A low-temperature valve inner leakage detection method based on multi-stage combination

Abstract

The application provides a low-temperature valve leakage detection method based on multi-stage combination, which is characterized by the following steps: a first monitoring valve group and a second monitoring valve group are arranged upstream and downstream of each low-temperature valve in a process system before commissioning; the method comprises the following steps: in a water pressure detection stage, a dew point detector is used to collect the water dew point difference ΔT of a single detection valve upstream and downstream of the first monitoring valve group to determine the water leakage of the low-temperature valve; in a purging and drying detection stage, a gas detector is used to collect the gas concentration difference ΔC of a single detection valve upstream and downstream of the second monitoring valve group to determine the gas leakage of the low-temperature valve; and in a precooling detection stage, a combustible gas detector is used to collect the natural gas concentration difference ΔF of a single detection valve upstream and downstream of the second monitoring valve group to determine the gas leakage of the low-temperature valve; the method can quickly and accurately complete the valve quality detection on the construction site through portable instruments and multi-index comprehensive analysis, and effectively avoids the problem of rework after commissioning.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of valve sealing performance detection, and particularly relates to a low-temperature valve internal leakage detection method based on multi-stage combination. BACKGROUND

[0002] As an important equipment of industrial pipeline system, the sealing performance of the valve is directly related to the safety and efficiency of the system. In low-temperature environments, especially in LNG (liquefied natural gas) gasification stations, liquid nitrogen storage and transportation systems, liquid hydrogen storage and transportation equipment, and other deep cold freezing systems, the valve needs to have excellent sealing performance to ensure the stable operation of the pipeline system. However, once the internal leakage of the valve occurs, not only will it cause energy waste, but also may cause safety accidents, and even lead to unplanned shutdown of the system.

[0003] In the prior art, the sealing performance detection of low-temperature valves mostly relies on detection platforms under factory experimental conditions. In the factory detection stage, the sealing performance of the valve is usually detected by simulating the working condition environment through a low-temperature test platform. However, after the valve is installed on site and debugged, its sealing performance may change due to transportation, installation and on-site working conditions. Due to the lack of efficient and convenient on-site detection methods, internal leakage problems during construction or before commissioning are difficult to be found in time. Once a failure occurs after commissioning, it usually needs to be repaired by returning to the factory, which seriously affects the construction progress and economic benefits.

[0004] The current on-site detection method of low-temperature valves is usually limited to the test of a single index (such as pressure decay or leakage rate), and its detection sensitivity and reliability are low, especially in complex construction environments, which is easily disturbed by external interference and leads to inaccurate detection results. In addition, the existing technology does not fully utilize the detection potential of the conventional operations (such as water pressure test, nitrogen purging and pre-cooling process) before the pipeline is put into production, which cannot meet the demand of multi-index comprehensive detection, resulting in a blind area in the judgment of internal leakage problems.

[0005] Before the LNG gasification station is put into production, it usually needs to sequentially perform water pressure test, nitrogen purging and low-temperature pre-cooling processes to meet the requirements of national and industry standards such as GB / T 24259-2020 Petroleum and Natural Gas Industry Pipeline Transportation System, GB / T 20801.5-2020 Pressure Pipeline Standard Industrial Pipeline Part 5: Inspection and Test, and Q / SY 08713-2020 Safety Technical Regulations for Trial Production of Liquefied Natural Gas Receiving Station. However, these processes are performed independently, and the valve internal leakage detection is not combined with the medium flow characteristics, the detection means is scattered and inefficient. In addition, although portable detection equipment has been gradually introduced into industrial sites, the current application is still mainly single-purpose, and a systematic multi-index comprehensive detection mode has not been formed, which limits its applicability and detection effect.

[0006] Therefore, developing a valve internal leakage detection technology based on a multi-stage comprehensive detection method, fully utilizing the operation characteristics of water pressure test, nitrogen purging and low temperature pre-cooling stage, and improving the detection accuracy and applicability through multi-index joint analysis are urgent technical problems to be solved. The present application proposes an innovative valve internal leakage detection method, which can significantly improve the detection efficiency and reliability, and is suitable for complex construction site conditions. SUMMARY

[0007] The present application proposes a low-temperature equipment valve internal leakage detection method based on multi-stage joint detection, which combines water pressure test, nitrogen purging and low temperature pre-cooling, etc. The dew point, nitrogen concentration and combustible gas concentration of the upstream and downstream of the valve are monitored by using a portable detection device, and the rapid diagnosis and determination of the sealing performance of the low-temperature valve are realized through multi-index comprehensive analysis. This method does not require complex experimental facilities and is suitable for construction site conditions, and has the advantages of convenient operation and high accuracy.

