Is it prohibited to use gate valves in oxygen pipelines?

In recent years, with the increase in oxygen consumption, large oxygen consumers have adopted oxygen pipeline transportation. Due to the long pipeline and wide distribution, coupled with the sudden opening or closing of valves, combustion and explosion accidents of oxygen pipelines and valves occur from time to time. Therefore, it is necessary to comprehensively analyze the hidden dangers and dangers of oxygen pipelines and cold doors, and take corresponding measures. is crucial.

In accordance with the provisions on valve materials in “GB 16912-1997 Oxygen and Related Gas Safety Technical Regulations”:

When the pressure is greater than 0.1MPa, it is strictly prohibited to use the gate valve.

When 0.1MPa<p≤0.6mpa, the valve disc is made of stainless steel.

When 0.6MPa<p≤10mpa, all stainless steel or all copper-based alloy valves.

When P>10MPa, all copper-based alloy.

Analysis of the causes of combustion and explosions of several common oxygen pipelines and valves:

1. Rust, dust, and welding slag in the pipeline rub against the inner wall of the pipeline or the valve port to generate high temperatures and burn.

This situation is related to the type, particle size and air flow speed of impurities. Iron powder is easy to burn with oxygen, and the finer the particle size, the lower the ignition point; the faster the gas speed, the easier it is to burn.

2. There are substances with low ignition points such as grease and rubber in pipelines or valves, which can ignite under local high temperatures.

The ignition points of several combustibles in oxygen (under normal pressure):

3. The high temperature generated by adiabatic compression causes combustible materials to burn.

For example, the pressure in front of the valve is 15MPa, the temperature is 20℃, and the pressure behind the valve is 0.1MPa. If the valve is opened quickly, the oxygen temperature behind the valve can reach 553℃ according to the adiabatic compression formula, which has reached or exceeded the ignition point of some substances.

4. The lower ignition point of combustibles in high-pressure pure oxygen is the cause of combustion of oxygen pipeline valves.

ygen pipelines and valves are very dangerous in high-pressure pure oxygen. Tests have proven that the detonation energy of fire is inversely proportional to the square of the pressure. These pose a great threat to oxygen pipelines and valves.

Precautions:

① To prevent fire, a copper-based alloy or stainless steel pipe 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.​

② Oxygen pipelines should have as few elbows and branches as possible. Oxygen pipeline elbows with a working pressure higher than 0.1MPa should be made of stamped valve-type flanges. The air flow direction of the branch head should be at an angle of 45° to 60° with the air flow direction of the main pipe.​

③ In the butt welding concave and convex flange, copper welding wire is used as the O-ring, which is a reliable sealing form for oxygen flanges with flame resistance.​

④ Oxygen pipelines should have good conductive devices, the grounding resistance should be less than 10Ω, and the resistance between flanges should be less than 0.03Ω.​

⑤ A diffuser pipe should be added to the end of the main oxygen pipe in the workshop to facilitate the purging and replacement of the oxygen pipe.

Before entering the workshop regulating valve, a filter should be installed.

Installation Precautions:

① All parts in contact with oxygen must be strictly degreased and blown clean with oil-free dry air or nitrogen after degreasing.

②Welding should use argon arc welding or arc welding.

Operation precautions:

① When opening and closing the oxygen valve, it should be done 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 for leaks and pressure.

③ Implement the operation ticket system and provide more detailed explanations and regulations on the purpose, methods, and conditions of the operation in advance.

④ Manual oxygen valves with a diameter greater than 70mm are only allowed to operate when the pressure difference between the front and rear of the valve is reduced to within 0.3MPa.

Maintenance precautions:

① Oxygen pipelines should be inspected and maintained regularly, derusted and painted every 3 to 5 years.

② The safety valves and pressure gauges on the pipeline must be calibrated regularly, once a year.

③Improve the grounding device.

④ Before hot work, replacement and purging should be carried out. The oxygen content in the blown gas is qualified when it is 18% to 23%.

⑤ The selection of valves, flanges, gaskets, pipes, and pipe 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, inspection and maintenance personnel.

Other security measures:

① Improve the attention paid to safety by construction, maintenance and operating personnel.

② Improve the vigilance of managers.

③Improve the level of science and technology.

④Continuously improve the oxygen delivery plan.

Article summary:

The real reason for the ban on gate valves is actually that the sealing surface of the gate valve will cause scratches and damage due to friction during relative movement (i.e. the opening and closing of the valve). Once damaged, “iron powder” will fall off from the sealing surface, and such fine particles will Iron powder can easily catch fire and burn, which is the real danger.

In fact, gate valves are prohibited on oxygen pipelines. Accidents can also occur with other stop valves. The sealing surface of the stop valve will also be damaged and danger may occur. The experience of many companies is that all oxygen pipelines use copper-based alloy valves instead of carbon steel. , stainless steel valves.

Copper-based alloy valves have the advantages of high mechanical strength, wear resistance, and good safety (no static electricity), so the real reason is that the sealing surface of the gate valve is easily worn and the iron filings produced are the culprit. As for the sealing performance Whether it goes down or not is not the key.

In fact, explosion accidents occur in many oxygen pipelines that do not use gate valves. They usually occur when the pressure difference on both sides of the valve is large and the valve opens quickly. Many accidents have also shown that ignition sources and combustibles are the final cause. Disabling the gate valve is just a means to control combustibles. It has the same purpose as regular rust removal, degreasing, oil ban, etc. As for controlling the flow rate and doing static grounding, it is to eliminate the source of ignition.

Personally, I think the valve material is the first factor. Similar problems also occur in hydrogen pipelines. The new regulations have removed the words “forbidden gate valves”, which is a clear proof. The key is to find the reason. In fact, many companies do not care about the operating pressure at all. Copper-based alloy valves are forced to be used, but explosion accidents still occur. Therefore, controlling the source of fire and combustibles, maintaining them carefully, and tightening the safety string are the most critical.