Pressure Relief Valves

Transcription

1 Pressure Relief Valves ANDERSON GREENWOOD flow control

2 Contents Pressure Relief Valves Pressure Relief Valve Revised May 1998 Catalog: PRVTM-US.97 Note 1. Some referenced figures, tables, equations, or paragraphs may not be included. Consult original document for complete text. General Information... 1, 2 Glossary... 3, 4 Valve Sizing Nomenclatures Valve Data Back Pressure and Subsonic Correction Gas and Vapor Steam Liquid Subsonic Flow Special Applications Reaction Forces Conversion Factors Fluid Data ANSI Flange Standards Valve Installations - Handling Procedures ASME Code Section I 1 Excerpts ASME Code Section IV 1 Excerpts ASME Code Section VIII 1 Excerpts API RP 520 Part I 1 Excerpts API RP 520 Part II 1 Excerpts NACE MR Excerpts API RP Noise Levels API RP Seat Leakage Requirements Section 16 Chemical Resistance for Metals Section 17 Chemical Resistance for Elastomers/Thermoplastics designs and specifications without notice. 3

3 General Information Intro Anderson Greenwood is a globally recognized leader in the field of pressure relief device technology. Fundamental to our ability to solve the most challenging application is our belief in understanding all application parameters. As a leader in the field of pressure relief device education, we proudly provide this manual for use as the finest technical source on our technology and the specialized area of these safety devices. The users of this manual will benefit from its completeness as a pressure relief device resource document. Copyright Notes 1. ASME: the American Society of Mechanical Engineers. 2. NACE: National Association of Corrosion Engineers. 3. API: American Petroleum Institute. Notes 1. Shop test procedure for temperature compensation available on request. 2. All shop orders will state 100 F [38 C] unless customer s purchase order states otherwise. 3. Inconel and Monel are registered trademarks of the International Nickel Company. 4. KYNAR is a registered trademark of the Pennwatt Chemical Corporation. 5. Hastelloy is a registered trademark of Haynes International. Body Materials Pressure relief valve standard body materials are ASME SA-216 grade WCB or WCC CS, or ASME SA-351 GR. CF8M SS. Also available at additional costs are bodies of special alloys, such as Hastelloy C, Monel, high temperature alloy, duplex SS, Titanium, alloy 20 and others. Castings Valve castings to meet requirements of radiography, magnetic particle, liquid penetrant examination and Charpy Impact tests are available on special order. Our documented quality control can provide complete chemical and physical analysis for all cast materials on request. Standard Flanged Connections (a) All steel flange ratings conform to ANSI B and are indicated on each orifice selector chart in the applicable product catalog. Heavier outlet flanges are available on application. For back pressure exceeding listed values, consult the factory for valve limitations. Steel raised face flanges are provided with a serrated finish on the flange face. (b) Standard Aluminum valves are manufactured with flat faced flange finish in accordance with commercial practice. The flanges are designated as Class 125 FF, with drilling equal to ANSI Class 150. (c) All iron flange ratings conform to ANSI B and to Flange Dimension Table (page 70). Iron flat face flanges are supplied with a smooth surface on flange face. (d) Bronze flange ratings conform to ANSI B and to Flange Dimension Table (page 86). Bronze flat face flanges are supplied with a smooth surface on flange face. (e) All ring joint flange facings comply with ANSI B ring groove. For ring joint facing dimensions, refer to the Flange Dimension Table (page 73). (f) Flange facings different from raised face can be furnished at additional cost. The standard surface finish roughness is AARH. DIN, JIS, or other flange finishes may be available on a product-by-product basis. Contact our sales department for availabilities. (g) Drilling of both inlet and outlet flanges always straddles center lines. Offset drilling is available with proper application. Special Flanges Anderson, Greenwood offers a variety of non-standard connection arrangements to meet the most exacting special flange requirement. Spring Materials Pressure relief valve standard spring materials are carbon steel aluminum painted. Spring materials of special alloys, such as tungsten steel, 316 SS, 302 SS, phosphor bronze, K-Monel and Inconel and others are all available in many models on request. Spring Assembly Corrosion Protection At additional cost, springs can be furnished with protective finishes of phenolic, plastic, epoxy resin, and nickel plate. Bellows Valves For easy field conversion, the conventional valve D Series may be changed to a bellows valve all orifice sizes from F to T by installing the bellows assembly and gaskets. Standard material for all bellows is Inconel 625. KYNAR coating, Monel and other materials available at additional cost. Bellows conversions in D and E orifices require a body adapter, stem, guide, gaskets and bellows. Bellows Coating The standard bellows is Inconel 625. KYNAR coating of the bellows for additional corrosion protection is available at additional cost. Seating Surfaces Armco 17-4 PH stainless steel hardened to hardfaced equivalence is an optional D Series disc material. The seating surfaces for other models with stainless steel trim can be hardfaced, when specified, at additional cost. designs and specifications without notice. 1

