Aubry Shackelford Inglenook Engineering, Inc. 664 County St; Attleboro, MA

An Apprach t Reducing Risk during Online Islatin, Remval and Maintenance f Relief Devices Using Engineering Assessment Aubry Shackelfrd Inglenk Engineering, Inc. 664 Cunty St; Attlebr, MA 02703 aubry@inglenkeng.cm Jeff Heil; Inglenk Engineering; jeff@inglenkeng.cm Ian Serg; Inglenk Engineering; ian@inglenkeng.cm Keywrds: Relief Device Islatin; Risk Reductin Abstract Overpressure prtectin must be prvided fr pressurized equipment by means f pressure relief devices, which smetimes require islatin and remval fr inspectin r repair. It wuld be advantageus t have a means f islatin withut shutting dwn the prcess, especially in light f the additinal risks assciated with shutdwn and startup; hwever, ding s may put the pressurized system at risk f being unprtected in the event f verpressure. Current recgnized practices f prviding dedicated persnnel fr mnitring and respnding t an verpressure event, r installatin f an independent spare, are nn-ideal in many instances. An engineering assessment can identify specific ways fr prviding temprary verpressure prtectin, including eliminatin f rt causes f verpressure events, prvisin f alternative pathways t verpressure prtectin, mdificatin f prcess peratins, and/r establishment f a means f equivalent r better risk reductin fr the duratin f the islatin. Guidelines and examples are prvided t illustrate the analysis, alng with recmmendatins fr dcumentatin. 1 Intrductin Overpressure prtectin must be prvided fr pressurized equipment by means f pressure relief devices, which refer t any device (mechanical r instrumented, passive r active) that acts autmatically and reliably t relieve material in respnse t an increase in system pressure. While pressure relief devices nrmally refer t pressure relief valves and bursting discs; explsin hatches, water seals, buckling pin devices, and pressure/vacuum breather valves are amng the devices that meet this requirement. These pressure relief devices smetimes require islatin and remval fr inspectin, maintenance, r repair, and it wuld be advantageus t have a means t d s withut shutting dwn the prcess, especially in light f the additinal risks assciated with shutdwn and startup; hwever, ding s may put the pressurized system at risk f being unprtected in the event f verpressure. The bjective f this paper is t prvide guidance n the engineering assessment fr islatin f pressure relief devices while maintaining

peratin f the prcess, with recmmendatins f actins that may be required t prvide temprary verpressure prtectin. ASME Sectin VIII nnmandatry appendix (NMA) M M.5.6 indicates that the use f dedicated persnnel t mnitr and respnd t a build-up f pressure within the vessel by means f clearly written prcedures is a means t ensure verpressure prtectin fr an at-risk pressurized system while its assciated pressure relief device is islated and the prcess is running. [1] There are several factrs that can make this a sub-ptimal slutin, including the elevated risks t persnnel, the speed at which verpressure may ccur relative t respnse time, and the limited availability f persnnel. Sme cmpanies will identify installatins where the safety, practicality, and/r ecnmics justify the installatin f a cmpletely independent spare that is capable f prviding sufficient relief capacity as an alternative means f verpressure prtectin. Prcedures shuld be in place t ensure the switchver between the pressure relief devices des nt allw the pressurized system t be unprtected fr any duratin beynd that t turn the valves. There are even specialized valves fr cmmn inlet line installatins which eliminate the duratin f nnprtectin while als minimizing inlet line pressure lsses. [2] Besides the bvius ecnmic dwnside t purchasing, installing, and maintaining a dedicated spare, there are ther less bvius drawbacks: the dedicated spare shuld nt be in peratin cncurrently with the primary relief device (vercapacity issue), sme wuld cntend that the dedicated spare shuld nt even be installed cncurrently (instead stred in a warehuse and installed as needed), the additinal piping may cause increased pressure drps if installed in cmmn with the primary relief device, and the ptential fr the spare t be affected by the same failure mdes as the primary relief device. The fllwing alternatives t these current practices can be used singularly r in cncert t ensure the at-risk pressurized system is adequately prtected fr the duratin f the verride: Temprarily eliminating rt causes f verpressure events Temprarily prviding alternative pathways t verpressure prtectin Temprarily mdifying the prcess peratin Temprarily ensuring equivalent r better risk reductin 2 Alternatives t Current Practices Overpressure Eliminatin. Each applicable cause f verpressure that affects the pressurized system being prtected by the pressure relief device t be islated may be evaluated t determine and eliminate the rt cause f the verpressure. Fr each cause f verpressure, identify the rt cause f the verpressure event, and determine if the rt cause f the verpressure event can be eliminated fr the duratin f the pressure relief device islatin. The fllwing techniques can be cnsidered fr sme select causes f verpressure: Prvisin f alternative/backup utility surces, such as an uninterruptable pwer supply (UPS)

