Safety valve calculation example

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Safety valve calculation example Safety valve is one of the safety accessories for pressure vessels to realize overpressure relief during operation. It is also a must-inspection item in the periodic inspection of online pressure vessels. It includes two series: anti-overpressure and anti-vacuum, that is, one is to discharge the overpressure medium inside the container to prevent the container from failing, and the other is to suck in the external medium to prevent the failure of the container's rigidity. All pressure vessels that comply with the applicable scope of the Capacity Regulations shall be equipped with safety valves according to the requirements of the design drawings. one. The selection method of the safety valve a) Determine the safety valve according to calculation. The nominal diameter must be such that the discharge capacity of the safety valve ≥ the safe discharge capacity of the pressure vessel. b) Determine the pressure level of the safety valve according to the design pressure and design temperature of the pressure vessel. c) For safety valves for steam with an opening pressure greater than 3MPa or safety valves for gases with a medium temperature exceeding 320°C, the form with a radiator (fin) should be selected; d) For flammable, extremely toxic or highly hazardous media, a closed safety valve must be used. If a lifting mechanism is required, a closed safety valve with a wrench should be used; e) When the safety valve is likely to withstand changes in back pressure and the variation exceeds 10% of the opening pressure, a safety valve with a bellows should be used; f) For non-hazardous media such as air, hot water above 60℃ or steam, a safety valve with a wrench should be used. g) Liquefaction tank (tank) trucks should use a built-in safety valve. h) Select the appropriate safety valve material according to the characteristics of the medium: for example, copper or copper-containing safety valves cannot be used for ammonia-containing media; safety valves containing 70% copper or red copper cannot be used for acetylene. i) For working conditions with large discharge volume, the full-open type should be selected; for stable working pressure and small discharge volume, the micro-open type should be selected; for high-pressure and large-discharge conditions, It is advisable to use non-direct starting type, such as pulse type safety valve. For containers with a length of more than 6m, two or more safety valves should be installed. j) For fixed vessels with low working pressure Pw, static weight (pressure cooker) or lever weight type safety valves can be used. Mobile equipment should use spring-type safety valves. k) For mediums that are thick and easy to block, a series combination relief device of a safety valve and a bursting disc should be used. l) The working pressure level of the spring is selected according to the nominal pressure of the safety valve. The relationship between the nominal pressure of the safety valve and the working pressure of the spring is shown in Table 1 m) The relationship between the nominal pressure PN of the safety valve and the working pressure of the spring Table 1 PN spring working pressure level 1.60.06～0.1>0.12>0.16～0.25>0.25～0.4>0.4～0.5 >0.5～0.6>0.6～0.8>0.8～1.0>1.0～1.3>1.3～1.62.5>1.3～1.6>1.6～2.0>2.0～2.5 Can only be used for greater than 1.3MPa6.4->1.3～1.6>1.6～2.0>2.0～2.5>2.5～3.2>3.2～4.0 >4.0～6.4 Can only be used for greater than 1.3MPa10>4～5>5～6.4>6.4～8>8～10 Can only be used for greater than 4.0MPa The safety valve should act sensitively and reliably. When the opening pressure is reached, the valve disc should open and fully rise in time to facilitate smooth discharge. At