4 General Information O-ring Seat Seals Anderson Greenwood offers the most complete line of pressure relief valves with O-ring seat seals. All valves are available up to the maximum pressure limits in a variety of O-ring materials. See O-ring Seals Section for complete details and specifications. Trim or Wetted Parts Trim refers to the nozzle or base and the disc in direct spring valves. Consult each product catalog for available materials. Operating and Set Pressure Differentials Optimum performance of a direct spring pressure relief valve protected system is available at operating pressures up to 90% of valve set pressure. Pump and compressor discharge pulsations are offset by the greatest allowable valve set pressure differentials. System pressure pulsations can cause valve malfunctions. Therefore, the pressure relief valve should be set as high as possible above the discharge line pressure. Applications requiring closer system-to-valve pressure differentials may be accommodated by soft seat seal, or Anderson, Greenwood high performance Series 80 or Pilot Operated Valves. Cold Differential Test Pressure Recommendations When pressure relief valves for high temperature service are tested at room temperature, a compensating adjustment is made in the set pressure. High temperature reduces set pressure lessens spring load via thermal expansion of spring, body and bonnet. Cold differential test pressure adjustments are also required on unbalanced valves when constant applied back pressure conditions exist. Cold differential test pressure adjustments are indicated on the valve nameplate, and are recorded on the functional test report. Set Pressure Lower Limits Minimum set pressure per valve series is listed in the applicable product catalog. Seat Tightness All metal seated pressure relief valves are tested for seat leakage per API STD 527 and ANSI B The Anderson Greenwood tightness standard for Series 80 and Pilot Operated pressure relief valves soft seated valves is: no leakage at 95% of set pressure for set pressures of 60 psig [4.13 barg] and higher, or no leakage at 3 psig [.21 barg] below the set pressure for set pressures below 60 psig [4.13 barg]. For all other direct spring, soft-seated pressure relief valves, seat tightness is: no leakage at 90% of set pressure for set pressures 15 psig [1.03 barg], or no leakage at 3 psig [.21 barg] below the set pressure for set pressures below 15 psig [1.03 barg]. Special Applications Many exacting process applications require specially built valves. When your valve requirements exceed catalog descriptions, Anderson Greenwood invites you to submit the specifications. Design data and quotations will be furnished. Valves for Corrosive Service A design advantage frequently overlooked in corrosive application is the full nozzle inlet option on many of our valve models. Until a valve discharges an infrequent occurrence the only contact surfaces are the wetted parts nozzle and disc. Where standard materials are susceptible to attack, corrosion resistant alloys are recommended. Valves for Low Temperature Service Anderson Greenwood has a wide range of pressure relief devices to meet service temperatures to -450 F [-267 C]. Steam Jacketed Valves To keep viscous fluids flowing or to prevent lading fluids from becoming solidified, heat is often applied to the valves. Application of heat, to the valves and piping, however, is often a problem. Steam tracing lines and insulation are frequently required, in addition to heating coils. Removal and reinstallation of a valve is expensive, time consuming and can create costly delays in a process application. Proper heat transfer to keep viscous fluids in their correct flowing state can be obtained by integrally jacketing the housing of the valve. Piping can be simplified, thus reducing maintenance time and permitting the use of many standard replacement parts. Contact your Anderson Greenwood representative or the factory for more information on Steam Jacketed Valves. Steam Jackets are available in integrally cast or bolt on type for the D and L Series only. Installation and Maintenance Complete installation and maintenance training manuals are available on request. Replacement Valves and Repair Parts Submit valve serial number for exact replacement. Anderson Greenwood will supply a valve with correct materials and dimensions. The serial number for most valves is located on the nameplate and stamped on the perimeter of the outlet or body flange. Proper replacement will be made for valves which have become obsolete. Iron and bronze valves may require the complete model number, located on the nameplate. Repair Tools For proper maintenance of Anderson Greenwood Pressure Relief Valves, nozzle wrenches, lapping discs and lapping plates are available, as are complete operating, installation and maintenance manuals Keystone/Anderson, Greenwood reserves Greenwood the right to change & Co. product designs and specifications without notice. 2

5 Glossary Accumulation is the pressure increase over the maximum allowable working pressure of the vessel during discharge through the pressure relief device, expressed in pressure units or as a percent. Maximum allowable accumulations are established by applicable codes for operating and fire contingencies. Actual Discharge Area is the measured minimum net area that determines the flow through a valve. Back Pressure is the pressure that exists at the outlet of a pressure relief device as a result of the pressure in the discharge system. Balanced Pressure Relief Valve is a spring-loaded pressure relief valve that incorporates a means for minimizing the effect of back pressure on the performance characteristics. Blowdown is the difference between the set pressure and the closing pressure of a pressure relief valve, expressed as a percent of the set pressure or in pressure units. Built-up Back Pressure is the increase in pressure in the discharge header that develops as a result of flow after the pressure relief device opens. Closing Pressure is the value of decreasing inlet static pressure at which the valve disc re-establishes contact with the seat or at which lift becomes zero. Cold Differential Test Pressure is the pressure at which the pressure relief valve is adjusted to open on the test stand. The cold differential test pressure includes corrections for the service conditions of back pressure or temperature or both. Conventional Pressure Relief Valve is a spring-loaded pressure relief valve whose performance characteristics are directly affected by changes in the back pressure on the valve. Curtain Area is the area of the cylindrical or conical discharge opening between the seating surfaces above the nozzle seat created by the lift of the disc. Design Gauge Pressure refers to at least the most severe conditions of coincident temperature and pressure expected during operation. This pressure may be used in place of the maximum allowable working pressure in all cases where the MAWP has not been established. The design pressure is equal to or less than the MAWP. Effective Discharge Area or Equivalent Flow Area is a nominal or computed area of a pressure relief valve used in recognized flow formulas to determine the size of the valve. It will be less than the actual discharge area. Huddling Chamber is an annular pressure chamber in a pressure relief valve located beyond the seat for the purpose of generating a rapid opening. Inlet Size is the nominal pipe size (NPS) of the valve at the inlet connection, unless otherwise designated. Leak-test Pressure is the specified inlet static pressure at which a seat leak test is performed. Lift is the actual travel of the disc away from the closed position when a valve is relieving. Maximum Allowable Working Pressure (MAWP) is the maximum gauge pressure permissible at the top of a completed vessel in its operating position for a designated temperature. The pressure is based on calculations for each element in a vessel using nominal thicknesses, exclusive of additional metal thicknesses allowed for corrosion and loadings other than pressure. The maximum allowable working pressure is the basis for the pressure setting of the pressure relief devices that protect the vessel. Maximum Operating Pressure is the maximum pressure expected during system operation. Nozzle Area is the cross-sectional flow area of a nozzle at the minimum nozzle diameter. designs and specifications without notice. 3