[0008] In order to solve the prior art, the technical scheme adopted by the present application is:

[0009] 1. A low-temperature valve internal leakage detection method based on multi-stage joint detection, the detection method is based on that a first monitoring valve group and a second monitoring valve group are arranged on the upstream and downstream of each low-temperature valve in the process system before commissioning; the method is to sequentially perform water pressure detection stage, purging and drying stage and pre-cooling stage internal leakage detection on each low-temperature valve in the process system before commissioning, comprising:

[0010] Water pressure detection stage: the water dew point difference ΔT of the upstream and downstream of the single detection valve of the first monitoring valve group is collected by a dew point detector to judge the water leakage of the low-temperature valve; the water dew point difference is:

[0011]

[0012] Wherein, ΔT is the water dew point difference of the first monitoring valve group, is the water dew point of the first monitoring valve group at time point t1; is the water dew point of the downstream of the valve at time point t2; t2 and t1 are two different time points, t2>t1, and the time interval satisfies the dew point difference determination;

[0013] Purging and drying detection stage: the nitrogen concentration difference ΔC of the upstream and downstream of the single detection valve of the second detection valve group is collected by a gas detector to judge the gas leakage of the low-temperature valve; the gas concentration difference is:

[0014]

[0015] Wherein, ΔC is the nitrogen concentration difference of the second monitoring valve group, The nitrogen concentration of the second monitoring valve group at time point t1; The nitrogen concentration of the second monitoring valve group at time point t2; t2 and t1 are two different time points, t2>t1, and the time interval satisfies the nitrogen concentration difference determination.

[0016] The pre-cooling detection stage: the valve leakage is judged by collecting the natural gas concentration difference ΔF of the single detection valve of the second detection valve group upstream and downstream through the combustible gas detector; the natural gas concentration difference is:

[0017]

[0018] Wherein, ΔF is the natural gas concentration difference of the second monitoring valve group, The natural gas concentration of the second monitoring valve group at time point t1; The natural gas concentration of the second monitoring valve group at time point t2; t2 and t1 are two different time points, t2>t1, and the time interval satisfies the nitrogen concentration difference determination.

[0019] Further, the water pressure detection stage for judging the water leakage of each low temperature valve includes:

[0020] 101, water is injected into the pipeline connected with the low temperature valve to be detected and gradually pressurized to a target value;

[0021] 102, when the water pressure rises to the target value, the pipeline and the internal of the low temperature valve are kept consistent in pressure for 10 minutes;

[0022] 103, the dew point value of the single valve of the first monitoring valve group upstream and downstream of the low temperature valve to be detected is collected by the dew point detector every 5 minutes, for 15 minutes, and the difference ΔT of the water dew point is calculated;

[0023] 104, whether the difference of the water dew point is greater than 0.5℃ is judged, if it is satisfied, the purge drying detection stage is entered, otherwise the low temperature valve is repaired and returned to step 102.

[0024] Further, the purge drying detection stage for judging the gas leakage of each low temperature valve includes: 201, the pipeline connected with the low temperature valve to be detected is purged for 10-15 minutes to ensure that there is no water vapor residue at the outlet of the low temperature valve to be detected;

[0025] 202, the water dew point value of the single valve of the first monitoring valve group upstream and downstream of the low temperature valve to be detected is collected by the dew point detector every 5 minutes, for 15 minutes, and the difference ΔT of the water dew point is calculated;

[0026] 203, whether the difference of the water dew point is greater than-20℃ is judged, if it is satisfied, the next step is entered, otherwise step 201 is returned;

[0027] 204. gradually injecting nitrogen into the pipeline connected to the low-temperature valve to be detected to a target value;

[0028] 205. every 5 minutes, collecting the nitrogen concentration value of each valve in the second monitoring valve group downstream of the low-temperature valve to be detected by a gaseous detector for 15 minutes, and calculating the nitrogen concentration difference ΔC;

[0029] 206. determining whether the nitrogen concentration difference ΔC is greater than 1%, if yes, entering the precooling detection stage, otherwise, returning to step 205 after repairing the low-temperature valve.

