Cryogenic valves for LNG applications

1. Choose a valve for cryogenic service 

Choosing a valve for cryogenic applications can be very complicated. Buyers must consider the conditions on board and in the factory. Moreover, the specific properties of cryogenic fluids require specific valve performance. Proper selection ensures plant reliability, equipment protection, and safe operation. The global LNG market uses two main valve designs.

The operator must reduce the size to keep the natural gas tank as small as possible. They do this through LNG (liquefied natural gas, liquefied natural gas). By cooling to approximately natural gas becomes liquid. -165 ° C. At this temperature, the main isolation valve must still work

2. What affects valve design?

Temperature has an important influence on the design of the valve. For example, users may need it for popular environments such as the Middle East. Or, it may be suitable for cold environments like polar oceans. Both environments can affect the tightness and durability of the valve. The components of these valves include the valve body, bonnet, stem, stem seal, ball valve and valve seat. Due to the different material composition, these parts expand and contract at different temperatures。

Cryogenic application options

Option 1:

Operators use valves in cold environments, such as oil rigs in polar seas.

Option 2:

Operators use valves to manage fluids that are well below freezing.

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.

3.Pressure

There is a build-up of pressure during normal handling of the refrigerant. This is due to the increased heat of the environment and subsequent steam formation. Special care should be taken when designing the valve / piping system. This allows pressure to build up.

4.Temperature

Rapid temperature changes can affect the safety of workers and factories. Due to the different material composition and the length of time they are subjected to the refrigerant, each component of the cryogenic valve expands and contracts at different rates.

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

In addition to the high temperature range, the valve must also meet considerable challenges. For liquefied helium, the temperature of the liquefied gas drops to -270 ° C.

5.Function

Conversely, if the temperature drops to absolute zero, valve function becomes very challenging. Cryogenic valves connect pipes with liquid gases to the environment. It does this at ambient temperature. The result could be a temperature difference of up to 300 ° C between the pipe and the environment.

6.Efficiency

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

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

7.Sealing

There is a very simple solution to this problem! You bring the plastic used to seal the valve stem to an area where the temperature is relatively normal. This means that the sealant of the valve stem must be kept at a distance from the fluid.

8.Three offset rotary tight isolation valve

These offsets allow the valve to open and close. They have very little friction and friction during operation. It also uses stem torque to make the valve more tight. One of the challenges of LNG storage is trapped cavities. In these cavities, the liquid can swell explosively more than 600 times. The three-rotation tight isolation valve eliminates this challenge.

9.Single and double baffle check valves

These valves are a key component in liquefaction equipment because they prevent damage caused by reverse flow. Material and size are important considerations because cryogenic valves are expensive. The results of incorrect valves can be harmful.

How do engineers ensure the tightness of cryogenic valves?

Leaks are very expensive when one considers the cost of first making the gas into a refrigerant. It's also 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 in relation to the body. If buyers choose the right valve, they can avoid the above problems.

Our company recommends using low temperature valves made of stainless steel. During operation with liquefied gas, the material responds well to temperature gradients. Cryogenic valves should use suitable sealing materials with a tightness of up to 100 bar. In addition, extending the bonnet is a very important feature because it determines the tightness of the stem sealant.


Post time: May-13-2020
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