Steel with nickel levels from 6 to 9% is able to remain ductile and crack resistant when containing materials of extremely low temperatures, including LNG. This temperature resistance is the result of the fine-grained structure of nickel-ferrite. Heat treatment makes this temperature resistance even more effective and effectively making these alloys very strong. Steel nickel alloys can be created with reduced wall thickness, creating cost-efficient solutions for constructing LNG storage and transportation tanks. Other parameters that play a role in the choice of material is the grain structure of the steel, brittleness, weldability of the steel and plastic deformation.<\/p>\n
It is also essential for LNG tanks to be constructed from materials that have excellent weldability. Nickel alloy levels from 6% to 9% are well known for their weldable properties and traits. Another important issue related to welding is temperature resistance. Welded joints need to maintain the highest levels of toughness and resilience. Otherwise, brittle fracture could result, allowing for the possible vaporization of the LNG. Fortunately, a great deal of research has been done to establish the safest materials for welding LNG tanks. Nickel alloys have withstood temperatures of -196\u00b0C (-320.8\u00b0F) without losing its toughness [1, 2].<\/p>\n
However, for stainless steel to have the best resistance to cracking, there are certain elements that need to be present. A minimum level of delta ferrite (this is a high temperature form of iron, formed on cooling low carbon concentrations in iron-alloy from the liquid state before transforming to austenite) is one. Stainless steel with higher levels of ferrite reduces the impact of low temperatures. When applied with sub-arc flux and covered electrode coatings, the stainless steel maintains a lower amount of micro-slag and higher ductility as a result. Stainless steel with these qualities makes an ideal partner for nickel in the construction of LNG tanks and containers.<\/p>\n
Kobelco of Japan [3] has given a good graph, shown in Figure 1, that shows the applicability of the various steels for cryogenic application.<\/p>\n
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Figure 1. The boiling points of various liquefied gases and applicable metals for storage tanks [3]<\/p>\n
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A wide range of regulations and standards define the design, construction, inspection, and maintenance of LNG tanks made of 9% Ni steel. Some of the relevant ASME, API, BS EN, and JIS standards are provided below.<\/p>\n
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- ASME Sec. VIII, Div. 1: Design and fabrication of pressure vessels; Div. 2: Alternative rules.<\/li>\n
- API Standard 620: Design and construction of large, welded, low-pressure storage tanks; Appendix Q: Low pressure storage tanks for liquefied hydrocarbon gases at temperature not lower than \u2212270\u00b0F (\u2212168\u00b0C).<\/li>\n
- BS EN 14620-1(2006): Design and manufacture of site built, vertical, cylindrical, flat-bottomed steel tanks for the storage of refrigerated, liquefied gases with operating temperatures between 0\u00b0C and \u2212165\u00b0C (\u2212265\u00b0F), Part 1: General.<\/li>\n
- JIS B8265(2010): Construction of pressure vessel \u2015 General principles; JIS B8267(2008): Construction of pressure vessel.<\/li>\n<\/ol>\n
One way of looking [4] at the viability of using lower nickel content steels is to use impact testing by the use of Charpy Impact vs Test Temperature Test. The Charpy Impact Test, also known as the Charpy V-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. Generally, low alloy nickel steels show rapid falloff of the Charpy impact resistance at low temperatures. Since the Ohio incident, rigorous laboratory tests showed that 3.5% Ni steel was not adequate for LNG temperatures.<\/p>\n
Another aspect that needs to be looked at is the steep rise in toughness even at very low temperature, as the nickel content of the alloy is increased to between 8 to 9%. This increased toughness shows that Charpy values well above 15 ft-lbs (20 Nm) minimum required by the ASME Boiler code could be met by these alloys. This search for a tough alloy that could be used at low temperatures and produced at reasonable cost seems to have paid off. This material, 9%Ni steel, eventually came to be the material of choice for LNG storage tanks.
\nSummary of Discussion<\/strong><\/p>\nFor LNG storage tanks, 9%Ni has proven to be a safe material of choice. Remember that the LNG storage tanks have an outer containment concrete shell and inner 9%Ni tank at atmospheric pressure. For LNG piping (no containment provision is provided), however, 36%NiFe is the preferred choice due to the diverse mechanical and physical properties such as low coefficient of expansion. The stress is lowest in 36%NiFe because of its extremely low coefficient of expansion, whilst the stress is high in 9%Ni, its high strength prevents this alloy from yielding because of the thermal strain of contraction.<\/p>\n
For LNG storage tank, the move to 7%Ni is purely due to cost reduction. However, the application of 7%Ni needs to have various approvals from certifying authorities (AMSE, Lloyds Register, ABS, DNV-GL, etc.).<\/p>\n
DNV-GL of Norway has not approved 7% Ni steel for LNG application. However, the IGC code (Table 6.3) provide an opening to use 5% Ni steel for temperatures down to \u2212165\u00b0C or \u2212265\u00b0F (LNG). It will accordingly be possible to also accept 7% Ni Steel on the same basis, i.e., provided impact testing at -196\u00b0C (\u2212320.8\u00b0F) show acceptable results (other tests may also be required based on the material properties and the chemical composition).<\/p>\n
If you are interested in LNG Shipping, LNG Storage, LNG Receiving Facilities and LNG Safety, we suggest attending our G4 (Gas Conditioning and Processing) or G4 with LNG Emphasis courses provided by PetroSkills\/JM Campbell.<\/p>\n
By: Dr. Jay Rajani, Senior PetroSkills Instructor
\nReferences
\n1.\u00a0\u00a0 \u00a0Development of Low-Nickel Steel for LNG Storage Tanks by Hitoshi Furuya et al of Nippon Steel and Sumitomo Metal Corporation.<\/p>\n