[0030] Further, the precooling stage includes the following steps for judging the leakage of each low-temperature valve:

[0031] 301. gradually injecting BOG evaporation gas into the pipeline connected to the low-temperature valve to be detected until the temperature is reduced to -110℃;

[0032] 302. every 5 minutes, collecting the natural gas concentration value of each valve in the second monitoring valve group downstream of the low-temperature valve to be detected by a flammable gas detector for 15 minutes, and calculating the nitrogen concentration difference ΔF;

[0033] 303. determining whether the natural gas concentration difference ΔF is less than 0.3%, if yes, entering the precooling detection stage, otherwise, returning to step 302 after repairing the low-temperature valve.

[0034] Advantages

[0035] 1. The detection result of the valve leakage problem has high determination accuracy. Through the joint analysis of the three indexes of dew point difference, nitrogen concentration difference and flammable gas concentration difference, the detection result deviation caused by single index misjudgment can be avoided. At the same time, the influence of environmental interference on the detection result can be overcome. In each detection stage, the sampling is not less than three times, and the average value is calculated to improve the detection accuracy and reliability.

[0036] 2. The present application can cover the whole process before the low-temperature industrial pipeline is put into production, especially combined with the requirements of national and industry standards such as GB 24259 and GB / T20801.5, while meeting the water pressure test, nitrogen purging and low-temperature precooling process, the valve sealing performance detection is completed at the same time, which improves the detection efficiency and reduces the cost.

[0037] 3. The present application is suitable for low-temperature valve leakage detection before LNG gasification receiving station is put into operation, and can be popularized to low-temperature industrial fields such as liquid nitrogen storage and transportation system, liquid hydrogen storage and transportation equipment and other deep cold freezing systems. In addition, the method can also be extended to chemical industry, natural gas transportation, power plant steam pipe network and other industries, and is widely applicable to valve sealing detection and verification in industrial field.

[0038] 4、The application adopts a multi-stage leak detection method, which can reasonably utilize the conventional operation steps before pipeline commissioning, does not need to add special detection equipment, can significantly shorten the detection period (more than 30% reduction), and can avoid the shutdown repair problem caused by the discovery of internal leakage after commissioning, thereby improving economic benefits and construction efficiency; at the same time, the application can discover and solve the internal leakage problem before commissioning, and avoid safety hazards in operation.

[0039] 5、The detection method of the application can be flexibly adjusted according to specific working conditions. For example, in the environment where BOG precooling is not applicable, nitrogen replacement and nitrogen precooling mode can be directly used to reduce the flammable gas concentration detection link, and adapt to more extensive field application requirements.

[0040] 6、The method of the application can be widely applied to the fields of low-temperature industries such as liquefied natural gas vaporization stations, liquid nitrogen storage and transportation systems, liquid hydrogen storage and transportation equipment, and deep freezing systems, and is suitable for industrial pipelines and equipment that need to be water pressure tested, nitrogen purged and low-temperature pre-cooled before commissioning.

[0041] 7、The detection technology of the application can be extended to the industries of chemical industry, natural gas transportation, power plant steam pipe network, etc., and is used for valve sealing performance verification and system internal leakage detection before commissioning, and has the characteristics of high efficiency, portability and wide applicability. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 A flow chart of a low-temperature equipment valve internal leakage detection method based on multi-stage combined detection of the application;

[0043] Figure 2 A commissioning control schematic diagram of a low-temperature equipment valve internal leakage detection method based on multi-stage combined detection of the application;

[0044] Figure 3 A flow chart of processing a low-temperature equipment valve by using the application;

[0045] Figure 4 A water pressure time curve schematic diagram involved in the application;

[0046] Figure 5 A purge pressure time curve schematic diagram involved in the application;

[0047] Figure 6 A replacement pressure time curve schematic diagram involved in the application;

[0048] Figure 7 A precooling pressure time curve schematic diagram involved in the application. DETAILED DESCRIPTION

[0049] The following will be described in combination with the drawingsFigure 1 ~ Attach Figure 7 The technical solutions of the present application are described in detail.

