Method for testing tightness of primary membrane in isothermal tank

Abstract

FIELD: sealed and heat-insulated membrane tanks.SUBSTANCE: invention relates to tanks for storing and / or transporting liquefied gases at low temperatures. The method for monitoring the tightness of the primary membrane of a membrane cryogenic tank includes feeding gas into the process cavity and detecting its exit on the opposite side. On the inner surface of the primary membrane, a hydrogen-selective indicator material is placed in advance in the form of continuous strips laid along the entire length of each weld seam and moulded elements. A hydrogen-nitrogen gas mixture is fed into the inter-barrier space and excess pressure is created in the specified space according to the following graph: 0-1 h - rise to 5 mbar, hold for 0.5 h; 1.5-2.5 h - rise to 10 mbar, hold for 0.5 h; 3-3.5 h - rise to 12.5 mbar, hold for 0.5 h; 4-6 h - rise to 15 mbar, hold for about 1.5 h. At the intersections of continuous bands, a visual assessment of the colour change is performed within the specified continuous bands and at the places of visual change in the colour of the hydrogen-selective indicator material, as well as in control areas, the volume fraction of hydrogen is measured using a portable hydrogen gas analyser.EFFECT: increased reliability in detecting microleaks in the primary membrane on a warm tank without the use of helium and ammonia, with a clear link between the defect and its location, increased test safety, and reduced labour intensity of testing.9 cl, 3 dwg

Description

[0001] Technical field

[0002] The invention relates to the field of sealed and heat-insulated membrane tanks, particularly to tanks for storing and / or transporting liquefied gases at low temperatures. The invention can be used to create tanks for transporting and storing liquefied natural gas (LNG) at a temperature of approximately -162°C under atmospheric pressure. The tanks can be installed onshore or on floating structures, including gravity-based structures. The proposed method is suitable for shipbuilding and onshore facilities during the construction, repair, and acceptance testing of membrane tanks for LNG. The procedure can easily be standardized into operating practices and quality control plans.

[0003] Technology Level

[0004] A prior art discloses a method for inspecting a gas leak in a hold of a liquefied natural gas (LNG) tanker by decompressing or compressing a first space and a second space of a hold installed in the LNG tanker and locating a gas leak location as a thermographic camera when the LNG leak is confirmed by a primary inspection. (Patent KR 101335699 B1)

[0005] The disadvantage of this technical solution is the dependence of the method's sensitivity on the tank's thermal conditions and surrounding conditions; the inability to clearly localize microdefects directly on the weld line, since the thermographic response is formed as a distributed cooling zone; the reduced reliability of the result when working on a "warm" tank, when temperature contrast is absent; and the need for thermal imaging equipment and preliminary operations to stabilize the structure's thermal balance.

[0006] A widely used ammonia method for leak testing of primary membrane welded joints in membrane LNG tanks is known in the art. It is used to meet the requirements of EN 14620-2 or other national standards. In this method, an ammonia-sensitive coating is applied to the inner surface of the primary membrane, after which a mixture of ammonia and an inert gas is introduced into the interbarrier or primary space of the tank. If a microdefect is detected, NH3 diffuses through it, causing a localized discoloration of the reagent layer.

[0007] The disadvantage of this technical solution is the toxicity and corrosivity of ammonia, high ventilation and protective equipment requirements, and the risk of residual contamination of structural components. The quantitative relationship between color intensity and leakage magnitude is ambiguous, complicating the comparison of results.

[0008] A known method for monitoring the primary membrane of an LNG membrane tank is based on the use of helium as a tracer gas (patent KR20100102886A, published September 29, 2010). According to this technical solution, helium is introduced into the inter-barrier space of the tank under controlled pressure, after which gas leaks are detected using a helium mass spectrometer leak detector. The helium flow passing through the primary membrane defect is recorded by a sensitive sensor, allowing for the detection of leaks.

[0009] The disadvantage of this technical solution is the difficulty of implementing the method due to the use of complex instrumentation; the lack of a clear visual indication of the defect "point"; sensitivity to background airflows when inspecting a large area; and the need for preparatory measures.

[0010] In addition, the above technical solutions do not include:

[0011] - supply of non-flammable hydrogen-nitrogen gas mixture directly into the interbarrier space (IBS) at regulated low excess pressures;

[0012] - hydrogen-selective visual indication on the inner surface of the primary membrane;

[0013] - mandatory quantitative measurement of the volume fraction of hydrogen using a portable gas analyzer in places where visual changes occur.

