Method for measuring pressure peak value in enclosure after damage of gas valve group

Abstract

A method for measuring a pressure peak value in an enclosure after a gas valve group is damaged comprises the following steps: calculating a gas volume flow rate at a leakage position of a gas pipeline, and calculating and deducing the gas volume flow rate when the gas valve group is damaged and leaked by combining possible leakage occurrence conditions and leakage sizes of the gas pipeline; and according to the ventilation design parameters in the enclosure and the design parameters of the gas pipeline cut-off system, the required pressure-bearing lower limit of the enclosure is calculated. Therefore, the ventilation type selection design and the structure design of the ship gas valve group enclosure place are effectively guided, and the safety of ship operation is ensured.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of ship operation design, and particularly relates to a method for measuring pressure peak in an enclosure after a gas valve group is damaged. BACKGROUND

[0002] With the continuous expansion of the application of natural gas as fuel in the field of ships, the importance of the safety of the gas system, especially the gas pipeline, is increasing. In order to meet the needs of safety design, on the basis of existing specifications, the classification society proposes the requirement that the pressure calculation of the enclosure where the gas valve group is located is referred to the gas double-wall pipe.

[0003] Compared with the technical development of land natural gas pipeline system, the development of marine natural gas pipeline system is relatively short, and the research on the calculation of the gas system is more inclined to the application level of equipment, and the calculation of the leakage safety more depends on the technical documents of the classification society. The classification society specification document only provides the calculation of the pressure peak of the gas leakage in the annular space between the inner pipe and the outer pipe of the double-wall pipe, but due to the great difference between the GVT enclosure space and the annular space between the inner pipe and the outer pipe of the double-wall pipe, the existing specification formula is difficult to apply, and therefore the calculation of the pressure peak generated by the gas leakage of the gas valve group enclosure is currently still lack of corresponding technical reference; although the finite element calculation can be effectively calculated, the time cost and the labor cost are relatively high. SUMMARY

[0004] To solve the above problems, the present application provides a method for measuring the pressure peak in the enclosure after the gas valve group is damaged, and the specific method is as follows: S1: According to the different damage positions, the damage area is determined.

[0005] (1) When the pipeline is damaged, the pipeline damage area S P is: .

[0006] .

[0007] In the formula: D is the inner diameter of the pipeline.

[0008] (2) When the gasket of the flange fails, the damage area .

[0009] In the formula: D is the inner diameter of the pipeline.

[0010] n is the thickness of the gasket.

[0011] (3) When the flange bolt connection fails, the damage area SB .

[0012] In the formula: D B This is the diameter of the bolt hole.

[0013] n is the thickness of the gasket.

[0014] S2: Determine the critical pressure ratio of the gas P C ; .

[0015] In the formula: k is the adiabatic coefficient.

[0016] P L The pressure of the gas in the pipeline is expressed in Pa.

[0017] S3: Determine the gas pressure PY.

[0018] When the ambient atmospheric pressure exceeds the critical pressure of the gas, the gas pressure at the leak point... .

[0019] When the ambient atmospheric pressure does not exceed the critical pressure of the gas, the gas pressure at the leak point is... .

[0020] In the formula, P0 is the ambient atmospheric pressure at the location, in Pa.

[0021] S4: Determine the gas leak velocity V Y .

[0022] When the ambient atmospheric pressure is greater than the critical pressure, i.e., P0 > P C hour, .

[0023] When the ambient atmospheric pressure is not higher than the critical pressure, i.e., P0 ≤ P C hour, .

[0024] In the formula: V Y The velocity at the gas leak outlet is m / s.

[0025] R con Let be the gas constant of the fuel gas, J / (kg·K).

[0026] T represents the temperature of the gas, in K.

[0027] S5: Determine the volume of gas leakage per unit time. .

[0028] .

[0029] In the formula: QY For gas leakage flow rate, m 3 / s.

[0030] S0 represents the damaged and leaking area. Considering the most severe scenario, the maximum value from the three cases in step S1 is taken, i.e.: .

