Carbon dioxide sniffing leak detection method for tank main membrane weld

Abstract

The present application relates to a carbon dioxide sniffing leak detection method for tank main film weld, in the detection process, vacuumizing in the main insulation space, then injecting the mixed gas of carbon dioxide and dry air which has been mixed in advance, making the pressure reach micro-positive pressure. Injecting dry air in the secondary insulation space, the pressure is slightly higher than that of the main insulation space. Connecting the gas sniffing head with carbon dioxide concentration instrument, small air pump and sound alarm instrument, slowly detecting the gas sniffing head close to the film weld, the concentration instrument displays the carbon dioxide concentration value in real time, and the sound alarm will be sent when the gas higher than the preset concentration is detected, and the leak is found. The method is environment-friendly, easy to implement, and the detection result is reliable. Meanwhile, using carbon dioxide as tracer gas, it is non-toxic, harmless, non-combustible and non-explosive compared with ammonia, the source is wide, the price is cheap, no need for post-processing, friendly to human body and environment.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon dioxide sniffing leak detection method for main membrane weld of storage tank, in particular, to a carbon dioxide sniffing leak detection method for main membrane weld of thin film type LNG storage tank. BACKGROUND

[0002] In the prior art, for sealed flammable and explosive, environmentally polluting, valuable medium, the system leakage rate must be less than the maximum allowable leakage rate index determined by environmental protection, safety and economy.

[0003] At present, the tracer gas leak detection method for thin film type LNG storage tank includes ammonia color developing leak detection method and helium mass spectrometry leak detection method. The main insulation space is pumped to vacuum, the outside of the main shielding weld is sprayed with ammonia sensitive developing paint, and then the main insulation space is filled with ammonia gas higher than atmospheric pressure. When there is a leak hole, ammonia gas escapes through the leak hole, so that the developing paint changes color, and thus the position of the leak hole can be found. The disadvantages of this method are: ammonia is toxic and flammable and explosive, special safety protection and post-processing are required; if repair welding is required, ammonia must be completely removed to prevent explosion and combustion. The helium leak detection method is to pump the main insulation space to vacuum, and then fill it with helium gas higher than atmospheric pressure. The sealing of the main shielding weld is detected by using a mass spectrometer suction gun outside the main shielding weld.

[0004] The disadvantages of the helium leak detection method are: helium is a scarce gas and expensive; the helium mass spectrometer is expensive.

[0005] Therefore, there is an urgent need for a new sniffing detection method for thin film type LNG storage tank. SUMMARY

[0006] The purpose of the present application is to provide an improved carbon dioxide sniffing leak detection method for main membrane weld of storage tank. Through the improvement of the step method, the detection method is safe to operate, has good detection effect, and is simple and inexpensive.

[0007] In order to achieve the above purpose, the technical scheme of the present application is: a carbon dioxide sniffing leak detection method for main membrane weld of storage tank, the detection method is used for thin film type LNG storage tank, and is characterized by comprising the following steps:

[0008] a. All valves are in closed state, the main insulation space is pumped to -700 mbar by using a mechanical pump through a gas suction port;

[0009] b. The bottled pure carbon dioxide and the dry air treated by a drying machine are injected into a mixing tank for pre-mixing, and a carbon dioxide concentration instrument is used for inspection, so that the carbon dioxide volume concentration of the mixed gas reaches 30%;

[0010] c. Injecting dry air into the secondary insulation space through the injection ports distributed on the main film to a pressure of +24 mbar, and injecting mixed gas into the primary insulation space to make the pressure of the primary insulation space reach +20 mbar;

[0011] d. Using the carbon dioxide concentration instrument to check whether the gas concentration at each position of the insulation space reaches the preset value of 20% through the sampling ports distributed on the main film. If the preset value is not reached, the sampling port should be exhausted, and the checking should be performed every 30 min until the concentration is satisfied.

[0012] e. Connecting the gas sniffer head with a small air suction pump, a carbon dioxide concentration instrument, and a sound alarm instrument, using a carbon dioxide standard leak hole to calibrate the carbon dioxide sniffer type leak detector, and setting the alarm concentration value.

[0013] The conversion relationship between the carbon dioxide concentration value and the leak rate is as follows:

[0014] The amount of gas is represented by the product of the pressure and the volume of the gas, PV,

[0015]

[0016] wherein m is the mass of the gas; M is the molar mass of the gas, which is a constant; R is the molar gas constant; and T is the thermodynamic temperature.

[0017] The gas mass m is,

[0018] m = ρVC

[0019] wherein ρ is the carbon dioxide density; V is the free volume of the primary insulation space; and C is the carbon dioxide concentration of the primary insulation space.