[0050] As Figure 1 , Figure 2 The present application provides a low-temperature valve internal leakage detection method based on multi-stage combination, which is based on the first monitoring valve group (D1, D2, D3, D4, D5, D6, D7, D8, D9, D10) and the second monitoring valve group (Z1, Z2, Z3, Z4, Z5) arranged on the upstream and downstream of each low-temperature valve (V1, V2, V3, V4, V5, V6, V7, V8, V9, V10) in the process system before commissioning. The method is to sequentially perform water pressure detection stage, purging and drying stage, and internal leakage detection in the precooling stage for each low-temperature valve in the process system before commissioning. In the present application, the first monitoring valve group and the second monitoring valve group are installed for each low-temperature valve in the process system of the LNG receiving and gasification station before commissioning, so as to realize online monitoring of the sealing performance of the low-temperature valve to water, nitrogen, combustible gas and other media. The first monitoring valve group and the second monitoring valve group of the present application can utilize the guide valve, vent valve, instrument sampling valve and the like of the process system. The present application can temporarily install tooling valves to realize that each low-temperature valve has the first monitoring valve group and the second monitoring valve group. The temporary tooling valves are removed after completing the internal leakage detection of the low-temperature valve.

[0051] In the water pressure test, the first monitoring valve group judges the sealing performance of the low-temperature valve to water under a certain pressure through the change of dew point. In the present application, the water dew point refers to the temperature at which the water vapor in the air or gas is cooled to saturation and begins to condense into dew drops under constant air pressure. The increase of the water dew point within a certain time in the first monitoring valve group indicates that water leaks from the high-pressure side of the low-temperature valve to the low-pressure side, and the low-temperature valve has internal leakage. In the present application, the sealing performance of the low-temperature valve to water is judged based on the monitoring of the change of dew point within a certain time to determine whether the valve has internal leakage of water. As Figure 3 shown, it includes:

[0052] S1, water pressure detection stage: the dew point detector is used to collect the water dew point difference ΔT of the single detection valve on the upstream and downstream of the first monitoring valve group to judge the water leakage of the low-temperature valve; the water dew point difference is:

[0053]

[0054] The dew point detector is used to detect the change of dew point of the low-temperature valve within a certain time; according to the size of the dew point difference ΔT within a certain time interval, the internal leakage condition is judged; when ΔT>0.5℃, it is determined that the valve has internal leakage; the present application utilizes Figure 4The water pressure-time curve is used to verify the pressure stability during the testing process, ensuring the accuracy of the test results; wherein: the water pressure testing stage includes the process of judging the leakage of each low-temperature valve, including:

[0055] 101. Inject water into the pipeline connected to the cryogenic valve to be tested and gradually increase the pressure to the target value;

[0056] 102. When the water pressure reaches the target value, continue for 10 minutes to ensure that the pressure inside the pipeline and the low-temperature valve is consistent.

[0057] 103. Every 5 minutes, the dew point value of the water in the first monitoring valve group of the low-temperature valve to be tested is collected by a dew point detector for 15 minutes, and the difference in water dew point ΔT is calculated.

[0058] 104. Determine if the difference in water dew point is greater than 0.5℃. If it is, proceed to the purging and drying test stage; otherwise, repair the low-temperature valve and return to step 102.

[0059] In the purging and drying stage of this invention, the sealing performance of the cryogenic valve under a certain pressure is determined by monitoring changes in the content of the vaporized gas in the second monitoring valve group. Nitrogen gas is used for drying in this invention. Therefore, the sealing performance of the cryogenic valve is accurately determined based on monitoring changes in nitrogen content over a certain period to identify any nitrogen leakage issues. Specifically:

[0060] S2. Purging and Drying Detection Stage: The gas concentration difference ΔC between individual valves in the second detection valve group is collected using a gas detector over a certain period to determine the leakage status of the cryogenic valves; the gas concentration difference is:

[0061]

[0062] like Figure 5 As shown, the nitrogen concentration changes upstream and downstream of the valve are detected using a nitrogen detector; the internal leakage is determined based on the magnitude of the nitrogen concentration difference ΔC; when ΔC > 1%, the valve is determined to have internal leakage; combined with... Figure 6 The purging pressure-time curve in the figure confirms the accuracy of the nitrogen purging effect and concentration change; wherein: the process of judging the leakage of each cryogenic valve in the purging and drying detection stage includes:

[0063] 201. Purge the pipeline connected to the cryogenic valve to be tested for 10 to 15 minutes to ensure that there is no water vapor residue at the outlet of the cryogenic valve to be tested;

[0064] 202. Every 5 minutes, the dew point value of the water in the first monitoring valve group of the low-temperature valve to be tested is collected by a dew point detector for 15 minutes, and the difference in water dew point ΔT is calculated.

