Comprehensive Procedure for Membrane Welding Tightness Testing (Ammonia Method) and LNG Tank Inspection

Conducting membrane tests for ammonia tightness is a critical procedure in ensuring the safety and efficiency of storage systems. These tests involve meticulous examination of the membrane’s integrity to detect any potential leaks that could lead to ammonia escape. Advanced techniques such as pressure testing and gas detection are employed to identify even the smallest breaches, thereby preventing environmental contamination and ensuring the safety of personnel.

Complementing the membrane tests, tank inspections are equally vital in maintaining the overall integrity of ammonia storage facilities. These inspections involve a thorough examination of the tank’s structure, welds and coatings to identify signs of wear, corrosion or damage. Regular inspections help in early detection of issues, allowing for timely repairs and preventing catastrophic failures. This dual approach of testing membranes and inspecting tanks ensures a robust and reliable ammonia storage system.

Membrane Tests

Principles

The containment system is tested with three different tests (Following diagram shows the sequence of tests.) (See Figure 1.):

1. Tightness test of the membrane welding: to check the tightness of the primary membrane welding.
2. Primary barrier global test: to check whether the primary barrier walls are perfectly tight when the tanks are loaded.
3. Secondary barrier tightness test: to define the global sealing property of the tank secondary barrier.

Tightness Test of the Membrane Welding (Ammonia Test)

This test consists in introducing a nitrogen ammonia mixture in the primary space (interbarrier spaces) in which a partial vacuum has been previously created. For this purpose, a distribution piping and connection pieces are inserted in Tank Installation: Bonding, Insulation and Final Quality Checkthe insulating system during installation.

The following diagram shows the different steps of the test. (See Figure 2.)

The mixture flows through any possible leaks existing in the welding of the membrane and reacts when coming into contact with a reactive paint, which has been previously spread on the seams to be checked. The main reaction of the paint is that it turns from yellow to violet when in contact with ammonia.

The paint reacts under a concentration of 0,5 % at least. To obtain this minimum concentration, it is necessary to inject a nitrogen ammonia mixture with 25 % +/- 5 % of ammonia.

To be sure of the presence of ammonia mixture behind the membrane on the whole surface, some reference leaks are distributed in the tank according to a precise diagram. They are produce by stopping the weld seam on 5 to 10 mm long. Key Characteristics of Membrane Tanks SystemsDrilling or punching the membrane is not allowed.

Reference leaks must are protected and sealed using adhesive e sealed and covered using a clear view panel adhered to membrane so as to:

• avoid an ammonia pollution of the tank;
• avoid the local formation of a turbulent zone where the composition of the mixture is not representative of an actual leak.

Figures 3, 4 and 5 show a reactive paint can, a welding seams painted with that paint and a defect.

Characteristics of the Tanks to be Tested

The followings are the characteristics of a typical tank:

• Free volume of primary space (space between membrane and secondary barrier) is about 15 liters per m² of membrane.
• Free volume of secondary space (between secondary barrier and inner hull) is about 20 liter per m² of membrane.
• Average length of welding seam is about 1,5 m per m² of membrane.

Ammonia

The principal characteristics of Ammonia (NH₃) are:

• Highly water solubility at normal temperature is equal to 800 times its volume.
• Pure ammonia gas, which does not attack steel is likely to attack copper alloys when in a humid medium.
• When dry, wood usually tolerates ammonia concentration of at least 30 %.
• Ammonia mixed with air in a proportion ranging from 15 % to 28 % per volume of air gives an inflammable mixture likely to explode at 20 °C under 1 bar pressure.
• Breathing large quantity of air loaded with ammonia is hazardous to human life.

Reactive Paint

The reactive paint is an aqueous solution composed of bromophenol blue and orthophosphoric acid with kaolin.

The color change occurs when the PH is higher than 3,3 by reaction of ammonia with orthophosphoric acid.

Preliminare Test Procedure

1) GENERAL

The tightness test is divided into three successive steps:

• Detection of leaks using ammonia tightness test.
• Repair and check to see whether all leaks have been repaired and therefore to check the quality of re-welding and to guaranty the complete tightness of the tank.
• Checking the closing area (side opening) and limited repair if any.