6 Glossary Opening Pressure is the value of increasing inlet static pressure at which there is a measurable lift of the disc or at which discharge of the fluid becomes continuous. Outlet Size is the nominal pipe size (NPS) of the valve at the discharge connection, unless otherwise designated. Overpressure is the pressure increase over the set pressure of the relieving device, expressed in pressure units or as a percent. It is the same as accumulation when the relieving device is set at the maximum allowable working pressure of the vessel, and the inlet pipe pressure loss is zero. Pilot Operated Pressure Relief Valve is a pressure relief valve in which the main valve is combined with and controlled by an auxiliary pressure relief device. Pressure Relief Device is actuated by inlet static pressure and designed to open during an emergency or abnormal conditions to prevent a rise of internal fluid pressure in excess of a specified value. The device also may be designed to prevent excessive internal vacuum. The device may be a pressure relief valve, a nonreclosing pressure relief device, or a vacuum relief valve. Rated Relieving Capacity is that portion of the measured relieving capacity permitted by the applicable code or regulation to be used as a basis for the application of a pressure relief device. Relief Valve is a spring-loaded pressure relief valve actuated by the static pressure upstream of the valve. The valve opens normally in proportion to the pressure increase over the opening pressure. A relief valve is used primarily with incompressible fluids. Rupture Disc Device is a nonreclosing differential pressure relief device actuated by inlet static pressure and designed to function by bursting the pressure-containing rupture disc. A rupture disc device includes a rupture disc and a rupture disc holder. Safety Relief Valve is a spring-loaded pressure relief valve that may be used as either a safety or relief valve depending on the application. Safety Valve is a spring-loaded pressure relief valve actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action. A safety valve is normally used with compressible fluids. Set Pressure is the inlet gauge pressure at which the pressure relief valve is set to open under service conditions. Simmer is the audible or visible escape of compressible fluid between the seat and disc at an inlet static pressure below the set pressure and at no measurable capacity. Spring-loaded Pressure Relief Valve is a pressure relief device designed to automatically reclose and prevent the further flow of fluid. Stamped Capacity is the rated relieving capacity that appears on the device nameplate. The stamped capacity is based on the set pressure or burst pressure plus the allowable overpressure for compressible fluids and the differential pressure for incompressible fluids. Superimposed Back Pressure is the static pressure that exists at the outlet of a pressure relief device before it actuates. It is the result of pressure in the discharge system coming from other sources and may be constant or variable. Relieving Conditions is the term used to indicate the inlet pressure and temperature on a pressure relief device at a specific overpressure. The relieving pressure is equal to the valve set pressure (or rupture disc burst pressure) plus the overpressure. (The temperature of the flowing fluid at relieving conditions may be higher or lower than the operating temperature.) designs and specifications without notice. 4

7 Valve Sizing Nomenclature Gas and Steam Flow Symbol Description Inch Metric Pounds Units A Orifice area or equivalent flow area. C The gas constant of gas, derived from the specific heat ratio, k. If C is unknown, use C = 315, a conservative value. Refer also to Physical Properties of Selected Gases. F Subsonic flow factor, based on the ratio of specific heats and pressure drop (differential) across the valve or nozzle. k The ratio of specific heats of gas, where k = C p /C v. When the value of k is unknown, use k = 1.001, a conservative value. Refer also to Physical Properties of Selected Gases. K The valve coefficient to be used where set pressure is 15 psig [1.03 barg] and greater, and in accordance with the requirements of Section VIII, Division 1 of the ASME Boiler and Pressure Vessel Code, ASME I, and ASME III. Valve coefficient K includes the required derating to 90% of actual average measured nozzle coefficient, K D, as required by the ASME Code. Please note that safety valve models available for gas and liquid applications will have differing nozzle coefficients. K b A back pressure correction factor for gas, used when the flow becomes subsonic, occurring when the pressure ratio across the valve nozzle exceeds the critical pressure, P CF /P 1. K N Steam flow correction factor, from the Napier equation. K SH Superheat correction factor for use in the steam formulas. M P P b Molecular weight of the flowing gas. Refer to Physical Properties of Selected Gases, or other resources, for listing of M. Set pressure in gauge units. All formulas herein are based on barg or psig. Back pressure, under relieving conditions, at valve outlet in gauge pressure units. square inch square centimeter (in 2 ) [cm 2 ] lb/in 2 gauge bar gauge (psig) [barg] lb/in 2 gauge bar gauge (psig) [barg] designs and specifications without notice. 5