[0065] 203、Judge whether the difference of water dew point is greater than -20 DEG C, if yes, go to next step, otherwise return to step 201;

[0066] 204, gradually inject nitrogen into the pipeline connected with the low-temperature valve to be detected to the target value;

[0067] 205, every 5 minutes, collect the nitrogen concentration value of each valve in the second monitoring valve group of the low-temperature valve to be detected by the gaseous detector, last for 15 minutes, and calculate the nitrogen concentration difference ΔC;

[0068] 206, judge whether the nitrogen concentration difference ΔC is greater than 1%, if yes, go to the precooling detection stage, otherwise return to step 205 after repairing the low-temperature valve.

[0069] The application implements a low-temperature precooling stage, in the second monitoring valve group, the sealing performance of the low-temperature valve to low-temperature combustible gas under a certain pressure is judged through the change of the combustible gas content.

[0070] S3, precooling detection stage: the natural gas concentration difference ΔF of each valve in the second detection valve group of the low-temperature valve is collected by the combustible gas detector within a certain time to judge the valve leakage; the natural gas concentration difference is:

[0071] ΔF=F t2 -F t1

[0072] The application uses BOG (boil-off gas) as test medium, monitors the natural gas concentration change of the valve through the combustible gas detector, judges the internal leakage according to the size of the upstream and downstream natural gas concentration difference ΔF, determines that the valve has internal leakage when ΔF>0.3%, and verifies the stability of the pressure and the low-temperature adaptability in the detection process by using the precooling pressure-time curve Figure 4

[0073] Wherein: the precooling stage judges the leakage of each low-temperature valve, including:

[0074] 301, inject BOG boil-off gas into the pipeline connected with the low-temperature valve to be detected gradually until the temperature is reduced to 110 DEG C;

[0075] 302, every 5 minutes, collect the natural gas concentration value of each valve in the second monitoring valve group of the low-temperature valve to be detected by the combustible gas detector, last for 15 minutes, and calculate the nitrogen concentration difference ΔF;

[0076] ​303、Judge whether the natural gas concentration difference ΔF is less than 0.3%, if satisfied, enter the precooling detection stage, otherwise maintain the low temperature valve and return to step 302.

[0077] The present application proposes two detection modes according to different pipeline commissioning modes:

[0078] 1) When the pipeline commissioning adopts BOG (boil-off gas) precooling, the detection includes three-stage combined detection of water pressure stage, drying stage and precooling stage to detect the low temperature valve leakage problem;

[0079] 2) When the pipeline commissioning adopts nitrogen replacement and nitrogen precooling, the detection includes combined monitoring of water pressure stage and drying stage to reduce the combustible gas related detection steps.

[0080] Embodiment 1: water pressure, drying, nitrogen replacement, BOG precooling

[0081] This embodiment is aimed at the working condition of using BOG (boil-off gas) as precooling medium, and combines water pressure test, drying treatment, nitrogen replacement and low temperature precooling steps to comprehensively detect the valve in multiple stages. The specific steps are as follows:

[0082] First stage: water pressure detection stage preparation:

[0083] Fill the pipeline with clean water and gradually increase the pressure to the design pressure P (usually 1.5 times the design pressure, such as 1.5P). In this stage, a portable dew point detector is used to monitor the dew point changes of the upstream and downstream of the valve in real time. Steady pressure step: after the water pressure is increased to the target pressure, the pressure is kept stable (error less than 0.05MPa) for 10 minutes to ensure that the pipeline system and the valve inside reach the pressure balance state.

[0084] Detection step: after the steady pressure is completed, the dew point changes of the upstream and downstream are continuously monitored for 15 minutes, and samples are taken every 5 minutes, a total of 3 times. After sampling, the average value of ΔT is calculated, and the internal leakage condition is determined.

