Sizing Pressure-Relief Devices
Contents
1. e on the reactor itself to protect the reactor vessel and the process side of the reflux condenser from overpressure e on the cooling water coils inside the reactor if hot liq uid is pumped into the reactor while the cooling water valves are closed the liquid inside the tubes will expand and likely damage the tubes on the coolant side of the reflux condenser because hot vapors from the reactor can cause the water to expand and create overpressure if the coolant valves are closed on the pump outlet as closing the outlet side valve may damage the pump and the discharge piping Define the overpressure scenarios For each piece of process equipment there are typically multiple scenarios that may result in overpressure An up to date piping and instrumentation diagram P amp ID is essential for identify ing these potential scenarios Reference 3 discusses the most common causes of overpressure for different types of process equipment When evaluating scenarios consider modes of operation other than normal operation such as startup and shutdown Once the potential scenarios have been defined the next step is to determine which are credible scenarios A Cooling Water Reflux Condenser Cooling Water Cooling Coils Positive Displacement Pump A Figure 3 This batch reactor requires relief devices in four locations on the reactor itself on the cooling water coils on the coolant side of the reflux condenser and2. which may damage the relief device and cause it to fail In addition the cost of an oversized relief system will be higher than one of the correct size It is important to note that the pressure relief system might never be used but it must operate effectively every time it is required Process operators must have confidence that the pressure relief system is designed sized and 68s www aiche org cep October 2013 CEP maintained properly Special considerations that impact the operability such as plugging must be addressed There are many different ways that high or low pres sure can be created in a process For instance high pressure may result from the failure of a control valve a reaction that is out of control thermal expansion of a liquid or even an external fire Each possible cause of overpressure is referred to as a scenario All potential overpressure scenarios must be carefully identified characterized as credible or non credible and documented prior to sizing the relief system Two phase relief discharge The mass discharged through the relief system is typically a gas liquid or a com bination of both A combination of gas and liquid is called two phase flow and its release is a frothy discharge similar to the discharge from a soda can that has been shaken and then opened Two phase sizing is considered only for those devices that two phase fluid enters For example if liquid propane enters a relief device and
3. CT TTTTIN TT ELE TTETTIN TO EEE TT TIN ELLE TT TIT TIS 0 65 aeean teenie pt tt tt tt 10 20 30 40 5 0 95 Backpressure Correction Factor w K 0 55 0 50 0 0 Backpressure gage Set Pressure gage x100 P Gage Backpressure A Figure 5 Use this diagram to determine the backpressure correction factor K for balanced bellows relief valves in liquid service The figure is drawn using the equation K 1 1165 0 01 P for P gt 17 and data from Ref 3 Viscosity Correction Factor v K 10 100 1000 10 000 100 000 R Reynolds Number A Figure 6 Use this diagram to determine the viscosity correction factor K for both conventional and balanced bellows relief valves in liquid service The figure is drawn using the equation In K 0 0857 0 9541 InR 35 571 F and data from Ref 3 CEP October 2013 www aiche org cep 73 Back to Basics viscous the value of K decreases and a larger relief area is required For a Reynolds number greater than 16 000 viscosity correction is not needed i e K 1 0 However in order to calculate the Reynolds number the relief area must be known This may require solving Eq 4 by trial and error In this case first assume a viscosity correction factor of K 1 0 and calculate the relief area then calculate the Reynolds number In most cases the Reynolds number will be
4. and Pressure Vessel Inspectors Code certification involves conducting flow tests under conditions specified in the ASME code The pressure vessel accumulation is the pressure increase above the MAWP usually expressed as a percentage of the MAWP When the relief device is at the MAWP the over pressure and accumulation are equal Backpressure is the pressure downstream of the relief device It includes the constant superimposed backpressure and the built up backpressure due to the discharge of fluid from the relief device through the downstream piping and or treatment system ASME Boiler and Pressure Vessel Code Section VIII requirements Figure summarizes the ASME Boiler and Pressure Ves sel Code BPVC Section VIII 6 requirements for pressure vessels and relief systems The left side of the figure shows code requirements for pressure vessels while the right side shows code requirements for typical relief devices The entire figure is relative to the MAWP which is assigned the arbitrary value of 100 As shown in Figure 1 the relief device s maximum allowable set pressure is equal to the vessel s MAWP This applies to vessels protected by a single relief device How ever if the vessel is protected by multiple relief devices then one relief device must be set no higher than the MAWP but the others can be set as high as 105 of the MAWP Typically relief devices are set to open at the MAWP The expected maximum operating pre
