Ammonia gas tightness detection method for main shield welding seam of liquefied natural gas film storage tank and film cabin

Abstract

The invention discloses an ammonia gas test method applied to detection of gas tightness of welding seams of a liquefied natural gas film storage tank and a marine film cabin. The ammonia gas test method comprises the following steps: 1) preparing detection equipment and chemical reagents; 2) cleaning a welding seam of the film storage tank and spraying a reaction type coating; (3) vacuumizing the main heat insulation layer, injecting a nitrogen-ammonia gas mixture, waiting for a certain reaction time, and periodically detecting the ammonia gas concentration and supplementing the ammonia gas during the period; 4) after the reaction time, emptying the nitrogen-ammonia gas mixture, observing from the inner side of the film storage tank, marking a weld leakage point, and repairing; and (5) after the leakage point is not found any more, post-processing is conducted on the heat insulation layer space of the storage tank. According to the gas tightness detection method, the precision and reliability of gas tightness detection of the welding seam are improved; the method is suitable for large-scale liquefied natural gas storage tank detection, and time and cost are saved; a detection blind area possibly existing in a traditional method is effectively avoided; and a detailed leakage repair and detection scheme is provided.

Description

Technical Field

[0001] The invention relates to the technical field of liquefied natural gas storage, and in particular to an ammonia test method for detecting the air tightness of welds of liquefied natural gas membrane storage tanks and marine membrane cabins. Background Art

[0002] LNG storage tanks usually use 9% nickel steel as the inner tank material. However, for LNG membrane storage tanks and ship membrane tanks with thin inner tank thickness, traditional non-destructive testing methods are difficult to effectively apply. Given that the safety of LNG storage tanks is directly related to the storage and transportation safety of LNG, it is crucial to test the airtightness of the welds of membrane storage tanks and ship membrane tanks.

[0003] In the prior art, commonly used weld air tightness detection methods such as gas leakage detection method and liquid penetration method have problems such as low detection accuracy, complex operation and high cost. Especially for the welds of liquefied natural gas membrane storage tanks and marine membrane tanks, due to the thin thickness of the welds, conventional non-destructive testing methods are difficult to find tiny leaks and cannot fully guarantee the air tightness of the tanks. Summary of the invention

[0004] The main purpose of the present invention is to overcome the deficiencies in the prior art and to provide an efficient, accurate and easy-to-operate method for detecting the ammonia gas tightness of the main shielding weld of a liquefied natural gas membrane storage tank and a marine membrane tank, so as to ensure the safety and reliability of the storage tank during use.

[0005] According to one aspect of the present invention, the present invention provides a method for detecting ammonia gas tightness of main shielding welds of liquefied natural gas membrane storage tanks and marine membrane tanks, comprising the following steps:

[0006] 1. Prepare testing equipment and chemical reagents, the testing equipment includes a vacuum pump, a pressure sensor, an isolation valve, a pressure reducing valve, a high conductivity injection pipe, a gas flow controller and a measuring device, an ammonia dilution water delivery system, a safety valve or a hydraulic protection device, etc.; the chemical reagents include a nitrogen-ammonia mixture with an ammonia content (volume percentage) of 25±5% and a reactive coating;

[0007] 2. From the inside of the membrane tank, that is, the inside of the main insulation layer, wipe the weld of the membrane tank with alcohol or acetone to clean it, then spray a layer of reactive coating, and then establish a vacuum state and purge it with dry nitrogen;

[0008] 3. First, the main insulation layer between the membrane storage tank and the outer shell, that is, the space between the outer tank and the inner tank of the membrane storage tank, is evacuated to -600mbar to -800mbar, and then a nitrogen-ammonia mixture is injected therein, and the pressure is kept rising to 15mbar to 35mbar, preferably 20mbar, and a certain reaction time is waited, during which the ammonia concentration is regularly detected and ammonia is supplemented;

[0009] 4. After the reaction time has passed, the nitrogen-ammonia mixture in the main insulation layer is emptied, and the weld leak is observed and marked from the inside of the membrane tank. When there is a leak, the reactive coating at the leak location changes color and can be easily observed. The detected leak is then repaired, and steps 1 to 3 are repeated for ammonia testing after the repair;

[0010] 5. After no more leaks are found, the insulation space of the tank is vented, replaced and purged to ensure that the residual ammonia does not exceed 1000ppm. At the same time, the reactive coating on the weld of the membrane tank is removed.