the same time, it should have good sealing performance. It should not only remain leak-proof during normal operation, but also require the valve disc to close in time and maintain sealing after opening and reset. Under exhaust pressure, the valve disc should reach the fully open position without oscillation, and ensure the specified air volume is discharged. two. Safety valve calculation example We require the safe discharge capacity of the safety valve to be selected or verified during pressure vessel design and periodic inspection. Based on the incomplete data in the past, large ones were often used instead of small ones, resulting in unnecessary waste. It is now planned to use GB150 Appendix B-B5.1 b) as a basis to select and calculate the safe discharge volume of the safety valve using different media, pressures and temperatures. Example 1: There is an air storage tank, DN1000㎜, volume V=5m3, the maximum working pressure is 0.8MPa, the working temperature is 30℃, the inlet pipe is φ57X3.5, determine the size of the safety valve. Solution 1) Determine the state conditions of the gas. Let Po - the outlet side pressure of the safety valve (absolute pressure) be 0.103MPa (approximately 0.1MPa) Then Pd-safety valve relief pressure (absolute pressure) is Pd=1.1Ps+0.1 =1.1×1.1Pw+0.1=1.068MPa (GB150 Appendix B4.2.1) When the outlet side of the safety valve is atmosphere: Po/Pd=0.103/1.068=0.0936 and (2/(k+1))k/(k-1) =(2/(1.4+1))1.4/(1.4-1)=0.53 ∴ Po/Pd＜(2/(k+1))k/(k-1) is a critical state condition. The discharge area A of the safety valve is calculated according to (B5) A≥ mm (B5) where: C gas characteristic coefficient, look up table B1 or C=520√k (2/(k+1)(k+1)/(k-1)) K—rated discharge coefficient of the safety valve, K=0.9 times the discharge coefficient (the discharge coefficient is provided by the manufacturer, generally 0.75); or selected according to the relevant provisions in Section 2 of Annex 5 of the "Capacity Regulations". 2) Calculation of the safe discharge volume of the container: The safe discharge capacity of containers containing compressed gas or water vapor shall be determined according to the following regulations: a. For compressor storage tanks or water vapor containers, take the maximum gas production of the compressor and water vapor generator respectively; b. The safe discharge capacity of gas storage tanks, etc. shall be calculated according to the formula (B1) Ws=2.83×10-3ρυd2 kg/h (B1) where ρ is the gas density under the discharge pressure. ρ=M (molecular weight) × Pw' (absolute discharge pressure) × 273/(22.4 × (273+t)) Air M=28.95 Absolute discharge pressure Pw'=10.68㎏/㎝2 Substituting into the above formula, we get ρ=28.95×10.68×273/22.4×303=12.44㎏/m3 υ—The gas flow rate of the inlet pipe of the container under working pressure m/s; look up Table 2 and get υ=10~15m/s Some common gas flow rate ranges Table 2 Fluid name/delivery pressure MPa flow rate range m/s Fluid name/delivery pressure MPa flow rate range m/s Compressed air 0~0.1 >0.1~<0.6 >0.6~<1.0 > 1.0~ 3.0~6.0 10~20 5.0~10.0 7.0~8.0 30~40 20~30 15~20 20~40 40~60 Saturated water vapor (main pipe) (branch pipe) Coal gas (initial pressure) 2KPa (initial pressure) 6KPa Ammonia ≤0.6 1.0~2.0 Liquid ammonia nitrogen 5~10 Acetylene gas hydrogen gas tap water (main pipe) (branch pipe) Flammable gas 40~60 35~40 0.75~3.0 3~12 10~20 3.0~8.0 0.3~1.0 2.0~5.0 2.0~8.0 ≤8.0 1.5~3.5 1.0~1.5 ≤1.0 Take υ=10m/s. Substitute the above ρ, ν, and d to get Ws=2.83×10-3×12.44×15×502 =1320.2㎏/h, then A= =205.4mm2. If a full-open safety valve with a wrench is used, A=0.785d02=205.4mm2 d0=(205.4/0.785)1/2=16.2㎜ According to statistical estimates, the ratio of the orifice diameter d0 of the full-lift safety valve to the nominal diameter DN is about 0.625, while