5. conventional spring operated relief devices in vapor service Figure 7 was constructed based on this equation K a bP o Range a b all 66 90 1 3026 1 137x10 6 1 3 63 90 1 294 1 1703x10 6 1 5 56 90 1 203 1 143x10 6 1 7 51 90 1 148 1 109x10 6 Table 3 For balanced bellows relief devices in vapor service this equation can be used to compute the backpressure correction factor and was used to construct Figure 8 K 1 a bG Overpressure a b Range 10 0 8707 4 724x10 30 50 20 0 9760 8 360x1077 30 50 Copyright 2013 American Institute of Chemical Engineers AIChE where both P p and P pay are in gage pressure units To use Figure 7 the backpressure and P ay must be converted to absolute pressure units with an equation such as P P 147 9 where P ax 18 in psig and P is in psia Gas sizing example A spring operated relief device must be sized for a pressure vessel containing an ideal hydrocarbon vapor The controlling scenario has a mass discharge rate W of 50 0 kg s Assume a vessel P p of 8 barg no super imposed backpressure a built up backpressure equal to 10 of set pressure and a process temperature of 473 K A review of normal operating pressures concludes that a set pressure P of 7 barg will be used The vapor has a molecular weight of 100 a heat capacity ratio of 1 3 and is ideal so z 1 0 This is an
6. partially vaporizes so that two phase flow occurs in the tailpipe only the tailpipe should be designed for two phase flow Two phase relief is common in reactive relief systems and systems with foamy or viscous materials The required relief area for two phase flow is typi cally two to ten times the area for single phase flow Two phase relief device design is more difficult than single phase design 2 4 experts with specialized knowledge should perform two phase relief design This article discusses only the sizing of relief devices for single phase flow either all gas or all liquid entering the relief device Copyright 2013 American Institute of Chemical Engineers AIChE Design and sizing terms and definitions The primary parameter used to characterize a pres sure vessel is the maximum allowable working pressure MAWP The MAWP is the maximum allowable pres sure at the top of a vessel at a designated temperature This designated temperature is known as the maximum allowable working temperature MAWT The MAWT is important because as the temperature increases the MAWP decreases since the strength of the metal is reduced In addition at very low operating temperatures approximately 20 F embrittlement may be an issue The vessel nameplate typi cally includes the MAWP MAWT and the minimum design metal temperature MDMT The MAWP MAWT and MDMT must not be exceeded under non emergency operat ing conditions The
7. set pressure of a relief device is the pressure at which the device operates For spring operated relief valves small amounts of leakage start to occur at 92 95 of the set pressure A relief device s overpressure is the pressure increase over its set pressure usually expressed as a percentage of the set pressure Pop acting relief valves do not immediately open completely to 100 lift Sufficient overpressure is necessary to achieve full lift ASME certified relief valves Pesaro sea ben Maximum allowable Maximum relieving pressure accumulation pressure fire sizing fire sizing 4 Maximum relieving pressure multiple reliefs Maximum allowable 11 accumulation pressure multiple reliefs Maximum relieving pressure single relief Maximum allowable accumulation pressure non fire sizing Maximum allowable set pressure multiple reliefs Maximum allowable working pressure MAWP Maximum allowable set pressure single relief Typical maximum allowable operating pressure A Figure 1 The ASME Boiler and Pressure Vessel Code Section VIII sets out requirements for standard pressure vessels left and the relief valves protecting them right as a percentage of the maximum allowable working pressure MAWP Copyright 2013 American Institute of Chemical Engineers AIChE are required to reach full rated capacity at 10 or less over pressure Relief valves are code certified by the National Board of Boiler
