Over the years, with the increase in the amount of oxygen, large oxygen users have used oxygen pipelines. Due to the long length of the pipeline, the distribution is wide, plus the quick opening or closing valve
According to the "GB 16912-1997 Oxygen and Related Gas Safety Technical Regulations", the provisions on valve materials:

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

0.1MPa
All stainless steel or all copper base alloy valves when 0.6MPa
When P>10MPa, all copper base alloy.

In recent years, with the increase in the amount of oxygen, large oxygen users have used oxygen pipelines. Due to the long length and wide distribution of the pipeline, coupled with the rapid opening or closing of the valve, the accident of burning and exploding of the oxygen pipeline and the valve occurs. Therefore, comprehensive analysis of the hidden dangers and dangers of the oxygen pipeline and the unpopular valve and corresponding measures are taken. is crucial.

I. Analysis of the causes of combustion and explosion of several common oxygen pipelines and valves

1. The rust, dust, and welding slag in the pipe are burned by the high temperature of the inner wall of the pipe or the valve port.

This situation is related to the type of impurities, particle size and gas flow rate. Iron powder is easy to burn with oxygen, and the finer the particle size, the lower the ignition point; the faster the gas velocity, the more likely the combustion occurs.

2. There are substances in the pipeline or valves with low-flammability points such as grease and rubber, which ignite at local high temperature.

The ignition point of several combustibles in oxygen (at atmospheric pressure):

Combustible name Burning point (°C)

Lubricating oil 273～305

Steel paper pad 304  

 Rubber 130～170

Fluororubber 474  

Trichloroethane 392

PTFE 507
                        
3.  The high temperature generated by adiabatic compression causes combustibles to burn
For example, the front of the valve is 15MPa, the temperature is 20 °C , and the valve is 0.1MPa after normal pressure . If the valve is quickly opened, the oxygen temperature after the valve is calculated to be 553 °C according to the adiabatic compression formula , which has reached or exceeded the ignition point of certain substances.

4. The reduction of the ignition point of combustibles in high pressure pure oxygen is the cause of the combustion of oxygen pipeline valves.
Oxygen pipelines and valves are very dangerous in high-pressure pure oxygen. Tests have shown that ignition detonation can be inversely proportional to the square of pressure, which poses a great threat to oxygen pipelines and valves.

Second, preventive measures

1. Design should comply with relevant regulations and standards
The design shall comply with the “Regulations on the Oxygen Pipe Network of Iron and Steel Enterprises” issued by the Ministry of Metallurgy in 1981, as well as the “Technical Regulations for the Safety of Oxygen and Related Gases” (GB16912-1997) and the Design Code for Oxygen Stations (GB50030-91). Requirements.
( 1) The maximum flow rate of oxygen in carbon steel pipes shall comply with the following table.
Maximum flow rate of oxygen in carbon steel pipes:
Working pressure ( MPa) ≤0.1 0.1～0.6 0.6～1.6 1.6～3.0 flow rate ( m/s) 20 13 10 8
( 2) In order to prevent fire, after the oxygen valve, a pipe of copper-based alloy or stainless steel with a length of not less than 5 pipe diameter and not less than 1.5 m shall be connected.
( 3) Oxygen pipelines should be as small as possible with elbows and splitting heads. Oxygen pipeline elbows with working pressure higher than 0.1MPa should be made by stamping into valve flanges. The direction of the airflow of the split head should be at an angle of 45° to 60° to the direction of the main airflow.
( 4) In the butt-weld flange, the copper wire is used as the O-ring, which is a reliable flame-resistant sealing form for oxygen.
( 5) The oxygen pipeline should have a good electrical conductivity, the grounding resistance should be less than 10Ω, and the resistance between the flanges should be less than 0.03Ω.
( 6) The end of the main oxygen pipeline in the workshop shall be provided with a discharge pipe to facilitate the purging and replacement of the oxygen pipeline. Before the longer oxygen pipeline enters the workshop regulating valve , a filter shall be provided.

2. Installation precautions
( 1) All parts in contact with oxygen should be strictly degreased, and then degreased and then blown off with dry air or nitrogen without oil. ( 2) Welding should be argon arc welding or arc welding.

3. Operational precautions
( 1) When switching the oxygen valve, it should be carried out slowly. The operator should stand on the side of the valve and open it once in place. ( 2) It is strictly forbidden to blow the pipeline with oxygen or test leaks and pressure with oxygen. ( 3) Implement the operating ticket system and make detailed explanations and regulations on the purpose, method and conditions of the operation in advance. ( 4) A manual oxygen valve with a diameter greater than 70 mm is allowed to operate when the pressure difference between the valve and the front is reduced to within 0.3 MPa.

4. Maintenance precautions
(1) Oxygen pipelines should be inspected and maintained frequently, and rust-painted and painted every 3 to 5 years. ( 2) The safety valve and pressure gauge on the pipeline shall be checked regularly, once a year. ( 3) Improve the grounding device. ( 4) Before the hot work, it should be replaced, purged, and the oxygen content in the gas is 18% to 23%. ( 5) The selection of valves, flanges, gaskets and pipes and fittings shall comply with the relevant provisions of the Technical Regulations for the Safety of Oxygen and Related Gases (GB16912-1997). ( 6) Establish technical files, training operations, maintenance, and maintenance personnel.

5. Other security measures
( 1) Improve the emphasis on safety of construction, maintenance and operation personnel. ( 2) Improve the vigilance of managers. ( 3) Improve the level of science and technology. ( 4) Continuously improve the oxygen supply program.
to sum up:
The reason why the gate valve is actually disabled is because the sealing surface of the gate valve is damaged by friction in the relative movement (ie, the switch of the valve). Once damaged, there is "iron powder" falling off from the sealing surface, so that the fine particles The iron powder is easy to burn, which is the real danger.
In fact, the gate valve is disabled on the oxygen pipeline, and other shut-off valves have the same accident. The sealing surface of the shut-off valve will be damaged. It is equally dangerous. Many companies have experience that all oxygen pipelines use copper-based alloy valves instead of carbon steel. Stainless steel valve.

Copper-based alloy valves have the advantages of high mechanical strength, wear resistance, good safety (no static electricity generation), so the real reason is that the iron filings generated by the sealing surface of the gate valve are extremely culprit, as for the sealing. Declining or not is not the key.
In fact, many oxygen pipelines that do not use gate valves have the same explosion accidents. Generally, the pressure difference between the two sides of the valve is large, and the valve opens faster. Many accidents also indicate that the ignition source and combustible materials are the ultimate cause. Disabling the gate valve is just a means of controlling combustibles, and the purpose of regularly removing rust, degreasing, and oil-free means is the same. As for controlling the flow rate and making electrostatic grounding, it is the source of ignition. Personally think that the valve material is the first factor, and similar problems appear on the hydrogen pipeline. The new specification has removed the word "disabled gate valve". This is a testimony. The key is to find the cause. Many companies do not care about the operating pressure at all. Forced use of copper-based alloy valves, but the same explosion accidents, so control of fire and combustibles, careful maintenance, tight and secure this string is the most critical.