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

According to GB 16912-1997 technical code for safety of oxygen and related gases on valve material:

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

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

When 0.6MPa < p ≤ 10MPa, all stainless steel or all copper base alloy valve

P> When the pressure is 10MPa, all copper base alloy is formed.

In recent years, with the increase of oxygen consumption, large oxygen users use oxygen pipeline transportation. Due to the long pipeline, wide distribution, and the quick opening or quick closing valve, the accidents of oxygen pipeline and valve combustion and explosion often occur. Therefore, it is very important to comprehensively analyze the hidden danger and danger of oxygen pipeline and cold door, and take corresponding measures.

1、 Analysis on the causes of combustion and explosion of several common oxygen pipelines and valves

1. The rust, dust and welding slag in the pipeline rub with the inner wall of the pipeline or the valve port to produce high temperature combustion.

Iron powder is easy to burn with oxygen, and the finer the particle size is, the lower the ignition point is; the faster the gas velocity is, the easier the combustion is.

2. There are grease, rubber and other substances with low ignition point in the pipeline or valve, which ignite at local high temperature.

The ignition points of several combustibles in oxygen (at atmospheric pressure) are as follows:

Fuel name and ignition point (℃)

Lubricating oil 273 ～ 305

Steel paper cushion 304

Rubber 130 ～ 170

Fluororubber 474

Trichloroethylene 392

Polytetrafluoroethylene 507

3. The high temperature produced by adiabatic compression makes the combustibles burn

For example, the oxygen temperature in front of the valve is 15MPa, the temperature is 20 ℃, and the normal pressure is 0.1MPa behind the valve. 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 combustible in high pressure pure oxygen is the inducement of oxygen pipeline valve combustion

Oxygen pipeline and valve are very dangerous in high-pressure pure oxygen. The test shows that the ignition energy is inversely proportional to the square of pressure, which poses a great threat to oxygen pipeline and valve.



2、 Preventive measures

1. The design shall comply with relevant regulations and standards

The design shall comply with the requirements of regulations on oxygen pipe network of iron and steel enterprises issued by the Ministry of metallurgy in 1981, technical code for safety of oxygen and related gases (GB 16912-1997), code for design of oxygen station (GB 50030-91).

(1) The maximum flow rate of oxygen in carbon steel pipe shall comply with the table below.

Maximum flow rate of oxygen in carbon steel pipe:

Working pressure (MPA) ≤ 0.10.1 ～ 0.60.6 ～ 1.61.6 ～ 3.0

Velocity (M / s) 20 13 10 8

(2) In order to prevent fire, a copper base alloy or stainless steel pipe with length not less than 5 times of pipe diameter and not less than 1.5m shall be connected behind the oxygen valve.

(3) Elbows and bifurcations shall be minimized for oxygen pipeline. Elbows of oxygen pipeline with working pressure higher than 0.1MPa shall be made by stamping into valve flange. The flow direction of bifurcation head should be 45 ° to 60 ° with the main flow direction.

(4) In the butt welding concave convex flange, the use of copper welding wire as O-ring is a reliable sealing form of flame resistance for oxygen flange.

(5) The oxygen pipeline shall have a good conductive device, the grounding resistance shall be less than 10 Ω, and the resistance between flanges shall be less than 0.03 Ω.

(6) The end of the main oxygen pipeline in the workshop should be equipped with a vent pipe to facilitate the purging and replacement of the oxygen pipeline. Before the long oxygen pipeline enters the workshop regulating valve, a filter should be set.


2. Installation precautions

(1) All parts in contact with oxygen shall be degreased strictly. After degreasing, dry air or nitrogen without oil shall be used to blow clean.

(2) Argon arc welding or electric arc welding shall 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 at one time.

(2) It is strictly forbidden to use oxygen to blow the pipeline or use oxygen to test leakage and pressure.

(3) The operation ticket system shall be implemented, and the operation purpose, method and conditions shall be specified in detail in advance.

(4) The manual oxygen valve with diameter greater than 70mm can only be operated when the pressure difference before and after the valve is reduced to less than 0.3MPa.


4. Maintenance precautions

(1) The oxygen pipeline should be inspected and maintained regularly, derusting and painting should be done once every 3-5 years.

(2) The safety valve and pressure gauge on the pipeline should be checked regularly once a year.

(3) Improve the grounding device.

(4) Before hot work, replacement and purging should be carried out, and the oxygen content in the blown gas is 18% - 23%.

(5) The selection of valves, flanges, gaskets, pipes and fittings shall comply with the relevant provisions of technical code for safety of oxygen and related gases (GB 16912-1997).

(6) Establish technical files, train operation, overhaul and maintenance personnel.



5. Other safety measures

(1) Improve the construction, maintenance and operator's attention to safety.

(2) Enhance the vigilance of management personnel.

(3) Improve the level of science and technology.

(4) Continuously improve the oxygen delivery scheme.

Conclusion:



The real reason why the gate valve is forbidden is that the sealing surface of the gate valve will be abraded and damaged due to friction in the relative movement (i.e. the opening and closing of the valve). Once it is damaged, the "iron powder" will fall off from the sealing surface. In this way, the fine iron powder is easy to ignite and burn, which is the real danger.

As a matter of fact, gate valves are not allowed in oxygen pipelines, and other stop valves have accidents. The sealing surface of the stop valve will be damaged and may be dangerous. The experience of many enterprises is that all oxygen pipelines use copper based alloy valves instead of carbon steel and stainless steel valves.

Copper based alloy valve has the advantages of high mechanical strength, wear resistance and good safety (no static electricity), so the real reason is that the iron filings produced by the extremely easy wear of the sealing surface of the gate valve is the culprit, and the decline of the sealing performance is not the key.

As a matter of fact, many oxygen pipelines without gate valve have explosion accidents, which usually occur at the moment when the pressure difference on both sides of the valve is large and the valve is opened quickly. Many accidents also show that the fire source and combustible are the ultimate causes. Banning gate valve is just a means to control combustible, and the purpose of regularly removing rust, degreasing and banning oil is the same, As for the control of flow rate, electrostatic grounding is to eliminate the fire source. I think valve material is the first factor, and there are similar problems in hydrogen pipeline. The word "forbidden gate valve" has been removed in the new specification, which is a clear proof. The key is to find out the reason. In fact, many enterprises force to use copper based alloy valves regardless of operating pressure, but there are also explosion accidents, so fire sources and combustibles are controlled and carefully maintained Safety is the key.