8. the data needed to determine the required relief flowrate and required relief area for each scenario For example a control valve failure scenario requires knowing the control valve s flow coef ficient and the upstream pressure If the overpressure is caused by a centrifugal pump then the pump curve and the size of the impeller are needed to calculate the flowrate To calculate the required relief area physical property Z2 wwwaiche org cep October 2013 CEP data such as heat capacity density heat of vaporization and vapor pressure are typically needed Determine whether single or two phase flow is likely At this point in the relief design process a determina tion must be made as to whether the material entering the relief device will be single phase or two phase flow when the device is at its relieving pressure If a chemical reac tion is involved two phase flow is very likely During a fire exposure two phase flow is possible but not likely unless the fluid is inherently foamy contains surfactants or has a high liquid viscosity The superficial velocity in a narrow diameter vessel may also result in two phase flow if the velocity is too high to allow for vapor liquid disengagement Determine the design basis for the device Adequate information is available at this point to determine the design basis for the relief device The design basis includes the mass discharge rate through the device and operat ing information for
9. the equation and constants in Table 2 and data from Ref 3 Z wwwaiche org cep October 2013 CEP the Reynolds number is almost certainly above 16 000 Thus assume K 1 0 The liquid is water at 70 F so G P P 1 0 1 0 110 psig 0 psig From Eq 4 Thus the minimum required relief orifice area would be 1 16 in in this example in psi 300 gpm 38 0 gpm 0 65 1 0 1 0 1 16 in Sizing for gas service For conventional spring operated relief devices in gas or vapor service choked flow through the relief orifice is assumed Choked flow through an orifice is represented by the following equation 2 v r 1 M Wak AP ZEZ 2 5 RT y l where W is the mass flowrate mass time K is the dis charge coefficient unitless P is the upstream relieving pressure for vapor service absolute pressure yis the heat capacity ratio of the gas or vapor unitless M is the molecu lar weight of the gas mass mol R z is the ideal gas constant pressure volume mol deg and T is the absolute tempera ture deg To simplify the calculation the term C which is a func tion of only the heat capacity ratio is defined as oO E E E E E E a See eee 0 5 10 15 20 2 30 35 40 45 50 Capacity with Backpressure Rated Capacity without Backpressure g ine Enn a a a E a x Backpressure gage 9 P Gage Backpressure Set Pressure gage x100 A
10. the relief device such as the relief set pressure overpressure and superimposed backpressure to name a few For rupture discs the burst temperature is also important The scenario that requires the largest area rather than the one with the highest mass flowrate is the controlling case for device sizing Liquid relief scenarios commonly have the highest mass flowrate but they are generally not the control ling scenarios when vapor scenarios also exist Relief device sizing The American Petroleum Institute s Recommended Practice for Sizing Selection and Installation of Pressure Relieving Systems in Refineries API 520 Part 1 3 is the most widely used manual for sizing relief devices in the chemical process industries The objective of the relief sizing calculation is to deter mine the required relief area for the relief device Almost every relief device installation has its own unique design issues and special considerations that will impact the relief sizing calculations The procedure outlined in the flowchart provided in Figure 2 must be followed to identify these issues The procedure described in this article is a generic sizing procedure and covers sizing calculations only for conventional spring operated devices in liquid and gas vapor service Sizing for liquid service For a spring operated pressure relief device in liquid ser vice the sizing procedure requires at a minimum a speci fication of the design bas
11. unfired pressure vessel so from Eq 8 the maximum allowable relieving pressure in gage units is P a 1 1 8 barg 8 8 barg The absolute maximum pressure is then P 8 8 1 013 9 813 bara 9 81x10 N m The backpressure is 10 of the set pressure 0 10 7 barg 0 7 barg To use Figure 8 to find the backpressure correction K the percent absolute backpressure is needed Backpressure abs Backpressure abs Set Pressure Overpressure abs _ 0 7 bara 1 013 bara 0 175 or 17 5 9 813 bara From Figure 7 at 17 5 absolute backpressure K 1 0 With z 1 0 y 1 3 T 473 K and M 100 kg kg mol from Eq 6 2 3 13 1 kg m s N 2 3 8314 N m kg mol K 1 3 1 kg kg mol K sN Assuming a discharge coefficient of K 0 975 the minimum required area is determined from Eq 7 50 kg s 1 2 kekemol K 975 9 81x10 Nim 7 32x10 A 7 32x10 473 K 1 0 100 kg kg mol 1 53x10 m CEP October 2013 www aiche org cep 75 Back to Basics and from the area the equivalent orifice diameter of the relief device is determined by 44 4 1 53x10 m T 3 14 0 140 m 14 0 cm This scenario requires a relatively large orifice size of 14 0 cm because the relief valve must handle a large volu metric discharge rate Wrapping it up Relief devices are safety systems that you hope are never activated However if they do activate they must b