[0011] Preferably, in the detection device, the pump displacement of the vacuum pump is 2 The measured surface area is not less than 1m 3 / h, and equipped with a vacuum degree display. The pump displacement refers to the vacuum displacement, and sufficient suction capacity must be ensured.

[0012] Preferably, the establishment of a vacuum state and purging with dry nitrogen in step 1 refers to establishing a vacuum state in the primary insulation layer space and the secondary insulation layer space according to the following requirements:

[0013] 1) The main insulation space is evacuated to remove air and moisture in the air, so as to facilitate uniform flushing of the nitrogen-ammonia mixture and prevent ammonia from being absorbed by moisture;

[0014] 2) Evacuate the space between the primary insulation layer and the secondary insulation layer to avoid applying excessive pressure to the bonding part of the secondary film;

[0015] 3) The vacuum degree of the secondary insulation space should be kept roughly the same as that of the primary insulation space, and the pressure difference between the two spaces should not exceed 30mbar;

[0016] 4) The two spaces shall be purged with dry nitrogen to further reduce the residual air and moisture. The purging shall be performed at least three times to make the relative humidity under atmospheric pressure less than or equal to 60%;

[0017] 5) During the purge process, three continuous pressure cycles of "rising from atmospheric pressure by 20 mbar and then dropping to atmospheric pressure" are carried out in the main insulation space;

[0018] 6) After nitrogen purging, the primary insulation layer space and the secondary insulation layer space are evacuated to -800 mbar.

[0019] Preferably, the reaction time in step 3 is 6-12 hours.

[0020] Preferably, the reactive coating is formed from a mixture of bromophenol blue, orthophosphoric acid and kaolin, and the pH value is adjusted to 2.8.

[0021] Beneficial Effects

[0022] The detection method according to the present invention has the following beneficial effects:

[0023] 1. Improve the accuracy and reliability of weld air tightness detection, and be able to accurately detect subtle leaks;

[0024] 2. Easy to operate, suitable for large-scale liquefied natural gas tank inspection, saving time and cost;

[0025] 3. Effectively avoids the possible blind spots in traditional methods and realizes comprehensive detection of welds;

[0026] 4. Provides detailed leakage repair and detection solutions to ensure the long-term safe use of storage tanks. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 This is a schematic diagram of the ammonia gas tightness test process.

[0029] Figure 2 This is a schematic diagram of the structure of a liquefied natural gas membrane storage tank.

[0030] Figure 3 for Figure 2 A partial enlarged view of the liquefied natural gas membrane storage tank (detailed view part B). DETAILED DESCRIPTION

[0031] Hereinafter, the present invention will be described in detail. Before describing, it should be understood that the terms used in this specification and the appended claims should not be interpreted as being limited to the general meaning and dictionary meaning, but should be interpreted according to the meaning and concept corresponding to the technical aspects of the present invention on the basis of the principle that the inventor is allowed to appropriately define the terms for the best interpretation. Therefore, the descriptions presented here are only preferred examples for illustrative purposes and are not intended to limit the scope of the present invention, so that it should be understood that other equivalents or improvements can be obtained therefrom without departing from the spirit and scope of the present invention.

[0032] Hereinafter, the preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terms used in the specification and the appended claims should not be interpreted as limited to the general and dictionary meanings, but should be interpreted based on the principle of allowing the inventor to appropriately define the terms for the best interpretation, based on the meaning and concept corresponding to the technical level of the present invention. Therefore, the description herein is only a preferred example for illustrative purposes, and is not intended to limit the scope of the present invention, so it should be understood that other equivalent implementations and modifications can be made without departing from the spirit and scope of the present invention.

[0033] In this document, the terms "include", "including", "have", "contain" or any other similar terms are open conjunctions, which are intended to cover non-exclusive inclusions. For example, a composition or product containing multiple elements is not limited to the elements listed herein, but may also include other elements that are not explicitly listed but are generally inherent to the composition or product. In addition, unless otherwise explicitly stated, the term "or" refers to an inclusive "or" rather than an exclusive "or". For example, any of the following situations satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist), A is false (or does not exist) and B is true (or exists), and A and B are both true (or exist). In addition, in this document, the interpretation of the terms "include", "including", "have", and "contain" should be considered to have been specifically disclosed and simultaneously cover closed or semi-closed conjunctions such as "consisting of" and "consisting essentially of".