8 Valve Sizing Nomenclature P 1 Absolute pressure at valve inlet connection under relieving conditions and equal to set pressure, p + overpressure + atmospheric pressure. Atmospheric pressure will be equal to standard sea level pressure, 14.7 psia [1.013 bara], unless otherwise specified. When a local plant site barometric pressure is mentioned, sizing for orifice area should be made with the stated local barometric pressure. P 2 Absolute pressure at valve outlet under relieving conditions; equal to back pressure, p b + atmospheric pressure (as expressed in previous paragraph). P CF /P 1 Critical pressure ratio. The critical pressure ratio is used to determine if the back pressure correction factor K b shall be applied to the sizing formula. t T V W Z Gas and Steam Flow Symbol Description Inch Metric Pounds Units Relieving temperature, to be evaluated at the valve inlet, under relieving condition. Absolute relieving temperature, equal to relieving temperature plus base temperature, where: T [ Rankin] = t [ F] and T [Kelvin] = t [ C] Gas flow capacity expressed in volumetric units per time unit. The formulas in this section are based on a sea level atmospheric pressure of 14.7 psia [1.013 bars] and a temperature base of 60 F or 0 C, respectively for metric and inchpound systems. Refer to Gas Flow Conversions for other pressure and temperature bases as well as other units of measure. Gas flow capacity expressed in weight units per time unit. Refer to Gas Flow Conversions for other units of measure. Compressibility factor, correcting for the difference between the physical characteristics of a theoretical gas and the actual gas under consideration. If Z is unknown, use Z = lb/in 2 absolute (psia) lb/in 2 absolute (psia) lb/in 2 absolute (psia) bar absolute [bara] bar absolute [bara] bar absolute [bara] degrees Fahrenheit ( F) degrees Rankin ( R) standard cubic feet per minute (scfm) pounds per hour (lb/h) degrees Celsius [ C] degrees Kelvin [ K] normal cubic meters per hour [Nm 3 /h] kilograms per hour [kg/h] Note 1. The formulas using this pseudo pressure ratio are valid only for the specified pressure units to the right and for the Series 90 low pressure pilot operated safety valves. P 1 Set pressure = 15 psig [1.03 barg] and higher P 1 Set pressure is less than 15 psig [1.03 barg] P 2 P 2 designs and specifications without notice. 6

9 Valve Sizing Nomenclature Notes 1. Relief valves certified for liquid applications with full lift at 10% overpressure, shall use K p = 1.00 at 10% and greater overpressure. The 1985 revision to ASME VIII required all liquid relief valves to have certified capacities at 10% overpressure. Therefore, the use of K p in the sizing formula would apply to non-asme Code valves only. 2. The maximum permitted values of overpressure for various types of liquid safety valves in this manual are as follows: Maximum Overpressure Liquid Safety Valve Type 10% Pilot operated. 10% Conventional and balanced direct spring operated, with certified full lift at 10% overpressure. 25% Conventional and balanced direct spring operated valves not meeting the above requirements. Note: Sizing may be done at 10% overpressure when the correction factor Kp is made equal to A G K K p K v K w p 1 p 2 Liquid Flow Symbol Description Inch Metric Pounds Units Orifice area Relative density of liquid at flowing temperature, referred to water at 68 F [20 C]. G water = Effective or certified nozzle coefficient. The certified nozzle coefficients, when given, are in accordance with the requirements of Section VIII, Division 1 of the ASME Boiler and Pressure Vessel Code, ASME I, and ASME III and include a derating to 90% of actual, as required by the Code. The effective nozzle coefficients, when given, also assume the same derating, but are not certified by the National Board of Boiler and Pressure Vessel Inspectors. Please note that safety valve models available for gas and liquid applications will have differing nozzle coefficients. Capacity correction factor due to lift characteristics of conventional and balanced spring operated valves, in liquid service, where full lift is achieved at 25% overpressure. Use K p = 0.60 for sizing these valve types at 10% overpressure, and K p = 1.00 for 25% and greater overpressure. 1 Capacity correction factor due to viscosity. For most applications, viscosity may not be significant, in which case use K v = Capacity correction factor for balanced bellows safety valves due to back pressure. Use K w = 1.00 for conventional (unbalanced) and pilot operated safety valves. Upstream pressure under relieving conditions. This is set pressure, plus overpressure. 2 Total back pressure, under relieving conditions, at valve outlet. R Reynolds Number. A dimensionless expression for the flow behavior of fluids and is used to determine the viscosity correction factor K v. µ Absolute viscosity of the liquid at the relieving temperature. Kinematic viscosity and/or viscosity expressed in other units of measure must be converted to absolute viscosity in centipoise. Most liquid applications need not consider viscosity and should therefore use a K v = The approximate viscosity of water under most conditions is 1 centipoise. When viscosity is given, it should be considered. W Liquid flow rate. square inch square centimeter (in 2 ) [cm 2 ] lb/in 2 gauge (psig) bar gauge [barg] lb/in 2 gauge bar gauge (psig) [barg] centipoise US gallons per min (US gpm) centipoise cubic meters per hour [m 3 /h] designs and specifications without notice. 7

10 Valve Sizing Nozzle Coefficients ASME Nozzle Coefficients Direct Spring PRVs Valve ASME I ASME III ASME IV ASME VIII Steam Hot Water Steam Gas, Vapor Liquids Steam Steam Gas, Vapor Liquids A Series D Series F Series G Series K Series L Series Y Series W.975 Model Model 63B.877 Model 83F.847 Model 81, Model Model 81P.720 API Nozzle Coefficients Direct Spring PRVs Valve ASME I ASME III ASME IV ASME VIII Steam Hot Water Steam Gas, Vapor Liquids Steam Steam Gas, Vapor Liquids A Series.878 D Series F Series G Series K Series L Series Y Series W.975 Model Model 63B.847 Model 83F.998 Model 81, Model Model 81P.720 Note 1. ASME nozzle coefficient is the actual coefficient recorded by the National Board of Boiler and Pressure Vessel Inspectors. It differs from the API nozzle coefficient. When sizing PRVs using the ASME coefficient, the ASME area must be used. The API nozzle coefficient is an effective coefficient to be used when sizing PRVs using API 526 orifice areas. designs and specifications without notice. 8