[0020] The amount of gas flowing through the leak hole per unit time is called the leak rate Q,

[0021]

[0022] f. Slowly detecting the gas sniffer head from the film weld 1-2 mm, and the concentration instrument displays the carbon dioxide concentration value in real time. When the gas concentration reaching the set alarm concentration value is detected, the sound alarm instrument will issue an alarm, indicating that a leak is found. Preferably, the primary insulation space is provided with an overpressure protection valve to prevent overpressure damage. In the e step, the tank space where the personnel move must be ventilated to protect the safety of the personnel.

[0023] Compared with the prior art, the technical scheme of the present application not only improves the overall technical scheme, but also improves many details. Specifically, the present application has the following beneficial effects:

[0024] 1. The improvement scheme of the application, vacuumizing in the main insulation space, then injecting the mixed gas of carbon dioxide and dry air which has been mixed in advance, making the pressure reach micro positive pressure, injecting dry air in the secondary insulation space, the pressure is slightly higher than that of the main insulation space, using carbon dioxide as tracer gas, which is non-toxic, harmless, non-combustible, non-explosive, widely available and cheap, and does not need post-processing, friendly to human body and environment;

[0025] 2. In the technical scheme of the application, the gas sniffer head is closely attached to the film weld for slow detection, the concentration instrument displays the carbon dioxide concentration value in real time, and the gas with a concentration higher than the preset concentration will issue a sound alarm to find the leakage, and the tank main shielding weld repair does not need to exclude the tracer gas, there is no risk of deflagration, and the process is simplified.

[0026] 3. The application uses a carbon dioxide concentration instrument and a standard leak hole for comparison, and the cost is small and the detection result is also convenient and reliable compared with the use of a helium mass spectrometer. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 It is a structural schematic view of the application.

[0028] Reference signs:

[0029] 1 LNG storage tank shell, 2 secondary insulation space, 3 secondary shielding film, 4 main insulation space, 5 main shielding film, 6 carbon dioxide concentration instrument. DETAILED DESCRIPTION

[0030] The technical scheme of the application will be described below in conjunction with the drawings. Obviously, the described embodiments are part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor belong to the scope of protection of the application.

[0031] The application discloses a sniffer type leakage detection method suitable for a main film weld of a thin film type LNG storage tank, which uses carbon dioxide as a tracer gas. The volume content of carbon dioxide in the atmosphere is 0.031%, which is an excellent background environment as a tracer gas. The method of the application is to vacuumize in the main insulation space, then inject the mixed gas of carbon dioxide and dry air which has been mixed in advance, making the pressure reach micro positive pressure. Dry air is injected in the secondary insulation space, and the pressure is slightly higher than that of the main insulation space. The gas sniffer head is connected to the carbon dioxide concentration instrument, a small air pump and a sound alarm instrument. The gas sniffer head is closely attached to the film weld for slow detection, the concentration instrument displays the carbon dioxide concentration value in real time, and the gas with a concentration higher than the preset concentration will issue a sound alarm to find the leakage. The method of the application is friendly to the detection environment, easy to implement, and the detection result is reliable.

[0032] EMBODIMENT

[0033] The application discloses a carbon dioxide sniffing type leakage detection method for a main membrane weld of a storage tank, and the detection method is used for a thin film type LNG storage tank which comprises an LNG storage tank shell 1, a secondary adiabatic space 2, a secondary shielding membrane 3, a main adiabatic space 4 and a main shielding membrane 5, and the detection method is characterized by comprising the following steps.

[0034] The carbon dioxide sniffing type leakage detection method for the main membrane weld of the storage tank is used for the thin film type LNG storage tank, and the detection method is characterized by comprising the following steps.

[0035] a. All valves are in a closed state, and the main adiabatic space is pumped to -700 mbar by using a mechanical pump through a gas extraction port;

[0036] b. A bottled pure carbon dioxide and dry air treated by a drying machine are injected into a mixing tank for pre-mixing, and a carbon dioxide concentration instrument is used for inspection, so that the carbon dioxide volume concentration of the mixed gas reaches 30%;

[0037] c. Dry air is injected into the secondary adiabatic space through injection ports distributed at each part of the main membrane to a pressure of +24 mbar, and the mixed gas is injected into the main adiabatic space, so that the pressure of the main adiabatic space reaches +20 mbar;

[0038] d. The carbon dioxide concentration instrument is used for checking whether the gas concentration of each part of the adiabatic space reaches a preset value of 20% through sampling ports distributed at each part of the main membrane, if the preset value is not reached, the sampling port is exhausted, and the checking is performed every 30 min until the concentration is satisfied;

[0039] e. A gas sniffing head is connected with a small air extraction pump, a carbon dioxide concentration instrument and a sound alarm instrument, the carbon dioxide sniffing type leakage detection instrument is calibrated by using a carbon dioxide standard leakage hole, and an alarm concentration value is set;

[0040] The conversion relationship between the carbon dioxide concentration value and the leakage rate is as follows:

[0041] The gas amount is represented by the product PV of the pressure and the volume of the gas,

[0042]

[0043] Wherein, m is the gas mass; M is the molar mass of the gas, which is a constant; R is the molar gas constant; and T is the thermodynamic temperature.

