7.15 Relief Valves—Determination Of Required Capacity

7.15 Relief Valves—Determination of Required CapacityOSHA CodesThe OSHA codes that relate to the design of pressure safetysystems are the following: OSHA 1910.106, Flammable and Combustible LiquidsOSHA 1910.110, Liquefied Petroleum GasesOSHA 1910.119, Process Safety Management975been sized to handle the largest of overpressure causes fromamong fire, run-away chemical reaction, loss of utilities, etc.,that is sufficient. It is not necessary to consider their simultaneous occurrence and size the PRV for the sum of two ormore of these causes.FIRE PROTECTIONCAUSES OF OVERPRESSUREOverpressure can be caused by fire and by nonfire processcauses. In the second category, there can be many potentialcauses. These will be discussed after the treatment of fireprotection that follows in the next paragraph. The potentialnonfire causes of overpressure include the following:1. Utility failures, which can be the failure of electricpower, instrument air, steam, coolant, or fuel2. Thermal expansion3. Blocked outlets4. Valve or process control failure5. Equipment failure6. Runaway chemical reaction7. Human errorIt should be emphasized that part of the goal of a safe plantdesign is the goal of minimizing the opportunities for humanerror.Substituting for Pressure Relief DevicesIn general it is not recommended to accept fail-safe instruments or the actions of automatic control loops as substitutesfor PRVs. This is because the reliability of control systemshas not advanced to the point where they are completelyreliable. Their reliability has increased substantially by theinvention of high integrity protective systems (HIPSs), emergency power supplies (EPSs), uninterruptible power supplies(UPSs), triple redundancy, and “two out of three” votingsystems. Yet, to date, there is no code or regulation thataccepts automatic process control as a substitute for the installation of PRVs.Similarly, the various safety oriented administrative procedures cannot be used as substitutes for PRVs. This isbecause human error can never be fully eliminated, and nomatter what the administrative procedure says, (according toMurphy’s Law) if it is possible to forget to drain a pipelineof its toxic or hazardous content, it will happen. One exception involves the use of car seals open (CSO) and car sealslocked (CSL) valves, which API RP 520 and 521 accept asmeans of guaranteeing that equipment will not be blocked in.In the overpressure analysis of a plant, the design engineer does not need to consider the possibility of the simultaneous occurrence of more than one cause of overpressure.In other words, if, for example, a chemical reactor PRV has 2003 by Béla LiptákThe ASME Unfired Pressure Vessel Code requires that pressure vessels covered by the code be adequately relieved.External fire is a potential overpressure source; therefore, forfire conditions, the code requires that relief devices be sizedsuch that, at maximum relieving conditions, the vessel pressure does not exceed the vessel design pressure by more than20%. This is referred to as 20% accumulation.For vessels with adequate liquid inventory, the requiredrelief capacity under fire conditions is a function of tank areaexposed to fire, of the heat flux per unit area of tank surface,and of the latent heat of the process fluid. These three factorswill be separately discussed below. The first to be discussedis the determination of the applicable heat flux.Gas-Filled TanksIf the vessel does not have an adequate liquid inventory, stressrupture can occur from overheated spots long before the internal pressure in the tank would reach the setting of the PRV.Therefore, gas-filled tanks and vessels with low liquid inventory require different approaches for protection against overpressure, and design engineers should consider API RP 521for determining the PRV size required when the tank is exposedto fire and some or most of its inner surface is not wetted.In most cases, PRVs sized in accordance with API RP520 cannot give overpressure protection to gas-filled tanks.Therefore, when the installation of hydrogen or other flammable gas-filled vessels is being designed, the followingoptions should also be considered:1. Bury the tank under ground and cover it with earth.2. Move the vessel away from the process area or surround it by a dike or a firewall.3. Elevate the vessel over the fire height (API, 25 ft,NFPA, 30 ft above grade).4. Use a water deluge system or install fireproof insulation.5. Provide automatic vapor depressurization.6. Provide automatic fire monitoring and automatic firefighting capability.Heat Absorption Across Unwetted Surfaces The effectivedischarge area of a relief valve required to protect a vessel thatis exposed to external fire and has unwetted surface area canbe determined by Equations 7.15(1) and 7.15(2).A (F′A′)/ Pu7.15(1)

