Cryogenic valves for LNG application – Part 2
In our second part of the cryogenic valve series I would like to responsive on questions like “What are the challenges of using cryogenic?”, “Selecting a valve for cryogenic service.”, “How can engineers ensure tightness of cryogenic valves?” and “What should engineers pay attention to during assembly of cryogenic valves?”.
What are the challenges of using cryogenic?
In cases where the gas is highly flammable, like natural gas or oxygen, the valve has to perform correctly also in the case of a fire.
There is the accumulation of pressure in the normal handling of cryogens. This is due to heat gain from the environment and consequent vapor formation. There needs to be a special consideration in designing the valve/piping system. This allows for pressure buildup.
Drastic temperature variations can compromise the safety of the worker and the plant. Each component of the cryogenic valve expands and contracts at different rates due to the different material compositions and the length of time they are subjected to the cryogen.
Another big problem when dealing with cryogens is heat gains from the surroundings. These heat gains are the reason why manufacturers insulate valve and piping.
Apart from the high-temperature range, the valve also has to cope with a considerable amount of challenges. The temperatures of liquefied gases go down to -270°C for liquefied helium.
Conversely, valve function becomes very challenging if the temperature plunges to absolute zero. A cryogenic valve connects the pipeline with the liquid gas to the environment. It does so at ambient temperature. The result can be a temperature difference of up to 300°C between the pipeline and the environment.
The temperature difference generates a heat flow from the warm to the cold area. It impairs the correct functioning of the valve. It also decreases the efficiency of the system under extreme circumstances. This is of particular concern if ice forms at the warm end.
But in cryogenic application, this process of passive heating is also used deliberately. This process is used in sealing the valve stem. Normally the valve stem is sealed with plastics. These materials cannot withstand the low temperature, but a high-performance metallic sealing of two parts, which are moving a lot in opposite directions, is just very expensive and nearly impossible.
There is a very simple solution for this problem. You bring the plastics for sealing of the stem to an area with relative normal temperatures. That means the sealant of the stem has to be in a distance to the fluid.
The bonnet is simply like a pipe. If the fluid is rising up this pipe it gets warmed form the outside temperature. When the fluid reaches the stem sealant, it is mostly at ambient temperature and gaseous. The bonnet also prevents that the handle gets frozen and is unable to be actuated.
Selecting a valve for cryogenic service
Choosing valves for cryogenic applications can be quite complicated. Buyers have to consider the conditions aboard ships and in plants. Also, the specific nature of low-temperature cryogenic fluids demands specific valve performance. Proper selection ensures plant reliability, protection of equipment, and operational safety. The global LNG market uses two main valve designs.
Valve designs acc. to the global LNG market
Triple offset rotary tight isolation valves
These offsets allow the valve to operate open and closed. There is minimal rubbing and friction in their operation. It also uses stem torque to make the valve more sealable. One of the challenges of LNG storage is trapped cavities. In these cavities, the liquid can expand over 600 times explosively. The triple rotary tight isolation valve eliminates this challenge.
Single and dual flapper check valves
These valves are critical components in liquefaction plants because they prevent damage from flow reversal. Material and sizing are important considerations because cryogenic valves are expensive. The results of an incorrect valve can be detrimental.
How can engineers ensure tightness of cryogenic valves?
When one contemplates the cost of making a gas into a cryogen in the first place, leakage is very expensive. It is also dangerous.
One big problem with cryogenics is the potential for seat leakage. Buyers often underestimate the radial and linear growth of the stem as it relates to the body. If buyers select the correct valve, they can avoid the problems mentioned above.
I recommend the use of cryogenic valves made of stainless-steel material. This material copes well with the temperature gradient during operation with liquefied gases. The cryogenic valves should have the right sealing materials, for a high tightness up to 100 bars.
Also the extended bonnet is a very important feature, as it determines the tightness of the stem sealant.
Keep in mind that suppliers test cryogenic valves down to -196°Celsius. These tests are in keeping with special international standards.
Testing of cryogenic valves
What should engineers pay attention to during assembly of cryogenic valves?
The cleanliness of the valve is very important in cryogenic application. Grease or lubricants get very hard at low temperatures. If particles lay down in sealing areas, like between the ball and the seats of a ball valve, there is no flat contact and sealant gets leaky. Therefore the valve has to be cleaned and degreased from the manufacturer’s side.
But also the buyer has a responsibility for the cleanliness of the valve. He has to ensure that the valve is assembled with clean tools, also all connections and piping parts have to be free of pollution of any kind.