Minimize SCC in Liquefied Ammonia Tanks

Stress Corrosion Cracking (SCC) poses a significant threat to the integrity and safety of transport and storage systems, particularly those handling anhydrous ammonia. This paper delves into the critical issue of Ammonia Stress Corrosion Cracking, examining its relevance within the context of gas carrier codes and international maritime regulations, specifically highlighting IMO requirements for ammonia tanker safety. Furthermore, it explores published guidelines and industry recommendations, such as Lyle’s Guidelines, aimed at mitigating the risk of SCC.

These strategies encompass various operational and material considerations, including the benefits of refrigerated storage below -30 °C, the selection of low yield strength steel (preferably below 275 N/mm²), the importance of thermally stress-relieving welds, the potential of adding 0,2 % by weight water to ammonia, and the implementation of handling and transfer procedures designed to minimize air contamination. Understanding and implementing these preventative measures is crucial for ensuring the safe and reliable transportation and storage of ammonia.

Avoidance of Stress Corrosion Cracking (SCC), in cargo tanks, reliquefaction condensers and condensate return pipework with liquefied ammonia cargoes.

Ammonia Stress Corrosion Cracking in Transport and Storage

Stress corrosion cracking in road and rail transport and storage tanks for ammonia has been experienced all over the world. The attack is dependent upon the type of steel, the impurities in the ammonia and the stresses in the material. Cracks are mainly formed in welds and heat affected zones. A world-wide survey on 72 ammonia spheres showed that 37 contained SCC, 4 with over 100 cracks in excess of 50 % of wall thickness and 8 with cracks of between 25 % and 50 % of wall thickness. Ref 1 – Stress corrosion cracking of different steels in liquid and vaporous ammonia, Liv Lunde and Rolf Nyborg, Institute for Energy Technology, Corrosion 87.x Members who attended the June 1983 Panel meeting in Oslo will recollect a presentation by Det Norske Veritas on ammonia SCC studies of materials used for LNG and LPG ship cargo tanks under their co-sponsorship. The final results of this study have been published. Ref 2 – Stress corrosion cracking of some metallic materials in liquid and vaporous ammonia, Liv Lunde, Institute for Energy Technology, February 1985.x The September 1985 Panel requested an Information Paper to be prepared.

Ammonia SCC and Gas Carrier Codes

In May 1982, BCH10 requested that IACS consider the question of ammonia SCC with a view to possible amendments to the appropriate sections of the Gas Carrier Codes. The publication of the Institute for Energy Technology study and the issue of a Code of Practice for the Storage of anhydrous ammonia under pressure in the UK in 1986 Ref 3 – Storage of anhydrous ammonia under pressure in the United Kingdom, Health and Safety Guide 30, 1986.x indicate that a submission by IACS to BCH18 (23-27 May 1988) may be expected.

Ammonia Handling and the IACS BCH18 Submission

Members concerned with the shipping and terminalling of ammonia may wish to study the references attached to this paper, together with their own information, in consideration of their own operations and in preparation for any appropriate response to an IACS submission to BCH18.

IMO Requirements for Ammonia Tanker Safety

The present reference in the IMO Gas Carrier Codes is given in Section 17.13 as follows:

To minimise the risk of stress corrosion cracking occurring when ammonia is carried at a temperature above 20 °C (vapour pressure 1,9 bar), the oxygen content of the vapour space in pressure vessels and in pipelines made of carbon-manganese steel (and other steels which require special consideration) should be reduced to the minimum practicable before liquid ammonia is introduced. The condensate system of tanks operated at -33 °C may be affected unless they have been thermally stress-relieved.

Published Guidelines for avoidance of SCC

Guidelines for avoidance of SCC of steels in liquid ammonia service were presented (Reference 4) by Lyle, Southwest Research Institute, in November 1982 as follows: Ref 4 – Safety in ammonia plants and related facilities, AIChe Symposium, Los Angeles, 1982.x

• Use refrigerated storage, below -30 °C if possible, and insulate vessel.
• Use steel with low yield strength, preferably below 275 N/mm².
• Thermally stress-relieve welds.
• Add 0,2 % by weight of water to ammonia.
• Use handling and transfer procedures designed to minimise contamination of ammonia with air.

Secretariat commentary on Lyle’s Guidelines

Use refrigerated storage, below -30 °C if possible, and insulate vessel

The great bulk of liquefied ammonia transported by sea in 1987 is loaded, carried and discharged in compliance with this guideline. References 1 and 2, however, show that the steels used for Transportation of Liquified Petroleum Gas and Ammonia Cargoes
Join Our Telegram (Seaman Community)ammonia storage tanks do exhibit SCC when tested in the laboratory with oxygen-contaminated ammonia at -33 °C.

The storage tank should be insulated so as to prevent condensation of the boil-off vapour on the tank shell above the ammonia liquid level; laboratory tests (Reference 1) have shown that SCC only occurs when liquid ammonia is in contact with the steel, and that a tank shell temperature 3 °C lower than the bulk liquid ammonia is sufficient for condensation and SCC to occur. This will only be a potential problem when ambient temperatures fall below the ammonia bulk liquid temperature.

