How It Works: Cryogenic Valves
Published: 01/09/2015
How It Works: Cryogenic Valves
Published: 01/09/2015
Understanding the cryogenic temperature range and other cold temperatures is important to valve selection.
The oil and gas industry generally accepts that the cryogenic temperature range is –238 degF [–150 degC] and below. Certain gases are considered cryogenic because compressing their volume requires more than just increasing pressure. Temperatures between –238 and 37 degF [–150 and 3 degC] are considered refrigeration.
Transportation of cryogenic media
Cryogen leakage is not only dangerous but also expensive, considering the cost to make a gas into a cryogen in the first place. The components of any valve will contract and expand at different rates because of different material compositions or the time that the valve has been exposed to cryogen.
Because heat gains from the environment are a constant battle when dealing with cryogens, valve and piping insulation are mandatory. Gas processing involves handling the physical properties of gases, such as LNG, nitrogen, oxygen, argon, and helium. These liquids are not stable at atmospheric pressure and, if allowed, can violently and instantaneously transform into the gas phase.
Pressure buildup is normal during cryogen processing because of this environmental heat gain and subsequent vapor formation. Special consideration should be given in the valve and pipe system design to accommodate the pressure buildup.
Other challenges with cryogenic services include seat leakage potential. The linear and radial growth of the stem in relation to the body often can be underestimated. Selecting the correct valve can help avoid these issues
Selecting cryogenic valves
While no strict guidelines are in place for valve type in cryogenic service, there is a trend to select quarter-turn ball valves for their tight-shutoff capabilities. This quarter-turn motion is also found in high-performance and triple offset butterfly valves. For larger valve sizes, gate valves are often used instead of ball valves because ball valves can be more costly. Ball valves can be more costly upfront but save costs long term through enhanced valve performance and longevity. Flange and welded configurations are used mostly for cryogenic piping connections, although customer preference typically determines which type is used.
Valves intended for cryogenic service should be assembled in a clean room and be lubricant free or utilize lubricants compatible with cold service. The valves should not have machine oil, grease, dirt, or any foreign material in the valve for the clean service. Any unnecessary matter can inhibit the valve performance at cryogenic temperatures. When buying a valve for cryogenic service, the operator should request documented procedures for cleaning to verify that the manufacturer followed proper procedures during the assembly process. Cameron rigorously prepares valve components for clean-room assembly and takes the precautions to keep components contaminant free to the standards required by the industry.