976Safety and Miscellaneous Sensorswhere2A the effective discharge area of the PRV valve, in.A′ the surface area of the vessel, which is exposed to2fire, ftPu the upstream relieving pressure, which is the sumof the atmospheric, the set pressure, and the overpressure, PSIAThe environmental factor F′ is calculated from Equation7.15(2) as follows:1.25F′ (0.1406/CKd)[(Tw T1)/T10.6506]7.15(2)whereC a constant that depends on the specific heat ratio ofthe particular gas. For the specific heat ratio (k) ofa number of gases, refer to Table 7.15a, and for acurve relating this ratio to coefficient C, refer toFigure 7.15b.Kd the coefficient of discharge of the PRV, which, forpreliminary sizing purposes, can be assumed to be0.975Tw the expected wall temperature of the tank, which,for carbon steel plate materials, can be assumed tobe 1,100 FT1 the gas temperature in R at the upstream relievingpressure (Pu). It is calculated by Equation 7.15(3)as follows:T1 PuTn/Pn7.15(3)wherePn the normal operating gas pressure, PSIATn the normal operating gas temperature, RLow Liquid Inventory TanksThe definition of what is “adequate” in terms of liquid inventory is a function of the time required to evaporate thatinventory during a fire vs. the response time of the fire fightersat the plant. Once the liquid is gone, the vessel becomes agas-filled tank.The minimum time is 10 to 15 min. This time period hasto take into account the location of the fire-fighting equipmentand the quality of the automatic fire monitoring instrumentation in the plant. A liquid inventory is usually considered“adequate” if it will last for at least 15 min during an externalfire. If it does not, the fire protection rules for gas-filled tanksshould be used.Heat Flux Across Wetted SurfacesWhen a vessel is exposed to external fire, the amount of heatabsorption will depend on the following:1. The wetted surface areas of the vessel and the connectedassociated piping 2003 by Béla Lipták2. The amount and quality of the insulation provided onthe tank and piping surfaces3. The quality and availability of the fire-fighting equipment in the plant4. The method applied to drain the flammable materialsaway from the tankTo determine the required relief capacity of a tank thatis exposed to external fire, it is necessary to determine theheat flux. This is the rate at which heat is transferred into thevessel or other process equipment. A number of heat fluxdetermination methods can be considered.The simplest technique assumes that the heat flux is fixedand does not depend on the type and size of the vesselinvolved. Under such an assumption, the heat flux is taken22to be a constant rate of 20,000 BTU/hr/ft (63 kW/m ). Otherapproaches relate the magnitude of the heat flux to the sizeof the vessel, reasoning that the larger the tank, the less likelyit is that it will be completely immersed in flames.API RP 521 provides one commonly used method fordetermining heat flux under fire conditions. This bulletingives two recommendations for calculating the heat flux inBTU per hour per square foot of total wetted surface of avessel that is exposed to fire. The two formulas are given inTable 7.15c, one for tanks with adequate drainage away fromthe tank, and the other for tanks without adequate drainage.A graphic representation of the formula for the adequatedrainage condition is shown in Figure 7.15d.Total Heat AbsorptionAPI Recommendation API RP 520, seventh edition (January2000), presents two widely used equations for determiningthe total heat absorption of wetted surface areas under fireconditions. Equation 7.15(4) is applicable if the means forboth prompt fire-fighting capability and adequate drainage offlammable materials away from the tank are provided (groundwith over 1% slope); Equation 7.15(5) is applicable if theyare not.0.827.15(4)0.827.15(5)Q 21,000F(A)Q 34,500F(A)whereQ total heat absorption of wetted surface area exposedto fire, BTU/hrF environmental factor, a constant having a value of1.0 or less2A the total wetted surface area of the tank, ftThe reader should consult the API Recommended Practice520 for additional details.NFPA Recommendations Another standard that is commonlyused in the determination of heat flux under fire conditions isin the recommendations of NFPA Bulletin No. 30, Flammable

7.15 Relief Valves—Determination of Required Capacity977TABLE 7.15aProperties of GasesMolecularWeightGasSpecific HeatRatio (k Cp /Cv)at 60 F and OneAtmosphereCritical FlowPressureRatio at 60 Fand icGravityat 60 Fand OneAtmospherePressure(psia)Temperature( F)CondensationTemperatureOneAtmosphere( F)0.554673 116 2595.0–15.01.05871890 1282.9–13.80.96974250 1552.7–34.81.522617206 442.1–9.51.453667197 542.8–10.82, 31Critical ConstantsFlammabilityLimits (volumepercent in 0.604.912304632345Air29.961.400.531.000547 221 313Ammonia17.031.300.530.5881636270 28Carbondioxide44.011.290.551.519107188 109References2, 32, 32, 32, 315.5–27.02, 32, 3Hydrogen2.021.410.520.0696188 400 4234.0–74.22, 3Hydrogensulfide34.081.320.531.1761306213 774.3–45.52, 3Sulfurdioxide64.041.270.552.2121143316142, 3Steam18.011.330.540.62232067062122, 3aEstimated.References:1. Physical Constants of Hydrocarbons C1 to C10 , ASTM Special Technical Publication No. 109A, Philadelphia, PA, 1963.2. International-Critical Tables, McGraw-Hill, New York.3. Engineering Data Book, Gas Processors Suppliers Association, Tulsa, OK, 1977.and Combustible Liquids. The National Fire Protection Association is an organization of insurance companies and regulatory organizations. Their recommendations are likely tomeet the requirements of most insurance companies, as theyare generally more conservative than the corresponding APIrecommendations.Table 7.15e lists the equations recommended by NFPAfor the determination of total heat absorption and for calculating the equivalent air flow for tanks exposed to external fire. 2003 by Béla LiptákLow-Pressure Tanks For above-ground tanks and storagevessels designed to operate from atmospheric pressure to 14PSIG (104 kPa) and used for the storage of flammable liquids,still another method of determining heat flux under fire condit

Excerpts from ASME Code UG-125(c)— All pressure vessels other than unfired steam boilers shall be protected by pressure-relieving devices that shall prevent the pressure from rising more than 10% or 3 psi, whichever is greater, above the maximum allowable working pressure except as permitted in (1) and (2). (See UG-134 for pressure settings.)