[0031] Therefore, we can conclude that: When P0 > P C hour, .

[0032] When P0≤P C hour, .

[0033] S6: The total gas volume V1 within the premises, calculated based on ambient atmospheric pressure, from the time a gas leak is confirmed until the gas supply is completely cut off.

[0034] .

[0035] .

[0036] .

[0037] .

[0038] .

[0039] In the formula, V A The volume of air in the location before the leak, at ambient atmospheric pressure, in meters. 3 .

[0040] V0 is the net volume of the location, in meters. 3 .

[0041] V G The volume of leaked gas (m³) when the gas shut-off alarm is triggered under ambient atmospheric pressure. 3 .

[0042] α is the lower alarm limit concentration set for the gas alarm device in the premises.

[0043] V LEAK Under ambient atmospheric pressure, the volume of leaked gas (m³) from the time the gas shut-off alarm is triggered until the gas supply is completely cut off. 3 .

[0044] t is the time required from triggering the shut-off alarm to the complete cut-off of gas, in seconds.

[0045] V EThe volume of gas discharged by the exhaust fan, in m³, during the time from triggering the gas shut-off alarm to the complete shut-off of the gas supply under ambient atmospheric pressure. 3 .

[0046] Q E The rated displacement of the exhaust fan is in meters (m). 3 / s.

[0047] S6: Determine the peak pressure at the enclosed area after the ship's gas valve assembly is damaged. .

[0048] .

[0049] .

[0050] When P0 > P C hour, .

[0051] When P0≤P C hour, .

[0052] Where D < 0.1, .

[0053] When D≥0.1, .

[0054] Furthermore, in the above-mentioned method for measuring the peak pressure inside the enclosure after a gas valve assembly is damaged, a combustible gas alarm device is installed inside the location of the gas valve assembly.

[0055] Furthermore, in the above-mentioned method for measuring the peak pressure inside the enclosure after a gas valve assembly is damaged, the lower alarm limit concentration set by the combustible gas alarm device is α.

[0056] The method for measuring the peak pressure within the enclosure after a gas valve assembly failure, as described above, can be further simplified in step S6 by making the total gas volume V1 as follows: .

[0057] This invention calculates the leakage velocity of gas pipelines to further determine the volumetric flow rate of the leaking gas, thereby calculating the pressure rise in the enclosed space of the gas valve assembly over a certain period of time. Because the calculation uses a volume-pressure approach, it avoids the situation where classification society calculations for double-walled pipe pressure peaks are inapplicable to enclosed spaces due to significant size differences. Therefore, it can serve as a design reference for the ventilation and structural design of such spaces on ships. Detailed Implementation

[0058] A method for rapidly calculating the peak pressure inside the enclosure after a gas valve assembly is damaged includes the following steps: (1) During ship operation, various factors such as humid climate, seawater corrosion, internal stress during pipeline installation, deformation after loading, and mechanical damage can all cause damage to the gas valve assembly and connected pipelines to a certain extent, leading to gas leakage. Based on different damaged locations and pipeline materials, the definition of the leakage mode and the formula for calculating the damaged area are as follows: (2) Pipeline damage: Damage to metal pipelines at any location other than flange connections. Based on experience, pipelines are susceptible to cracks or breaks due to corrosion, mechanical damage, etc., and the damaged area is usually no more than 100 mm². 2 For small-diameter pipes, the damaged area will be correspondingly smaller. The formula for calculating the damaged area is defined as follows: .

[0059] .

[0060] In the formula: S P This represents the area of ​​pipeline leakage.

[0061] D is the inner diameter of the pipe.

[0062] (3) Flange gasket failure: Flange seal failure is caused by wear, misalignment, etc. Since the leakage area varies depending on the degree of wear or misalignment of the flange gasket, the average leakage area calculation formula is defined as follows: .

[0063] In the formula: S F This represents the leakage area of ​​the flange gasket.

[0064] D is the inner diameter of the pipe.

[0065] n is the thickness of the gasket.

