Selection and installation precautions of oxygen valve
Based on the analysis of the characteristics of oxygen and the causes of combustion and explosion of oxygen pipes and oxygen valves, this paper puts forward suggestions on the selection and installation of oxygen valves.
With the rapid development of industrial modernization and the continuous emergence of new processes and devices, the parameter range of oxygen valves is also constantly improving, and the requirements for oxygen valves are increasingly strict. Oxygen is a combustion promoter, especially high-temperature and high-pressure oxygen can not meet any spark, so anti-static performance is an important guarantee for stable operation of oxygen valve. The anti-static measures include the selection, design, structure, materials and other aspects of the oxygen valve. The design aspects include the size, shape, internal surface of the flow channel, material selection, sealing surface design, anti rotation structure of the valve core and valve stem, and dust prevention measures for the valve stem nut. Especially for the structural design of high-temperature and high-pressure oxygen valves, the requirements for material selection and processing are more important and strict.
Analysis on the causes of combustion and explosion of oxygen pipes and valves
- 1) Rust, dust and welding slag existing in the pipeline rub with the inner wall of the pipeline or valve internals to produce high temperature and cause combustion. This situation is related to the type of impurities, particle size and oxygen flow rate. 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 more likely combustion will occur.
- 2) Pipe or valve degreasing is unacceptable. There is grease or rubber or other substances with low ignition point found in the internal seals, which may cause combustion under local high temperature.
- 3) The high temperature generated by adiabatic compression makes combustibles burn. For example, under the working condition of 15MPa in front of the valve and 20 ℃ in temperature, and 0.1MPa in normal pressure 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 ignition point of combustibles in high-pressure pure oxygen will be reduced, which is one of the inducements for the combustion of oxygen pipeline valves.
- 5) Quick opening and closing of the valve during operation will cause sudden change of flow rate; At the same time, high temperature will be generated due to adiabatic compression, reaching the ignition point of some materials.
Selection principle of oxygen valve
The structure of the selected oxygen valve shall eliminate all factors generating static electricity as far as possible:
- 1) Anti static structure shall be set.
- 2) Select a reasonable sealing form, master the minimum sealing surface width that can withstand the necessary sealing force, and understand the influence of various factors on the sealing performance.
- 3) The required materials shall be reasonably selected according to the maximum speed requirements of different materials in the impact occasions under the operating pressure of the valve, as well as the environment, medium, temperature, cost, etc.
- 4) Generally, the oxygen regulating valve is plunger type single seat valve (sleeve or labyrinth type valve can be selected for large differential pressure field).
- 5) The oxygen shut-off valve shall be ball valve (with anti fly out design), and the gate valve shall not be used for the oxygen pipeline.
- 6) Ball valves and plug valves can be made of highly elastic/polymer materials (that is, soft seats can be used), but attention must be paid to adiabatic compression (temperature conditions).
- 7) Oxygen shut-off valves are generally required to enter SIS system, so oxygen shut-off valves shall have SIL level certification based on IEC61508 (GB/T20438) and other standards. Generally, internationally recognized certification bodies include TUV Rheinland.
Specific type selection requirements
Valve structure requirements
The flow direction of ordinary valve medium is from bottom to top, that is, the medium flow enters from the valve body passage below the valve disc and flows out from the valve body passage above the valve disc after passing the valve seat. Under the high temperature and high pressure working conditions, the flow path of the valve is different from that of the ordinary valve. The flow direction of the oxygen medium is up and down, that is, the high temperature and high pressure oxygen flow enters from the valve body flow path above the valve flap and flows out from the valve body flow path below the valve flap after passing the valve seat.
The surface of the inner cavity of the flow passage through which the oxygen medium flow beam passes shall be smooth and smooth, and the undercut shall be chamfered; The sharp corners of the internal parts of the valve shall be rounded off without edges, sharp corners, unevenness and sudden changes, so as to ensure that there is no sudden change in the pressure and direction of the flow beam of the medium during the flow process, and there is no solid in the inner cavity of the valve body, so as to prevent the generation of static electricity; It shall have a good anti-static structure, and the grounding resistance shall not be greater than 10 Ω, and the resistance between flanges shall not be greater than 0.03 Ω.
Set protective devices. It mainly includes the protection of the valve stem stuffing box part and the valve stem thread part, mainly to prevent dust from sticking to the valve stem, and then entering the stuffing box during the valve stem movement, and further entering the oxygen medium to generate static electricity. Therefore, it is necessary to protect the valve stem from dust pollution, prevent the generation of electrostatic sparks, isolate the exposed part of the valve stem from the atmosphere, and prevent dust from entering the lower part of the valve stem through the trapezoidal thread, thus entering the packing culvert during the rotation of the valve stem.