12. Back to Basics Sizing Pressure Relief Devices DANIEL A CROWL MICHIGAN TECHNOLOGICAL UNIV Scott A TIPLER THE Dow CHEMICAL COMPANY Although relief devices may never be activated they must be designed and sized to function correctly every time they are necessary This article provides an introduction to sizing pressure relief devices for liquid and vapor service pressure relief device protects process equipment Am the hazards of high or low pressure in a rocess It operates by opening at a designated pres sure and ejecting mass from the process The ejected mass contains energy the removal of the energy reduces the process pressure A previous article in CEP 1 provided a basic overview of pressure relief systems That article stressed the impor tance of properly locating selecting designing and main taining pressure relief devices to ensure that they operate when their service is required This article follows up with an introduction to sizing pressure relief devices to ensure that they function properly More detailed information on relief sizing is provided in Refs 2 4 Relief device basics The purpose of relief sizing is to determine the proper discharge area of the relief device and diameter of the asso ciated inlet and outlet piping If the relief device is under sized high pressure and equipment failure may result If the relief device is oversized the relief may become unstable during operation
13. Figure 8 Use this plot to determine the backpressure correction factor Ky for balanced bellows relief devices in vapor service It is drawn using the equation and constants in Table 3 and data from Ref 3 Copyright 2013 American Institute of Chemical Engineers AIChE re Equation 5 is modified by adding a compressibility fac tor z to account for nonideal gas behavior and the back pressure correction K to account for backpressure With these adjustments Eq 5 can be solved for the relief area 42 W TXz 7 CK PK V M Kand K are normally provided by the valve manufacturer For preliminary sizing purposes a discharge coefficient K of 0 975 is used The backpressure correc tion factor K is determined by Figure 7 or 8 depending on whether the relief device is a conventional spring operated device or a balanced bellows valve Data for Figure 7 were derived from the equation in Table 2 and data for Figure 8 were derived from the equation in Table 3 For Figure 8 K is a function of the ratio of the back pressure and the set pressure For Figure 7 K is a function of the ratio of the backpressure and the maximum allow able relieving pressure P which is determined from the allowable accumulation P ax 1 1X P 4 for unfired pressure vessels P ax 1 21 P wp for vessels exposed to fire 8 P a 1 33X P awp for piping Table 2 These factors can be used to compute the backpressure correction factor for
14. P A where A has units of in Q has units in U S gal min and AP has units of psi Equations 1 3 model liquid discharge through an orifice with fully turbulent flow Equation 3 must be adjusted for the viscosity of the fluid a fluid with a higher viscosity requires a larger orifice Equation 3 must also be adjusted for back pressure if a balanced bellows relief valve is selected Incorporating these adjustments into Eq 3 results in the following equation in psi Q G 4 38 0 gpm K K K VP P where K is the adjustment factor for backpressure unit less K is the adjustment factor for viscosity unitless G is the specific gravity of the liquid referenced to water at 70 F which is equal to P P ef P is the upstream reliev ing pressure gage pressure which is the set pressure plus allowable overpressure and P is the total backpressure gage pressure Copyright 2013 American Institute of Chemical Engineers AIChE For conventional relief valves in liquid service use K 1 0 For balanced bellows relief valves in liquid service K must be obtained from the manufacturer For preliminary sizing K can be determined from Figure 5 K is the unitless viscosity correction factor and can be determined from Figure 6 The viscosity correction fac tor is a function of the fluid s Reynolds number As the Reynolds number decreases i e the liquid becomes more PT IN tt PT TIN TTT ELLIE TIN TTT
15. Routing to the suction vessel is recommended to reduce the risk of dangerous heating of the fluid caused by liquid recirculating through a blocked pump Overpressure hazards of process lines are sometimes overlooked Any blocked liquid filled line poses a risk of overpressure if the fluid can be heated by ambient tempera ture or sun exposure Heat traced liquid filled lines typically require a relief device or an open outlet to allow for liquid expansion Long lines 7 e more than about 300 ft that are not in continuous service generally need protection from liquid thermal expansion as do loading and transfer lines and other lines that extend beyond the battery limit i e the property line Finally lines with a history of overpressure for example as indicated by gasket blowout generally require a relief device The coolant side of a heat exchanger may or may not require a relief device depending on the service Mainte nance isolation valves are typically present on the coolant lines Therefore if the valves are closed and the coolant side is exposed to heat from the hot side of the exchanger damage might occur However piping systems such as cooling water lines are seldom isolated in an operating plant even when the plant is down If the coolant side is drained whenever the exchanger is isolated then a relief may not be required Table 1 These general guidelines provide advice on where relief devices are required All
