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High Purity Gas Pipeline Valves and Fittings
2019-03-16
Introduction of Valves and Fittings for High Purity Gas Pipeline
The quality of high purity gas pipeline system will directly affect the purity, cleanliness and dryness of high purity gas. Therefore, it is of great significance to strictly control the design, material selection, manufacturing, cleaning treatment, installation and use of high purity gas transmission system.
Valve fittings
Selection of Valves for High Purity Gas Pipeline
The tightness of valves used in high purity gas system is very strict. At present, the valves of high purity gas pipeline in domestic clean workshop are basically made of SS304 or SS316L stainless steel. The valve types are diaphragm valves, bellows valves and ball valves. Corrugated pipe valves have better tightness than ball valves. When gas flows through the valves, there is no filler in contact with the outside environment, so there is no leakage phenomenon: besides the tightness of the diaphragm valves, they also have small dead volume of the valve body, easy to blow out and less pollution, so they are suitable for gases with very strict requirements for gas purity and production process or with high risk.
1. High purity gas is usually welded by manual and automatic argon arc welding. Socket-socket welding and butt welding are usually used in welding type. The advantage of socket welding is that the pipeline is in the middle of the welding pipe, which is convenient for welding. However, the disadvantage is that there is a gap between the pipeline and socket, and there will be "dead space" which is not easy to blow out impurities, thus affecting the quality of high purity gas. Therefore, for highly pure gas, butt welding should be adopted and no weld on the inner surface should be required, that is, stainless steel wire should not be used in construction, and the weld should be filled by melting of the base metal itself.
2. In order to ensure the quality of the gas transported to the gas-using equipment, stainless steel metal hose should be used to connect the high-purity gas pipeline with the gas-using equipment, and non-metal hose should not be used.
3. The connection of high purity gas pipeline with accessories such as valves should be made by special joints which are sealed and leak-proof. There are two common joints: VCR (Vacuum Coupling Retainer) and SWaglok (Swaglok). VCR uses excellent metal cushions and uses longitudinal pressure to compress. Therefore, the leakage rate is very low, about 10-9A cc/s, and the pressure resistance is high. They are often used for gas impurity content of 1.0 *10-9. Class I high purity gas conveying system, while SWG is not as good as VCR, low pressure resistance, basically the safety of these two joint modes can replace the welding mode.
Connection Types and Characteristics of High Purity Gas Pipeline
Threaded connection
The threaded connection is used for gas pipeline with large volume of gas and low purity requirement. When galvanized steel pipe is used, the threaded connection is adopted. The sealing material of threaded connection is PTFE sealing belt, and the traditional oil flax is not allowed to be used. The sealing tape should be properly wound. If the winding is too thick, too long, too thick or too much beyond the groove of the end of the pipe thread, the sealing belt will be bitten off by the thread when the thread is tightened, which will form a larger contaminant in the pipe. Therefore, it is necessary to leave 2-3 convex teeth from the groove to avoid winding the sealing belt.
Butt welding, socket welding or sleeve joint
1. For pipelines requiring high purity, butt welding without scar on the inner wall is adopted. High purity argon shielded welding.
2. For pipelines with relatively low purity requirements, socket welding or casing welding can be used, because the gas residual space of socket welding is relatively large. It is difficult to blow clean, and now casing welding is widely used. Casing welding should make the casing and the main pipe have the same material and inner surface grade, the gap between the joint should not be too large, the overlap part should not be too long, and the suitable overlap length should be within 5 mm. When processing the inner pipe of the casing, the main pipe should be interference, so that the main pipe is tightly wrapped by the casing and welded, and the welding slag will not fall into the pipe. Moreover, the dead space of gas is small and easy to blow out.
Storage Requirements for Components of High Purity Gas Pipeline
Store in zones according to varieties, materials, specifications and batches; stainless steel and carbon steel, low alloy steel pipe components shall not be contacted; outdoor storage of pipe components shall be provided with brackets and cushions; construction site storage materials shall be placed neatly, clearly marked, and special materials. When distributing pipeline components, the materials, specifications, models, quantities and labels shall be checked. Material should be labeled and transplanted before cutting.
Three Forms of High Purity Gas Supply
1. On-site gas production and pipeline gas supply;
2. Purchase liquid gas and supply gas by pipeline after gasification.
3. Purchase gas cylinders and supply gas with gas cylinders (groups). The method of a project is determined by comparing the production scale, gas consumption, gas quality, and gas market supply in the city or region where the plant is located.