The material of oxygen valve shall comply with the GB/T16912 standard, and shall be determined comprehensively according to the medium pressure, temperature, flow rate (there are certain restrictions on the flow rate of oxygen according to the flammability of different metals. If the flame retardant alloy can be flame retardant under the design pressure of the pipeline, the flow rate limit need not be considered at this time), purity and other parameters. For the pressure bearing materials, there shall be a warranty and quality certificates (certificate of origin).
1) The selection principle of materials for oxygen valves at normal temperature and pressure: titanium alloy is prohibited for metal parts (components); Polyimide is not allowed to be used as the sealing element for non-metallic parts (components), and titanium containing stainless steel, such as 1Cr18Ni9Ti (321), 0Cr19Ni10Ti, should be avoided as far as possible.
The main body of special valves used in high-pressure and high-purity oxygen pipelines at room temperature shall be made of stainless steel with no oxidation, rust and low carbon content, such as forgings: F304, F304L, F316, F316L, etc; Bars: S31600, S30400316L, 316304, 304L, etc; Castings: CF8, CF3, CF8M, CF3M, CF8C, etc. When stainless steel casting is selected, pickling must be carried out to remove impurities such as sand and oxide in the inner cavity of the valve.
2) When the pressure is greater than 15Mpa, good flame retardancy of the material shall be considered, and nickel base alloy with higher safety level can be selected as the material of the valve. For example, nickel base alloy: ASTM B164 Monel 400, Monel K500, M35-1, ASTM A494CY-40; Inconel alloy: ASTM B164-Ainconel 600, inconel 625, inconel 718; Hastelloy: Hasalloyc-276.
3) When the pressure is not more than 15Mpa, copper and copper alloy can be selected as pressure bearing parts, and red copper, soft aluminum, PTFE (F4), polytetraethylene spiral wound gasket, flexible graphite, flexible graphite spiral wound gasket can be selected as sealing elements, or corrugated pipes made of stainless steel (such as 304316304L, 316L) can be selected for sealing.
The seal ring can be 3160, 3180, 5160, 8380, 8360, FM-2D, Karez (perfluoro rubber), etc.
Oils and oils can be 78057804, FL-10, fluoro oil, etc. 7805 is recommended.
According to the investigation, the internal parts of liquid oxygen valve and low-pressure gas oxygen valve of ENN Group are made of SUS316; The valve bodies/trim of liquid oxygen above 1500LB of Ningmei Group are Monel series, most valve bodies below 1500LB are CF8M (316), and the trim is StelliteCr 21 Stellite alloy, gas oxygen valve body internals are mainly Monel series; The liquid oxygen valve of Shaanxi Shenmu Chemical Industry Co., Ltd. belongs to the low-pressure category. The valve body is made of stainless steel, the internals are 1.4541 (equivalent to 06Cr18Ni11Ti), the valve seat of the gas oxygen internals is Stellite cobalt based alloy, and the other internals are Monel400/Monel500 series.
Applicable scope and acceptance criteria of main degreasers
Scope of application
Degreasers for oxygen pipelines and valves include industrial carbon tetrachloride, dichloroethane, distilled ethanol, industrial alcohol, etc. The scope of application is as follows:
1) Carbon tetrachloride: It is applicable to degreasing of metal and non-metal parts. It is prohibited to degrease nonferrous metals with carbon tetrachloride.
2) Dichloroethane: suitable for degreasing various non-metallic pipes and accessories, not metal parts.
3) Distilled ethanol: applicable to degreasing of metal materials, not non-metallic materials.
4) Industrial alcohol (concentration not less than 95.6%): it is applicable to degreasing of instruments, control valves, valves and pipes with low requirements, and can also be used as supplementary scrubbing fluid for degreasing parts. Precautions are as follows:
- (1) The degreasing solvent shall not be mixed and shall not contact with concentrated acid and alkali.
- (2) When carbon tetrachloride and dichloroethane are used for degreasing, the degreased parts shall be dry and free of water.
- (3) Tools, appliances and instruments contacting degreased parts must be degreased in advance according to the same requirements for degreased parts.
- (4) When degreasing by soaking, the soaking time is 1h~2h.
- (5) When degreasing by wiping, cotton yarn should not be used. Cloth and silk that are not easy to shed fiber should be used. After degreasing, careful inspection must be carried out. It is strictly prohibited to attach fibers to the degreased surface. The degreased parts must be sealed and stored after degreasing.