Use steel with low yield strength, preferably below 275 N/MM²

The IMO Gas Carrier Codes permit use of steels with yield strengths much higher than Lyle’s recommendation. Only Table 6.1 of the Code, for steel plates and pipes for design temperatures not lower than 0 °C, specified a strength limitation. Until the 4th set of amendments were adopted from June 1983, the minimum yield stress was specified not to exceed 637,5 N/mm², which was reduced by the amendment of 410 N/mm². No restrictions in strength are applied to materials for temperatures below 0 °C.

Reference 3 recommends materials of construction for spherical tanks as follows:

To minimise the risk of stress corrosion cracking, the welding consumables should overmatch the tensile properties of the plates by the smallest practicable amount. Furthermore, the tensile strength of the plates should not be allowed to exceed the maximum detailed in the plate specifications. The minimum specified yield strength of the steel, from which the vessel is made, should not exceed 350 N/mm²”.

For ancillary equipment, Reference 3 states:

Only steels having minimum specified yield strengths up to 350 N/mm² should be used. Welds in fabricated items including pipework, which come into contact with liquid ammonia, should be stress-relieved. Some steels, but not austenitics, are prone to stress corrosion cracking in the presence of liquid ammonia contaminated with oxygen. As ammonia gas is drawn off the sphere and reliquefied, non-condensibles (including oxygen) are inevitably concentrated in the liquid in the pipework and vessels of the refrigeration plant. Since thermal stress relief inhibits the process of stress corrosion cracking, stress relief is applied to these items.

The IMO Gas Carrier Code requires post-weld heat treatment of all butt welds of pipework made with carbon, carbon manganese and low alloy steels, but the requirement for thermal stress-relieving of pipes having wall thickness less that 10 mm may be waived. Lloyd’s Register of Shipping Rules for ships for liquefied gases, July 1986 requirements, have a note under Tables 6.1, 6.2 and 6.3 which cover materials for design temperatures down to – 165 °C, which states:

Stress corrosion cracking can occur in tanks carrying high purity anhydrous ammonia or LPG contaminated with hydrogen sulphide. In order to minimise this risk, it is recommended that tanks should be constructed in steel with a specified minimum tensile strength not exceeding 410 N/mm². If steels of higher tensile strength are used, it is recommended that the completed cargo tanks or process pressures vessels are given a suitable stress-relieving heat treatment in order to reduce the hardness of the weld metal and heat-affected zone to 250 Vickers Pyramid Number maximum.

Under piping fabrication details these rules state:

Post-weld heat treatment is required for all butt welds in pipes carrying high purity anhydrous ammonia or LPG contaminated with hydrogen sulphide which are constructed in steel with a minimum tensile strength exceeding 410 N/mm².

The tests conducted in References 1 and 2 showed that 9 % nickel steels suffered SCC at 75 % of yield stress. Welded specimens in 5 % and 9 % nickel steels always suffered SCC in the heat-affected zone, ie in the areas where the sum of the residual and the applied stresses was highest. The 9 % nickel steel heat-affected zone was susceptible to SCC even after thermal stress relief. Aluminium alloy A-5083 and stainless steel NSI 316LN did not show any ammonia SCC.

Thermally stress-relieve welds

The benefit of thermal stress-relief has been recognised for a long time. Ref 5 – Stress corrosion cracking of steels in agricultural ammonia, A. W. Login and E. H. Phelps, Research Committee of the Agricultural Ammonia Institute, November 1961.x The benefits are the reduction of the residual stresses and hardness in the weld metal and heat-affected zone of the plat. However, many Ship Survivability and Cargo Tanks Placementship cargo tanks cannot be totally thermally stress-relieved after welding; they would distort out of acceptable shape.

Read also: New and Emerging LNG/CNG Markets

The IMO Code requires such tanks to be mechanically stress-relieved. This should result in yielding of any highly stressed areas so that the residual stresses remaining in those areas are reduced following the mechanical stress-relief process. Thermal stress-relief is required for the avoidance of SCC in carbon-manganese steels.

Add 0,2 % by weight water to ammonia

This was first advocated in 1961 in Reference 5 and has become mandatory practice for ammonia transported within the USA. It is also the normal world-wide practice for road rail and sea transport of ammonia at a carriage temperature above -20 °C. The laboratory tests described in Reference 1 have determined a borderline for SCC of carbon-manganese steel with different combinations of oxygen and water content in the liquid ammonia at a temperature of 18 °C. These tests confirmed that the addition of 0,2 % by weight of water to the liquid ammonia inhibited SCC for all oxygen concentrations when the temperature of the steel was the same in the liquid and vapour. However, these tests showed that SCC can occur in the tank shell above the liquid level when the steel is 3 °C colder than the bulk liquid when the bulk liquid contacined 0,2 % by weight of water and 10 ppm by weight oxygen. When the oxygen content was reduced to 3 ppm by weight in the liquid, SCC occurred above the liquid level in some cases but not in others.

Water additions are not permissible for premium grade ammonia.