16. e designed and sized properly to ensure that they work every time Nomenclature equivalent orifice area area simplification term defined by Eq 6 specific gravity of the liquid at the flowing tem perature referenced to water at 70 F unitless g gravitational constant length mass force time QAD 7 adjustment factor for backpressure in vapor gas service unitless K discharge coefficient unitless K adjustment factor for viscosity in liquid service unitless adjustment factor for backpressure in liquid service unitless M molecular weight mass mol IP upstream relieving pressure gage pressure in Eq 4 and absolute pressure in Eq 7 total backpressure gage pressure E percent gage backpressure unitless maximum overpressure for vapor service gage pressure maximum allowable working pressure MAWP gage pressure set pressure gage pressure pressure drop across the orifice force area volumetric discharge rate volume time ideal gas constant pressure volume mol deg absolute temperature deg average discharge velocity of the fluid through the relief orifice distance time mass flow rate mass time no max 5 MAWP NAOR x N compressibility factor for nonideal gases unitless y heat capacity ratio of the gas or vapor unitless p density of the fluid mass volume Pref reference density mass volume WG wwwaiche or
17. ection Expertise Area Leader for The Dow Chemical Co Midland MI Phone 989 638 6634 Email SATipler Dow com He has been employed by Dow Chemical ina variety of manufacturing and engineering roles since 1985 He has over 20 years of overpressure protection design experience and assumed his current role in 2007 He received a BS in chemical engineering from the Michigan Technological Univ a LITERATURE CITED 1 Kelly B D What Pressure Relief Really Means Chem Eng Progress 106 9 pp 25 30 Sept 2010 2 Crowl D A and J F Louvar Relief Sizing Chapter 10 in Chemical Process Safety Fundamentals with Applications 3rd ed Prentice Hall Englewood Cliffs NJ pp 459 503 May 2012 3 American Petroleum Institute Recommended Practice for the Sizing Selection and Installation of Pressure Relieving Systems in Refineries API RP 520 Part 1 8th ed API Washington DC 2008 4 Fisher H G et al Emergency Relief System Design Using DIERS Technology American Institute of Chemical Engineers New York NY 1992 5 Anderson Greenwood and Crosby Technical Service Manual www andersongreenwood com literature asp Aug 2013 6 American Society of Mechanical Engineers Boiler and Pressure Vessel Code Section VIII Rules for Construction of Pressure Vessels ASME New York NY 2013 ADDITIONAL RESOURCES American Institute of Chemical Engineers Guideline
18. g cep October 2013 CEP DANIEL A CROWL is the Herbert H Dow Professor for Chemical Process Safety at Michigan Technological Univ Houghton MI Phone 906 487 3221 Email crowl mtu edu He has been involved with process safety education and research since 1985 He is the author or editor of several books on process safety has produced many modules and student certificate programs for the Safety and Chemical Engineering Education SAChE program and is a past editor of AIChE s quarterly journal Process Safety Progress He worked for two years as a process control engineer for St Regis Paper Co and then spent 16 years teaching at Wayne State Univ in Detroit before joining Michigan Tech in 1993 His research is in flammability and reactivity and he consults with industry in these areas He is a member of the AIChE SAChE Com mittee the 11a Committee on Loss Prevention the Chem E Car Rules Committee and the ASTM E27 Committee on the Hazard Potential of Chemicals He is a Fellow of AIChE the Center for Chemical Process Safety CCPS the ACS Div of Chemical Health and Safety and the National Speleological Society He has received numerous awards including most recently the Merit Award from the Mary Kay O Connor Process Safety Center at Texas A amp M Univ He received a BS in fuel sci ence from the Pennsylvania State Univ and an MS and PhD in chemical engineering from the Univ of Illinois SCOTT A TIPLER is the Overpressure Prot
19. is This includes the volumetric discharge flowrate through the relief device the set pressure and the overpressure For a balanced bellows relief valve the backpressure must also be known Copyright 2013 American Institute of Chemical Engineers AIChE For liquid service the sizing calculation is based on the fundamental equation for liquid discharge through an orifice 2 u K re 1 where u is the average discharge velocity of the fluid through the relief orifice in units of distance time K is the effec tive discharge coefficient unitless g is the gravitational constant distance mass force time AP is the pressure drop across the orifice force area and p is the density of the fluid mass volume The unitless discharge coefficient K is normally provided by the valve manufacturer It can also be obtained from the ASME National Board of Boiler and Pressure Vessel Inspectors for code certified devices For preliminary sizing a value of K 0 65 is assumed The volumetric flow of liquid Q volume time is equal to the discharge velocity u multiplied by the orifice area A Substituting and solving for the area gives Hs 2 A working equation with fixed units is derived from Eq 2 by replacing the density with the specific gravity P P e and using water as the reference material and mak ing the appropriate substitutions for unit conversions The result is Eq 3 n o o flea 3 38 0 gpm K A
