Cryogenic valves for LNG application – Part 1
One of our customers needed a primary isolation valve for a pressure measurement on a container ship. The ship uses natural gas power. The valve had to meet the Fire Safe API requirements. It also had to pass a cryogenic test with liquid nitrogen (-196 ° C). The container ship rides under the American flag. Thus, an independent examiner had to administer the test. This examiner came from the American Bureau of Shipping (ABS) and the US Coast Guard.
There were many points to be considered in this project. For this reason, I decided to write a series of articles about cryogenic valves in March.
Special features in the shipping industry
Shipbuilders are very minimalist. Thus, space is rarely available on ships. The operators had to reduce the volume to keep the natural gas tanks as small as possible. They did this by liquefying natural gas (LNG, Liquefied Natural Gas). Natural gas becomes liquid by cooling to approx. -165 ° C. At this temperature, the primary isolation valve must still work.
What influences the valve design?
The temperature has a significant influence on the design of a valve. For example, the user might need it for a hot environment like the Middle East Region. Alternatively, it might be for a cold environment like the polar sea. Both environments affect the tightness and durability of a valve. The components of these valves include the body, bonnet, stem, stem seals, ball, and seats. These components expand and contract at different temperature rates because of varying material composition.
Options for an application at low temperature
Option 1:
The operator uses the valve in cold surroundings like an oil rig in the polar sea.
Option 2:
The operator uses the valve to manage a fluid, which has a temperature far below the freezing point.
What is the difference of that 2 options?
The difference between these two options is the temperature of the fluid.
For the first possibility, the fluid is not significant for the working temperature. The temperature of the fluid has just a little influence on the performance of the valve. The working temperature is defined by a cold surrounding. For this kind of application the temperature goes down to -50°C.
For the second possibility, the temperature goes down to -100°C and below. In this kind of application, the fluid determines the working temperature. The fluid is the source of the cold temperature. The cold comes from the inside of the valve, from the fluid and not from the environment.
In this post, I would like to elaborate on the second option. For that application, you need what is known as cryogenic valves.
What are cryogenic valves?
The oil & gas industry considers anything below -238° F (-150° C) to be cryogenic. The industry also considers certain gases “cryogenics.” It takes more than increased pressure to compress the volume of these gases. The industry considers anything above the cryogenic range stated above up to 37° F (3° C) to be “refrigeration.”
Knowing where cryogenic temperatures begin is critical to valve selection. The buyer has to understand many things about freezing temperatures. This knowledge is essential for selecting a valve to work for an extended period.
Where are cryogenic valves used?
Quality and safety are the two critical considerations when choosing a valve. In cryogenic application the valve has to be made out of the right materials. There are a few kinds of materials, which can cope with the very low temperature. These chosen materials are able to withstand the pressure at these temperatures. This applies to the parts of the valve, which are in direct contact with the fluid and the temperature.
Transport and storage
Operators use cryogenic valves in the production of liquefied gases. They also use the valves for the transport and storage of these gases. The gases include liquid nitrogen, argon, oxygen, hydrogen, natural gas, and helium. In the liquefied condition the gases are a lot better to handle.
LNG application
One big part in the use spectrum for cryogenic valves is the LNG application. Transport ships use natural gas for fuel because it produces less pollution than conventional fuels like diesel. LNG is thus the future fuel for ships.
Cryogenic valves for LNG application – Part 2
In the next post I will discuss the following topics:
- What are the challenges of using cryogenic?
- Selecting a valve for cryogenic service.
- How can engineers ensure tightness of cryogenic valves?
- What should engineers pay attention to during assembly of cryogenic valves?
(Image source: © alexyz3d / Fotolia & AS-Schneider)