The Selection of the Valve for the Oxygen Pipeline

A valve is an important device on the oxygen pipeline and it is also a frequent source of accidents. Therefore, the selection of the oxygen valve should be very careful. The dedication characteristics of oxygen valves are as follows: The valve body that is in contact with oxygen should be made from copper alloys or nickel alloys which have good combustion resistance and do not produce sparks during friction and impact. Generally, silicon brass is selected. Stainless steel, cast steel or ductile iron can be used for oxygen pipelines with low pressure. Flame-retardant materials should be used for sealing boxes such as graphitized asbestos, polytetrafluoroethylene and expanded graphite. The valve needs to be strictly degreased and well packaged to prevent pollution and the words "for oxygen only" should be marked. The valve must be subjected to strict and standardized strength tests and air tightness tests to prevent leakages. An oxygen valve with a large diameter is generally equipped with a bypass valve with small average pressure to facilitate operation and ensure safe opening of the valve.
 

1. For oxygen pipelines with working pressure greater than 0.1MPa, gate valves are strictly prohibited. The reason is that rust easily accumulates for the stool and it is not closed well. The rust of the stool is squeezed when the valve is operated, which is easy to form excitation energy. The airflow entrains rust, and particles rub against and impact the pipe wall, which may easily cause a blast. There have been many lessons from such accidents. Moreover, the gate valve with a double disc is easy to lose thallium, which is very unsafe.

 
2. The manual oxygen valve with nominal pressure greater than and equal to 1.0MPa and a nominal diameter greater than and equal to 150 mm should be provided with a bypass, because this kind of valve belongs to an oxygen valve with higher pressure and larger diameter. If there is no bypass valve with small average pressure, the main valve will be opened too violently. The great pressure difference causes the oxygen's pressure on the downstream side of the valve outlet to rise sharply, and there will be friction and temperature increases for the high-speed oxygen flow, forming adiabatic compression, and igniting the oxygen pipeline. If there is a bypass valve with small average pressure, slowly open this small valve to equalize the pressure. The pressure on the downstream side of the outlet will be slowly increased to reduce the pressure difference with the inlet side to avoid excessively fast flow rates, excessive temperature rise, form adiabatic compression, and finally cause ignition. When the pressure difference between the upstream and downstream sides of the main valve is less than and equal to 0.3MPa, open the main valve again to ensure safety. The higher the pressure of the oxygen valve and the larger the diameter are, the more dangerous it is, and you should choose a valve with a small bypass with small average pressure. This is a safety requirement. Europe, the United States, and Japan have already formulated regulations, and China also has related successful experience.
 
3. The selection of materials of oxygen valves (Table 5). We should consider not only strength, economy, but also safety. Table 5 is quoted from the GB 16912 standard.
 
Table 5 The material selection of oxygen valves 

 
The following notes should be paid attention to:
(1) The material used for the pressure or flow regulating valve whose working pressure is above 0.1MPa should be stainless steel, copper alloys or a combination of the two.
(2) The sealing packing of the valve should be made from polytetrafluoroethylene or flexible graphite.
 
Table 5 divides the working pressure into the following 3 kinds:
① Pressure less than and equal to 0.6MPa: The oxygen pressure is low, and the risk is low. Malleable cast iron, ductile iron or cast steel can be used for the valve body and bonnet. Higher requirements are put forward for the valve stem, valve disc and other moving parts, and stainless steel is used. 
② Pressure between 0.6 and 10MPa: the oxygen pressure is high with a wide range, and the application range is wide. All industries are involved. The requirements are high, and the valve is made from stainless steel, copper alloys or a combination of stainless steel and copper alloys (Copper alloy is preferred.), nickel and nickel alloys. Stainless steel does not rust; copper alloys, nickel and nickel alloys do not catch fire when exposed to friction of oxygen. It has good flame retardancy and good safety. The higher the oxygen pressure is, the more copper alloy, nickel and nickel alloy valves should be used. 
③Pressure greater than 10MPa: it belongs to high-pressure oxygen and is very dangerous. The valve material can only be copper alloys, nickel and nickel alloys. For pressure or flow control valves with working pressure greater than 0.1MPa, because their throttling and expansion conditions are impact occasions, the risk is high. Therefore, the valve material should be at least stainless steel, and the combination of stainless steel and copper alloys should be used when the oxygen pressure is relatively high. If the oxygen pressure is high, a copper alloy valve should be used. The sealing packing of the valve should be made from flame-retardant polytetrafluoroethylene or flexible graphite.
 
4. Frequently operated oxygen valves with nominal pressure greater than and equal to 1.0MPa and a nominal diameter greater than and equal to 150 mm are oxygen valves with high pressure and larger diameters. Pneumatic remote control valves should be used to operate remotely to avoid injury in accidents. At the same time, the labor intensity of the operator can be reduced.