20. much greater than 16 000 However if the Reynolds number is less than 16 000 use Figure 6 to determine a new viscosity correction factor and calculate a new relief area and Reynolds number Repeat this procedure until the solu tion converges on a Reynolds number Liquid sizing example A detailed study has determined the required relief flow through a conventional spring operated relief valve on a pro cess vessel to be 300 gpm of water Assume a set pressure of 100 psig a maximum overpressure of 10 a temperature of 70 F and no backpressure With a set pressure of 100 psig and an overpressure of 10 of the set pressure or 10 psig the upstream relieving pressure P is 110 psig For a conventional relief valve no backpressure cor rection is necessary K 1 0 The volumetric discharge rate Q through the relief valve is given as 300 gpm The discharge coefficient K is not specified for a preliminary estimate assume K 0 65 The Reynolds number through the relief valve is not known However at a volumetric discharge rate of 300 gpm Capacity with Backpressure Rated Capacity without Backpressure oO O lt 0 70 80 90 100 Backpressure abs Set Pressure Overpressure abs x100 P Absolute Backpressure A Figure 7 Use this plot to determine the backpressure correction factor K for conventional spring operated relief devices in vapor service It is drawn using
21. ns depend on the type of relief device selected There are many different types of relief devices The most common are spring operated relief valves balanced spring operated relief valves balanced using a bellows or balanced piston rupture discs pilot operated relief valves and rupture pin relief devices Combinations of these devices may also be desirable i e a rupture disc located under a relief valve Details on these different types and the advantages and disadvantages of each are discussed in Refs 2 and 5 One must have a good understanding of the process in order to select the right type of relief device for that process Rupture discs and buckling pin type relief devices do not reclose after activation Therefore they should not be the first choice for most applications Preference should be given to reclosing relief devices for both safety and reliability The routing of the relief device effluent will also influ ence the selection of the type of relief If the device dis charges to the atmosphere then a conventional spring relief valve is generally the best and most economical choice If the discharge is routed to a containment e g pipe or vessel or treatment e g scrubber or flare header sys tem then a balanced or pilot operated relief valve is more appropriate due to the backpressure from the downstream system Acquire data for relief device sizing The next step in the relief design procedure is to acquire
22. on the pump outlet Copyright 2013 American Institute of Chemical Engineers AIChE credible scenario is typically one that involves a single failure Scenarios that require multiple independent failures are typically not considered when sizing individual relief devices Credibility is based on risk which is a function of probability and consequence Formal hazard identification procedures such as layers of protection analysis LOPA hazard and operability HAZOP analysis fault tree analy sis FTA or failure modes and effects analysis FMEA can be used to assess risk Most relief system designs involve compromises A particular relief device might work fine for the primary design i e controlling scenario but pose a performance risk for other scenarios An example of this is a relief valve that is sized for a vapor scenario but must also work effec tively in a liquid scenario The liquid scenario will often present some mechanical instability risks which must be tolerable to the plant owner e g it may require lower pres sure to reclose in liquid service It is possible that no causes for overpressure can be identified for a particular piece of equipment in the process The ASME Boiler and Pressure Vessel Code Section VIII 6 requires all pressure vessels to have protection from overpressure regardless of whether there are any credible overpressure scenarios In such cases a relief device is typically installed to meet
23. pressure vessels require overpressure protection All low pressure storage tanks require pressure and vacuum relief for normal operation e g pumping in and out tank breathing caused by temperature changes Tanks must also be protected from any emergency events that could create an abnormally high venting load e g fire exposure procedural failure during line blowing etc Positive displacement pumps compressors and turbines require relief devices on the discharge side for deadhead protection Segments of liquid filled piping that have a high risk of overpressure due to thermal expansion e g unloading lines should have relief devices Piping that can be overpressured due to process control failure e g high pressure steam letdown control into a low pressure steam header need relief devices A vessel jacket is usually considered a distinctly separate pressure vessel and requires its own overpressure protection This list is not exhaustive Copyright 2013 American Institute of Chemical Engineers AIChE Batch chemical reactor example Figure 3 depicts a batch chemical reactor in a pilot plant A positive displacement pump is used to load the vessel contents Cooling coils inside the reactor control the reaction temperature and a reflux condenser provides additional temperature control In this example relief devices may be required at four locations to provide overpressure protection