[0066] (4) Flange bolt connection failure: Gas leaks from the gasket due to the loss of preload on the fastening bolts. Considering a single failure mode, i.e., only one bolt loses preload, the formula for calculating the leakage area is defined as follows: .

[0067] In the formula: S B This represents the leakage area of ​​the gasket when the bolt fails.

[0068] D B This is the diameter of the bolt hole.

[0069] n is the thickness of the gasket.

[0070] (5) Based on the determined damaged area, in order to further calculate the volume of leaked gas, it is also necessary to calculate the gas leakage velocity through the damaged opening. Considering the actual situation, the pipeline damage can be regarded as a small-diameter leak.

[0071] Normally, the rate of gas leakage through a small orifice is directly proportional to the gas temperature and pressure. However, when the gas pressure reaches the critical pressure, the increase in gas pressure will no longer affect the gas leakage rate, and the maximum gas leakage rate will not exceed the local speed of sound. When the gas pressure does not exceed the critical pressure, considering that the orifice or crack is usually small in area, the gas leaking through the orifice can be regarded as isentropic flow, and is affected by ambient pressure, gas temperature, and gas pressure.

[0072] Therefore, calculating the gas leakage velocity requires first determining the gas critical pressure. The gas critical pressure is directly proportional to the gas pressure in the pipeline system. The proportionality coefficient is the gas critical pressure ratio, which is only related to the gas adiabatic coefficient. The calculation formula is as follows: .

[0073] In the formula: k is the adiabatic coefficient.

[0074] (6) Due to the direct proportional relationship between the critical pressure of the gas and the pressure of the gas in the pipeline, its calculation formula is as follows: P C =βP L , .

[0075] In the formula, P C The critical pressure is expressed in Pa.

[0076] P L The pressure of the gas in the pipeline is expressed in Pa.

[0077] β is the critical pressure ratio.

[0078] Combining the formula for the critical pressure ratio of gases in (5), the following expression can be derived: .

[0079] (7) Since the leakage velocity at the orifice is related to the gas pressure at the leak point, the gas pressure at the leak point needs to be considered in two cases. That is, when the ambient atmospheric pressure exceeds the critical pressure of the gas, the expression for the gas pressure at the leak point is as follows: .

[0080] In the formula: P0 is the ambient atmospheric pressure at the location, in Pa.

[0081] When the ambient atmospheric pressure does not exceed the critical pressure of the gas, the expression for the gas pressure at the leak point is as follows: .

[0082] In the formula: P Y The pressure of the gas at the leak point is expressed in Pa.

[0083] Combining the formula for the critical pressure of gas in (6), the following expression can be derived: .

[0084] (8) Because the gas flow velocity at the leak hole is relatively high, the gas does not have enough time to complete the heat exchange with the surrounding environment. Therefore, the instantaneous gas flow at the leak hole is approximately isentropic one-dimensional flow, that is, the gas flow velocity along the pipe axis is at the inlet side of the small hole, and the gas leakage velocity is at the outlet side of the small hole. In addition, for ship gas valve assemblies, the gas pressure is generally lower than 1.6 MPa, so it can be regarded as an ideal gas. Therefore, the calculation formula for the gas leakage velocity can be derived as follows: .

[0085] In the formula: V Y The velocity at the gas leak outlet is m / s.

[0086] V L The velocity of the gas along the axial direction of the pipeline leak outlet is expressed in m / s.

[0087] R con Let be the gas constant of the fuel gas, J / (kg·K).

[0088] T represents the temperature of the gas, in K.

[0089] (9) Because when a leak occurs, the gas velocity along the pipeline axis is much smaller than the gas leakage velocity, i.e., relative to V Y V L It can be ignored. Therefore, equation (8) can be simplified to: .

[0090] (10) Combining the two different cases of ambient atmospheric pressure and gas critical pressure in (7), the following simplified formula can be derived: When the ambient atmospheric pressure is greater than the critical pressure, i.e., P0 > P C hour, .

[0091] When the ambient atmospheric pressure is not higher than the critical pressure, i.e., P0 ≤ P C hour, .