[0085] Judgment standard: determine the internal leakage condition by the dew point difference ΔT. When ΔT>0.5℃, it is determined that the valve is leaking.

[0086] Unqualified treatment: if ΔT>0.5℃ is determined as unqualified, the following measures need to be taken:

[0087] 1) Reduce the pressure and check the sealing of the connection part of the pipeline and the valve;

[0088] 2) Repair or replace the related parts of the leakage point;

[0089] 3) Re-perform water pressure detection to ensure that ΔT≤0.5℃.

[0090] Second stage: drying stage (air)

[0091] Purge time is 10 to 15 minutes until no water vapor remains at the outlet and the dew point value is stable at -20℃

[0092] Verification step: Monitor the drying effect by a portable dew point detector to ensure that the dew point value meets the standard.

[0093] Unqualified treatment: If the dew point value does not reach below -20℃, extend the purge time or adjust the air flow until the detection is qualified.

[0094] Nitrogen replacement stage preparation: Inject high-purity nitrogen (purity ≥ 99.9%) from the pipeline inlet and gradually increase the pressure to the target pressure (usually 0.6P to 1.1P). A portable nitrogen detector is used to monitor the upstream and downstream nitrogen concentration changes in real time, as shown in Figure 5 .

[0095] Replacement step:

[0096] 1) Divide the pipeline into several sections and replace nitrogen in each section separately;

[0097] 2) Monitor the oxygen content at the outlet in real time until the oxygen content is less than 0.5%.

[0098] Detection step: After completion of replacement, continuously monitor the upstream and downstream nitrogen concentration for 10 minutes, taking samples every 5 minutes for a total of 3 samples. Judgment criteria: Determine the internal leakage by the nitrogen concentration difference ΔC. When ΔC > 1%, determine that the valve is leaking.

[0099] Unqualified treatment: If ΔC > 1% is determined to be unqualified, nitrogen purge should be performed again and re-detection should be performed until ΔC ≤ 1%.

[0100] Third stage: Pre-cooling detection stage preparation: Inject BOG (boil-off gas) from the pipeline inlet and gradually reduce the pipeline temperature to -110℃. A portable combustible gas detector is used to monitor the natural gas concentration of the cryogenic valve.

[0101] Cooling step: Control the cooling rate within 10℃ / min to avoid damage to the pipeline due to thermal stress.

[0102] Detection step: After the temperature is stable, continuously monitor the upstream and downstream natural gas concentration for 15 minutes, taking samples every 5 minutes for a total of 3 samples.

[0103] Judgment criteria:

[0104] Determine the internal leakage by the natural gas concentration difference ΔF. When ΔF > 0.3%, determine that the valve is leaking.

[0105] Unqualified treatment: if AF>0.3% is judged as unqualified, the precooling operation should be suspended, and the valve should be repaired or replaced, and then the detection is carried out again. Embodiment two: water pressure, drying, nitrogen replacement, nitrogen precooling

[0106] The main difference between the present embodiment and embodiment one is that low-temperature nitrogen is used as the cooling medium in the precooling stage. The specific operation steps are as follows:

[0107] Preparation work of nitrogen precooling detection stage:

[0108] Low-temperature nitrogen is injected into the pipeline, and the pipeline temperature is gradually reduced to-110℃, while the portable dew point detector and nitrogen detector are used to monitor the upstream and downstream dew points and nitrogen concentration.

[0109] Cooling step: the cooling rate is controlled within 10℃ / min to ensure the safety of the system.

[0110] Detection step: after the temperature is stable, the upstream and downstream dew points and nitrogen concentration are continuously monitored for 10 minutes, and samples are taken every 5 minutes, a total of 3 times.

[0111] Judgment standard:

[0112] According to the comprehensive analysis results of dew point difference AT and nitrogen concentration difference AC, the internal leakage condition is judged.

[0113] Unqualified treatment: if the detection is unqualified, nitrogen should be injected again and detection should be carried out until AT and AC meet the standard.

[0114] It should be noted that the protection scope of the present application is not limited to the following description, but covers all improvements and modifications based on the technical solutions.