2) SAFETY RULES

Adequate ventilation must be able to rapidly and efficiently ensure the sweeping of polluted atmosphere.

Detectors will be arranged at various points inside the tank to enable the personnel to check the ammonia concentration at any time.

Maximum threshold limit value (TLV) for ammonia concentration, based on an eight (8) hours per day exposure, is 50 ppm (35 mg/m³).

The threshold limit value of ammonia concentration, which is hazardous after an hour exposure is equal to 500 ppm.

Threshold limit value of breath, which is immediately dangerous to life is 5 000 ppm.

The olfactive threshold ranges about 20 ppm. Therefore ammonia is often detected by smelling before it becomes noxious.

3) RECOMMENDATIONS ABOUT THE MEMBRANE

The tank should be kept as clean as possible. All the persons having access to the tank and more specially those in charge of cleaning the weld, applying the paint and finding the leaks should wear clean clothes, gloves and non rigid soled shoes.

The gloves and clothes must be free from dirt and alkaline product, which might spoil the membrane or alter the test paint.

4) CLEANING OF THE TANK

The tank must be clean.

Test paint PH is to be checked daily. It is to be confirmed that there are no alkaline traces left on the membrane, in particular oil, grease, smoke deposits, weld slag. Dirt is to be cleaned and neutralized by sponging with slightly acid water. For this purpose, a solution of 5 to 8 % of nitric acid per volume of distilled water will be used.

Cleaning with slightly acidic water should be done always need, otherwise it may have a dtrimental effect. Acid passing through the leakage opening might impregnate the insulation and the ammonia injected would be neutralized before reacting with the paint. That would lengthen the color changing and reduce the test sensitivity.

Cleaning will be followed by drying with dry rag or paper.

5) PAINT APPLICATION, CONDITIONING OF THE TANK

Paint should not be applied for at least 24 hours after completion of cleaning and drying.

Quality control of paint application is directly related to paint temperature, proper agitation, nozzle cleanliness and spray pattern evenness.

In order to obtain correct paint stability for the test duration, it is necessary to keep the tank atmosphere maintained at the following conditions:

• Temperature above 20 °C on membrane sheets.
• Relative humidity lower than 70 % (50 % being the optimum).

These conditions must be obtained prior to start paint application and kept throughout the reading of the test.

The PH nominal value of 2,8 is to be checked daily.

Particular attention is to be paid to control the immediate drying of the paint. When the paint dries rapidly the influence of atmosphere is less sensitive.

The paint has to be regularly spread.

Over-thickness of painting is unfavorable for leak detection because:

• The thickness of the material makes the reaction less visible.
• The experience shows that thick spread paint has a self discoloration and often turns to a brownish color.

Paint application can be accomplished by means of:

• combined gun (for painting and for drying);
• “airless” spray gun;
• aerosol method with pressurized container.

The last method is the most commonly used due to portability and ease of application.

Nitrogen Ammonia Mixture Injection

This operation must be carried out with extreme care as the success of this test depends on the uniform distribution of the nitrogen-ammonia mixture within the primary insulation space. (See Figure 6.)

Nitrogen filling commonly is commenced after both primary and secondary insulation spaces have been evacuated into a negative pressure (vacuum) condition of – 800 millibar.

In the primary space, it is additionally necessary to extract any moisture, to insure a uniform scouring of the nitrogen-ammonia mixture and avoid NH₃ absorption by moisture.

In the secondary space, care is to be exercised to not place an excessive pressure on the non bonded parts of the secondary barrier curved joints.

Secondary space must be kept approximately at the same level of vacuum than the primary space. The pressure difference between these two paces should not exceed 30 millibar.

Then, both spaces are purged with dry nitrogen to reduce the residual air and remaining moisture traces. The purging operation is performed three times and can be repeated additional times if needed to obtain a relative humidity equal to or lower than 60 % at atmospheric pressure.