[0092] (11) The volume of gas leakage per unit time is equal to the product of the leakage rate and the leakage area, that is: .

[0093] In the formula: Q YFor gas leakage flow rate, m 3 / s.

[0094] S0 represents the damaged and leaking area. Based on the most severe case, the maximum value of the three cases (1), (2), and (3) is taken, namely: .

[0095] Combining the formula for gas leakage rate derived in (10), we can conclude that: When P0 > P C hour, .

[0096] When P0≤P C hour, .

[0097] (12) Unlike the annular space of a double-walled pipe, the space where a gas valve assembly is usually located is relatively large. After a gas leak, the velocity will drop sharply. Therefore, the pressure peak fluctuation caused by the leak is negligible. The increase in the gas volume in the space is the fundamental reason for the pressure rise in the gas valve assembly space. Therefore, it is necessary to calculate the increase in the atmospheric pressure gas volume in the space after the gas leak.

[0098] After a gas leak, there are a total of three types of gas in the premises: (13) Air volume in the location before leakage: This volume is equal to the location volume under ambient atmospheric pressure, i.e.: .

[0099] In the formula: V A The volume of air in the location before the leak, at ambient atmospheric pressure, in meters. 3 .

[0100] V0 is the net volume of the location, in meters. 3 .

[0101] (14) The volume of leaked gas when the gas shut-off alarm is triggered: According to the regulations, combustible gas alarm devices are required in the premises where gas valve assemblies are located. When the gas concentration in the premises reaches a certain level, the gas shut-off alarm is triggered. The volume is calculated according to the following formula based on the relationship between the net volume of the premises and the concentration: .

[0102] In the formula: V G The volume of leaked gas (m³) when the gas shut-off alarm is triggered under ambient atmospheric pressure. 3 .

[0103] α is the lower alarm limit concentration set for the gas alarm device in the premises.

[0104] (15) The volume of leaked gas from the time the gas shut-off alarm is triggered until the gas is completely shut off: The above volume can be calculated based on the gas leakage rate of the gas pipeline rupture and the time from alarm to shut-off. .

[0105] In the formula: V LEAK Under ambient atmospheric pressure, the volume of leaked gas (m³) from the time the gas shut-off alarm is triggered until the gas supply is completely cut off. 3 .

[0106] t is the time required from triggering the shut-off alarm to the complete cut-off of gas, in seconds.

[0107] (16) According to the requirements of relevant regulations, the location of the gas valve assembly is usually equipped with an exhaust device. Therefore, the volume of gas discharged by the exhaust fan during the time from triggering the shut-off alarm to the complete shut-off of the gas is: .

[0108] In the formula: V E The volume of gas discharged by the exhaust fan, in m³, during the time from triggering the gas shut-off alarm to the complete shut-off of the gas supply under ambient atmospheric pressure. 3 .

[0109] Q E The rated displacement of the exhaust fan is in meters (m). 3 / s.

[0110] (17) From (13)-(16) above, the total gas volume in the premises, considering the ambient atmospheric pressure, after the gas leak until the gas supply is completely cut off, can be calculated as follows: .

[0111] The above equation can be simplified to: .

[0112] In the formula: V1 represents the total gas volume in the location at ambient atmospheric pressure when the gas supply is completely cut off, in meters. 3 .

[0113] (18) Since the premises are under normal pressure and temperature, they can be calculated as ideal gases. Therefore, considering that the net volume of the premises remains constant, while the gas pressure inside the premises changes due to the increase in the total gas volume, the following formula can be used for calculation: .

[0114] Combining (17) with the above equation, we can obtain: .

[0115] In the formula: ΔP is the peak pressure increase in the location when the gas supply is completely cut off, in Pa.

[0116] (19) Based on the two types of formulas derived in (11), the formula for calculating the peak pressure in the enclosed area after the ship's gas valve assembly is damaged can be obtained: when hour, .

[0117] when hour, .

[0118] , where D < 0.1.

[0119] , where D≥0.1.