[0044] The gas mass m is,

[0045] m = ρVC

[0046] Wherein, p is the carbon dioxide density; V is the main adiabatic space free volume, C is the main adiabatic space carbon dioxide concentration.

[0047] The amount of gas flowing through the leak per unit time is called the leak rate Q,

[0048]

[0049] f, the gas sniffer head is slowly detected away from the film weld 1-2mm, the concentration instrument real-time displays the carbon dioxide concentration value, the sound alarm instrument will issue an alarm when the gas concentration reaches the set alarm concentration value, indicating that the leak is found. In the specific operation, the main adiabatic space is provided with an overpressure protection valve to prevent overpressure damage. In step e, the tank space where personnel move must be ventilated to protect personnel safety.

[0050] The carbon dioxide sniffer type leak detection method for the main film weld of the thin film type LNG storage tank has the following beneficial effects:

[0051] (1) The carbon dioxide is used as the tracer gas in the present application, which is non-toxic, harmless, non-combustible and non-explosive, and does not need post-processing, and is friendly to the human body and the environment.

[0052] (2) The tracer gas does not need to be removed for the main shielding weld repair welding of the storage tank, and there is no risk of deflagration, and the process is simplified.

[0053] (3) The carbon dioxide is used as the tracer gas in the present application, which is widely available and inexpensive compared with helium.

[0054] (4) The carbon dioxide concentration instrument and the standard leak are used for comparison in the present application, which is less expensive and the detection result is also convenient and reliable compared with the use of a helium mass spectrometer.

[0055] The above content is a further detailed description of the present application in combination with the specific preferred embodiments, and cannot be considered as limiting the specific implementation of the present application to the above description. For ordinary skilled persons in the technical field to which the present application belongs, some simple deductions or substitutions can be made without departing from the concept of the present application, and all of them should be considered as belonging to the protection scope of the present application.

Claims

1. A method for detecting a leak in a weld of a main membrane of a storage tank by carbon dioxide sniffing, the method being used for a thin-film LNG storage tank, characterized in that: The detection method comprises the following steps: a. All valves are closed, and the main adiabatic space is pumped to -700 mbar by using a mechanical pump through the air outlet; b. The bottled pure carbon dioxide and the dry air treated by the drying machine are injected into the mixing tank for pre-mixing, and the carbon dioxide concentration instrument is used for inspection to make the carbon dioxide volume concentration of the mixed gas reach 30%; c. The dry air is injected into the secondary adiabatic space through the injection ports distributed at each part of the main film to reach a pressure of +24 mbar, and the mixed gas is injected into the main adiabatic space to make the pressure of the main adiabatic space reach +20 mbar; d. The carbon dioxide concentration instrument is used to check whether the gas concentration at each part of the adiabatic space reaches the preset value 20% through the sampling ports distributed at each part of the main film, if not, the sampling ports are exhausted, and the checking is performed every 30 min until the concentration is satisfied; e. The gas sniffer is connected with a small air pump, a carbon dioxide concentration instrument and a sound alarm instrument, the carbon dioxide sniffer type leak detector is calibrated by using a carbon dioxide standard leak hole, and the alarm concentration value is set; The conversion relationship between the carbon dioxide concentration value and the leak rate is as follows: The gas amount is represented by the product PV of the pressure and the volume of the gas, Wherein, m is the gas mass; M is the molar mass of the gas, which is a constant; R is the molar gas constant; T is the thermodynamic temperature; The gas mass m is, m = ρVC Wherein, ρ is the carbon dioxide density; V is the free volume of the main adiabatic space, and C is the carbon dioxide concentration of the main adiabatic space; The gas amount flowing through the leak hole per unit time is called the leak rate Q, f. The gas sniffer is slowly detected at 1-2 mm away from the film weld, the carbon dioxide concentration value is displayed in real time by the concentration instrument, when the gas concentration reaching the set alarm concentration value is detected, the sound alarm instrument will give an alarm, indicating that the leak is found.

2. The method for carbon dioxide sniffer leak detection of a tank main film weld according to claim 1, characterized in that: The main adiabatic space is provided with an overpressure protection valve to prevent damage caused by overpressure.

3. The method for carbon dioxide sniffer leak detection of a tank main film weld according to claim 1, characterized in that: In steps d-f, in order to protect the safety of personnel, the tank space where the personnel move must be ventilated.

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