During this purging operation, three successive pressure cycles of atmospheric pressure (PA) + 20 millibar, to PA can be performed. The purpose of this test is to detect any weld weaknesses by stressing the weld of the primary membrane.

Following the last nitrogen purge, the spaces are vacuum pumped to – 800 millibar. During vacuum operations, pressure is measured at bottom and top of the tank.

Next operation is to fill the primary interbarrier space with nitrogen ammonia mixture while filling the insulation space with nitrogen (pressure in the secondary space must rise at the same time and in the same pressure conditions):

1. injection of nitrogen ammonia mixture from –800 millibar to +20 millibar;
2. the pressure in the primary space will be increased in two steps at different rate:

• Initial injection rate is to raise pressure not to exceed 200 millibar/h from – 800 millibar up to the atmospheric pressure.
• Second stage, pressure in the interbarrier space is to be increased at a rate not to exceed 10 millibar/h from atmospheric pressure to a maximum pressure of + 20 millibar. During this time, tank weld surfaces are to be monitored continuously to prevent ammonia contamination of tank atmosphere due to large leak should one be found.

The secondary space is always kept with an excess pressure of 5 to 10 millibar with respect to the pressure in the primary space during the pressure rise.

Safety devices are to be fitted on the two spaces and set at 25 millibar in order to insure that the pressure in the spaces never exceeds this value.

From this time and after checking the ammonia concentration at the inlet and at the Type “B” Prismatic Tanks Design and Analysisoutlet of tank, the paint application may start.

The filling of nitrogen ammonia mixture must be in accordance with the following conditions:

a) For a duration of 6 hours, the primary space is to be kept under a pressure of 20 millibar, keeping a continuous flow of nitrogen ammonia mixture by regulating both outlet valves located on the liquid and gas domes. The mixture flow rate must remain with the range 10 to 20 m³/h.

b) For a 6 hours text duration, the inter barrier space is to be maintained under the same condition of pressure as described above. The inlet gas flow will be reduced with regard to the preceding phase (mixture flow rate must remain within the range of 5 to 10 m³/h) and monitored so as to maintain the 20 millibar pressure.

c) Repeated pressure cycles (PA + 20 millibar to PA) in the primary space are performed. Three successive pressure increases/decreases will be carried out, pressure level being maintained during few minutes (if these cycles were not performed during the purging operations).

d) All above test cycle should last approximately 24 hours. When reference leaks have reacted, the control phase will start and will be maintained for approximately 20 hours with the 20 millibar pressure maintained for the duration of the control (about 20 hours). If any reference leak does not react the 20 millibar pressure is maintained until it is detected. Only at this point, the 20 hours period will start.

To reduce the operation time, paint control can start on one complete level providing that level above and under this one will be reacted (top level for instance includes top of the tank, upper part of the aft and forward faces whole or part of the upper chamfers).

A final examination of the entire surface will be carried out and, in case of control duration up to 24 hours, periodical control of the entire surface will be carried out on 24 hours basis before final control.

During these operations it is advisable to check the injected nitrogen ammonia mixture so as to have a permanent control of the concentration and to be able to correct any possible deviation.

Locating and Repair of the Leaks

1) LOCATING AND MARKING LEAKS

This operation has to be:

• rapid to avoid tank pollution;
• synchronized with the beginning of the primary space filling operation.

2) REPAIRS OF LEAKS

Once all the leaks have been detected, they must be repaired.

Before repairs can be commenced, vacuum is re-established in the primary space, which is then re-rinsed with nitrogen. Pressure within the secondary space is to be maintained at exact conditions of pressure.

Repairs are carried out with the primary space full of dry nitrogen at atmospheric pressure. This is critical to avoid any possibility of fire damage.

Repairs are carried out by approved welders.

3) CONTROL AND TEST AFTER REPAIR

According to the results of the ammonia test, there are two possibilities:

• If number of leaks found is less than or equal to four (4) per 1 000 m² of membrane, the ammonia test need not be repeated and the tightness testing may be verified as follows:

1. visual inspection;
2. vacuum box inspection;
3. dye penetrant test.