[0014] The technical problem solved by the claimed invention is to increase the reliability of monitoring the tightness of the primary membrane in an isothermal tank.

[0015] Disclosure of the essence of the invention

[0016] The technical results include increased reliability of primary membrane microleak detection on a "warm" tank without the use of helium and ammonia, with a clear reference to the defect location; increased test safety due to the use of a non-flammable gas mixture of 5% H2 / 95% N2 and low excess pressure in the interbarrier space (up to 20 mbar); reduced labor intensity of testing due to the absence of multiple studies with a mass spectrometer and expensive helium, the absence of toxic and corrosive substances (NH3); increased reproducibility due to a double evaluation criterion (visual hydrogen-selective indication + quantitative measurement of the volume fraction of H2 with a portable gas analyzer); completeness of welded joints control: ensured by 100% coverage of the seams with indicator material, including intersections and corners.

[0017] The specified technical results are achieved in a method for monitoring the tightness of the primary membrane of a membrane cryogenic tank, including feeding gas into a process cavity and detecting its exit on the opposite side, while on the inner surface of the primary membrane, a hydrogen-selective indicator material is placed in advance in the form of continuous strips laid along the entire length of each weld and molded elements, a hydrogen-nitrogen gas mixture is fed into the interbarrier space and excess pressure is created in the specified space according to a step schedule, at the intersections of the continuous strips, a visual assessment of the change in color within the said continuous strips is performed and at the places of a visual change in the color of the hydrogen-selective indicator material, and also at control sections, using a portable hydrogen gas analyzer, the volume fraction of hydrogen is measured.

[0018] An additional feature is that excess pressure is created according to a step schedule with dwell times within 3-20 mbar.

[0019] An additional feature is that the volume fraction of hydrogen in the hydrogen-nitrogen gas mixture is no more than 5%.

[0020] An additional feature is that the connections of continuous strips and formed elements are made with an overlap, while the maximum permissible unclosed gap along the edge of the strip is no more than 5 mm in any section 100 mm long.

[0021] An additional feature is that the total duration of exposure at pressure increase levels does not exceed 6 hours, with further exposure for the study, and upon completion of the test, the inter-barrier space is purged with nitrogen to the background concentration of hydrogen.

[0022] An additional feature is that a multilayer tape on a polymer substrate is used as a hydrogen-selective indicator material, including polyimide or polyethylene terephthalate, containing a gas-chromic layer based on WO3 and / or MoO3 with a thickness of 200-800 nm, modified with palladium and / or platinum 0.1-10 wt.%, with a protective diffusion coating of microporous SiO2 / Al2O3 or fluoropolymer with a thickness of 0.5-5 μm and an acrylate adhesive layer.

[0023] An additional feature is that a sensor layer based on Nb2O5 and / or Ta2O5 with catalytic additives of palladium and / or platinum 0.1-5 wt.% is used as a hydrogen-selective indicator material.

[0024] An additional feature is that the hydrogen-selective indicator material is designed to produce a visually distinguishable color change when exposed to 100 ppm hydrogen for no more than 20 minutes at 23°C and a relative humidity of 20-80%, with reversible restoration of the original color upon ventilation.

[0025] An additional feature is that the hydrogen volume fraction measurements are carried out directly above the strip and in the adjacent zone with a radius of at least 50 mm.

[0026] Brief description of drawings

[0027] Fig. 1. Graph of the step change in excess pressure in the interbarrier space (X-axis - time, h; Y-axis - pressure, mbar).

[0028] Fig. 2. Schematic diagram: 1 - H2 / N2 gas mixture source; 2 - feed regulating element; 3 - fine adjustment element; 4 - interbarrier space (IBS); 5 - IBS excess pressure meter; 6 - outlet / ventilation valve; 7 - ventilation duct.

[0029] Fig. 3. Scheme of continuous control of welded seams: 8 - welded seam; 9 - controlled zone (continuous strip of indicator material), including formed elements at intersections.

[0030] Implementation of the invention

[0031] A control gas mixture is fed into the interbarrier space (IBS) - hydrogen in nitrogen with a volume fraction of hydrogen of no more than 5% (the rest is nitrogen), classified as non-flammable.

[0032] In IBS, a small excess pressure is created according to a regulated schedule (Fig. 1).

