Method for determining remaining maintenance time of LNG tank based on safe take-off pressure correction

Abstract

The application discloses a kind of LNG tank box remaining maintenance time determination method based on safe take-off pressure correction, including calculating the real-time gas-liquid total mass of LNG tank box;Calculate the density of tank inflation ρ z ;Calculate the tank inflation pressure P z ;Compare tank inflation pressure and safety valve take-off pressure P sv And correct safety valve take-off pressure;If P sv >P z , safety valve take-off pressure is corrected as P sv =P z , then P z Corresponding state is calculated to the cumulative heat leakage of safety valve take-off, and further calculate the remaining maintenance time of LNG tank box;Otherwise, still with P sv Corresponding state calculates the remaining maintenance time of LNG tank box;This method improves the safety control level in the process of LNG tank box transportation.Meanwhile, for short-distance transportation, according to the longest possible transportation cycle, the safety valve take-off pressure is corrected, the filling rate, volume utilization rate and transfer efficiency of LNG tank box are improved, and the logistics turnover cost is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of engineering prediction, and particularly relates to a method for determining the residual maintenance time of an LNG tank based on safety take-off pressure correction. BACKGROUND

[0002] LNG tank water-land (waterway, railway and highway) intermodal transportation is a third new LNG logistics mode parallel to pipeline transportation and LNG bulk carrier transportation. The residual maintenance time of the LNG tank is a key parameter related to the safe transportation of the LNG tank. The existing laws, regulations and related technical rules require that the safety valve of the LNG tank does not take off to cause leakage of flammable gas during the transportation of the LNG tank on a ship and when the vehicle carrying the LNG tank passes through a tunnel or culvert. Therefore, only by accurately predicting the residual maintenance time of the LNG tank can the above-mentioned accidents be avoided. The existing methods for predicting the residual maintenance time of the LNG tank are basically calculated by the ratio between the total heat leakage required by the LNG tank to reach the state corresponding to the safety valve take-off pressure and the static daily evaporation rate measured in the LNG tank type test by the LNG tank manufacturer.

[0003] However, based on the existing experience in the industry and the static experiment data of the LNG tank, when the filling rate of the LNG tank is greater than 91%, the safety valve take-off pressure of the LNG tank is smaller than the set pressure due to the volume expansion of the liquid phase, so that the actual residual maintenance time of the LNG tank is much smaller than the residual maintenance time predicted based on the safety valve take-off pressure. This will cause great safety hazards to the transportation of the LNG tank. Therefore, the International Maritime Dangerous Goods Code also makes a principle limitation on the filling rate of the LNG tank, but also makes a principle exemption for short-distance transportation.

[0004] Therefore, the application provides a method for determining the residual maintenance time of the LNG tank based on safety take-off pressure correction, realizes accurate prediction of the residual maintenance time of the LNG tank, improves the perception of the state information of the LNG tank, and improves the safety control level in the transportation process of the LNG tank. Meanwhile, for short-distance transportation, the safety valve take-off pressure is corrected according to the longest possible transportation period, the filling rate of the LNG tank is as high as possible under the premise of safety, the volume of the LNG tank is effectively utilized, the transportation efficiency of the LNG tank is improved, and the logistics turnover cost is reduced. SUMMARY

[0005] The application aims to make up for the defects that the calculation results of the prior art have large errors compared with the actual maintenance time, and provides a method for determining the residual maintenance time of the LNG tank based on safety take-off pressure correction based on the volume expansion of the liquid phase, so as to realize more accurate prediction of the residual maintenance time of the LNG tank and higher utilization efficiency of the LNG tank.

[0006] In order to achieve the above-mentioned purpose, the present application relates to a LNG tank remaining maintenance time determination method based on safe take-off pressure correction, comprising the following steps:

[0007] Step one, through the method of polynomial simulation simplifying intermediate conversion, calculate the real-time gas-liquid total mass m0 of LNG tank;

[0008] Step two, taking 98% of the effective volume of the tank as the reference, calculate the tank density p z ;

[0009] Step three, through the method of polynomial simulation simplifying intermediate conversion, calculate the tank pressure P z ;

[0010] Step four, compare the tank pressure P z with the safe valve take-off pressure P sv and correct the safe valve take-off pressure P sv .

[0011] Step five, calculate the cumulative heat leakage Q according to the corrected safe valve take-off pressure.