• If number of leaks found during the test is greater than four (4) per 1 000 m² of membrane, a full second ammonia test is to be carried out.

Draining-Rinsing-Cleaning of the Tank

Once the leak detection test is finished, the primary and secondary spaces must be drained and rinsed before being filled wit dry nitrogen.

This rinsing should be done at least three or four times in succession. Analysis of ammonia concentration is to be performed with rinsing operations being repeated until the remaining concentration of ammonia does not exceed 1 000 ppm.

It is advantageous to utilize the highest possible vacuum, in order to minimize the number of draining and rinsing operations required being most desirable to create in the spaces the highest as possible vacuum in order to carry out the minimum repeated draining and rinsing operations.

The same procedure applies to the secondary space.

The last operation of this ammonia test consists in a thoroughly cleaning of the reactive paint off the membrane.

Local Test

1) TEST AFTER CLOSING THE SIDE OPENING

When all the operations for the closing of the side opening are completed, a last tightness test is carried out. Considering the small length of welding (less than 40 m) the ammonia test is not requested.

The last test will include the following steps:

• Visual examination.
• Vacuum box test.

2) TEST AFTER LIMITED REPAIR

In case of any damage to the membrane after completion of the ammonia test ( e. g. as a result of scaffolding removal) the damages are to be examined and tested to similar criteria as when number of repairs is less than or equal to four (4) repairs per 1 000 m² of membrane.

Author

Olga Nesvetailova

Freelancer

Literature

1. The Society of International Gas Tanker and Terminal Operators (SIGTTO). Liquefied Gas Handling Principles on Ships and in Terminals (LGHP4) / 4^th Edition: 2021.
2. The international group of liquefied natural gas importers (GIIGNL). LNG custody transfer handbook / 6^th Edition: 2020-2021.
3. American Gas Association, Gas Supply Review, 5 (February 1977).
4. ©Witherby Publishing Group Ltd. LNG Shipping Knowledge / 3^rd Edition: 2008-2020.
5. CBS Publishers & Distributors Pvt Ltd. Design of LPG and LNG Jetties with Navigation and Risk Analysis / 4^th Edition.
6. NATURAL GAS PROCESSING & ITS ENERGY TRANSITION ROLE: LNG, CNG, LPG & NGL Paperback – Large Print, November 14, 2023.
7. American Gas Association, Gas Supply Review, 5 (February 1977).
8. The Society of International Gas Tanker and Terminal Operators (SIGTTO). Ship/Shore Interface / 1^st Edition, 2018.
9. Department of Transportation, US Coast Guard, Liquefied Natural Gas, Views and Practices Policy and Safety, p. IV-3.
10. Department of Transportation, US Coast Guard, Liquefied Natural Gas, Views and Practices Policy and Safety, p. IV-4.
11. Federal Power commission, Trunkline LNG Company et al., Opinion No. 796-A, Docket No s. CP74-138-140 (Washington, D. C.: Federal Power Commission, June 30, 1977).
12. Federal Power Commission, Final Environmental Impact Statement Calcasieu LNG Project Trunkline LNG Company Docket No. CP74-138 et al., (Washington, D. C.: Federal Power Commission, September 1976).
13. Federal Power Commission, «FPC Judge Approves Importation of Indonesia LNG».
14. OCIMF, ICS, SIGTTO & CDI. Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases / 1^st Edition, 2013.
15. Federal Power Commission, «Table of LNG imports and exports for 1976», News Release, June 3, 1977, and Federal Energy Administration, Monthly Energy Review, March 1977.
16. Office of Technology Assessment LNG panel meeting, Washington, D. C., June 23, 1977.
17. Socio-Economic Systems, Inc., Environmental Impact Report for the Proposed Oxnard LNG Facilities, Safety, Appendix B (Los Angeles, Ca.: Socio-Economic Systems, 1976).
18. «LNG Scorecard», Pipeline and Gas Journal 203 (June 1976): 20.
19. Dean Hale, «Cold Winter Spurs LNG Activity»: 30.