24. s for Pres sure Relief and Effluent Handling Systems Center for Chemi cal Process Safety CCPS New York NY 1998 Hellemans M The Safety Relief Valve Handbook Elsevier Oxford U K 2009 Malek M Pressure Relief Devices Mc Graw Hill New York NY 2005 Copyright 2013 American Institute of Chemical Engineers AIChE
25. ssure should be low enough to prevent relief device activation during routine operations The difference between the set pressure and the maximum operating pressure is known as the operating margin The required operating margin depends on the type of relief device and the pressure control capability of the process The allowable accumulation for pressure vessels protected by a single relief device is 110 as shown in Figure 1 The exception to this is fire exposure scenarios for which the allowable accumulation is 121 of the MAWP When multiple relief devices are used for non fire scenarios the allowable accumulation is 116 The relief design procedure Figure 2 is a flowchart for the pressure relief design procedure Define the protected system The first step in the design procedure is to define the protected system and understand the operation of the process The protected system may include several pieces of equipment For example if a relief CEP October 2013 wwwaiche org cep 69 Back to Basics is provided on the top of a distillation column it may protect the column reboiler condenser and accumulator The design procedure is more complex for processes involving connected equipment with different pressure ratings The relief designer needs to have a good understanding of the operation of the process in order to assess the overpressure protection needs Locate the relief devices Next determine where over pressure protec
26. the code requirements Even if no credible scenarios exist a small relief device is good insur ance against overpressure Mixing vessel example In Figure 4 a mixing vessel s relief valve is shown in red This vessel has four potential overpressure scenarios The nitrogen regulator could fail exposing the vessel to Relief Device High Pressure Nitrogen Cooling Water A Figure 4 A mixing vessel with a high pressure nitrogen supply system whose relief valve is shown in red is subject to several overpressure scenarios CEP October 2013 wwwaiche org cep 71 Back to Basics the pressure of the nitrogen supply system e A leak could develop in the cooling water coils expos ing the vessel to the pressure of the coolant system or pos sibly resulting in a reactive scenario if the process materials react with the coolant e A procedural failure could cause the pump to continue operating while the vessel outlet is closed exposing the ves sel to the discharge pressure of the pump e The vessel could be exposed to fire which may heat the vessel s contents and result in overpressure Note that these are scenarios only for the mixing ves sel Scenarios would also need to be identified for all other pressure vessels in this process which are not included in Figure 4 Choose the type of relief device The next step is to select an appropriate relief device for the specific application because the sizing calculatio
27. tion may be needed The codes and standards set different requirements for different types of equipment For example all pressure vessels must be protected from overpressure per ASME code Protection may also be needed for other equipment in the process where damage or personnel injury can be caused by excessive pressure Table 1 contains general guidelines on where relief devices are required although there are likely to be other special cases in any process where they may be necessary Low pressure storage tanks require both vacuum and pressure relief devices since these vessels typically are not designed for full vacuum Relief devices for these tanks are designed only for vapor relief and not for liquid discharge due to overfilling instrumentation and or procedural safeguards are used to mitigate the overfilling scenario Define Protected System Locate Relief Devices Define Overpressure Scenarios Choose Relief Device Types Acquire Data Two Phase Flow Single Phase Flow Specify Design Basis Design Relief System A Figure 2 The relief device sizing procedure involves these steps ZO wwwaiche org cep October 2013 CEP Pumps can be an overpressure hazard to downstream equipment All positive displacement pumps should be reviewed as a potential source of overpressure Normally the relief device on a pump discharges to the inlet side of the pump either to the pump suction pipe or to the suction vessel
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