[0012] Step six, calculate the daily average heat leakage Qd according to the corrected safe valve take-off pressure.

[0013] Step seven, calculate the LNG tank remaining maintenance time t.

[0014] Further, characterized in that, the method for calculating the real-time gas-liquid total mass m0 of LNG tank in step one is:

[0015] m0 = p li × V × Φ + p gi × V × (1-Φ) (1-1)

[0016] In the saturated homogeneous model, there is a fixed relationship between the gas phase density, the liquid phase density and the pressure, through the method of polynomial simulation simplifying intermediate conversion process, according to this method, the real-time gas phase density p gi and the real-time liquid phase density p li are calculated as follows:

[0017] p li = a z％ P i 6 + b z％ P i 5 + c z% P i 4 + d z% P i 3 + e z% Pi 2 +f z％ P i +g z％ (1-2)

[0018] ρ gi =A z% P i 6 +B z% P i 5 +C z% P i 4 +D z% P i 3 +E z% P i 2 +F z％ P i +G z %(1-3)

[0019] m0—total mass of gas and liquid in the tank, kg

[0020] ρ gi —density of gas in the tank, kg / m 3

[0021] ρ li —density of liquid in the tank, kg / m 3

[0022] V—effective volume of the tank, m 3

[0023] Φ—filling rate, %

[0024] P i —pressure in the tank, MPa

[0025] a z％ , b z％ , c z％ , d z％ , e z％ , f z％ , g z％ —polynomial coefficients of pressure converted into gas density under a specific LNG component.

[0026] A z％ , B z％ , C z％ , D z％ , E z％ , F z％ , G z％ —polynomial coefficients of pressure converted into liquid density under a specific LNG component.

[0027] Further, the method for calculating the tank rising density ρ z in step two is as follows:

[0028]

[0029] ρ z —liquid phase density during tank rising, kg / m 3 3 ;

[0030] Further, the method for calculating the tank rising pressure P z in step three is as follows:

[0031] P z = k Z％ ρ z 3 + l z％ ρ z 2 + m z％ ρ z + n z％

[0032] wherein k z％ , l z％ , m z％ , n z％ —polynomial coefficients for converting the liquid phase density ρ z into pressure; in the saturated homogeneous model, there is a fixed relationship between the gas phase density, the liquid phase density and the pressure, and here the intermediate conversion process is simplified by polynomial simulation.

[0033] Further, the method for comparing the tank rising pressure P z with the safety valve take-off pressure P sv and correcting the safety valve take-off pressure in step four is as follows: if P sv > P z , then the safety valve take-off pressure is corrected as P sv = P z ; otherwise, P sv is not corrected.

[0034] Further, the method for calculating the cumulative heat leakage Q according to the corrected safety valve take-off pressure in step five is as follows: calculate the sum of the gas-liquid internal energy inside the LNG tank at the time of safety valve take-off and the real-time gas-liquid internal energy under the corrected state, and take the difference between the two as the cumulative heat leakage Q.

[0035] ​Further, the method for calculating the daily heat leakage Qd in step six according to the corrected safety valve take-off pressure is: calculating the measured daily heat leakage Qd2 according to the state parameters measured in the LNG tank standing test, and taking the average value of the stable data before the test to obtain the daily heat leakage Qd.

[0036] Further, if the number of measured daily heat leakages is insufficient, the measured daily heat leakage Qd2 of the missing data days can be replaced by the daily heat leakage Qd1 based on the static daily evaporation rate calculated according to the static daily evaporation rate a20 filled on the tank nameplate.

[0037] Further, the method for calculating the remaining maintenance time of the LNG tank in step seven is:

[0038]

[0039] In the formula: k is a correction coefficient.

[0040] Overall, compared with the prior art, the above technical solutions conceived by the present application can achieve the following beneficial effects:

[0041] (1) The existing LNG tank safety valve take-off remaining maintenance time prediction algorithm is improved and optimized, further considering the situation that the safety valve take-off pressure of the LNG tank is less than the set pressure due to the volume expansion of the liquid phase, which causes the actual remaining maintenance time of the LNG tank to be much less than the remaining maintenance time predicted based on the safety valve take-off pressure, avoiding the large safety hazard caused by the LNG tank transportation due to the tank expansion.

