unit KO drum

Safety valvesunit 1

Safety valvesunit 2

Safety valvesunit n

Flare KO

drum


2/6CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 2015 79

using combustion gas or nitrogen

from the upstream end toward theKO drums to avoid ingress of air intothe system.

Optimized closed systemsThe calculation criteria for sizing theflare KO drums and process-unit KOdrums result in very large vessels.

This implies the need to install the

collection headers very high abovegrade level, since they must drainto the KO drums. Equipment, suchas air coolers that must be mountedabove the process unit headers areconsequently also very high. Thisrequires long stretches of processpiping to and from the equipment.

Figure 2 shows such a situation.

If it was possible to change the de-sign criteria for the process-unit KOdrums, the process-unit flare headerand the air coolers may be installedat a lower level with considerablylower installation costs as a result ofthe use of less structural steel andprocess- and flare-header piping.

FIGURE 2. This large, horizontal process-unit KO drum requires the air coolersto be mounted very high

FIGURE 3. A large, horizontal flare-unit KO drum can require a very high pipingarrangement

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At the flare unit, the KO drum iseven larger than the process unitdrums, and as a result, the main flareheader at the inlet to the vessel isvery high, as can be seen in Figure 3.Consequently, the main flare header

at the flare-unit battery limits (BL) isalso very high. Since the flare headerin large, new, petroleum refineries isnormally very long (about 2 km), itmeans that at the farthest point fromthe flare unit the header is at least5 m higher than at the flare-unit bat-tery limits. This installation requires alot of structural steel to maintain theflare header at the required height,and consequently high investmentcosts for the pipe rack are required(Figure 4).

The criteria used to size the KOdrums for carryover of droplets thatare 600 m in diameter is, accordingto API-521, to eliminate the possibil-ity of incomplete combustion with ex-cessive smoking, possible burningrain, and even flame-out of the flare.

It is clear that the flare unit KOdrum must be sized according tothis limitation as it is upstream of theflare. However, this limitation is notnecessary for the process unit ves-sels, as these are upstream of the

flare-unit KO drum. In this case, iflarge droplets are carried over fromthe process-unit KO drums, theflare-unit KO drum will retain themand maintain adequate conditionsfor the flare.

API-521 item 6.4.3.6.7 presents aclear explanation of the design pa-rameters for these vessels:

Some flare systems require a flareknockout drum to separate liquidfrom gas in the flare system and tohold the maximum amount of liquidthat can be relieved during an emer-gency situation.

Knockout drums are typically lo-

cated on the main flare line upstreamof the flare stack or any liquid seal. Ifthere are particular pieces of equip-ment or process units within a plantthat release large amounts of liquidto the flare header, it is desirable to

have knockout drums inside the bat-tery limits to collect these liquids.This reduces the sizing requirementsfor the main flare knockout drum, aswell as facilitates product recovery.

In general, a flare can handle smallliquid droplets. However, a knockoutdrum is required to separate drop-lets larger than 300 m to 600 min diameter in order to avoid burn-ing liquid outside the normal flameenvelope. If unit knockout drums areprovided upstream of the main flare

knockout facilities, these drums maybe sized to separate droplets typi-cally greater than 600 m in diam-eter. The use of unit knockout drumseffectively reduces the sizing require-ment for the main flare knockoutdrum and facilities, See 7.3.2.1.

The liquid hold-up capacity of aflare knockout drum is based onconsideration of the amount of liquidthat can be released during an emer-gency situation without exceedingthe maximum level for the intended

degree of liquid disengagement. Thishold-up should also consider anyliquid that can have previously ac-cumulated within the drum that wasnot pumped out. The hold-up timesvary between users, but the basicrequirement is to provide sufficientvolume for a 20 min to 30 min emer-gency release. Longer hold-up timesmight be required if it takes longer tostop the flow. It is important to realizeas part of the sizing considerationsthat the maximum vapor releasecase might not necessarily coincidewith the maximum liquid. Therefore,the knockout drum size should be

determined through considerationof both the maximum vapor releasecase as well as the release case withthe maximum amount of liquid.[2]

Analyzing the above, we can con-clude the following:

Process unit KO drums are not1.mandatory.There is no size limit for droplet2.

carryover of process-unit KOdrums larger than 600 m indiameter is permitted.Process-unit KO drums, if in-3.stalled, are provided to collectliquid.Flare-unit KO drums must be4.sized in order to retain dropletslarger than 600 m, as it is up-stream of the flare.Process-unit KO drums should5.be designed to provide sufficientvolume for 2030 min emergency

liquid release unless the expectedresponse time is longer.

