Stem Technology
During the opening and closing process of the valve, the stem bears tension, pressure and torsion forces, and contacts with the medium directly. At the same time, there is relative friction movement between the stem and the filler. Therefore, when choosing the stem material, it must ensure that it has sufficient strength, good impact toughness, abrasion resistance and corrosion resistance at the specified temperature.
There should be an anti-static mechanism at the joint of the stem and the sphere and at the contact of the stem and the valve body to prevent the static electricity from gathering on the sphere. Stem safety design should prevent "blow-out" under working pressure, and the flange on the stem should be disposed of as a ring ring to reduce friction coefficient.
Characteristics of Traditional Stem Processing Technology
1. The screw flying cutter is used in traditional valve stem threads, which leads to inaccurate thread accuracy, while the traditional chip-free rolling die is easy to be damaged, deformed and distorted.
2. Grinding machine is not accurate and time-consuming.
3. Double clamping and pull-down seal is used in the reverse part, which is poor in concentricity and easy to leak.
4. The end part is indexed by milling machine indexing plate, which is slow in processing, multi-process and time-consuming.
Characteristics of the new valve stem manufacturing process:
1. The new type of valve stem thread is extruded by self-made, which has high thread strength and accurate accuracy.
2. The concentricity of the threaded part and the polished rod part overcomes the problem of twisting and fragility of the rolling die which has troubled the domestic rolling process for a long time.
3. The precision of light rod position achieves mirror effect, easy to open and close, and good sealing.
4. NC technology is used to form the reverse part at one time, which has high precision and good sealing performance.
5. The T-groove at the end is forged with high efficiency and low cost.
6. The whole production process of valve stem has reached the international advanced level.
Under the strong corrosive, volatile and toxic and harmful process conditions, once the stem seal is destroyed, the strong corrosive, volatile and toxic and harmful process medium leaks out from the control valve stem, which will bring serious consequences to the surrounding environment and personal safety. Using bellows valve stem seal is a way to solve the above problems. Bellows are usually made of stainless steel. This special valve cover structure protects the packing box of the control valve from contact with the fluid. Once the bellows break, the packing box structure above the bellows will prevent serious consequences when the bellows break down and fail. In engineering practice, the pressure rating of bellows should be fully considered in the selection of bellows seal form. The pressure rating of bellows seal will decrease with the increase of temperature. There can be no solid particles in the fluid, and the longest cycle operating life of bellows material. In places where stainless steel is not resistant to strong corrosion of some process media, such as wet chlorine gas, trace hydrochloric acid in wet chlorine gas will cause stainless steel bellows to be corroded quickly.
Material commonly used in valve stem
Copper alloy
Generally, the material grades are QA19-2 and HPb59-1-1. Suitable for low pressure valves with nominal pressure less than or equal to 1.6 MPa and temperature less than or equal to 200 degrees.
carbon steel
Carbon steel is generally A5, 35 steel, after nitriding treatment, suitable for ammonia valves with nominal pressure less than or equal to 2.5 MPa, low and medium pressure valves with water, steam and other media. A5 steel is suitable for valves whose temperature does not exceed 300 degrees; 35 steel is suitable for valves whose temperature does not exceed 450 degrees. (Note: The actual route proves that carbon steel nitriding can not solve the problem of corrosion resistance and should be avoided. )
Alloy steel
Generally, 40Cr, 38CrMoA1A, 20CrMo1V1A and other materials are selected. After chromium plating treatment, 40Cr is suitable for water, steam, oil and other media whose nominal pressure is less than or equal to 32 MPa and temperature is less than or equal to 450 degrees. 38CrMoA1A is nitrided and can withstand 10 MPa pressure at 540 degrees. It is commonly used in power station valves. 20CrMo1V1A can withstand 14MPa pressure at 570 degrees, which is often used in power station valves.
Stainless steel
Generally, 2Cr13, 3Cr13, 1Cr17Ni2, 1Cr18Ni12Mo2Ti and other materials are selected. 2Cr13 and 3Cr13 stainless steels are suitable for water, steam and weak corrosive media with nominal pressure less than or equal to 32MPa and temperature less than or equal to 450 degrees. They can be strengthened by chromium plating and high frequency quenching. 1Cr17Ni2 stainless steel valve, low temperature valve, corrosion resistant medium. 1Cr18Ni9Ti and 1Cr18Ni12Mo2Ti stainless steel are used in high temperature valves with nominal pressure less than 6.4MPa and temperature less than 600 degrees, stainless steel valves with temperature less than - 100 degrees and low temperature valves. 1Cr18Ni9Ti can resist corrosive media such as nitric acid and 1Cr18Ni12Mo2Ti can resist corrosive media such as acetic acid. When 1Cr18Ni9Ti and 1Cr18Ni12Mo2Ti are used in high temperature valves, nitriding treatment can be used to improve the scratch resistance.
Bearing chromium steel
GCr15 is suitable for ultra-high pressure valves with nominal pressure less than or equal to 300 MPa and temperature less than or equal to 300 degrees.
There are many materials for making valve stem, such as 4Cr10Si2Mo martensitic heat-resistant steel, 4Cr14Ni14W2Mo austenitic heat-resistant steel, etc.