[0120] In the formula: P0 is the ambient atmospheric pressure at the location, in Pa.

[0121] P L The pressure of the gas inside the pipeline is Pa.

[0122] k is the adiabatic coefficient of the gas.

[0123] R con Let be the gas constant of the fuel gas, J / (kg·K).

[0124] T represents the temperature of the gas, in K.

[0125] S0 is the area of ​​the leak, in meters. 2 .

[0126] t is the time required from triggering the gas alarm to the gas emergency shut-off, in seconds.

[0127] Q1 is the air volume of the mechanical exhaust at the gas valve assembly location, in meters. 3 / s.

[0128] α is the lower alarm limit concentration set by the gas alarm device in the premises.

[0129] V0 is the effective volume within the premises, in m³. 3 .

[0130] D is the inner diameter of the gas pipeline, in meters (m).

[0131] D B The diameter of the flange bolt hole for the gas pipeline is in meters (m).

[0132] n is the thickness of the gasket, m.

[0133] According to the above method, for a gas valve assembly with a pressure of 6.5 bar and a pipeline diameter of DN250, located in a 185 m³ chamber equipped with a 1.81 m³ / s mechanical exhaust fan, gas leakage caused by pipeline damage to the gas valve assembly will not cause an increase in pressure within the enclosed space before the gas is cut off.

[0134] The formula for calculating the pressure borne by the enclosure after a ship's gas valve assembly is damaged is derived from the above: when hour, .

[0135] when hour, .

[0136] D < 0.1.

[0137] , D≥0.1.

[0138] The meanings of the letters in the formula and the assumed parameters are as follows: P0 is the ambient atmospheric pressure at the location: 101325 Pa.

[0139] P L The pressure of the gas inside the pipeline is 650,000 Pa.

[0140] k is the adiabatic coefficient of the gas, which is 1.33.

[0141] R con The gas constant for the combustion gas is 431.95 J / (kg·K).

[0142] T represents the temperature of the gas: 353K.

[0143] t is the time required from triggering the gas alarm to the emergency gas shut-off: 30s.

[0144] Q1 is the air volume of the mechanical exhaust at the gas valve assembly location: 1.81m³ 3 / s.

[0145] α is the lower alarm limit concentration set for the gas alarm device in the premises: 0.03.

[0146] V0 represents the effective volume within the premises: 185m³ 3 .

[0147] D is the inner diameter of the gas pipeline: 0.250m.

[0148] D B The diameter of the flange bolt hole for the gas pipeline is 0.024m.

[0149] n represents the thickness of the gasket: 0.003 μm.

[0150] The calculation results are as follows: According to the calculation results: P0 = 101325 < 650000 × [2 / (1.33+1)] 1.33 / (1.33-1) =351237, select the second type of calculation formula.

[0151] Since the inner diameter of the gas pipeline is 0.25m, therefore: S0=MAX(0.0001, 3.14×0.25×0.003 / 4, 3.14×0.024×0.003 / 4).

[0152] =MAX(0.0001, 0.000589, 0.000057)=0.000589 m 2 .

[0153] Pa.

[0154] Considering that in actual design, the cabin is equipped with natural air inlets for mechanical exhaust, the above calculation result of negative value should be corrected to 0, which means that it will not actually cause an increase in pressure in the space.

[0155] This invention calculates the leakage velocity of gas pipelines to further determine the volumetric flow rate of the leaking gas, thereby calculating the pressure rise in the enclosed space of the gas valve assembly over a certain period of time. Because the calculation uses a volume-pressure approach, it avoids situations where classification society calculations for double-walled pipe pressure peaks are inapplicable to enclosed spaces due to significant size differences. Therefore, it can serve as a design reference for the ventilation and structural design of such spaces on ships.