[0034] In this article, all features or conditions defined in the form of numerical ranges or percentage ranges are only for brevity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be deemed to have covered and specifically disclosed all possible secondary ranges and individual values ​​within the range, especially integer values. For example, the range description of "1 to 8" should be deemed to have specifically disclosed all secondary ranges such as 1 to 7, 2 to 8, 2 to 6, 3 to 6, 4 to 8, 3 to 8, etc., especially secondary ranges defined by all integer values, and should be deemed to have specifically disclosed individual values ​​such as 1, 2, 3, 4, 5, 6, 7, 8, etc. within the range. Unless otherwise specified, the above interpretation method applies to all contents of the entire present invention, regardless of whether the range is broad or not.

[0035] If the quantity or other numerical value or parameter is expressed as a range, a preferred range or a series of upper and lower limits, it should be understood that all ranges consisting of any upper limit or preferred value of the range and the lower limit or preferred value of the range have been specifically disclosed herein, regardless of whether these ranges are disclosed separately. In addition, if a numerical range is mentioned herein, unless otherwise specified, the range should include its endpoints and all integers and fractions within the range.

[0036] like Figure 1 As shown, the present invention provides a method for detecting ammonia gas tightness of the main shielding weld of a liquefied natural gas membrane storage tank and a marine membrane tank, which mainly includes the following steps:

[0037] First, prepare the testing equipment and chemical reagents. The vacuum pump should have sufficient suction capacity to ensure that the vacuum state can be established quickly. 2 The measured surface area, pump displacement should not be less than 1m 3 / h, and equipped with a vacuum display. The pressure sensor is used to monitor the pressure changes in the insulation layer space in real time. The isolation valve and the pressure reducing valve control the gas inflow and outflow. The high conductivity injection pipe has good sealing and pressure resistance. The gas flow controller and the measuring device accurately measure the injection amount and flow rate of the nitrogen-ammonia mixture. The ammonia dilution water delivery system is used to safely handle ammonia and prevent environmental pollution. The safety valve or hydraulic protection device prevents overpressure to ensure the safety of equipment and personnel. The chemical reagent uses a nitrogen-ammonia mixture with an ammonia content of 25±5% as the detection gas. The reactive coating is formed by a mixture of bromophenol blue, orthophosphoric acid and kaolin, the pH value is adjusted to 2.8, and the color changes when it encounters ammonia.

[0038] Next, clean and paint the tank welds. Wipe the weld surface with alcohol or acetone to ensure that there is no grease and foreign matter. Spray the reactive paint evenly on the inside of the tank to cover the weld and keep the coating even. Create a vacuum state to extract the air and moisture in the tank to prevent ammonia from being absorbed by moisture. Use dry nitrogen to purge the insulation space to ensure that the nitrogen-ammonia mixture is evenly distributed.

[0039] Next, conduct an ammonia test. Inject a nitrogen-ammonia mixture into the main insulation layer, and increase the pressure from -80kPa to +2kPa to ensure that the gas is evenly distributed. Regularly test the ammonia concentration to ensure that the ammonia concentration in the test area meets the requirements. Check the weld surface to find and mark the leak.

[0040] Then, repair the detected leaks. During the repair process, evacuate and purge with nitrogen to ensure that the repair area is free of moisture and impurities. Visual inspection, dye penetrant testing, and vacuum chamber testing are used to test the quality of the repair. After the repair is completed, record the location of the leak and the repair status for subsequent maintenance and inspection. Re-perform the ammonia test to ensure the repair effect.

[0041] Finally, after the test is completed, the insulation space of the tank is vented, replaced, and purged. The venting operation discharges the nitrogen-ammonia mixture in the insulation space. Use dry nitrogen replacement to ensure that the ammonia concentration does not exceed 1000ppm. Multiple vacuuming and nitrogen replacement cycles are performed to minimize residual ammonia. The insulation space is thoroughly cleaned to ensure that there is no residual paint and contaminants. Use a brush and a clean rag to remove reactive paint, wipe the paint residue area with a slightly damp sponge, rinse with distilled water and wring out the sponge to ensure thorough cleaning.

[0042] The ammonia gas tightness detection method of the main shielding weld of the liquefied natural gas membrane storage tank and the marine membrane cabin of the present invention uses ammonia as the detection medium, combined with reactive coatings and vacuum detection technology, and can efficiently and accurately detect subtle leakage points of the weld. The method is easy to operate, suitable for large-scale liquefied natural gas storage tank detection, can effectively avoid detection blind spots, and realize comprehensive detection. At the same time, the present invention also provides a detailed leak repair and detection scheme to ensure the long-term safe use of the storage tank. Compared with the prior art, the present invention greatly improves the detection accuracy and reliability, saves time and cost, and is of great significance for ensuring the safety of liquefied natural gas storage tanks.