[0033] Step graph of pressure in IBS (excessive relative to atmospheric):

[0034] 0-1 h - rise to 5 mbar, hold time 0.5 h;

[0035] 1.5-2.5 hours - rise to 10 mbar, hold time 0.5 hours;

[0036] 3-3.5 hours - rise to 12.5 mbar, hold time 0.5 hours;

[0037] 4-6 h - rise to 15 mbar, hold for about 1.5 h.

[0038] 6-24 h - conducting research.

[0039] The internal gas environment of the tank is maintained close to atmospheric; therefore, gas flow through the defect is ensured by the IBS overpressure itself. Equivalent adjustments to the stage durations are permissible while maintaining the 5 / 10 / 12.5 / 15 mbar levels.

[0040] If a localized leak occurs, gas from the IBS passes through the defect toward the internal volume of the tank, where an indicator material has been placed that changes color upon contact with hydrogen. The color change must be confirmed by measuring the volume fraction of H2 using a portable hydrogen analyzer (threshold no worse than 10 ppm).

[0041] The method is implemented using standard means (see Fig. 2):

[0042] 1 - H2 / N2 gas mixture source; 2 - IBS feed control element; 3 - fine flow rate adjustment element; 4 - interbarrier space (IBS); 5 - IBS excess pressure meter (0-50 mbar, accuracy class no worse than 0.25%); 6 - IBS outlet / vent valve to the safe path; 7 - ventilation duct / hood / scrubber (stationary H2 monitoring is desirable). The control circuit withstands pressure stages; interlocks: upper limit P_IBS (e.g. 20 mbar) with forced release after 6; exposure timer; signal on the H2 content in duct 7.

[0043] Work is performed on a "warm" tank; no structural changes to the tank are required

[0044] The indicator material (tape / plate) contains a multi-layer structure:

[0045] Base: Polyimide 25-50μm or PET 50-125μm.

[0046] Sensor (gas chromic) layer: tungsten oxide WO3 and / or molybdenum oxide MoO3 with a thickness of 200-800 nm, modified with palladium and / or platinum 0.1-10 wt.% (preferably 0.5-5 wt.%).

[0047] Protective diffusion coating: microporous SiO2 / Al2O3 or fluoropolymer (PVDF-HFP) 0.5-5 μm, porosity 10-40% (permeable to H2, protects against abrasion / dust).

[0048] Adhesive layer: low-halogen acrylate, adhesion 10-20 N / 25 mm, residual tack after 24 hours ≤ 0.5 N / 25 mm; dismantling without traces.

[0049] Liner: siliconized paper / PET, removable before installation.

[0050] Standardized properties:

[0051] Visual detection threshold (ΔE) at 23°C: ≤ 100 ppm H2 for 1-5 min; 10-50 ppm for 5-15 min (control areas).

[0052] Reversibility: restores the original color when aired for 10-60 minutes; reuse is allowed up to 3 cycles.

[0053] Operating conditions: 0…40°C; relative humidity 20-80%; no direct humidification.

[0054] Compatibility: no chlorine-containing plastics or corrosive additives.

[0055] In an alternative embodiment, a sensor layer (equivalent) is used as an indicator layer: Nb2O5, Ta2O5 with Pd / Pt nanoparticles 0.1-5 wt.% (for special flexibility / strength conditions).

[0056] On a warm membrane tank, a controlled zone 9 in the form of a continuous strip of indicator material is placed along all roof and wall seams, on the inner surface of the primary membrane, and along the entire length of each weld seam 8 (Fig. 3). Molded elements (corner segments and central inserts) are installed at the seam intersections, connected to the strips with an overlap of at least 10 mm, eliminating "blind spots."

[0057] Requirements: strip width 20-30 mm (or ≥ 2× the width of the heat-affected zone of the seam + 10 mm); the maximum allowable uncovered gap along the edge is no more than 5 mm over any 100 mm long section; the strip is positioned symmetrically to the seam axis 8, overlapping the seam and the adjacent area. The corners of the strip are formed with radii to prevent wrinkles and delamination.

[0058] The interbarrier space is inerted with nitrogen. The pressure in the IBS is maintained according to the graph in Fig. 1. No visual changes were detected at 10 mbar. At 15 mbar, two localized color changes were noted after 12-18 minutes; a portable gas analyzer indicates 55-70 ppm over the corresponding areas. The areas are marked. After releasing the mixture through valve 6 and purging the IBS with nitrogen, the indicator material is removed.