11 Valve Sizing Nozzle Coefficients ASME Nozzle Coefficients Pilot Operated PRVs Valve ASME VIII Steam Gas, Vapor Liquids 223, , , , , , 473, 673, 873, / /93T API Nozzle Coefficients Pilot Operated PRVs Valve ASME VIII Steam Gas, Vapor Liquids 223, , , , , , 473, 673, 873, / /93T designs and specifications without notice. 9

12 Valve Sizing Orifice Areas Orifice Areas Direct Spring PRVs Valve Orifice ASME Area API Area Designation (in 2 ) [cm 2 ] (in 2 ) [cm 2 ] A Series D A Series E A Series F A Series G A Series H A Series J D Series D D Series E D Series F D Series G D Series H D Series J D Series K D Series L D Series M D Series N D Series P D Series Q D Series R D Series T Note 1. The ASME area is the actual flow area certified by the National Board of Boiler and Pressure Vessel Inspectors. The API area is the flow area defined per API 526. When sizing PRVs, care should be exercised to not mix API and ASME values. designs and specifications without notice. 10

13 Valve Sizing Orifice Areas Orifice Areas Direct Spring PRVs Valve Orifice ASME Area API Area Designation (in 2 ) [cm 2 ] (in 2 ) [cm 2 ] K Series F K Series G K Series H K Series J K Series K K Series L K Series M K Series N K Series P K Series Q L Series L Series C L Series V L Series G Model Model 63B Model 63B Model 81, 83, 81P Model 81, Model 81, 83, 81P Model 81, 83 F Model 81, 83, 81P G Model 81, 83 H Model 81, 83, 81P J designs and specifications without notice. 11

14 Valve Sizing Orifice Areas ASME Orifice Areas Pilot Operated PRVs Valve Size Orifice Area, in 2 [cm 2 ] Type X33 1 Type X23 1 Type X73 1 Type 226/526 Type 576 Type x ( D ) [0.98] /2 x ( E ) [1.71] ( F ) [2.05] 1 1 /2 x ( G ) [3.86] ( H ) [5.27] [8.62] [8.62] 1 1 /2 x ( G ) [3.86] ( H ) [5.27] ( H ) [5.27] 2 x ( G ) [4.06] ( G ) [4.07] ( J ) [8.62] [16.32] ( J ) [8.62] [16.32] ( H ) [6.33] ( H ) [6.28] ( J ) [10.55] ( J ) [10.55] 3 x ( J ) [9.34] ( L ) [19.26] [42.91] ( L ) [19.26] [42.91] ( K ) [14.82] ( K ) [13.95] ( L ) [22.95] 4 x ( L ) [22.95] ( L ) [22.66] ( M ) [29.06] ( M ) [28.19] ( P ) [42.91] [62.12] ( P ) [42.91] [62.12] ( N ) [35.00] ( N ) [32.61] ( P ) [51.04] ( Q ) [89.12] 6 x ( Q ) [79.68] ( R ) [107.45] ( R ) [107.45] [138.84] ( R ) [129.03] ( RR ) [148.32] 6 x Dual [138.84] 8 x ( T ) [174.90] ( T ) [174.90] [285.03] ( T ) [209.68] 8 x Dual [189.81] 8 x Dual 10 8 x Single [285.03] [285.03] Notes 1. Series 200/300/400/600/800/ Except for liquid service. 3. Threaded body only. designs and specifications without notice. 12

15 Valve Sizing Orifice Areas API Orifice Areas Pilot Operated PRVs Valve Size Orifice Area, in 2 [cm 2 ] Type X33 1 Type X23 1 Type X73 1 Type 226/526 Type 576 Type x ( D ) [0.71] ( F ) [1.98] 1 1 /2 x ( E ) [1.26] 1 1 /2 x ( G ) [3.24] ( H ) [5.06] [8.11] [8.11] 1 1 /2 x ( G ) [3.24] ( H ) [5.06] ( H ) [5.06] 2 x 3 3 x ( G ) [3.24] ( H ) [5.06] ( J ) [8.30] ( K ) [11.86] ( L ) [18.41] ( G ) [3.24] ( J ) [8.30] [15.35] ( J ) [8.30] [15.35] ( H ) [5.06] ( J ) [8.30] ( J ) [8.30] ( L ) [18.41] [40.37] ( L ) [18.41] [40.37] ( K ) [11.86] ( L ) [18.41] 4 x ( M ) [23.22] 6.38 ( P ) [41.16] [58.44] 6.38 ( P ) [41.16] [58.44] 4.34 ( N ) [28.00] ( L ) [18.41] 3.60 ( M ) [23.22] 4.34 ( N ) [28.00] 6.38 ( P ) [41.16] 6 x ( Q ) [71.29] ( R ) [103.22] ( R ) [103.22] ( Q ) [71.29] ( R ) [103.22] 6 x Dual [130.61] 8 x ( T ) [167.74] ( T ) [167.74] [268.13] ( T ) [167.74] 8 x Dual [178.55] 8 x Dual 10 8 x Single [268.13] [268.13] Notes 1. Series 200/300/400/600/800/ Except for liquid service. 3. Threaded body only. designs and specifications without notice. 13