[0043] The following examples are only listed as examples of embodiments of the present invention and do not constitute any limitation to the present invention. It can be understood by those skilled in the art that modifications within the scope of the essence and concept of the present invention fall within the scope of protection of the present invention. Unless otherwise specified, the reagents and instruments used in the following examples are all commercially available products.

[0044] Example 1

[0045] Experimental preparation

[0046] 1. Storage tank cleaning should meet the following requirements:

[0047] 1) Before the test, the welds on the film should be cleaned and wiped with alcohol or acetone. There should be no foreign matter left on the contact surface between the film and ammonia to avoid clogging of the leak point. The tank wall on the ammonia side should be kept clean, and there should be no grease on the welds;

[0048] 2) The film should not contain any alkaline substances such as oil, grease, smoke, welding slag, tap water, etc. The film should be washed with weakly acidic nitric acid water, and the volume content of nitric acid should be (5-8)%. The weakly acidic water should be used for cleaning to prevent reducing the sensitivity of the test;

[0049] 3) Wipe the tank wall dry with a dry rag or paper. The pH value of the dry rag or paper should be suitable for the relevant reactive coating;

[0050] 4) Wait at least 24 hours before spraying reactive paint.

[0051] 2. Paint spraying and tank adjustment should meet the following requirements:

[0052] a. The spraying of paint should comply with the following requirements:

[0053] a1. After the tank is cleaned until the end of the test, the temperature and humidity should be kept stable. The film surface temperature should be higher than 20°C and the relative humidity should be lower than 70%, for example, kept at about 50%;

[0054] a2. Apply the reactive coating to the weld to be inspected using a gas spray gun, spray can or brush;

[0055] a3. Each liter of reactive paint can spray about 5m 2 The film weld is sprayed with the weld as the width and the length is about 50m;

[0056] b. Preparation of reactive coating: The reactive coating is a mixture of bromophenol blue, orthophosphoric acid and kaolin and a diluent, which are mixed evenly and the pH value is adjusted to 2.8.

[0057] Reactive coating spraying should meet the following requirements:

[0058] 1) The paint should be sprayed on the completely dry film surface;

[0059] 2) After spraying, the paint should be dried quickly to prevent environmental impact;

[0060] 3) The paint should be sprayed evenly on the weld.

[0061] Reactive paint spraying can generally be done by three methods: combination spray gun, spray gun only and spray can.

[0062] Combined spray gun spraying operations should meet the following requirements:

[0063] a. Use a compressor to supply the required compressed air to the spray gun (the air passes through the heating resistor). After preheating, use the spray gun to spray evenly at a distance of 25 to 30 cm from the tank wall. After coating the weld seam about 50 cm long, stop supplying paint;

[0064] b. The spray gun only sprays hot air to dry and purge the area just sprayed.

[0065] Spraying operations using only a spray gun should meet the following requirements:

[0066] a. Operate with a normal spray gun and connect the spray gun directly to the dry dust-free air generator unit;

[0067] b. Install a high efficiency filter before connecting the spray gun and air dryer.

[0068] The vacuum state should be established in the primary insulation layer space and the secondary insulation layer space according to the following requirements:

[0069] a. Evacuate the space in the main insulation layer. Extract the air and moisture in the air to facilitate uniform flushing of the nitrogen-ammonia mixture and prevent ammonia from being absorbed by moisture;

[0070] b. Evacuate the space of the secondary insulation layer to avoid applying excessive pressure to the bonding part of the secondary membrane;

[0071] c. The vacuum degree of the secondary insulation space should be kept roughly the same as that of the primary insulation space: the pressure difference between the two spaces should not exceed 30mbar;

[0072] d. Use dry nitrogen to purge the two spaces to reduce residual air and moisture;

[0073] e. The purging operation should be performed at least three times to make the relative humidity under atmospheric pressure less than or equal to 60%;

[0074] f. During the purge process, three consecutive pressure cycles of "AP rising to +20 mbar and then falling to AP" (AP is atmospheric pressure) should be carried out in the main insulation space;

[0075] g. After nitrogen purging, the vacuum should be evacuated to -800mbar.

[0076] 3. Fill the primary insulation space with nitrogen-ammonia mixture and fill the secondary insulation space with nitrogen. The pressure of the primary insulation space and the secondary insulation space should be kept consistent during the injection process. In order to fill the primary insulation space with nitrogen-ammonia mixture, the gas delivery pipeline and connectors should be installed in the insulation layer during the installation of the insulation system. Inject the nitrogen-ammonia mixture to increase the pressure in the tank from -800mbar to +20mbar.