[0059] Retest after local repair at 15 mbar shows no color change or excess concentration; the area is considered leak-proof.

[0060] The test section is considered defective if the following are simultaneously observed: - a persistent change in the color of the indicator material within band 9; - an excess of the measured H2 volume fraction above the instrument's background level. The absence of both signs after holding at 10-15 mbar levels indicates the section is leak-tight.

[0061] Thus, in contrast to known technical solutions (helium and ammonia), the claimed invention provides:

[0062] Using a non-flammable mixture of hydrogen in nitrogen (H2≤ 5%) as a control gas environment, supplied directly into the inter-barrier space of the membrane tank.

[0063] Conducting tests at low excess pressures in the interbarrier space according to a regulated step schedule of 5 / 10 / 12.5 / 15 mbar.

[0064] Placement of hydrogen-selective indicator material on the inner surface of the primary membrane with the requirement for continuous (solid) coverage of all welded seams, including intersections and corners, with standardized overlaps and permissible gaps.

[0065] Mandatory instrumental confirmation of the result by measuring the volume fraction of hydrogen with a portable gas analyzer (threshold no worse than 10 ppm) directly above the area of ​​visual change and at control points.

[0066] It is this combination - H2 / N2 in the interbarrier space → mbar levels on a graph → hydrogen-selective visualization on the membrane side with continuous seam coating → mandatory quantitative measurement - that ensures the achievement of the stated technical results and distinguishes the solution from helium (mass spectrometric, without visualization of the site) and ammonia (toxic, with ambiguous visual interpretation) approaches.

Claims

1. A method for testing the tightness of the primary membrane of a membrane cryogenic tank, including feeding gas into a process cavity and detecting its exit on the opposite side, characterized in that a hydrogen-selective indicator material is placed in advance on the inner surface of the primary membrane in the form of continuous strips laid along the entire length of each weld and molded elements, a hydrogen-nitrogen gas mixture is fed into the interbarrier space and excess pressure is created in the specified space according to the schedule: 0-1 h - increase to 5 mbar, hold for 0.5 h; 1.5-2.5 h - increase to 10 mbar, hold for 0.5 h; 3-3.5 h - increase to 12.5 mbar, hold for 0.5 h; 4-6 h - increase to 15 mbar, hold for about 1.5 h., at the intersections of continuous stripes, a visual assessment of the color change is performed within the specified continuous stripes and at the places of visual change in the color of the hydrogen-selective indicator material, and also at the control sections, using a portable hydrogen gas analyzer, the volume fraction of hydrogen is measured.

2. The method according to paragraph 1, characterized in that the excess pressure is created in stages, within the range of 3-20 mbar.

3. The method according to paragraph 1, characterized in that the volume fraction of hydrogen in the hydrogen-nitrogen gas mixture is no more than 5%.

4. The method according to paragraph 1, characterized in that the connections of continuous strips and molded elements are made with an overlap, while the maximum permissible unclosed gap along the edge of the strip is no more than 5 mm in any section 100 mm long.

5. The method according to paragraph 1, characterized in that the total duration of exposure at pressure increase levels does not exceed 6 hours, with further exposure for conducting the study, and upon completion of the test, the interbarrier space is purged with nitrogen to a background concentration of hydrogen.

6. The method according to claim 1, characterized in that the hydrogen-selective indicator material is a multilayer tape on a polymer substrate including polyimide or polyethylene terephthalate, containing a gas-chromic layer based on WO3 and / or MoO3 with a thickness of 200-800 nm, modified with palladium and / or platinum 0.1-10 wt.%, with a protective diffusion coating of microporous SiO2 / Al2O3 or fluoropolymer with a thickness of 0.5-5 μm and an acrylate adhesive layer.

7. The method according to claim 6, characterized in that a sensor layer based on Nb2O5 and / or Ta2O5 with catalytic additives of palladium and / or platinum 0.1-5 wt.% is used as the hydrogen-selective indicator material.

8. The method according to paragraph 6, characterized in that the hydrogen-selective indicator material is designed with the possibility of a visually distinguishable change in color when exposed to 100 ppm hydrogen for a period of no more than 20 minutes at 23 °C and a relative humidity of 20-80%, with reversible restoration of the original color upon ventilation.

9. The method according to paragraph 1, characterized in that the measurements of the volume fraction of hydrogen are carried out directly above the strip and in the adjacent zone with a radius of at least 50 mm.

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