16 Valve Sizing Orifice Areas Orifice Areas - Types 93/93T Valve ASME API Size (in 2 ) [cm 2 ] (in 2 ) [cm 2 ] Orifice Areas - Types 91/94 Valve ASME API Size (in 2 ) [cm 2 ] (in 2 ) [cm 2 ] Orifice Areas - Type 95 Valve ASME API Size (in 2 ) [cm 2 ] (in 2 ) [cm 2 ] Orifice Areas - Type 96A Valve ASME API Size (in 2 ) [cm 2 ] (in 2 ) [cm 2 ] Orifice Areas - Series 9000 Valve ASME API Size (in 2 ) [cm 2 ] (in 2 ) [cm 2 ] designs and specifications without notice. 14

17 Back Pressure and Subsonic Flow Correction Factor for Section I and Section VIII Sizing Formulas for Gas, Steam and Liquid designs and specifications without notice. 15

18 Valve Sizing Evaluating Back Pressure Correction K b Factor for Gas and Steam When any back pressure exists, a test for subsonic flow should be made. If the absolute outlet-to-inlet flowing pressure ratio (P 2 /P 1 ) is greater than 0.30 a back pressure correction factor K b may be required subject to the additional commentary below. The required values of K b as a function of P outlet/p inlet are determined from the curves presented in this section for both conventional spring operated and pilot operated pressure relief valves. General Whenever flow through a pressure relief valve occurs under sonic conditions, the value of K b is When a pressure relief valve discharges directly to the atmosphere and the set pressure is 15 psig [1.03 barg] or greater, flow is considered to be sonic, therefore K b remains equal to If the pressure relief valve discharges into any piping where the back pressure at the valve outlet under relieving conditions exceeds a definitive limit, flow will be subsonic. The orifice area calculation of a pressure relief valve, flowing under these conditions, must be mathematically adjusted using the back pressure correction factor K b. K w Factor for Liquids Whenever back pressure is encountered in bellows and pressure balanced spring operated liquid relief valves, a reduction in flow capacity due to reduced valve lift can result. A K w factor to correct for this reduction is included in the liquid capacity equation. The required values of K w based upon P 2 /P 1 are determined from the curves at the back of this section. Sources of Back Pressure A pressure relief valve whose outlet is discharging into vent piping or to another pressure vessel or system will encounter one or two types of back pressure: superimposed and/or built-up. Superimposed back pressure may come from the vent system due to the discharge of other pressure relief valves into a common manifold or due to the nature of other processes that affect the downstream pressure. The presence of superimposed back pressure may not necessarily create subsonic flow. However the outlet pressure may rise further, due to flow from the pressure relief valve, and may be sufficient to cause subsonic flow. Built-up back pressure occurs as a result of the discharge of fluid through a flowing pressure relief valve with connected downstream piping or equipment. In some instances, relatively short sections of piping connected by the outlet of a pressure relief valve and venting to the atmosphere will be sufficient to create back pressure during a relieving cycle that will cause flow to be subsonic. The result will be a reduction of flow capacity. If this is less than the required relieving capacity, the inlet pressure may rise sufficiently to exceed the permissible accumulation for the application. The problem is compounded when there is also some superimposed back pressure, since built-up back pressure will be additive. Conventional Direct Spring Operated Pressure Relief Valves If a conventional, direct spring operated pressure relief valve is to be applied where any built-up back pressure will be developed, the maximum permissible built-up back pressure shall not exceed 10% of set pressure. Under this limit, no back pressure correction factor need be applied, except as follows: When a conventional pressure relief valve is set to open with a superimposed back pressure sufficiently high to create subsonic flow, the back pressure correction factor may be applied (assuming that the pressure ratio exceeds the critical ratio). If the valve is known to be tolerant to a greater amount, the back pressure correction factor may be applied. Balanced Pressure Relief Valves The balanced bellows valve is balanced against superimposed back pressure. It is also resistant to a moderate amount of built-up back pressure. Apply the back pressure correction factor K b. When balanced bellows valves are used, the maximum permissible built-up back pressure should not exceed 40%. Pilot Operated Pressure Relief Valves A properly selected and installed pilot operated pressure relief valve will operate effectively under all combinations of superimposed and built-up back pressure, limited only by the valve pressure rating and practical considerations. Apply the back pressure correction factor K b, if applicable. Solving for K b The critical pressure ratio is a function of the value of k, the specific heat ratio of the gas. The value of P CF /P 1 varies from to for a range of k between 1.00 and When sizing valve designs for set pressures below 15 psig covered under API 2000, the P outlet/p inlet ratio may be calculated and compared directly to the correct P critical for the gas or vapor k value. The k values for selected gases, the P critical vs. k equation and a set of P critical vs. k curves for frequently encountered k values are presented in the Fluid Data section of this manual. designs and specifications without notice. 16

19 Valve Sizing Superimposed Back Pressure Correction Factors Direct Spring PRVs/Vapor and Gases ASME Section VIII Constant Back Pressure Correction Factor K b 1.00 Back Pressure Correction Factor K b Back Pressure Percentage = Back pressure, psia or bara Flowing pressure, psia or bara x 100 Example: Set pressure = 200 psig [13.79 barg] Constant back pressure = 160 psig [11.03 barg] Back pressure percentage (absolute) = x 100 = 74% or = 74% Factor K b = 0.91 (follow dotted line from curve) Capacity with back pressure = 0.91 x rated capacity without back pressure designs and specifications without notice. 17