Fr thse situatins invlving a specific individual valve, prviding a mechanical lcking mechanism and/r administrative cntrls may be an effective means f eliminating the rt cause f individual valve failure. See NMA M fr additinal infrmatin. Fr thse situatins invlving the inadvertent pening f a specific valve, prviding a mechanical lcking mechanism (Chain Lck Clsed r mechanical stp) r cmpletely discnnecting the surce f pressure is an effective means f eliminating the rt cause f individual valve failure pening. Fr thse situatins invlving the inadvertent clsure f a cntrl valve, cnsideratin may be given t peratin f a minimum r fixed flw bypass arund the cntrl valve. Fr thse situatins invlving the failure f an autmatic cntrl valve, cnsideratin can be given t temprarily fixing the actuatr t prevent full pening r clsing f the cntrl valve. In these cases, restricting the actuatr shuld be cnsidered a mechanical lcking device. Fr situatins invlving heat exchanger internal failure (e.g., tube failure r hle in plate f plate exchanger), the surce f high pressure can be eliminated by the islatin and bypass arund the high pressure side. Fr situatins invlving reverse flw (such as check valve(s) failure r backflw), the surce f high pressure can be eliminated by the islatin f the dwnstream high pressure side. Fr situatins invlving accidental mixing, the means f intrducing the ht r vlatile fluid t the system can be prevented, r the presence f the ht r vlatile reservir can be eliminated. Fr situatins invlving a thermal r hydraulic expansin, the islatin f the fluid and/r the ptential fr heat t be added t the islated fluid can be eliminated. Fr a thermal relief valve in nn-hazardus service, perating prcedures that ensure drainage and prper islatin may be cnsidered adequate risk mitigatin fr the remval f the pressure relief device. Fr a thermal relief valve in hazardus service, the fllwing shuld be cnsidered adequate risk mitigatin fr the remval f the pressure relief device: Mechanical lcking elements (fr example, car seals) n the piping islatin valves that prvide a means f preventing the valves being clsed withut authrizatin and thus prevent the liquid-filled piping frm being islated, and Visible placard at the lcatin f the islatin valve that indicates the valve is nt t be clsed. Cnsideratin may be given t preventing the system frm being liquid-filled when islated. This can be effectively dne by creating a gas pcket that the liquid is capable f expanding int. Fr situatins where the pressure transients are caused by clsure f a fast-acting valve, cnsideratin may be given t temprarily slwing r stpping the clsure f the valve.