Claims

1. A method for measuring the peak pressure within the enclosure after a gas valve assembly is damaged, characterized in that, The specific method is as follows: S1: Determine the damaged area based on the location of the damage; (1) When the pipeline is damaged, the damaged area S P for: ; ； In the formula: D is the inner diameter of the pipe; (2) When the flange gasket fails, the damaged area ; In the formula: D is the inner diameter of the pipe; n is the thickness of the gasket; (3) When the flange bolt connection fails, the damaged area S B = ; In the formula: D B The diameter of the bolt hole; n is the thickness of the gasket; S2: Determine the critical pressure ratio of the gas P C ; ； In the formula: k is the adiabatic coefficient; P L The gas pressure in the pipeline, in Pa; S3: Determine the gas pressure P Y ; When the ambient atmospheric pressure exceeds the critical pressure of the gas, the gas pressure at the leak point... ; When the ambient atmospheric pressure does not exceed the critical pressure of the gas, the gas pressure at the leak point is... ; In the formula, P0 is the ambient atmospheric pressure at the location, in Pa; S4: Determine the gas leak velocity V Y ; When the ambient atmospheric pressure is greater than the critical pressure, i.e., P0 > P C hour, ; When the ambient atmospheric pressure is not higher than the critical pressure, i.e., P0 ≤ P C hour, ; In the formula: V Y The gas leakage outlet velocity is denoted as ρ, in m / s. R con Let be the gas constant of the fuel gas, J / (kg·K); T is the temperature of the gas, in K; S5: Determine the volume of gas leakage per unit time. ; ； In the formula: Q Y For gas leakage flow rate, m 3 / s; S0 is the area of ​​the damage and leakage, from which we can conclude: When P0 > P C hour, ； When P0≤P C hour, ； S6: The total gas volume V1 within the premises, calculated based on ambient atmospheric pressure, from the time a gas leak is confirmed until the gas supply is completely cut off. ； ； ； ； ； In the formula: V A The volume of air in the location before the leak, at ambient atmospheric pressure, in meters. 3 ; V0 is the net volume of the location, in meters. 3 ; V G The volume of leaked gas (m³) when the gas shut-off alarm is triggered under ambient atmospheric pressure. 3 ; α is the lower alarm limit concentration set by the gas alarm device in the premises; V LEAK The volume of leaked gas (m³) between the triggering of the gas shut-off alarm and the complete shut-off of the gas supply under ambient atmospheric pressure. 3 ; t is the time required from triggering the gas shut-off alarm to the complete gas shut-off, in seconds; V E Under ambient atmospheric pressure, the volume of gas discharged by the exhaust fan from the time the gas shut-off alarm is triggered until the gas supply is completely cut off, in cubic meters. 3 ; Q E The rated displacement of the exhaust fan is in meters (m). 3 / s; S6: Determine the peak pressure ΔP in the enclosed area after the ship's gas valve assembly is damaged; ； ； When P0 > P C hour, ； When P0≤P C hour, ； in, When D < 0.1, ; When D≥0.1, .

2. The method for measuring the peak pressure within the enclosure after a gas valve assembly is damaged, as described in claim 1, is characterized in that... The location of the gas valve assembly is equipped with a combustible gas alarm device.

3. The method for measuring the peak pressure within the enclosure after a gas valve assembly is damaged, as described in claim 1, is characterized in that... The lower alarm limit concentration set for the combustible gas alarm device is α.

4. The method for measuring the peak pressure within the enclosure after a gas valve assembly is damaged, as described in claim 1, is characterized in that... In step S5, considering the most stringent case, S0 takes the maximum value of the three cases in step S1, that is: .

5. The method for measuring the peak pressure within the enclosure after a gas valve assembly is damaged, as described in claim 1, is characterized in that... The total gas volume V1 in step S6 can be simplified to: .

6. The method for measuring the peak pressure inside the enclosure after a gas valve assembly is damaged, as described in claim 1, is characterized in that... In step S4, calculating the gas leakage velocity requires first determining the critical pressure of the gas. The critical pressure of the gas is directly proportional to the gas pressure in the pipeline system. The proportionality coefficient is the ratio of the gas critical pressure, which is only related to the gas adiabatic coefficient. The calculation formula is as follows: ； In the formula: k is the adiabatic coefficient.

Images