[0042] (2) The present application proposes a LNG tank remaining maintenance time determination method based on the safety take-off pressure correction based on the volume expansion of the liquid phase, to realize more accurate LNG tank remaining maintenance time prediction. Thus, the perception degree of the LNG tank state information is improved, and the safety control level in the LNG tank transportation process is also improved.

[0043] (3) The International Maritime Dangerous Goods Code 4.2.2.7 limits the filling rate of the LNG tank, but mentions that the filling rate can be exceeded for short-distance transportation under the condition of ensuring safety, but there is no specific implementation measure at present. According to the method proposed in the present application, the safety valve take-off pressure can be corrected for the longest transportation period of the LNG tank for short-distance transportation, the filling rate of the LNG tank is increased as much as possible under the premise of ensuring safety, the LNG tank volume is effectively utilized, the LNG tank transfer efficiency is improved, and the logistics turnover cost is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 The logical flowchart of the preferred embodiment of the present application. DETAILED DESCRIPTION

[0045] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and should not be used to limit the present application. In addition, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0046] Please refer to Figure 1 A method for determining the remaining maintenance time of an LNG tank based on a safe take-off pressure correction, comprising the following steps:

[0047] Step one, calculate the real-time gas-liquid total mass m0 of the LNG tank by a polynomial simulation method to simplify the intermediate conversion process. The specific method for calculating the real-time gas-liquid total mass m0 of the LNG tank is as follows:

[0048]

[0049] In the saturated homogeneous model, there is a fixed relationship between the gas phase density, the liquid phase density and the pressure. By polynomial simulation to simplify the intermediate conversion process, according to this method, the real-time gas phase density ρ gi and the real-time liquid phase density ρ li are calculated as follows:

[0050] ρ li = a z％ P i 6 +b z％ P i 5 +c z％ P i 4 +d z％ P i 3 +e z％ P i 2 +f z％ P i +g z％ (1-2)

[0051] ρ gi =A z％ P i 6 +B z％ P i 5 +C z％ P i 4 +D z％ P i 3 +Ez％ P i 2 +F z％ P i +G z %(1-3)

[0052] m0—total mass of gas and liquid in the tank, kg

[0053] ρ gi —density of gas in the tank, kg / m 3

[0054] ρ li —density of liquid in the tank, kg / m 3

[0055] V—effective volume of the tank, m 3

[0056] Φ—filling rate, %

[0057] P i —pressure in the tank, MPa

[0058] a z％ , b z％ , c z％ , d z％ , e z％ , f z％ , g z％ —polynomial coefficients for converting pressure into density of gas at a specific LNG component.

[0059] A z％ , B z％ , C z％ , D z％ , E z％ , F z％ , G z％ —polynomial coefficients for converting pressure into density of liquid at a specific LNG component.

[0060] Step two, calculate the rising density ρ z based on 98% of the effective volume of the tank.

[0061] The calculation method for calculating the rising density ρ z is as follows:

[0062]

[0063] ρ z —density of liquid when the tank is rising, kg / m 3 , V—effective volume of the tank, m 3 ;

[0064] Step three, calculate the rising tank pressure P by the method of polynomial simulation simplifying intermediate conversion z ;

[0065] Specifically, P z = k Z% ρ z 3 + l z% ρ z 2 + m z％ ρ z + n z%

[0066] Wherein, k z％ , l z％ , m z％ , n z％ - the polynomial coefficient of the liquid density ρ z converted into pressure under specific LNG components; in the saturated homogeneous model, there is a fixed relationship between the gas phase density, the liquid phase density and the pressure, and here the intermediate conversion process is simplified by polynomial simulation.

[0067] Step four, compare the rising tank pressure P z with the safety valve take-off pressure P sv and correct the safety valve take-off pressure P sv ; if P sv > P z , the safety valve take-off pressure is corrected to P sv = P z ; otherwise, P sv is not corrected.

[0068] Step five, calculate the cumulative heat leakage Q according to the corrected safety valve take-off pressure.

[0069] Specifically, calculate the sum of the gas-liquid internal energy in the LNG tank at the time of safety valve take-off and the real-time gas-liquid internal energy under the corrected state, and take the difference between the two as the cumulative heat leakage Q.