While the temprary eliminatin f the rt cause f verpressure is ideal, it is ften impractical, particularly fr systems affected by multiple causes f verpressure. Alternative Overpressure Prtectin Pathway. A mre cmmn apprach is t prvide a pathway t an alternative means f verpressure prtectin, which is nt nly indicated as an acceptable practice in ASME Sectin VIII NMA M, but als a practice with precedence in industry with a single relief device prviding verpressure prtectin t multiple pieces f equipment in a given system. There are many installatins where an alternative path can be prvided, and the P&ID is useful in the identificatin f thse paths. It is, f curse, permissible t have mre than ne alternative path fr difference causes f verpressure. In general, the acceptable path(s) with the minimal pressure drps shuld be used. Fr multiple relief device installatins, it is preferred that nly ne pressure relief device in that installatin be islated at a time. Credit may be taken fr the peratin f the ther pressure relief devices that remain nline. There shuld nt be any means f autmatic islatin in the alternative path. In the event a means f autmatic islatin exists and the alternative pressure relief device is the nly practical slutin, then further risk assessment may be needed. Mechanical lcking elements and administrative cntrls may prevent the autmatic islatin frm ccurring fr the limited duratin f the islatin event. Blck (islatin) valves in the path may warrant mechanical lcking elements and administrative cntrls t prevent unacceptable peratin. Any pressure relief device prviding the alternative prtectin shuld be sized fr the cause f verpressure t ensure adequate capacity in the event verpressure was t ccur. In many cases, the cause f verpressure fr the at-risk pressurized system may cincide with a cause f verpressure fr the alternative pressurized system. An engineering evaluatin shuld be perfrmed t determine the apprpriate cumulative required relief rate t use fr these cases. In the event the alternative pressure relief device is prviding verpressure prtectin fr a limiting maximum allwable wrking pressure (MAWP) less than the MAWP f the at-risk pressurized system, the relief pressure shuld be based n the allwable accumulatin f the weaker system. Any pressure relief device prviding the alternative prtectin shuld meet the fllwing installatin requirements: The set pressure f the pressure relief device shall be less than r equal t the limiting MAWP f the pressurized system. The crss-sectinal area available fr flw within the alternative path shall be greater than r equal t the crss-sectinal area f the inlet t the pressure relief device(s) prviding verpressure prtectin. The pressure drp between the at-risk pressurized system and the alternative pressure relief device shuld nt adversely affect the verpressure prtectin. If the alternative pressure relief device is a pressure relief valve, then the inlet pressure drp f the alternative pressure relief valve shuld be limited t the blwdwn f the alternative pressure relief valve. A cnsistent apprach t the identificatin f alternative paths t verpressure prtectin is useful. Fr each applicable cause f verpressure affecting the system f interest, identify the

surce f verpressure and any pressure relief device between the surce f verpressure and the system. If a pressure relief device can be fund alng this path, then that relief device is a preferred alternate prvided it meets the sizing and installatin criteria. Any pressure relief devices prviding verpressure prtectin fr the pressurized system dwnstream f the system f interest shuld be limited t the same fluid phase that nrmally travels in the alternative path. In the event a different fluid phase is present, further analysis shuld be perfrmed t ensure the different fluid phase will nt adversely affect the pressurized systems nr cmprmise the verpressure prtectin. Pressure relief devices that are upstream f the system f interest and upstream f the pressure-generating surce are nt preferred alternatives, but may be justified with sufficient analysis. Once the alternative pressure relief devices have been identified, each device shuld be evaluated t determine if they can prvide adequate capacity and satisfy the installatin requirements. The inspectin reprts fr the alternative relief device(s) shuld be reviewed t ensure that the maintenance f the pressure relief device(s) is current. Finally, the evaluatin and the actins required shuld be dcumented t ensure the alternative verpressure prtectin path(s) are established crrectly and perated safely. Prcess Mdificatin. An additinal means f prviding adequate verpressure prtectin is t mdify the prcess peratin, ften with the gal f reducing relief requirements t match available capacity r installatin requirements. A detailed review f the input parameters fr a given verpressure scenari can ften identify leverage pints, such as the fllwing: Fr rate-dependent types f scenaris - unit thrughput Rtating equipment maximum discharge pressure - suctin pressure External fire n wetted vessels - liquid level Heat input - heating medium flw Inadvertent valve pening r cntrl valve failure - upstream pressure, maximum pening This tactic is ften used in cnjunctin with the alternative pathway, particularly when the capacity f the alternative relief device is the limiting factr. Equivalent Risk Reductin. One final means f prviding adequate temprary verpressure prtectin is t prvide a means f equivalent risk reductin. This apprach is nt cdified in current Recgnized and Generally Accepted Gd Engineering Practices (RAGAGEP), but is a lgical extensin f ther risk-based effrts and is alluded t in API Standard 521 (6th Editin) 4.4.16. [3] Assessment f the risks assciated with the islatin f a pressure relief device is useful in determining whether r nt the prpsed means f mitigating the remval f verpressure prtectin is adequate, and a Layer f Prtectin Analysis (LOPA) [4] can be an apprpriate means f assessing these risks. The alternative means f prviding the level f risk reductin that is nrmally prvided by the pressure relief device being islated shuld be identified, nting that credit fr Independent Prtectin Layers (IPLs) that have already been identified as prviding a required level f risk reductin fr the pressurized system shuld nt be taken when identifying alternative means f prviding the level f risk reductin needed.