[0077] 1) The pressure increase in the main space will be achieved in two stages:

[0078] a. The pressurization speed should be less than 200mbar / h, and the pressure in the insulation space should rise from -800mbar to AP;

[0079] b. The pressurization rate should be 10mbar / h, raising the pressure of the insulation space from AP to A.P+20mbar. Within two hours of the pressurization operation, the tank should be checked quickly to avoid contamination of the tank due to large leaks. Inspections should be allowed during this stage, and the previous stage should be as short as possible to prevent the mixture from being diluted by air flowing in from the leak;

[0080] 2) During the pressure-boosting operation, the pressure in the secondary insulation space should always be (5-10) mbar higher than the pressure in the primary insulation space;

[0081] 3) Safety devices should be installed in both spaces to ensure that the pressure in the space does not exceed the safety value of 25mbar;

[0082] 4) Install sampling holes on the main shielding membrane and start the paint spraying operation after completing the ammonia concentration check of the sampling holes.

[0083] 4. Regularly inject nitrogen-ammonia mixture:

[0084] 1) The filling of nitrogen-ammonia mixture should be carried out in the following order:

[0085] a. The main insulation space should be continuously injected with a nitrogen-ammonia mixture, which should be introduced into the dome outlet valve and kept flowing continuously. The time should not be less than 6 hours, the pressure should be maintained at 20mbar, and the flow rate should be maintained at (10-20)m 3 / h;

[0086] b. Further reduce the flow rate and continue to inject the nitrogen-ammonia mixture and monitor it. The time should not be less than 6 hours, the pressure should be maintained at 20mbar, and the flow rate should be maintained at (5~10)m 3 / h;

[0087] c. All the above operations last for about 12 hours in total. When all sampling holes have reacted, pressure operation control is carried out and maintained for at least 24 hours. During the coating inspection, the pressure will be maintained at 20mbar;

[0088] 2) The sampling hole should be sealed to prevent ammonia pollution and can display the status of the color strip. The sampling hole is an observable sign to verify whether ammonia is filled into the main insulation space. If the color strip at the sampling hole changes color, it proves that ammonia has been correctly filled into the main insulation space. In addition, the sampling hole should not become a channel for ammonia to enter the inner tank from the main insulation space, so the sampling hole should also be sealed when the color strip is placed to prevent ammonia from seeping into the inner tank from the sampling hole;

[0089] 3) In order to prevent the final leak detection from exceeding 24 hours, all weld surfaces are tested periodically according to standard procedures before the final test. During these operations, the concentration of ammonia in the insulation space is adjusted and controlled by continuous analysis and control of the injected nitrogen-ammonia mixture and sampling, and deviations caused by flow or temperature changes are corrected. The required concentration is:

[0090] a. At the injection point (tank bottom area): the volume concentration of ammonia is (25±5)%

[0091] b. At the tank outlet (dome area): the volume concentration of ammonia is 1% (minimum).

[0092] 4) Use an ammonia concentration detector to perform intermittent testing and analysis as follows: During the injection of the mixture at a stable or reduced flow rate (pressure 0 to +20 mbar), and before the visual inspection, measure the ammonia concentration every 6 hours and record the results; During the visual inspection: measure the ammonia concentration every 2 hours and record the results.

[0093] 5. Location and repair of leaks

[0094] The repair of leaks should meet the following requirements:

[0095] 1) After all leaks have been detected and marked, repair work should be carried out. Before repairing, the main insulation space should be evacuated and then flushed with nitrogen;

[0096] 2) The pressure in the sub-insulation space should be monitored under the same conditions;

[0097] 3) The repair of leaks should be carried out under atmospheric pressure in the main insulation space filled with dry nitrogen. Before repairing all leaks found, visual inspection and dye penetration inspection should be carried out;

[0098] 4) After the leak is repaired, the label on the film should be removed and recorded at the corresponding position on the drawing.

[0099] The second test shall meet the following requirements:

[0100] 1) During the test, the number of leaks found in the film was every 1000m2 When the number is not more than 4, the airtightness test will include visual inspection, dye penetrant testing, and vacuum chamber test;

[0101] 2) During the test, the number of leaks found in the film was every 1000m 2 If the number of leaks is greater than 4, the ammonia air tightness test shall be conducted again after repairing the leaks found in the first air tightness test.

