What are the measures for safe maintenance of pipelines and valves? Why are gate valves prohibited for oxygen piping

What are the measures for safe maintenance of pipelines and valves? Why are gate valves prohibited for oxygen piping

1. Before maintenance, it is necessary to confirm that the valve, pipeline and system are reliably isolated, and the system where the valve and pipeline are located can be repaired only after discharging water and lowering pressure to zero. When cutting a pipe, the cut pipe must be blocked in time to prevent debris. When beveling, the staff should wear protective glasses, and the Mars splashing direction should not stand. The replaced material should be confirmed by the material and have the qualification certificate before it can be used to prevent the wrong material. In the maintenance of soda and wind powder pipeline, the old pipeline is cut, the new pipeline, expansion joint and elbow are installed and lifted, and the lower part is strictly prohibited to stand and pass when welding, so as to avoid injury, burn and scald.
2, release the valve electric head power, pneumatic head air source, and hang a warning sign. When removing the electric and pneumatic head, loosen the bolt and slowly hoist it down with the hoist, and place it. After DISASSEMBLY, THE VALVE parts should be placed in the designated position, and the ground should be laid with rubber pad to prevent damage to the equipment and the ground. When LOOSENING THE VALVE flange BOLT, the WORKER IS NOT allowed to stand on the joint surface of the flange, in case there is residual soda spray injury. Before the pulverized coal pipeline stops running, the pulverized coal in the pipeline should be blown clean to prevent spontaneous combustion of powder accumulation
3. Before assembly, the joint surface of valve core and valve seat shall be inspected and qualified before assembly. When assembling, we should clean up the accepted parts and components, strictly in accordance with the process sequence, to prevent leakage and misloading. The grinding of valve seat and spool shall strictly implement the process standards of coarse, fine and fine. When replacing the flat grinding sandpaper, the grinding tools shall be put forward first to prevent the nuts and other sundry objects from falling into the valve shell.
Technical measures for pipeline and valve maintenance
1. After local maintenance of the valve, hydraulic test is carried out together with the boiler body. Tooth pad plane should be smooth without silk marks, cracks, etc., forming filler should be smooth and non-destructive. The joint shall be 45°, staggered 1200 placement. Clean and check the valve stem and packing box, polish with sandpaper, packing box wall should be clean, packing seat should be clean without scars, the ellipticity should be <0.05mm joint surface should be free of scars, pitting roughness should be above 0.8.
2, the shell should be intact, the joint surface should be smooth, no grooves, scars, eliminate pitting, pits, silk marks, to achieve bright consistent, smooth, roughness of 0.2, sealing surface without defects. The bearing seat surface is smooth and non-destructive, no rust. The steel ball is intact and should rotate flexibly. The valve seat and the inner wall of the copper sleeve should be smooth and burr free, and the ellipticity of the sleeve should be <0.2mm. The ellipticity of the quadrate ring and washer should be <0.1mm.
3, the gap between the valve stem and the bottom cover bushing under the spool is 0.2mm, the gap between the valve stem and the valve seat is 0.3-0.4mm, the gap between the valve stem and the packing cover and the packing seat is 0.15-0.20mm, the gap between the valve cover and the valve seat is 0.2-0.3mm, the gap between the washer and the valve cover and the valve seat is 1.0-1.2mm, The gap between gland and seat is 0.5-1.0mm.
4, the valve stem in the spool rotation should be flexible, up and down loose 0.05mm, the spool in the valve seat flexible rotation. There shall be no obvious wrong mouth in the pipeline, and the interior of the pipeline shall be smooth and the weld seam shall be filled. When disassembling, the wire mouth should be prevented from being damaged, the switch is flexible, and there is no internal leakage. The contact belt should be continuous and uniform to more than 2/3 of the width, and there is a continuous sealing line. The tetra ring should be placed in the groove, and the gap between the four sides should be even. The disc in the valve seat series moving clearance should be maintained in 1-1.5 mm.
Why are gate valves prohibited for oxygen piping
In accordance with the “GB 16912-1997 Oxygen and related gas safety Technical regulations” on the valve material: pressure is greater than 0.1mpa, it is forbidden to use gate valve, 0.1mpAP0.6mpa, the valve disc is stainless steel, 0.6mpAP10mpa, all stainless steel or all copper alloy valve,
P
When 10 mpa
, all copper base alloy.
In recent years, with the increase of oxygen consumption, large users of oxygen are using oxygen pipeline delivery. Due to the long pipeline, wide distribution, coupled with the rapid opening or closing valve, resulting in oxygen pipeline and valve combustion accidents occur from time to time, so, *** analysis of oxygen pipeline and cold door existing hidden dangers, dangers, and take corresponding measures is crucial.
First, several common oxygen pipeline, valve combustion cause analysis
1. The rust, dust and welding slag in the pipeline friction with the inner wall of the pipeline or valve port, resulting in high temperature combustion.
This situation is related to the type of impurities, particle size and airflow speed. 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 it is to burn.
2. There are grease, rubber and other substances with low ignition point in the pipeline or valve, which will ignite at local high temperature.