the ratio of the orifice diameter d0 of the micro-lift safety valve to the nominal diameter DN is about 0.8. ∴Select a full-lift safety valve with a wrench with a nominal diameter of DN32. Table 3 of the relationship between the nominal diameter and the orifice diameter of the safety valve Safety valve nominal pressure \ nominal diameter DN 15, 20, 25, 32, 40, 50, 80, 100 Full-open PN (MPa) 1.0, 2.5, 4.0, 6.4 do 20, 25, 32, 50, 65 PN (MPa) 10.0 20, 25, 32, 40, 50 PN (MPa) 16, 32 12, 20 Micro-start PN (MPa) 1.6, 2.5, 4.0, 6.4 do 12, 16, 20, 25, 32, 40, 65, 80 PN (MPa) 16, 32 do 8 12 PN (MPa) 16, 32 14/16 Example 2. Change the medium in Example 1 to steam. untie: In pressure vessels, the ratio of the outlet side pressure of most safety valves to its relief pressure, Po/pd, is less than the theoretical value of 0.528. (This value is obtained by using air as the test medium Po/pd=0.528) It belongs to a critical state. Pd——Release pressure of safety valve (absolute pressure) Pd=1.1×Ps＋0.1MPa=1.1×1.1Pw＋0.1=1.21×0.8＋0.1=1.068MPa. It is found that ρ=5.388Kg/m3. K=455°(t=182℃) ∴WS=2.83×10-3ρνd2=2.83×10-3×5.388×25×502=953Kg Minimum discharge area AA= The compression coefficient Z of steam at operating temperature and pressure. It can be calculated according to the formula in Question 71 of "Technical Issues on Pressure Vessel Safety" edited by Gao Honghua.: (Note 1) Z= = =0.93 where: R——848Kg·m/Kmol·KT——Absolute temperature of steam K ν——Specific volume of steam M——Molecular weight. Steam k=1.135 ∴A= =245.7mm2 do= = =17.7㎜ Take a safety valve with DN=32. (do=20㎜) Note 1. The medium compressibility can be calculated according to GB150 Appendix Chapter B. The critical characteristics of some commonly used media can be found in Table 4. The main physical characteristics of certain gases can be found in Table 4. Table 4 Name Molecular weight Critical temperature t℃ Critical pressure Patm (absolute pressure) K=Cp/Cv Hydrogen H2 2.02 -239.9 12.8 1.407 Oxygen O2 32 -118.8 49.71 1.4 Air 29 -140.8 37.25 1.4 Nitrogen oxide NO 30 -94 67.2 1.4 Carbon dioxide CO2 44 31.1 72.9 1.30 Water vapor H2O 18.2 374.1 225.4 1.3 (superheat) 1.135 Ammonia NH3 17.03 132.4 111.5 1.29 Hydrogen sulfide H2S 34.09 100.4 88.9 1.3 Fluorine-12 CF2Cl2 120.09 111.7 39.6 1.14 Chlorine Cl2 70.91 144.0 76.1 1.36 Propane C3H8 44.09 96.84 42.01 1.133 Butane C4H10 58.12 152.01 37.47 1.094 Benzene C6H6 78.11 287.6 48.7 1.18 Acetylene H2C2 26.04 36.3 61.6 1.238 Isobutane 58.12 58.12 36.00 1.079 The above working conditions. It can also be calculated using equation (B7). This formula omits the calculation of the complicated locking coefficient Z when calculating. When Pd≤10Mpa, A= = =242.8mm2 do= = =17.6㎜. As we all know, the compression coefficient Z reflects the difference between real gas and ideal gas in the relationship between pressure, temperature and specific volume. At normal temperatures and low pressures, there is little difference between real gases and ideal gases. That is, the compression coefficient Z≈1, and the adiabatic index K of commonly used diatomic gases, such as air, oxygen, nitrogen, hydrogen and carbon monoxide, is 1.4. Therefore, the safety valve displacement calculation formula is simplified to the following formula: W=27KPdA Using the working conditions of Example 1, after substitution, A= =205.4mm2 Do= = =16.2㎜ is very different from the detailed calculation of Example 1. On the other hand, it should be noted that, such as the safety valve of ammonia synthesis cycle machine, due to the high pressure on the outlet side. Therefore, the pressure ratio Po/Pd>(2/(k+1) )k/(k-1) is in a subcritical state, and equation (B6) is used to calculate the relief valve discharge. However, the safety valve selection calculation for the boiler