20 Valve Sizing Superimposed and Built Up Back Pressure/Subsonic Flow Correction Factors Direct Spring PRVs - Model Designations - Bellows Valves Only/Vapors and Gases ASME Section VIII Capacity with back pressure Rated capacity without back pressure Kb = Variable Back Pressure Correction Factor K b % Overpressure psig [3.44 barg] and over 15 psig [1.013 barg] Back pressure percentage = Back pressure, psig or barg Set pressure, psig or barg x 100 Example: Set pressure = 100 psig [6.89 barg] Back pressure = 0 to 35 psig [2.41 barg] 35 Back pressure percentage (gauge) = x 100 = 35% max. 100 Factor K b = 0.94 (follow dotted line from curve) Capacity with back pressure = 0.94 x rated capacity without back pressure Capacity with back pressure Rated capacity without back pressure Kb = Variable Back Pressure Correction Factor K b % Overpressure psig [3.44 barg] and over psig [1.013 barg] Back pressure percentage = Back pressure, psig or barg Set pressure, psig or barg x 100 Example: Set pressure = 100 psig [6.89 barg] Back pressure = 0 to 35 psig [2.41 barg] 35 Back pressure percentage (gauge) = x 100 = 35% max. 100 Factor K b = 0.99 (follow dotted line from curve) Capacity with back pressure = 0.99 x rated capacity without back pressure designs and specifications without notice. 18

21 Valve Sizing Superimposed and Built Up Back Pressure/Subsonic Flow Correction Factor (K b ) Back Pressure Correction Factor for Piston Pilot Operated PRVs - Gas, Vapor, or Steam Back Pressure Factor, K b k = 1.0 k = 1.2 k = 1.4 k = 1.6 k = 1.8 k = P 2 P 1 = Absolute Pressure Ratio at Valve Inlet The above curves are applicable for all pressure ranges and overpressures and accurately predict the reduction on capacity for full lift, API orifice valves. For full bore valves, multiply above K b values by designs and specifications without notice. 19

22 Valve Sizing - Back Pressure Correction Factor for Type 727 Gas, Vapor or Steam k = k = 1.2 Back Pressure Factor, K b k = 1.4 k = k = k = P /P = Absolute Pressure Ratio at Orifice 2 1 designs and specifications without notice. 20

23 Valve Sizing - Liquid Flow Back pressure percentage = Back pressure, psig or barg Set pressure, psig or barg x 100 Example: Set pressure = 100 psig [6.89 barg] Back pressure = 0 to 24 psig [1.65 barg] 24 Back pressure percentage (gauge) = x 100 = 24% max. 100 Factor K w = 0.95 (follow dotted line from curve) Capacity with variable back pressure = 0.95 x rated capacity Based on differential pressure F d Direct Spring PRVs - Model Designation - Bellows Valves Only/Liquid Service Capacity with variable back pressure Rated capacity based on Fd Kw = Variable or Constant Back Pressure Correction Factor K w Curve to Evaluate Liquid Back Pressure for Series 81P Correction Factor k w 1.00 Kw = Back Pressure Correction Factor Based on 10% Overpressure Correction Curve for Types 81P - G and 81P - J Correction Curve for Types 81P - 4 and 81P Percentage Back Pressure = Back Pressure, psig [barg] Set Pressure, psig [barg] x 100 designs and specifications without notice. 21

24 Valve Sizing Gas and Vapor Equations ASME VIII Gas Flow (Set Pressure 15 psig [1.03 barg]) U.S. Weight Flow (lb/h) Formula 1 W TZ A = CK P 1 K b M U.S. Volumetric Flow (scfm) Formula 2 V MTZ A = 6.32 CK P 1 K b Metric Weight Flow [kg/h] Formula 1M W TZ A = CK P 1 K b M Metric Volumetric Flow [Nm 3 /h] Formula 2M V MTZ A = CK P 1 K b Sizing Information ASME Section I and VIII After system capacity has been determined, a properly sized pressure relief valve is determined by the following method. A. From the formulas in this section calculate required orifice area as a function of capacity. The orifice sizes for steam, air, or water may be obtained from the capacity tables catalog. B. Identify the required orifice letter designation, such as D, E, F, etc. Always choose an orifice which is equal to, or greater than the required orifice area. C. Specifications exceeding Anderson Greenwood standard catalog descriptions should be referred to our sales department. D. When selecting orifice areas and nozzle coefficients, either select the ASME area and nozzle coefficient, or the equivalent API area and nozzle coefficient. Mixing ASME and API values is incorrect and may result in a dangerous sizing error. designs and specifications without notice. 22

25 Valve Sizing Gas and Vapor Flow (English Units) Example 1 - ASME VIII Gas Given: Butane, with a required flow rate of 16,000 scfm, set at 88 psig, 10% overpressure, gas temperature of 60 F, discharging to a closed header system. The back pressure (maximum) is 40 psig. Find: The required orifice area. Examples for Steam and Gas Applications Solution: Use Formula 1 V MTZ A = 6.32 CK P 1 K b 16,999 (58.12)(520)(1) A = = 6.32 (326)(111.5) K K b K K b V = 16,000 M = C = 326 T = = 520 Z = 1.0 P 1 = 88 (1.1) = K b = 40 = 45% 88 Bellows Valve K b = 0.77 Pilot Valve = 49% K b = 0.98 For the bellows valve, the correct orifice selection would be an 8T10 (26.0 in 2 ). For the case of selecting a POSRV, the correct orifice selection would be 6R8 (16.00 in 2 ). Case 1 - Select a DB Series with K = 0.971, K b = 0.77, A = in 2. Case 2 - Select a POSRV with K = 0.86, K b = 0.98, A = in 2. designs and specifications without notice. 23

