Technical Articles
Design and Manufacturing Standards of Cryogenic Valves Wall Thickness, Valve Seat and Anti-static
Posted: 04/19/2017 16:39:21  Hits: 32
Cryogenic valves refer to valves applicable to the medium temperature between -40 ℃ and -196 ℃. The cryogenic valve includes the cryogenic ball valve, cryogenic gate valve, cryogenic globe valve, cryogenic safety valve, cryogenic check valve, cryogenic butterfly valve, cryogenic needle valve, cryogenic throttle valve, cryogenic pressure reducing valve, etc. It is mainly used for ethylene and liquefied natural gas (LNG) plant, liquefied petroleum gas(LPG) storage tank, satellite station, aircraft equipment, petrochemical tail gas separation equipment, liquid oxygen, liquid nitrogen, liquid argon, carbon dioxide cryogenic storage tank and tanker, pressure swing adsorption(PSA) oxygen-making devices, etc. The output liquid cryogenic medium such as ethylene, liquid oxygen, liquid hydrogen, liquefied natural gas, liquefied petroleum products, not only are flammable and explosive, but also will gasify when the temperature raise and their volume will become hundreds of times larger during the process of gasification. The application of cryogenic valves will help to control the temperature and prevent the explosion, leakage and other hidden dangers.
 
The commonly used cryogenic valves are: cryogenic gate valve, cryogenic globe valve, cryogenic check valve, cryogenic ball valve, cryogenic butterfly valve, etc. The central cavity of cryogenic gate valve and cryogenic ball valve is between the valve plate and the ball and both central cavities are with "pressure hole". All cryogenic valves are sealed in one way and the medium flow direction is cast or marked on the valve bodies.

1.Minimum wall thickness: The wall thickness in ASMEB16.34 standard is not acceptable for the "minimum thickness" of cryogenic valves' bodies, bonnets and other shell parts. The minimum wall thickness of the gate valves shall not be less than the specification of  API 600, globe valves shall not be less than the specification of BS1873 and check valves shall not be less than the minimum wall thickness specified in BS 1868 standard. The diameter of valve rod should meet the requirements of API 600 or BS 1873.

2. Valve seat: The cryogenic valves sealing pairs can be designed to metal-PTFE soft seal or metal-metal hard seal according to the working temperature and nominal pressure of the medium. However, PTFE is only suitable for the condition when the working temperature of the medium is higher than -73 ℃ since PTFE will become brittle at too low temperature. Except this, PTFE can not be used when the pressure stage is equal to or exceeds CL1500, because it will produce "cold rheology" if the pressure exceeds CL1500, which will affects the valve sealing. The seat of cryogenic gate valves, globe valves and check valves are made by overlaying Co-Cr-W hard alloy on the valve bodies, making the valve bodies and valve seats become entireties so as to prevent leakage caused by the valve seat deformation at low temperature and ensure the reliability of the sealing between the valve seats and the valve bodies.

 3.Anti-static Design: For flammable and explosive low temperature medium, if valve packing or gasket and seal are both made of insulation materials such as PTFE, it will produce static electricity when the valve opens and closes and static electricity is very terrible for the flammable and explosive low temperature medium. Therefore, the valve should be designed with an anti-static device.

 Material selection of cryogenic valves:
1.Valve body and bonnet:LCB(-46℃), LC3(-101℃), CF8(304)(-196℃).
2.Valve plate: Welded cobalt-based stainless steel hard alloy.
3.Valve seat: Welded cobalt-based stainless steel hard alloy.
4.Valve rod: 0Cr18Ni9.

The standard and structure of cryogenic valve:
1.Design: API6D, JB/T7749.
2.Routine examination and test: According to the API598 standard.
3.Cryogenic examination and test: According to the JB/T7749 standard.
4.Drive mode: Manual, bevel gear drive and electric drive devices.
5.Valve seat structure: Welded structure, welded cobalt-based hard alloy seal surface to guarantee the sealing of valve.
6.Adopt elastic-structured valve plate and design a releasing-pressure hole on the pressure entering side.
7.Mark the flow direction on the valve bodies of one-way sealed valves.
8.Cryogenic ball valve, gate valve, globe valve and butterfly valve adopt the long-neck structure to protect the packings.
9.Standard of cryogenic ball valve: According to JB/T8861-2004 standard.