The ignition point of several combustibles in oxygen (at atmospheric pressure) :
Name of fuel Ignition point (℃)
Lubricating oil 273 ~ 305
Vulcanized fiber mat 304
Rubber 130 ~ 170
Fluorine rubber 474
Cross-linked with 392 b
Teflon 507
3. The high temperature generated by adiabatic compression causes combustibles to burn
For example, before 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 after the valve can reach 553℃ according to the adiabatic compression formula, which has reached or exceeded the ignition point of some substances.
4. The ignition point reduction of combustible material in high pressure pure oxygen is the inducement of oxygen pipeline valve combustion
Oxygen pipeline and valve in high pressure pure oxygen, the risk is very great, the test has proved that the fire of the *** can be inversely proportional to the square of pressure, which poses a great threat to the oxygen pipeline and valve.
Second, preventive measures
1. The design shall comply with relevant regulations and standards
The design should comply with the 1981 Ministry of Metallurgy issued by the Iron and Steel Enterprise oxygen pipe network of several regulations, as well as the oxygen and related gas safety technical regulations (GB16912-1997), “Oxygen station design code” (GB50030-91) and other regulations and standards.
(1) The large flow rate of oxygen in carbon steel pipe should conform to the following table.
Large flow rate of oxygen in carbon steel pipe:
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, a section of copper base alloy or stainless steel pipe with a length not less than 5 times the pipe diameter and not less than 1.5m should be connected behind the oxygen valve.
(3) Elbow and bifurcation head should be set as few as possible in oxygen pipeline. Elbow of oxygen pipeline with working pressure higher than 0.1mpa should be made of stamped valve type flange. The airflow direction of the bifurcation head shall be 45 to 60 angles from the direction of the main airflow.
(4) In the butt welding of concave-convex flange, copper welding wire is used as O-ring, which is a reliable sealing form of oxygen flange with flammability.
(5) The oxygen pipeline should have a good conductive device, the grounding resistance should be less than 10, the resistance between flanges should be less than 0.03.
(6) The end of the main oxygen pipeline in the workshop should be added with a release pipe to facilitate the purging and replacement of the oxygen pipeline. Before the long oxygen pipeline enters the regulating valve in the workshop, a filter should be set.
2. Installation Precautions
(1) all parts in contact with oxygen should be strictly degreased, degreased with dry air or nitrogen without oil.
(2) Welding shall be argon arc welding or arc welding.
3. Precautions for Operation
(1) When switching on and off 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 prohibited to use oxygen to brush the pipeline or use oxygen to test leakage and pressure.
(3) The implementation of the operation ticket system, in advance of the operation of the purpose, method, conditions to make a more detailed description and provisions.
(4) Manual oxygen valves with a diameter greater than 70mm are allowed to operate only when the pressure difference between the front and back of the valve is reduced to less than 0.3mpa.
4. Precautions for maintenance
(1) Oxygen pipeline should be regularly checked and maintained, rust removed and painted, every 3 to 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. When the oxygen content in the blown gas is 18% ~ 23%, it is qualified.
(5) Valve, flange, gasket and pipe, pipe fitting selection should comply with the “Oxygen and related gas safety technical regulations” (GB16912-1997) relevant provisions.
(6) Establish technical files, train operation, overhaul and maintenance personnel.
5. Other security measures
(1) Improve the importance of construction, maintenance and operation personnel to safety.
(2) improve the vigilance of management personnel.
(3) Raising the level of science and technology.
(4) Continuously improve the oxygen delivery plan.
Conclusion:
The reason why the gate valve is banned is actually because the sealing surface of the gate valve in the relative movement (that is, the valve switch) will cause abrasion damage due to friction, once damaged, there is iron powder from the sealing surface off, such fine particles of iron powder is easy to burn, this is the real danger.
In fact, the oxygen pipeline is forbidden to gate valve, other stop valves have accidents, the sealing surface of the stop valve will be damaged, as likely to be dangerous, the experience of many enterprises is that the oxygen pipeline all use copper alloy valve, not carbon steel, stainless steel valve.
Copper alloy valve has the advantages of high mechanical strength, wear resistance, good safety (does not produce static electricity), so the real reason is because the sealing surface of the gate valve is easy to wear and produce iron is the main culprit, as for the decline in sealing is not the key.
In fact many gate of oxygen pipeline is not used as an accident, usually appear on both sides of the valve pressure difference is bigger, the valve opens faster, many accidents also shows that the ignition source and fuel is the cause of the end, disable gate valve is only a means to control fuel, and rust, degreasing, banned oil on a regular basis are the same, the purpose of As for the control of flow rate, do a good job of electrostatic grounding is to eliminate the source of fire. Personally think that the valve material is factors, on the hydrogen pipe also appear similar problems, the new specifications have words will disable gate removed, is a testament, the key to find the reason, many companies are simply regardless of operating pressure, are forced by the copper alloy valve, but as some accidents occur, so controlling the fire and fuel, careful maintenance, The key is to tighten the safety string.