system must be calculated based on the formulas and coefficients given in the "Boil Regulations". Example 3: Liquefied petroleum gas storage tank, inner diameter Di=1600mm, length L=6000mm, wall thickness δn=16mm, V=13.3m3, head shape is elliptical, medium components are: propylene 50%, propane 15%, n-isobutylene 15%, n-isobutane 15%, residual liquid 5%. The components of liquefied petroleum gas are shown in Table 5. Liquefied petroleum gas single component components and latent heat of vaporization table 5 Weight component untie: For pressure vessels without a thermal insulation layer, the safe discharge capacity is calculated according to (B3). W=2.55×105FAt×0.82/q kg/h where: F—coefficient, for containers on the ground, F=1 At—heated area of the container, horizontal storage tank with elliptical head At=πD0 (L+0.3 D0)=3.14×1.632 (6.916+0.3×1.632)=37.9㎡ 50℃ vaporization potential r=∑ Calculation of the relief capacity of the safety valve. When the barrel length of the storage tank is ≥ 6m, two safety valves should be installed. In general, it is more reasonable to calculate half the value. This will prevent the safety valve from being too large and causing waste. The minimum exhaust area A of the safety valve is A= = =577.3mm2. In the formula, Pd=1.1P+0.1=1.1×1.8+0.1=2.08MPa M molecular weight = 44 (based on the main component propane) ∴do= = =27.12㎜ Choose two DN40A42H-4.0 full-open safety valves. (The minimum nominal diameter of this safety valve is 40) The gas is centrally transported from two or more devices to a storage tank (concentrated tank). Or when one piece of equipment transports gas to several storage tanks (gas distribution tanks). See Example 4 for the calculation of the safe discharge capacity of the storage tank. Example 4: Two air compressors simultaneously deliver gas to a gas tank with a volume of V=100m3. Its gas transmission pressure is Pw=1.0MPa ； t is normal temperature. The intake pipe is φ108×4. The safe discharge amount of the storage tank at this time. ∵At Pw=1.0Mpa ； When t=20℃, ρ=12.87㎏/m3. Take the gas flow rate of the air inlet pipe as ν=15m/s ∴The safe discharge volume W of the storage tank is W'=7.55(ρo)Vd2 =7.55×1.293×15×1002× =6.55×103kg/h where: ρo——density of gas in standard state kg/m3 ； P=12.87㎏/m3 of air under standard conditions Pd——Discharge pressure of the container MPa (absolute) T——Discharge temperature of the container (absolute) K d——Inner diameter of the total air inlet pipe of the container ㎜ In fact, W＇=7.55ρoVd2 is equivalent to W=28×10-3ρVd2. The difference is that there is no need to find the density ρ (㎏/m3) of the gas in the discharge state. In addition to the above-mentioned common storage tanks, we also encounter evaporators, reactors, etc. The pressure increases due to the evaporation of the liquid in the tank due to heat, or the medium vaporizes due to chemical reactions. As the volume increases and the internal pressure increases, the safe discharge amount should be determined based on the heat released by the input heat carrier or the maximum amount of gas that may be generated by the chemical reaction in the device, as well as the time required for the reaction. In addition, JB/T4750-2003 "Pressure Vessels for Refrigeration Devices" B.4 Caliber of Safety Valve and Bursting Disk introduces the calculation method of the caliber of the safety valve equipped on the container, which we can refer to and select in the design. d=C1 (B.1) In the formula, C1=35 (B.2) D0 - the outer diameter of the container m L - the length of the container m P - is the design pressure MPa. When two or more containers are connected, the caliber of the safety valve is calculated by substituting the sum of the D0L values ​​of the respective containers into the formula (B.1).