Technical Articles
How to Choose Valves Used under Low Temperature Conditions?
Posted: 06/16/2017 11:33:18  Hits: 20
In engineering, the common low temperature medium environments can be classified into 3 categories:
1. -50℃-0℃, the main material of the valve is low temperature carbon steel.
2. -70℃--51℃, the main material of the valve is austenitic stainless steel or nickel-based low-temperature steel.
3. -196℃--101℃ (often referred as super-low temperature environment), the main material of the valve is austenitic stainless steel, but the material of nickel-based low-temperature steel can also be chosen under appropriate conditions.
 
When we choose the valves used under low temperature conditions, we should give consideration to the following principles.
 
The types of valves
1. We should first choose valves which will not form the closed valve cavity, such as globe valves, butterfly valves, single seat floating ball valves. It does not mean that valves which can form the closed valve cavity should be definitely refused to be used. When they are adopted, appropriate measures must be taken to relieve the pressure in the valve cavity.
 
2. We prefer to select valves with closed parts which have good thermal expansion and contraction compensation. The thermal expansion and contraction compensation capacity of commonly used shut-off valves from high to low is: globe valves, gate valves, ball valves, mechanically balanced plug valves, metal sealed butterfly valves.

3. When non-metallic sealed valves are used, attention should be paid to the temperature limit of the non-metallic materials used under the low temperature conditions. We can consult the valve manufacturers about the specific operation temperature range of the materials.

4. Valves which are sealed with the help of sealant can not be chosen.
 
The structures of valves
1. We should give priority to valves with the welded connection structure, to reduce the difficulty of cold and heat insulation. When the heat treatment after welding is required, the effect of the heat treatment on the valve seat's or valve clack's sealing performance should be evaluated for valves with DN40 and below DN40. If necessary, the short pipe connection structure or the flange connection structure should be adopted.

2. Lengthen the valve bonnet or stem, if it is necessary.

3. Valves which will form the closed valve cavity should be provided with the self-release devices and the flowing direction marks.

4. For valves with the media whose temperatures are between 0℃ and 50℃, there is no need to lengthen the valve bonnet or stem under normal circumstances. However, for valves which have 90 degree rotary switches, the possibility of collisions between the hand wheels or handles and the thermal or cold insulation materials should be evaluated and if necessary, a request for lengthening the valve bonnets or stems should be made. The lengthening length should be decided based on the principle that the collisions will not happen between the hand wheels or handles and the thermal or cold insulation materials.

5. Under the conditions with a combination of low temperature and flammable media, the non-metallic sealed valves should have the fire safety and the anti-static structures.
 
The materials of valves
1. For valves' pressure-bearing parts, the low temperature shock test shall be carried out in accordance with ASME B31.3 or GB/T 20801.

2. Under low temperature conditions, the impact toughness of the material is sensitive to manufacturing defects. Therefore, the surface defects, internal defects, non-metallic inclusions and other aspects of the materials of valves' pressure-bearing parts must be appropriately controlled.   

3. The materials of the valve bolts' should meet the manufacturing standard of low temperature materials and should have the shock test in accordance with  ASMEB31.3 or GB/T20801.

4. The adaptability of the valve internal parts' materials, the valve bonnets' gaskets and stem packing to the low temperature conditions should be evaluated and determined.

5. For the main materials of valves, the internal parts' materials (including possibly used non-metallic sealing rings), valve bonnets' gasket, stem packing, etc., they not only have to meet the requirements of low temperature working conditions, but also should be able to adapt to the high temperature conditions. For example, when the cryogenic pipe needs to be purged with steam, the above materials should be able to accommodate the temperature conditions of the purge vapor.