It is nt uncmmn fr sme equipment t have islatin and/r depressuring systems which are functinal yet nt SIL rated. T take credit fr islatin and/r depressuring systems as an alternative means f verpressure prtectin fr a shrt time frame, these systems shuld have apprpriate means f activatin and shuld be inspected t ensure an apprpriate likelihd t wrk as intended. T evaluate credit fr islatin systems, the islatin valves shuld be tightshutff. T evaluate credit fr depressuring systems, the capacity f the depressuring valve shuld exceed the required relief rates. In either case, the ptential credit fr these systems shuld be evaluated n a risk basis. Dcumentatin. Regardless f the apprach taken, the analysis, results, and prcedure fr islatin f a pressure relief device need t be dcumented. Cmpanies ften have a set f safeguard verride prcedures and dcumentatin that may be apprpriate fr the islatin f a pressure relief device, as it is a critical piece f engineered cntrls at the facility. This dcumentatin shuld include: The actins required t mitigate the risks assciated with the islatin f a pressure relief device. A summary f each applicable cause f verpressure fr the at-risk pressurized system and the means by which the cause f verpressure is mitigated. P&ID(s) that identify the elements f the piping systems that require sme actin while the pressure relief device is islated shuld be prvided, if applicable. Fr risk mitigatin prvided by means f alternative verpressure prtectin paths, these additinal items shuld be included fr this dcumentatin: P&ID(s) that indicate the alternative paths. Calculatins that demnstrate the alternative pressure relief device(s) are capable f prviding adequate verpressure prtectin. Fr risk mitigatin as identified by means f risk assessment, this additinal item shuld be included fr this dcumentatin: Risk assessment reprt that indicates the risk mitigatin measures fund t be apprpriate. 3 Cnclusin In summary, when pressure relief devices require islatin and remval fr inspectin r repair during peratin, it is pssible t perfrm an engineering assessment t identify a means fr prviding temprary verpressure prtectin. These means may include any cmbinatin f eliminating rt causes f verpressure events, prviding alternative pathways t verpressure prtectin, mdifying the prcess peratin, and/r ensuring a means f equivalent r better risk reductin fr the duratin f the islatin. Regardless f the appraches taken, the time during which the relief device is islated shuld be minimized, reinfrcing the need fr planning and preparatin fr effective and efficient cntrl f this nn-rutine wrk prcess.

4 References [1] ASME, Nnmandatry Appendix M - Installatin and Operatin, In 2010 ASME Biler and Pressure Vessel Cde, 2011a Addendum, American Sciety f Mechanical Engineers, July 2011, pp. 660-665. [2] D.B. DeMichael and M.F. Edwards. Prperly Islate Pressure Relief Devices. Chemical Engineering Prgress, 95(11), Nvember 1999, pp. 57-64. [3] API Standard 521 (6th Editin), Pressure-relieving and Depressuring Systems, American Petrleum Institute, January 2014, p. 60. [4] CCPS. Layer f Prtectin Analysis - Simplified Prcess Risk Assessment. Center fr Chemical Prcess Safety, American Institute f Chemical Engineers, New Yrk, NY, Octber 2001.