Claims

1. A method for detecting cryogenic valve internal leakage based on multi-stage joint, characterized in that, The detection method is based on the first monitoring valve group and the second monitoring valve group arranged upstream and downstream of each cryogenic valve in the process system before commissioning; The method is to sequentially perform water pressure detection stage, purging and drying stage and pre-cooling stage for each cryogenic valve in the process system before commissioning, comprising: The water pressure detection stage: the dew point detector is used to collect the water dew point difference ΔT of the single detection valve upstream and downstream of the first monitoring valve group to judge the water leakage of the cryogenic valve; the water dew point difference is: ； wherein, is the water dew point difference of the first monitoring valve train, is the water dew point of the first monitoring valve train at the time point is the water dew point downstream of the valve at the time point and are two different time points, the time interval satisfies the dew point difference determination;​​ The purging and drying detection stage: the gaseous detector is used to collect the nitrogen gas concentration difference ΔC of the single detection valve upstream and downstream of the second detection valve group to judge the gas leakage of the cryogenic valve; the nitrogen gas concentration difference is: ； wherein is the nitrogen concentration difference value, is the nitrogen concentration of the second monitoring valve group at the time point ; is the nitrogen concentration of the monitoring valve group at the time point ; and are two different time points, , the time interval satisfies the nitrogen concentration difference value determination; The pre-cooling detection stage: the combustible gas detector is used to collect the natural gas concentration difference ΔF of the single detection valve upstream and downstream of the second detection valve group to judge the gas leakage of the valve; the natural gas concentration difference is: ； wherein is the difference in natural gas concentration of the second monitored valve train, is the natural gas concentration of the second monitored valve train at the time point ; and is the natural gas concentration of the second monitored valve train at the time point ; and and are two different time points, the time interval satisfies the nitrogen concentration difference determination.

2. The method of claim 1, wherein, The water pressure detection stage for judging the water leakage of each cryogenic valve, comprising: 101 water is injected into the pipeline connected with the cryogenic valve to be detected and gradually pressurized to a target value; 102 when the water pressure rises to the target value, the pipeline and the internal cryogenic valve are kept consistent for 10 minutes; 103 the dew point detector is used to collect the water dew point value of the single valve in the first monitoring valve group upstream and downstream of the cryogenic valve to be detected every 5 minutes, for 15 minutes, and the water dew point difference ΔT is calculated; 104 it is judged whether the water dew point difference is greater than 0.5℃, if yes, the purging and drying detection stage is entered, otherwise the cryogenic valve is repaired and returned to step 102.

3. The method of claim 1, wherein, The purging and drying detection stage for judging the gas leakage of each cryogenic valve, comprising: 201 nitrogen is injected into the pipeline connected with the cryogenic valve to be detected for 10-15 minutes to ensure that there is no water vapor residue at the outlet of the cryogenic valve to be detected; 202 the dew point detector is used to collect the water dew point value of the single valve in the first monitoring valve group upstream and downstream of the cryogenic valve to be detected every 5 minutes, for 15 minutes, and the water dew point difference ΔT is calculated; 203 it is judged whether the water dew point difference is greater than -20℃, if yes, the next step is entered, otherwise step 201 is returned; 204 nitrogen is gradually injected into the pipeline connected with the cryogenic valve to be detected and pressurized to a target value; 205 the gaseous detector is used to collect the nitrogen gas concentration value of the single valve in the second monitoring valve group downstream of the cryogenic valve to be detected every 5 minutes, for 15 minutes, and the nitrogen gas concentration difference ΔC is calculated; 206 it is judged whether the nitrogen gas concentration difference ΔC is greater than 1%, if yes, the pre-cooling detection stage is entered, otherwise the cryogenic valve is repaired and returned to step 205.

4. The method of claim 1, wherein, The pre-cooling stage for judging the gas leakage of each cryogenic valve, comprising: 301 BOG evaporation gas is gradually injected into the pipeline connected with the cryogenic valve to be detected to reduce the temperature to -110℃; 302 Every 5 minutes, the natural gas concentration value of each valve in the second monitoring valve group downstream of the low-temperature valve to be detected is collected by a flammable gas detector for 15 minutes, and the nitrogen concentration difference value is calculated ; 303 determine the difference of natural gas concentration whether it is less than 0.3%, if yes, go to the precooling detection stage, otherwise, maintain the low temperature valve and return to step 302.

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