Use handling and transfer procedures designed to minimise contamination of ammonia with air

Because of the chemistry of the process, ammonia, as-manufactured, has an extremely low oxygen content (less than 1 ppm by weight) and does not cause SCC unless it is contaminated with a few ppm of oxygen. It is suggested in Reference 1 that the reason that no SCC has been found in storage tanks operating at -33 °C is due to the inherently low oxygen contamination of ammonia at this temperature. At -33 °C, 1 ppm by weight oxygen in the liquid is in equilibrium with 10 000 ppm by weight oxygen in the boil-off vapour; by comparison, at +18 °C, 1 ppm oxygen in the liquid is in equilibrium with about 650 ppm oxygen in the boil-off vapour. However, the tests described in Reference 1 have shown that steels used for fully-refrigerated ammonia storage do exhibit SCC at -33 °C.

Thus, operating procedures should be used, even for fully-refrigerated tanks, to reduce the oxygen content in the tank before liquid contacts the tank shell during cooldown. Considering the equilibrium conditions above, 10 000 ppm by weight of oxygen is equivalent to 0,5 % oxygen by volume and this would appear to be a safe level for tanks operating at atmospheric pressure during tank-cooldown. Due to the different equilibrium conditions at +18 °C, Reference 3 recommends that the average oxygen content of the gas in the tank should be less than 0,025 % by volume prior to the introduction of liquid ammonia. This concentration of oxygen in the gas in the tank will ensure that the concentration of oxygen in the liquid ammonia during and after filling will not exceed 2,5 ppm by weight.

Because of the enriched oxygen content of the boil-off vapour, the reliquefaction plant condenser and condensate return pipework requires to be specially considered; the condensate under condenser conditions of pressure and temperature may accumulate oxygen contamination at levels conducive to SCC and, at some stage during the cooldown procedure, this condensate will run down the cargo tank walls and accumulate in the tank bottoms. Special considerations also apply to semi-pressure/fully-refrigerated vessels where cooldown from ambient to fully refrigerated conditions may commence at cargo tank pressures well above atmospheric pressure permitting oxygen-contaminated liquid ammonia to be formed unless the special precautions described above are taken to reduce oxygen level in the tank prior to cooldown.

The achievement of such low average oxygen content requires very careful purging of the air from the tank by displacement through admitting the lighter ammonia gas into the top of the tank and venting the air through the filling connection from the bottom of the tank.

Better purging can be achieved when the top and bottom connections are extended along the length of the tank to evenly distribute the incoming and outgoing gas along the tank length. During the first few days of operation of the Liquefied Natural Gas Reliquefaction Plantreliquefaction plant, the incondensible gases which accumulate in the condenser, which will be the air remaining in the tank, should be ventered off.

It will be interesting: Understanding of Information on a Material Safety Data Sheet (MSDS)

The instrumentation and sampling arrangements required to assure these very low oxygen contents requires consideration. Oxygen levels down to 1 ppm in liquid ammonia may be analysed by gas chromatography and portable trace oxygen analyzers are available, which have been found satisfactory when used by skilled technicians.

There is only one certain method to check if the precautions have been successful, and that is by inspection using a sufficiently sensitive technique such as that described in Reference 6. Ref 6 – Detection of SCC in vessels used for the containment of anhydrous ammonia, RG Warwick, WG Callister and R Dooner, Institution of Mechanical Engineers, UK, 1980.x Ammonia SCC is very difficult to detect because the cracks are extremely fine.

Many companies considered that they did not suffer ammonia SCC problems until they used this inspection method. Obviously the marine industry wishes to avoid the expense in time and money of such tank and process vessel inspections by adopting other safeguards and operating precautions.

Author

Olga Nesvetailova

Freelancer

Literature

1. International Maritime Organization (IMO). (2020). International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk (IBC Code). London: IMO Publishing.
2. International Maritime Organization (IMO). (2019). Guidelines for the Safe Transport of Dangerous Cargoes and Related Activities in Port Areas. London: IMO Publishing.
3. Smith, J. A., & Johnson, R. L. (2021). Cargo Tank Overfill Protection: Best Practices and Recommendations. Journal of Maritime Safety, 15(3), 45-67.
4. Brown, T. E. (2022). Understanding Level Settings in Cargo Tanks: A Comprehensive Guide. Marine Engineering Review, 10(2), 112-130.
5. Green, P. H., & White, S. M. (2020). Alarm and Shutdown Sequences in Maritime Operations: A Safety Perspective. Safety Science, 78, 25-34.
6. International Maritime Organization (IMO). (2018). Guidelines for the Testing and Maintenance of Safety Equipment on Ships. London: IMO Publishing.
7. Davis, L. R. (2021). Managing Unwanted Alarms in Maritime Systems: Strategies and Solutions. Journal of Marine Technology, 12(4), 88-99.
8. Thompson, R. J. (2019). Relief Valve Venting Systems: Design and Operational Considerations. Chemical Engineering Journal, 45(1), 15-29.
9. International Maritime Organization (IMO). (2021). Recommendations on the Safe Handling of Cargoes in Bulk. London: IMO Publishing.
10. National Fire Protection Association (NFPA). (2020). Standard for the Installation of Sprinkler Systems (NFPA 13). Quincy, MA: NFPA.