- (6) It is recommended to clean metal parts (components) with industrial alcohol; Clean non-metallic parts with carbon tetrachloride.
Degreasing acceptance can be inspected by any of the following methods
- 1) Ultraviolet light method: use ultraviolet light with a wavelength of 320nm~380nm to check the surface of degreased parts. If there is no oil fluorescence, it is qualified.
- 2) Filter paper method: wipe the surface of degreased parts with clean and dry white filter paper, and it is qualified if there is no grease trace on the paper.
- 3) Camphor method: use oil-free steam to blow the surface of degreased parts, take its condensate, put a small particle of pure camphor with a diameter of no more than 1mm, and the camphor particle is qualified if it keeps rotating.
- 4) Analytical method: the degreaser that passes the sampling inspection shall be qualified if the oil content does not exceed 350mg/L.
Precautions for oxygen valve installation
- 1) Before installing the valve, the inside of the pipeline shall be cleaned to remove iron filings and other impurities to prevent foreign matters from being mixed in the valve sealing seat. The degreasing shall be qualified, and the valve shall be closed during installation.
- 2) If the valve has been degreased before delivery and has not been polluted after delivery to the site, it can be installed directly. It is recommended that all parts in contact with oxygen should be strictly inspected, and the ultraviolet light method and filter paper wiping method can be used for inspection.
- 3) The overhauled valve must be strictly degreased. After the degreasing is qualified, it shall be purged with oil-free dry air or nitrogen. Before installation, it shall be wrapped with clean silk cloth to prevent secondary pollution.
- 4) Degreasing personnel shall wear protective equipment during degreasing. Some degreasing agents are toxic reagents. Maintenance personnel shall wear silk gloves. Direct contact with degreased parts is prohibited.
- 5) Argon arc welding or electric arc welding shall be adopted for the welding of companion installation flanges of valves, and flaw detection shall be carried out after welding of large diameter valves.
- 6) In butt welding concave convex flange, red copper welding wire is used as O-ring, which is a reliable sealing form of flame resistance of flange for oxygen.
- 7) The oxygen pipeline shall be equipped with a good conductive device, and the grounding resistance shall be less than 10 Ω. The flange connection and threaded connection on the pipeline and valve shall be bridged with metal wires, and the resistance shall be less than 0.03 Ω.
- 8) Flange gasket selection. When the pressure is less than 3Mpa, PTFE gasket and flexible graphite composite gasket can be selected; For 3Mpa~10Mpa, spiral wound gasket, annealed softened copper gasket, nickel and nickel alloy gasket can be selected; When the pressure is greater than 10Mpa, annealed and softened copper gasket, nickel and nickel alloy gasket can be selected.
- 9) When the oxygen valve is heavy, it should be well supported on the pipes before and after the valve. The oxygen control valve group should be equipped with an independent valve chamber or protective wall.
Based on the analysis of oxygen characteristics and the causes of combustion and explosion of oxygen pipes and valves, this paper concludes that the following aspects should be taken into consideration when designing and selecting oxygen valves:
- 1) The design shall be anti-static, anti rotating and dust-proof.
- 2) Materials with good flame retardancy and corrosion resistance shall be used to prevent rust and sparks.
- 3) Under high content or high pressure, nickel base alloy (such as Monel, Inconel) internals shall be considered.
- 4) In processing, it is smooth without sharp edges and corners.
- 5) Technically, strict degreasing treatment is required.
- 6) Select the manufacturer and refer to the performance. At present, manufacturers of imported oxygen control valves with good performance include Masoneilan, Flowserve, fisher, etc; Manufacturers of imported oxygen shut-off valves include Tyco, Perrin, Argus, etc; Domestic oxygen control valves include: Wu Zhong, Chuanyi, Altam, etc; Domestic oxygen shut-off valves include: Antway, the 11th Institute of Aerospace, Chuanyi, etc.
- 7) At the same time, the leakage rate grade and sealing surface form of the valve shall be selected according to the design data sheet, and the opening degree of the valve under different loads and the maximum flow rate under working conditions shall be checked and calculated, whether the opening and closing speed of the valve meets the process requirements, the maximum nominal pressure that it can withstand and other parameters shall be checked and calculated.
Author: Zheng Zhiming
Source: China Oxygen Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Stainless Steel Valves, Stainless Steel Flanges, Stainless Steel Pipe Fittings, and Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels, and other industries.)
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