Why Is It Forbidden to Use Gate Valves in Oxygen Pipelines?

According to the regulations of valve materials in GB 16912 Oxygen and Related Gas Safety Technical Regulations, when the pressure is greater than 0.1MPa, it is strictly forbidden to use gate valves. When the pressure is between 0.1 and 0.6MPa, the valve disc should be made from stainless steel. When the pressure is between 0.6 and 10MPa, valves made from all stainless steel or copper-based alloys should be used. When the pressure is greater than 10MPa, valves should be made from all copper-based alloys.
 
In recent years, with the increase in oxygen consumption, most oxygen users have used oxygen pipelines for oxygen transportation. Burning and exploding accidents of oxygen pipelines and valves happen from time to time due to the long pipelines and wide distribution, coupled with the sudden opening or quick closing of the valves. Therefore, comprehensively analyze the hidden danger and danger of the oxygen pipeline and taking corresponding measures is crucial.
 
Cause analysis of combustion and explosion of several common oxygen pipelines and valves

1. The friction between the rust, dust and welding slag in the pipeline and the inner wall of the pipeline or the valve port will cause high temperature and burn. This situation is related to the type, particle size and airflow velocity of impurities. Iron powder and oxygen are easy to combust. The finer the particle size is, the lower the ignition point becomes; the faster the gas velocity is, the easier the combustion becomes.

                 
2. There are low ignition point substances such as grease and rubber in the pipeline or valve, which will ignite under partial high temperature.

The ignition points of several combustibles in oxygen under normal pressure are as the following:

 
3. The high temperature generated by adiabatic compression burns combustibles.
For example, the pressure in front of the valve is 15MPa and the temperature is 20°C; the pressure behind the valve is 0.1MPa. If the valve is opened quickly, the oxygen temperature behind the valve can reach 553°C according to the adiabatic compression formula, which have reached or exceeded the ignition point of some substances.
 
4. The lower ignition point of combustibles in high-pressure pure oxygen is the inducement of the combustion of oxygen pipelines and valves.
Oxygen pipelines and valves are extremely dangerous in high-pressure pure oxygen. Tests have shown that the detonation energy of fire is inversely proportional to the square of the pressure, which poses a great threat to oxygen pipelines and valves.
  
Preventive measures
1. The design should comply with relevant regulations and standards.
The design should comply with the requirements of regulations such as Several Regulations for Oxygen Pipelines of Iron and Steel Enterprises issued by the Ministry of Metallurgy in 1981, Oxygen and Related Gas Safety Technical Regulations (GB16912-1997) and Design Specifications of Oxygen Stations (GB50030-91).

(1) The maximum flow rate of oxygen in carbon steel pipes should meet the following standards: When the pressure is less than or equal to 0.1MPa, the flow rate should be 20m/s. When the pressure is between 0.1 and 0.6MPa, the flow velocity is 13m/s. When the pressure is between 0.6 to 1.6MPa, the flow velocity is 10m/s. When the pressure is between 1.6 to 3.0MPa, the flow velocity is 8m/s.          　
(2) In order to prevent fire, a section of copper-based alloys or stainless steel pipelines with a length of not less than 5 times the pipe diameter and not less than 1.5m should be connected behind the oxygen valve.　　
(3) Oxygen pipelines should have as few elbows and bifurcations as possible. Oxygen pipeline elbows with working pressure higher than 0.1MPa should be stamped. The airflow direction of the bifurcation head should be at an angle of 45° to 60° with the airflow direction of the main pipe.　　
(4) In the butt-welded concave-convex flange, the red copper welding wire is used as the O ring seal, which is a reliable sealing for flame resistance of the oxygen flange.　　
(5) The oxygen pipeline should have a good electrical device. The grounding resistance should be less than 10Ω, and the resistance between the flanges should be less than 0.03Ω.　　
(6) A vent tube should be installed at the end of the main oxygen pipeline in the workshop to facilitate the purging and replacement of the oxygen pipeline. A filter should be installed before the longer oxygen pipeline enters the workshop regulating valve.
 
2. Installation considerations
(1) All parts being in contact with oxygen should be strictly degreased, and after degreasing, use oil-free dry air or nitrogen to blow.
(2) Argon arc welding or arc welding should be used for welding.
 
3. Operation precautions
(1) The oxygen valve should be opened and closed slowly. The operator should stand on the side of the valve and open it all at once.
(2) It is strictly forbidden to use oxygen to blow the pipeline or use oxygen to test leakage and pressure.
(3) The operation purposes, methods and conditions shall be explained and stipulated in detail in advance.
(4) Manual oxygen valves with a diameter greater than 70mm are allowed to be operated when the pressure difference between the front and back of the valve is reduced to within 0.3MPa.
 
4. Precautions for maintenance
(1) The oxygen pipeline should be checked and maintained frequently, rust removed and painted once every 3 to 5 years.
(2) The safety valve and pressure gauge on the pipeline should be checked regularly, once a year.
(3) Perfect the grounding device.
(4) Before flame operation, replacement and purging should be carried out. The oxygen content in the blown gas should be between 18% and 23%.
(5) The selection of valves, flanges, gaskets, pipes and fittings should comply with the relevant regulations of the Oxygen and Related Gas Safety Technical Regulations (GB16912-1997).
(6) Establish technical files, and train operation and maintenance personnel.
 
5. Other safety measures
(1) Make the construction, maintenance and operation personnel to pay attention to safety.
(2) Improve the vigilance of managers.
(3) Improve the level of science and technology.
(4) Continuously improve the oxygen delivery program.
 
Conclusion
The real reason for forbidding the gate valve is actually that the sealing surface of the gate valve will be damaged by friction due to relative movements of sealing surfaces (that is, the opening and closing of the valve). Once the sealing surface is damaged, there will be iron powder falling from the sealing surface. Such tiny iron powder is easy to catch fire, and this is the real danger.
 
In fact, gate valves are forbidden on oxygen pipelines. Other valves like globe valves are also subject to accidents. The sealing surface of the globe valve will also be damaged, and danger may also occur. The experience of many companies is that valves made from copper-based alloys are adopted for oxygen pipelines instead of carbon steel and stainless steel valves.
 
Copper-based alloy valves have the advantages of high mechanical strength, wear resistance and good safety (no static electricity). The real reason is that the sealing surface of the gate valve is very easy to wear, resulting in iron filings. As for decrease in sealing performance is not the problem.

In fact, many oxygen pipelines that do not use gate valves also have explosion accidents, which generally occurs at the moment when the pressure difference between the two sides of the valve is great and the valve is opened quickly. Many accidents have also shown that ignition sources and combustibles are the ultimate cause. Forbidding to use the gate valve is just a means to control combustibles, and the purpose is the same as the regular removal of rust, degreasing and oil prohibition. As for controlling the flow rate and improving static grounding are to eliminate the source of ignition. In my opinion, the material of the valve is the first factor. Similar problems occur on hydrogen pipelines. The new specification has removed the words of prohibiting gate valves, which is a clear proof. The key is to find the reason. Many companies actually do not care about the operating pressure, but adopt copper-based alloy valves. Explosion accidents will also occur. Therefore, controlling the fire source and combustibles, carefully maintaining the pipeline and paying attention to the safety are the most critical.