Taking into consideration theabove conclusions, the process-unitKO drums can be sized consider-ing basically only the liquid hold-uptime. The flare-unit KO drums, lo-cated downstream, will collect liquiddroplets larger than 600 m in diam-eter. Therefore, the criteria for siz-ing process-unit KO drums can bechanged from separation of dropletsgreater than 600 m in diameter to

liquid hold-up.As there is no worry about droplet

carryover, it is possible to considerthe use of vertical KO drums in theprocess units instead of a horizontalvessel, as they present several ad-vantages when designed only for thecollection of liquid as seen below:

Smaller vesselHas a smaller footprint and canbe installed closer to the piperack

The height of the process unit

header is lower, which saves onstructural steel in the pipe rack

The arrangement of the processunit header can be simplified, re-sulting in a smaller total length

Air coolers, if installed, can belowered, reducing process pip-ing to and from the equipment

Reduced weight of the pipe rackand KO drum reduces founda-tion requirements

These vertical KO drums can bedesigned without internals, andwith the outlet flare nozzle at 180deg from the inlet nozzle and at thesame elevation, as the liquid droplet

FIGURE 4. This sketch depicts a very high pipe rack to support the main flare header



carryover is not in question. How-ever, the designer should avoid verylarge droplet carryover, which resultsin vessels with a smaller length-to-diameter ratio than usual for verticalgas-liquid separation vessels. Thereason to remove very large drop-lets in the process units is not tooverload, with liquid, the new main

header proposal presented below.As can been seen in API-521 item

6.4.3.6.7 (quoted earlier), process-unit KO drums are not mandatory.But, because condensation alwaysoccurs in flare headers, it is recom-mended to maintain the process-unitKO drum unless this header can bedrained to the main flare header out-side the battery limit (OSBL).

This change of design criteria for theprocess-unit KO drums will reducethe vessel volume by up to 80%, re-

sulting in a considerable investment-cost reduction for the inside the bat-tery limit (ISBL) relief system.

OSBL cost reductions may be ob-tained for the main flare header byreducing the elevation above gradelevel of this very large (diametersaround 80 in.) and long pipe. This

FIGURE 5. This flare-unit KO drum is equipped with horizontal inlet connections

Vapor outlet

Front viewTop view

Vapor inlet

Vapor inlet

FIGURE 6. This schematic shows a closed pressure-relief system using the alternative suggested here

1:500 1:500 1:500

1:500 1:500 1:500

Main flare header

Flarestack

Unit BL Unit BL Unit BL

1:500

Unit KO drum Unit KO drum Unit KO drum

Safety valvesunit 1

Safety valvesunit 2

Safety valvesunit n

Flare KOdrum

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can be done in two steps thefirst of which is to reduce the headerheight at the flare unit battery limits.This may be done by a simple al-teration of the headers inlet pipingarrangement to the very large, hori-zontal flare-unit KO drum whichcan be over 8-m dia. by changing

the vertical inlet connections to hori-zontal ones, as indicated in Figure 5.In large petroleum refineries, this al-teration to the inlet connections canresult in a reduction of the headerheight at the flare unit battery limitsby more than 4 m.

The second step is to reduce thelarge increase in height of the mainflare header from the flare unit tothe farthest process unit becauseof the required slope of 1:500. Thismay be achieved by installing a ves-

sel and pumps along the pathwayto collect condensate, thus dividingthe header into two approximatelyequal parts. The first part is fromthe farthest process-unit drains tothe header collection vessel, andthe second is from this vessel to theflare-unit KO drum (an intermediate

main-header KO drum).This suggestion is based on API-

521 item 7.3.1.3.8, which states:A small drain pot or drip leg can benecessary at low points in lines thatcannot be sloped continuously to theknockout or blowdown drum. [2]

An alternative to the installation

of a second main-header KO drumand pumps, which require addi-tional investment costs, is to in-tegrate the process units with themain flare header by carefully de-signing the process-unit KO drumsand headers.