26 Valve Sizing Gas and Vapor Flow - [Metric Units] Examples for Gas and Steam Applications Solution: Use formula 1M and the physical properties found on pages P 1 = 5 x (1.1) = bara T = = 288 K M = 58.12, C = 326 Z = 1.00 (used when no value is given) K b = 1.00 (when back pressure equals atmospheric) 9, x 288 x cm 2 A [cm 2 ] = = x 326 x K x x 1.00 K Example 2 Given: Butane, with a required flow rate of 9,000 Nm 3 /h, set at 5 barg, 10% overpressure, relieving temperature of 15 C, discharging to atmosphere. Find: The required orifice area for a typical conventional safety valve and the orifice selected. A = = cm Selecting a valve with a K = 0.971, the orifice to be selected is a 4P6 (41.16 cm 2 ). Solution: The same data is used as in example 2, except use a nozzle coefficient K = (from page 9) in formula 2M. 9, x 288 x 1.00 A [cm 2 ] = = cm x 326 x x x 1.00 From page 12, the next larger available orifice is cm 2 corresponding to a fullbore, 3-inch x 4-inch valve, Series 273 or 473. Note in this example, ASME not API coefficients are used. Example 3 Given: Same as example 2. Find: The appropriate size valve for a piston type (Series 200, 300, etc.), pilot operated safety valve. Solution: The back pressure represents 44% of the set pressure (2.2/5.0). For a Direct Spring SRV, K b = 0.78, for a 200 Series Pilot Operated PRV, the back pressure represents 49% of the absolute pressure ratio: x Therefore, the K b for the Series 200 = (from page 19). Again using formula 2M, for the Direct Spring Valve: 9, x 288 x 1.0 A = = cm x 326 x 971x x.78 Example 4 Given: The same as example 2, except with a built-up back pressure of 2.2 barg. Find: The appropriate size to meet the relieving conditions. Selecting a direct spring would result in a 6Q8 (71.29 cm 2 ). For the POSRV: 9, x 288 x cm 2 A = = x 326 x K x x.985 K Case 1 - Select a Type 223, K = 0.830, A = cm 2. Case 2 - Select a Type 273, K = 0.809, A = cm 2. Selecting a pilot valve could either be a 4P6 (42.91 cm 2 ) or a full bore 3 x 4 (42.91 cm 2 ). designs and specifications without notice. 24

27 Valve Sizing Steam Flow Sizing Information ASME Section I and VIII After system capacity has been determined, a properly sized pressure relief valve is determined by the following method. A. From the formulas in this section calculate required orifice area as a function of capacity. The orifice sizes for steam, air, or water may be obtained from the capacity tables catalog. B. Identify the required orifice letter designation, such as D, E, F, etc. Always choose an orifice which is equal to, or greater than the required orifice area. C. Specifications exceeding Anderson Greenwood standard catalog descriptions should be referred to our sales department. ASME I Sonic Steam Flow (Set Pressure 15 psig [1.03 barg]) A = U.S. Units (lb/h) Formula 3 W K P 1 K SH K N K b K N = 1.00 for P 1500 psig K N = P P where 1500 psig < P < 3200 psig Metric Units [kg/h] Formula 3M ASME VIII Sonic Steam Flow (Set Pressure 15 psig [1.03 barg]) U.S. Units (lb/h) Formula 4 A = W K P 1 K SH K N K b K N = 1.00 for P barg K N = P P where barg < P < barg Metric Units [kg/h] Formula 4M D. When selecting orifice areas and nozzle coefficients, either select the ASME area and nozzle coefficient, or the equivalent API area and nozzle coefficient. Mixing ASME and API values is incorrect and may result in a dangerous sizing error. A = W 51.5 K P 1 K SH K N K b K N = 1.00 for P 1500 psig K N = P P where 1500 psig < P < 3200 psig A = W 52.5 K P 1 K SH K N K b K N = 1.00 for P barg K N = P P where barg < P < barg designs and specifications without notice. 25

28 Valve Sizing Steam (English Units) Examples for Steam Applications Solution: Use Formula 3 A = A = W K P 1 K SH K N K b = K (.97)(1)(34.7)(1) K W = 4050 K SH = 0.97 K N = 1.00 K b = 1.00 P 1 = = 34.7 Selecting a valve with a K = (K Series), A = 2.66 in 2. This orifice corresponds to an L orifice (2.853 in 2 ). Note the overpressure for set pressures between 15 psig and 70 psig is 3% or 2 psig minimum. Example 5 - ASME I Steam Given: Steam, with a required flow rate of 4,050 lb/h, set at 18 psig, is required for an ASME I boiler application. Steam temperature is 420 F. Find: = The required orifice area K Solution: Use Formula 4 W A = 51.5 K P 1 K SH K N K b A = 84,000 = (K)(1)(1)(454.7)(1) K W = 84,000 K SH = 1.00 K N = 1.00 K b = 1.00 P 1 = 400 (1.1) = Example 6 - ASME VIII Steam Given: Steam, with a required flow rate of 84,000 lb/h, set at 400 psig, is required for an unfired pressure vessel application. Steam temperature is 448 F. Find: The required orifice area and approximate valve capacity. Selecting a valve with a K = (D Series), A = 3.69 in 2. This orifice corresponds to a 4N6 orifice (4.34 in 2 ). The approximate flowing capacity can be estimated from the ratio of actual area to required area as follows: W = 84,000 x 4.34 = 98,796 lb/hr 3.69 designs and specifications without notice. 26