Selecting Pipe Fittings for Oxygen Pipelines
1. Selection of elbows, reducers and tees on oxygen pipelines
The elbows, reducers, and tees on the oxygen pipeline are parts that are prone to oxygen flow impact and severe friction. Some impact occasions are the frequent sources of burning and explosion. The rolled products with smooth inner walls, uniform wall thicknesses and regular grooves should be used to avoid accidents.
(1) Folded elbows are strictly prohibited for the oxygen pipeline, because the oxygen flow at the folding part is turbulent, and the impact and friction are severe, which is the part where accidents occur frequently. When cold bending or hot bending is adopted to make carbon steel elbows, the bending radius should not be less than 5 times the nominal diameter so that slow transition and turning of the oxygen flow can be obtained. There should not be folding for elbows when they are put into use to reduce friction and impact. When using rolled butt welded elbows, long-radius elbows should be used, that is, the bending radius is greater than or equal to 1.5 times the nominal diameter of the pipe, which becomes a less dangerous non impact occasion. For steel coil welded pipes with working pressure less than and equal to 0.1MPa, miter elbows are allowed to be used due to low pressures and low risk. Pipe fittings that are greater than and equal to two-section pipes in the middle should be adopted for 90° elbows to try to make the turning smoothly. The inner wall of the elbow should be smooth, free of sharp edges, burrs and weld beading to reduce friction and impact.
The elbows, reducers, and tees on the oxygen pipeline are parts that are prone to oxygen flow impact and severe friction. Some impact occasions are the frequent sources of burning and explosion. The rolled products with smooth inner walls, uniform wall thicknesses and regular grooves should be used to avoid accidents.
(1) Folded elbows are strictly prohibited for the oxygen pipeline, because the oxygen flow at the folding part is turbulent, and the impact and friction are severe, which is the part where accidents occur frequently. When cold bending or hot bending is adopted to make carbon steel elbows, the bending radius should not be less than 5 times the nominal diameter so that slow transition and turning of the oxygen flow can be obtained. There should not be folding for elbows when they are put into use to reduce friction and impact. When using rolled butt welded elbows, long-radius elbows should be used, that is, the bending radius is greater than or equal to 1.5 times the nominal diameter of the pipe, which becomes a less dangerous non impact occasion. For steel coil welded pipes with working pressure less than and equal to 0.1MPa, miter elbows are allowed to be used due to low pressures and low risk. Pipe fittings that are greater than and equal to two-section pipes in the middle should be adopted for 90° elbows to try to make the turning smoothly. The inner wall of the elbow should be smooth, free of sharp edges, burrs and weld beading to reduce friction and impact.
(2) The reducer of the oxygen pipeline should adopt rolled butt welded pipe fittings. When welding is adopted, the length of the reducing part should not be less than 3 times the different valve between the outer diameters of the pipes at both ends, so that the oxygen flow changes smoothly and slowly. The inner wall of the reducer should be smooth, free of sharp edges, burrs and weld beading to reduce friction and impact and prevent an explosion.
(3) Rolled butt welding should be adopted for the tee used for the oxygen pipeline. When it is not available, it should be prefabricated in the factory or on-site, and the part should be processed till there are no sharp angles, no protruding parts and weld beading to ensure safety. It is forbidden to make holes on site, because it is difficult to maintain no sharp angles, protruding parts, weld beading and welding slags for the tee, which may cause accidents easily.
2. Selection of flanges and gaskets on oxygen pipelines
The strength and processing accuracy should be ensured for flanges on the oxygen pipeline, and they should be selected in strict accordance with the current national and industrial standards. No combustibles should be used for flanges and gaskets of pipes, and the good sealing performance is required. The higher the oxygen pressure is, the higher the risk and the stricter the requirements for gaskets become (Table 4). Table 4 is quoted from the GB 16912 standard.
Table 4 Gaskets of flanges of oxygen pipelines
(3) Rolled butt welding should be adopted for the tee used for the oxygen pipeline. When it is not available, it should be prefabricated in the factory or on-site, and the part should be processed till there are no sharp angles, no protruding parts and weld beading to ensure safety. It is forbidden to make holes on site, because it is difficult to maintain no sharp angles, protruding parts, weld beading and welding slags for the tee, which may cause accidents easily.