This alternative is to use some,two or three, of the process-unit KOdrums to receive condensate fromthe OSBL main-flare header. In thiscase, it is important to make surethat the liquid hold-up capability of

the selected process-unit KO drumsconsiders this additional servicerequirement and that they are ad-equately sized. It is also necessaryto make sure that the response timeused to size all the process-unit KOdrums is adequate and that largequantities of liquid will not be carried

over to the main flare header. Provi-sions must be made to permit iso-lation of the process-unit KO drumsused for this service from the pro-cess units during shutdown.

This installation collects conden-sate formed in the main flare headeralong its extension, reducing the

amount carried over to the flare-unitKO drum and permitting a reductionin its size.

Figure 6 shows a schematic de-sign of a closed pressure-relief sys-tem using the alternatives suggestedin this section.

Further integration of the ISBL andOSBL flare projects can bring gainsby considering the pressure profileof the main flare header determinedby the refinery-flare design case. Thepressure at the battery limit of the

process unit farthest from the flareunit will be higher than the processunit closest to the flare unit. In thetraditional approach, the maximumpressure at the battery limits of allthe process units is defined as aconstant value and is the same forall process units.

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Taking this into consideration,this profile permits that the processunits nearest to the flare unit can re-duce the diameter of the ISBL flareheaders until the maximum permit-ted back pressure is reached for themost critical pressure-safety valve(PSV). Normally the most critical PSV

is the valve with the lowest set pres-sure. All ISBL headers should be de-signed for the maximum possible ve-locity and values of over 35% of theMach number should be pursued,but limited to 50%, and approxi-mately maintained along the ISBLheader, by adjusting the diameter tominimize header costs.

The main flare header should alsobe sized carefully, to minimize thediameter, by considering the maxi-mum possible process-units bat-

tery-limit pressures defined by theback pressure of the PSVs. Oncemore, the diameter of the headershould be adjusted to maintain thevapor velocity for the design case,which is approximately constantfrom the farthest process unit tothe flare unit.

Cost savingsAs can be seen from the abovediscussion focused on petroleumrefineries, fairly simple project con-siderations can reduce the cost ofconstruction of a closed pressure-relief system. It is possible to sig-nificantly reduce the size of the

process-unit KO drums, while atthe same time save considerablestructural steel used for the ISBLand OSBL pipe racks. It is also in-dicated that by careful calculationsof the closed flare system with in-tegration of the ISBL with OSBL,it is possible to reduce the flareheader diameters.

In comparison with the traditionalapproach, this new manner to proj-ect the pressure relief system offersa reduction of about 30% in the

height of the main flare header andaround 20% in the height of the unitflare headers. This together with themuch smaller unit KO drums, re-duced header diameters and lessprocess piping for the lowered aircoolers, permits an investment costreduction for the relief system of up

to 20% as compared with the tradi-tional project. n

Edited by Dorothy LozowskiReferences1. Mukherjee, S., Pressure-Relief System Design, Chem.Eng., November 2008, pp.4045.

2. Pressure-Relieving and Depressuring Systems, APIStandard 521, Fifth Edition, January 2007 and Ad-dendum, May 2008.

AuthorPeter Cain is a process con-sultant for Petrobras in Brazil(Phone: 55-21-98211-0627;Email: [email protected]). He has more than 40 yearsof experience in positions includ-ing process engineer, job leader,technical coordinator, technicalmanager, principal partner andconsultant in the petrochemical,

petroleum, chemical, industrial waste and nuclearfields. For the past 16 years, he has worked withPetrobras on the installation of several industrial unitsin areas such as hydrogen generation, hydrocrackerand hydrotreatment units, power generation, cooling-water towers, crude and vacuum distillation and oth-ers that are parts of modern petroleum refineries. Heis also participating in the basic engineering and FEEDprojects of two, new 300,000-bbl/d refineries and haspresented several project revisions to reduce Capexand Opex. He has also realized the clean up of sev-eral areas contaminated with oil as principal partnerand founder of a waste treatment company. Cain holdsan honors degree in applied physics from Bath Univer-sity in England.

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optimizing pressure relief systems

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