[0070] Step six, calculate the daily average heat leakage Qd according to the corrected safety valve take-off pressure; the specific method is: calculate the measured daily heat leakage Qd2 according to the state parameters measured by the LNG tank standing test, remove the unstable data in the early stage of the test, and take the average value to obtain the daily average heat leakage Qd; wherein, if the number of measured daily heat leakages is insufficient, the measured daily heat leakage Qd2 of the missing data days can be replaced by the daily heat leakage Qd1 based on the static daily evaporation rate a20 calculated from the liquid nitrogen static daily evaporation rate a20 filled on the tank nameplate;

[0071] Step seven, calculate the remaining maintenance time t of the LNG tank.

[0072] Specifically,

[0073] wherein: k - correction factor.

[0074] Those skilled in the art can easily understand that the above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for determining a remaining maintenance time of an LNG tank based on a safe take-off pressure correction, the method comprising: calculating a safe take-off pressure of the LNG tank; determining a remaining maintenance time of the LNG tank based on the safe take-off pressure. Comprising the following steps: Step one, calculate the LNG tank real-time gas-liquid total mass m0 by the method of polynomial simulation simplification intermediate conversion; Step two, calculate the tank density based on 98% of the tank's effective volume ; Step three, calculate the surge tank pressure by the method of polynomial simulation simplifying intermediate conversion ; Step four, compare the tank pressure P z to the safety valve lift pressure P sv and correct the safety valve lift pressure P sv ; Step five, calculate the cumulative heat leakage Q according to the corrected safety valve take-off pressure; Step six, calculate the daily average heat leakage Qd according to the corrected safety valve take-off pressure; Step seven, calculate the LNG tank remaining maintenance time t; The step four compares the rising tank pressure P z with the safety valve take-off pressure P sv and corrects the safety valve take-off pressure if P sv >P z , then the safety valve take-off pressure is corrected to P sv =P z ; otherwise P sv is not corrected. The method for calculating the LNG tank remaining maintenance time in step seven is: In the formula: k---correction coefficient.

2. The LNG tank remaining maintenance time determination method based on safety take-off pressure correction according to claim 1, characterized in that In the present application, The method for calculating the LNG tank real-time gas-liquid total mass m0 in step one is: In the saturated homogeneous model, there is a fixed relationship between the gas phase density, the liquid phase density, and the pressure. The intermediate conversion process is simplified by polynomial simulation. According to this method, the real-time gas phase density ρ gi and the real-time liquid phase density ρ li are calculated as follows: （1-2） （1-3） In the formula: m0---existing gas-liquid total mass in the tank, kg p gi - gas phase density in the tank, kg / m3 p li - density of the liquid phase in the tank, kg / m3 V---effective volume of the storage tank, m³ Φ---filling rate, % P i In-tank pressure, MPa a z% , b z% , c z% , d z% , e z% , f z% , g z% —polynomial coefficients for pressure converted to gas phase density at specific LNG components; A z% , B z% , C z% , D z% , E z% , F z% , G z% Polynomial coefficients for pressure converted to liquid density at specific LNG components.

3. The method of claim 1, wherein the method is characterized by: The step two calculates the density of the rising tank The calculation method is: p z - density of liquid phase at the beginning of the filling, kg / m3, V - effective volume of the tank, m3.

4. The method of claim 1, wherein the method is characterized by, The method for calculating the surge pressure in step three is: ​ where: k z% , l z% , m z% , n z% —liquid density at specific LNG component polynomial coefficients of conversion to pressure; in the saturated homogeneous model, there is a fixed relationship between gas density, liquid density, and pressure, which is simulated here by a polynomial to simplify the intermediate conversion process.

5. The method of claim 4, wherein the method is characterized by, The method for calculating the cumulative heat leakage Q according to the corrected safety valve take-off pressure in step five is: calculate the sum of the LNG tank internal energy under the corrected state and the real-time gas-liquid internal energy when the safety valve takes off, and take the difference between the two as the cumulative heat leakage Q.

6. The method of claim 5, wherein the method is characterized by, The method for calculating the daily average heat leakage Qd according to the corrected safety valve take-off pressure in step six is: calculate the measured daily heat leakage Qd2 according to the measured state parameters of the LNG tank static test, remove the unstable data before the test, and take the average value to obtain the daily average heat leakage Qd.

7. The method of claim 6, wherein the method is characterized by, If the number of measured daily heat leakages is insufficient, the measured daily heat leakage Qd2 of the missing data days can be replaced by the daily heat leakage Qd1 based on the static daily evaporation rate calculated by the liquid nitrogen static daily evaporation rate a20 filled on the tank nameplate.

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