Low temperature valves for liquefied natural gas applications

Characteristics of the shipping industry
The shipyard is very simple. Therefore, there is very little space on board. The operator must reduce the volume to keep the gas tank as small as possible. They make it this through LNG (liquefied natural gas). The natural gas is turned into a liquid by cooling. The main isolation valve must still operate at -165 °C.

What will influence valve design?
Temperature has an important influence on the design of the valve. For example, users may need it for use in tropical environments such as the Middle East. Or it may be suitable for cold environments like the polar ocean. Both environments affect the seal and durability of the valve. The components of these valves include the valve body, bonnet, valve stem, stem seal, valve ball and seat. These components expand and contract at different temperatures due to differences in material composition. Users may need valves for use in tropical environments such as the Middle East. Or, it may need to be applied to a cold environment like the polar ocean.

Low temperature application option
Option 1:
Operators use valves in cold environments, such as oil rigs in polar seas.
Option 2:
The operator uses valves to manage fluids at temperatures well below freezing.

Low temperature valves for liquefied natural gas applications

In the case of highly flammable gases, such as natural gas or oxygen, the valve must also operate correctly in the event of a fire.

There is a build up of pressure in the normal handling of the cryogen. This is due to an increase in heat in the environment and subsequent steam formation. Special care must be taken when designing the valve/pipe system. This allows pressure to accumulate.

Dramatic temperature changes can affect the safety of workers and factories. Each component of the cryo-valve expands and contracts at a different rate due to the different material compositions and the length of time they are subjected to the cryogen.

Another big problem when handling refrigerants is the increase in heat from the surrounding environment. This increase in heat is the reason for manufacturers to isolate valves and pipes.

In addition to the high temperature range, valves must also deal with considerable challenges. For liquefied hydrazine, the temperature of the liquefied gas is lowered to -270 °C.

Conversely, if the temperature drops to absolute zero, the valve function becomes very challenging. A cryogenic valve connects the tubing and the liquid gas to the environment. If it does this at ambient temperature, the result may be a temperature difference of up to 300 °C between the pipeline and the environment.

The temperature difference creates a heat flow from the warm zone to the cold zone. It can damage the normal function of the valve. It also reduces the efficiency of the system in extreme cases. This is of particular concern if the ice forms at the warm end.

However, in low temperature applications, this passive heating process is also deliberately used. This process is used to seal the valve stem. Typically, the stem is sealed with plastic. These materials cannot withstand low temperatures, but the high-performance metal seals of the two components, which move a lot in the opposite direction, are very expensive and almost impossible.

There is a very simple solution for this problem. Bring the plastic used to seal the stem to a relatively normal temperature zone. This means that the sealant of the valve stem must be kept at a distance from the fluid.

The hood is like a tube. If the fluid rises from this pipe, it will warm from the outside. When the fluid reaches the stem sealant, it is primarily at ambient temperature and is gaseous. The hood also prevents the handle from freezing and malfunctioning. 

Choose a valve for cryogenic service
Selecting valves for low temperature applications can be very complicated. Buyers must consider the conditions on board and at the factory. Moreover, the specific properties of cryogenic fluids require specific valve performance. Proper selection ensures plant reliability, equipment protection and operational safety. The global LNG market uses two main valve designs.

Low temperature valves for liquefied natural gas applications

Valve design meets the global LNG market

Three offsets rotating tight isolating valve
These offsets allow the valve to open and close. The friction and friction are small during operation. It also uses stem torque to make the valve more sealed. One of the challenges of LNG storage is the trapped cavity. In these cavities, the liquid can expand explosively more than 600 times. Three rotating tight isolating valves eliminate this challenge.

Single and double flap check valves
These valves are a key component in liquefaction equipment because they prevent damage from flow reversal. Materials and dimensions are important considerations because cryogenic valves are expensive. The result of an incorrect valve can be harmful.

How do engineers ensure the tightness of cryogenic valves?
Leakage is very expensive when one considers the cost of first making a gas into a refrigerant. This is also very dangerous.

A big problem with cryogenic technology is the possibility of valve seat leakage. Buyers often underestimate the radial and linear growth of the stem associated with the body. If the buyer chooses the right valve, they can avoid the above problem.

I recommend using a cryogenic valve made of stainless steel. This material responds well to temperature gradients during operation with liquefied gases. The cryogenic valve should be of a suitable sealing material with a seal of up to 100 bar.

In addition, the extended bonnet is a very important feature because it determines the tightness of the stem sealant.
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