2. Selection of flanges and gaskets on oxygen pipelines
The strength and processing accuracy should be ensured for flanges on the oxygen pipeline, and they should be selected in strict accordance with the current national and industrial standards. No combustibles should be used for flanges and gaskets of pipes, and the good sealing performance is required. The higher the oxygen pressure is, the higher the risk and the stricter the requirements for gaskets become (Table 4). Table 4 is quoted from the GB 16912 standard.
Table 4 Gaskets of flanges of oxygen pipelines
| Working pressure p /MPa | Gaskets |
| P being less than and equal to | PTEF gaskets and flexible graphite composite gaskets |
| P being between 0.6 and 3.0 | Spiral wound gaskets, PTEF gaskets and flexible graphite composite gaskets |
| P being between 3.0 and 10 | Spiral wound gaskets, anneal softened copper gaskets, nickel and nickel alloy gaskets |
| P being greater than 10 | Anneal softened copper gaskets, nickel and nickel alloy gaskets |
The working pressure of oxygen is divided into 4 different kinds in Table 4, which is consistent with the pressure range of the oxygen pipe's material in Table 3. It is scientific, reasonable, convenient and applicable, and compatible with each other.
The different kind of working pressure is as follow:
① For low-pressure oxygen pipelines with pressure less than and equal to 0.6MPa, PTFE gaskets and flexible graphite composite gaskets can be adopted for the flange.
② Pressure between 0.6MPa and 3.0MPa is a commonly used oxygen pressure range in steel, metallurgy, shipbuilding, and machinery industries. The oxygen pressure is slightly higher. In addition to the aforementioned two types of gaskets, you'd better use spiral wound gaskets for pipelines.
③ The pressure range between 3.0MPa and 10MPa is the oxygen pressure range commonly used in the chemical industry. The oxygen pressure is high. Spiral wound gaskets can be used for this kind of pipe. When the oxygen pressure is close to the upper limit, it is best to use annealing softened copper gaskets or nickel and nickel alloy gaskets.
④ The oxygen pressure greater than 10MPa is mostly used for cylinder filling and a few chemical industries. The pressure is very high and the risk is great. Annealing softened copper gaskets or nickel and nickel alloy gaskets can only be used for the pipelines.
3. Connections of oxygen pipelines
The oxygen pipeline should be welded, and the connection should be firm to reduce leakages. However, flanges or threads can be used for connecting equipment and valves to facilitate connections. Teflon film should be used as the filler for the threaded connection. It is strictly forbidden to use lead-coated linen, cotton silk or other grease containing materials as the filler, because these materials are flammable and will ignite when oxygen leaks and becomes the source of ignition.
4. Filters of oxygen
A filter that can be cleaned regularly should be installed in front of the oxygen compressor's inlet and the oxygen regulating valve. A filter is installed at the inlet of the oxygen compressor to prevent rust, debris and particles from entering the cylinder or casing, which may damage the equipment and cause a fire. A filter should also be installed before the oxygen regulating valve. Because the diameter of the regulating valve is generally 1 to 2 levels smaller than that of the pipe, the oxygen is throttled, and the flow rate is high, close to subsonic speed. It is necessary to prevent debris, rust, and particles from friction and impact when flowing through the regulating valve at high speeds, which may cause a fire. The oxygen filter should be cleaned regularly, and the cleaning cycle is determined by the user according to the actual operating conditions. If it is not cleaned, rust, debris and particles will accumulate in the filter, resulting in increased resistance, forming eddy currents, heat due to generation of the friction or impact and causing the damage of the filer and an explosion. Such accidents have occurred many times.
The housing of the oxygen filter should be made from stainless steel, copper and copper alloys with high strength and no rust properties, and the internal parts of the filter should be made from copper and copper alloys that do not catch fire during friction and impact. In addition to meeting the filtering function, the filter should also have enough strength to prevent the filter from breaking and losing the filtering function; the broken pieces of the filter will also affect the normal operation of the oxygen compressor or the regulating valve. The filter should be made from non-flammable and high-impact nickel copper alloys, followed by non-flammable copper alloys, except aluminum containing copper alloys. The mesh size should be 60 to 80 meshes, which is the preferred data proved by practice. If the hole is too large, the filtering effect will not be good. If the hole is too small, the metal wire will be too thin; the strength will be low, and it will be fragile. The effective filtration area of the filter element should be more than 2 times the cross-sectional area of the pipe to ensure that the filter element can pass the design flow when partially blocked.
