Cryogenic valve testing

摘要

Are we actually testing or just complying with written standards? With LNG (liquefied natural gas) becoming popular as a clean fuel, there have been tremendous investments in the LNG plants and huge investments are expected in the future too. Of course, valves are always at the heart of every plant's operating and isolation philosophy and especially so when the discussion is about LNG, which is highly flammable. Curious about the applicability of cryogenic valve testing standards, the authors have spent a great deal of time researching this subject and contacting industry experts for their opinions and experiences. The shortcomings of the current standards are best illustrated in an incident related by Ewald Soller, Team Leader Mechanical/ Valves at TGE Gas Engineering GmbH: "Ball valves were purchased for an LNG terminal project and successfully passed a witnessed factory acceptance testing (FAT) to BS6364. In the field the required leakage rates could not be achieved once installed. Trouble shooting led to the test method itself. The test method required by BS6364 cools from the outside in whereas under real conditions the process cools from the inside out. Therefore during the test the body is contracting around the trim where in the field the trim contracts first. As a result some types of valves can seal during the test but cannot replicate their results in the field. In this case the standards allowed a valve to pass FAT that could not seal in the field. Although additional insulation was added and extra stabilization time allowed to try to cool the trim adequately, eventually it was necessary to replace the position seated ball valves with torque seated ball valves in order to get the required shutoff. This led to changing the internal requirements for ball valves above and beyond meeting BS6364 to ensure the required results were achieved."