[0102] 6. Discharge, replacement and purging of the insulation space of the storage tank

[0103] After the leak detection is completed, the primary insulation space and the secondary insulation space should be vented first, and a cycle of vacuuming and injection of dry nitrogen should be performed. The highest possible vacuum degree should be established in the insulation space to minimize repeated replacement.

[0104] Continuous displacement operations should be performed with dry nitrogen and sampled and analyzed until the residual ammonia level does not exceed 1000 ppm. A thorough analysis of these spaces will ensure the effectiveness of the venting, displacement, and purge operations. The same process should be used for the secondary insulation layer space.

[0105] After the replacement purge is completed, the removal of the coating on the film should meet the following requirements:

[0106] 1) After brushing off the paint, wipe off the dust, and then use a dust collector to completely remove the dust.

[0107] 2) Use a slightly damp clean sponge to remove the paint.

[0108] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for detecting the ammonia gas tightness of the main shielding weld of a liquefied natural gas membrane storage tank and a marine membrane tank, comprising the following steps: 1) Prepare detection equipment and chemical reagents, the detection equipment includes a vacuum pump, a pressure sensor, an isolation valve, a pressure reducing valve, a high-conductivity injection pipe, a gas flow controller and a measuring device, an ammonia dilution water delivery system, a safety valve or a hydraulic protection device, etc.; the chemical reagents include a nitrogen-ammonia mixture with an ammonia content (volume percentage) of 25±5% and a reactive coating; 2) From the inside of the membrane tank, that is, the inside of the main insulation layer, wipe the weld of the membrane tank with alcohol or acetone to clean it, then spray a layer of reactive coating, and then establish a vacuum state and purge it with dry nitrogen; 3) First, the main insulation layer between the membrane storage tank and the shell, that is, the space between the outer tank and the inner tank of the membrane storage tank, is evacuated to -600 mbar to -800 mbar, and then a nitrogen-ammonia mixture is injected therein, and the pressure is kept rising to 15 mbar to 35 mbar, preferably 20 mbar, and a certain reaction time is waited, during which the ammonia concentration is regularly detected and ammonia is supplemented; 4) After the reaction time has passed, the nitrogen-ammonia mixture in the main insulation layer is emptied, and the weld leak is observed and marked from the inside of the membrane tank. When a leak exists, the reactive coating at the leak location changes color and can be easily observed. The detected leak is then repaired, and steps 1 to 3 are repeated to perform an ammonia test after the repair; 5) When no more leaks are found, the insulation space of the tank is vented, replaced and purged to ensure that the residual ammonia does not exceed 1000ppm, and the reactive coating on the weld of the membrane tank is removed at the same time.

2. The airtightness detection method according to claim 1, characterized in that: In the detection equipment, the pump displacement of the vacuum pump is 20m 2 The measured surface area is not less than 1m 3 / h, and equipped with a vacuum degree display. The pump displacement refers to the vacuum displacement, and sufficient suction capacity must be ensured.

3. The airtightness detection method according to claim 1, characterized in that: The establishment of a vacuum state and purging with dry nitrogen in step 1 refers to establishing a vacuum state in the primary insulation layer space and the secondary insulation layer space according to the following requirements: 1) The main insulation space is evacuated to remove air and moisture in the air, so as to facilitate uniform flushing of the nitrogen-ammonia mixture and prevent ammonia from being absorbed by moisture; 2) Evacuate the space between the primary insulation layer and the secondary insulation layer to avoid applying excessive pressure to the bonding part of the secondary film; 3) The vacuum degree of the secondary insulation space should be kept roughly the same as that of the primary insulation space, and the pressure difference between the two spaces should not exceed 30mbar; 4) The two spaces shall be purged with dry nitrogen to further reduce the residual air and moisture. The purging shall be performed at least three times to make the relative humidity under atmospheric pressure less than or equal to 60%; 5) During the purge process, three continuous pressure cycles of "rising from atmospheric pressure by 20 mbar and then dropping to atmospheric pressure" are carried out in the main insulation space; 6) After nitrogen purging, the primary insulation layer space and the secondary insulation layer space are evacuated to -800 mbar.

4. The airtightness detection method according to claim 1, characterized in that: The reaction time in step 3 is 6-12 hours.

5. The airtightness detection method according to claim 1, characterized in that: The reactive coating is formed from a mixture of bromophenol blue, orthophosphoric acid and kaolin, and the pH value is adjusted to 2.8.

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