HFI-welded steel pipe for cryogenic service
Steel grade X8Ni9 replaces stainless steel in LNG transportation
More than 90 percent of the natural gas produced worldwide is distributed via pipelines. An alternative to this is transportation by sea using LNG carriers. With this mode, the natural gas is liquefied at -163 °C and thus reduced to one 600th of its original volume. This places extreme demands on the pipes used for this purpose, which Salzgitter Mannesmann Line Pipe satisfies with the new steel grade X8Ni9.
To minimize energy requirements during transportation, the pipes must be given a close-to-perfect thermal insulation. The "FW-KAMMER PIPE"(Kammer is the German word for chamber) launched by FW-FERNWÄRME-TECHNIK in 2012 has proved ideal for this application. It is a triple pipe-in-pipe system made up of a fully insulated inner pipe, a chamber pipe and a casing pipe. The chamber is under permanent vacuum, and the inner pipe features a high-grade cold insulation coating. The first product generation had an inner pipe of stainless steel for LNG transportation at -162 °C.
HFI-welded pipes in grade X8Ni9 steel make our FW KAMMER PIPE more economical and more attractive for our customers.Volkwart Harders, Owner of FW FERNWÄRME GmbH
Improvement approach Economic efficiency
To make the product more economical and attractive, Salzgitter Mannesmann Line Pipe developed a new steel grade for this application together with Salzgitter Mannesmann Flachstahl. Valentina Berger from the Technical Customer Service (TCS) department of Salzgitter Mannesmann Line Pipe in Siegen: "We have profited from the experience of our sister company, where high-nickel alloys have been produced for years."
The hot wide strip was processed to pipe of the dimensions 219.1 x 5.0 mm at Salzgitter Mannesmann Line Pipe in Hamm. The specified product properties were achieved by a custom-optimized quench-and-temper treatment.
New grade replaces stainless steel
FW-FERNWÄRME-TECHNIK installed the X8Ni9 pipes as inner pipe in its FW-KAMMER PIPE. For the chamber pipe, material numbers 1.4301/1.4307 were used and for the casing pipe P355NL with a polyethylene corrosion protection coating. The components of FW-KAMMER-PIPE also have different coefficients of thermal expansion, given the different materials and the temperature profile under operating conditions: inner pipe: -196 °C; chamber pipe: -17 °C to +5 °C, and casing pipe: -3 °C to +8 °C.
"If the KAMMER-Pipe were made entirely of 1.4301/1.4307 steel, the pipeline would undergo extreme contraction under operating conditions," explains Project Manager Michael Stille, who was responsible for the test track on the FW FERNWÄRME-TECHNIK premises. "In practice, this would lead to significantly higher costs as we would have to buy compensating elements. The use of dissimilar materials for the pipes has substantially reduced contraction between the chamber pipe and the inner pipe."
The new inner pipes in grade X8Ni9 steel have much higher strength levels than the previously used stainless steel ones. This allows for increased operating pressures despite thinner pipe walls.
Simulation track with 100 measuring points
For test purposes, FW-FERNWÄRME-TECHNIK laid an almost 50 m long simulation pipeline on its premises. This was filled with liquid nitrogen and cooled to an operating temperature of ‑196 °C over several weeks. The entire pipeline – chamber, casing, pipe bends, fixing points, bearings and tangent lengths – was fitted with measuring sensors.
Michael Bick, who cooperated in the development of the quench-and-temper treatment at Salzgitter Mannesmann Line Pipe in Hamm, was more than satisfied with the test results: "At ‑196 °C, the new inner pipes revealed a toughness of over 80 J. The specified level was 40 J. At about 600 MPa, the yield strength was also significantly above the required 490 MPa."
Michael Stiller of FW FERNWÄRME-TECHNIK confirms: "The outcome of the tests is that we can in future substitute HFI-welded pipes in X8Ni9 for the stainless steel pipes previously used on the LNG pipelines. This makes the product more economical and more attractive for our customers."
Yet another good example of how innovative product improvements can lead to more sustainable solutions.