Knowledge of Forged Steel Cryogenic Globe Valves

Knowledge of Forged Steel Cryogenic Globe Valves

Feb 15, 2021


Material selection of forged steel cryogenic globe valves
Selection of materials for gaskets and packing of forged steel cryogenic globe valves: As the temperature drops, the fluoroplastics shorten greatly, which will result in a decline in the sealing function and cause leakages. Asbestos packing cannot prevent permeability leakages. Rubber has swelling properties to LNG and cannot be used at low temperatures.
 
Selection of valve bonnets of cryogenic globe valves: long neck bonnets should be used. The reasons for choosing a long neck bonnet are as follows:

(1) The long neck bonnet has the function of protecting the stuffing box of the cryogenic valve, because the tightness of the stuffing box is one of the keys to the cryogenic valve. If the stuffing box leaks, it will reduce the cooling effect and cause the liquefied gas to gasify. In the low temperature state, as the temperature decreases, the elasticity of the packing gradually disappears, and leakage proofing performance decreases. The leakage of the medium causes the packing and valve stem to freeze, which affects the normal operation of the valve stem. At the same time, the valve stem moves up and down, and the packing will be scratched, causing serious leakages. Therefore, it is necessary to ensure that the temperature of the filling part is above 8°C.
 
(2) The valve bonnet with long neck structure is convenient to wind the cold insulation material to prevent the cold energy loss of the cryogenic valve.
 
(3) The long neck structure of the cryogenic valve facilitates quick replacement of the main part of the cryogenic valve by removing the valve bonnet. Since the process pipelines and valves in the cold section of the equipment are often installed in the cold box, the long neck valve bonnet can extend through the wall of the cold box. When the main part of the cryogenic valve needs to be replaced, only the valve bonnet needs to be disassembled instead of the valve body. The valve body and pipeline are welded into one body, which reduces the leakage of the cold box as much as possible and ensures the sealing of the cryogenic valve.
 
Material selection of valve stems and fasteners of forged steel cryogenic globe valves: When the temperature is higher than -100°C, the stem and bolt material should be Ni, Cr-Mo and other alloy steel. After proper heat treatment, the tensile strength is improved and thread scratches are prevented. When the temperature is lower than -100℃, austenitic stainless acid-resistant steel should be selected.
 
The forged steel globe valve made from acid-resistant steel has poor hardness, which will cause the valve stem and packing to scratch each other, resulting in leakages at the packing. Therefore, the surface of the valve stem must be plated with hard chromium (The thickness of the coating is 0.04 to 0.06mm), or nitriding and nickel-phosphorus plating treatment is necessary to improve the surface hardness. In order to prevent the nut and bolt from being stuck, Mo steel or Ni steel is generally chosen for the nut. At the same time, coat the surface of the thread with molybdenum disulfide.
 
Features of forged steel cryogenic globe valve
A forged steel cryogenic globe valve is suitable for the management system of cryogenic liquid storage and transportation equipment. It has the characteristics of flexible opening and closing as well as reliable sealing. It can also be used for other low temperature and cryogenic media management systems. Normal temperature globe valves refer to valves whose media have a operating temperature of between -40℃ and +500℃, and the material for the valve body is also different. Cryogenic globe valves mainly refer to globe valves in the low temperature liquid medium system whose medium operating temperature is between -196℃ and -150℃.
 
Low temperature valves, especially ultra-low temperature valves have extremely low operating temperatures. When this type of valve is planned, in addition to following general valve planning guidelines, there are also some special requirements. Cryogenic valves have the following requirements based on operating conditions:

1. Cryogenic valves should have the ability to operate for a long time with low temperature media and ambient temperature.

2. Cryogenic valves should not become a significant heat source for the cryogenic system. This is because the inflow of heat not only reduces the thermal efficiency, but it will cause the internal fluid to evaporate rapidly if the inflow is too much, resulting in an abnormal pressure rise, posing a risk.

3. The low temperature medium should not have harmful effects on the operation of the hand wheel and the seal function of the packing.

4. The valve components which are directly in contact with the cryogenic medium should have explosion-proof and fire-proof structure.

5. The valve part that operates at low temperatures are not smooth, so structural measures are required to avoid scratches by friction parts.
 
 
 

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