Materials for sealing surfaces
The sealing surface is the most critical working surface for the valve. The quality of the sealing surface is related to the service life of the valve. Usually, the sealing surface's material should consider factors such as corrosion resistance, scratch resistance, erosion resistance, and oxidation resistance, which are divided into the following two categories:
Soft sealed materials
The sealing surface is the most critical working surface for the valve. The quality of the sealing surface is related to the service life of the valve. Usually, the sealing surface's material should consider factors such as corrosion resistance, scratch resistance, erosion resistance, and oxidation resistance, which are divided into the following two categories:
Soft sealed materials
- Rubber including nitrile rubber and fluorine rubber
- Plastics such as polytetrafluoroethylene and nylon
Hard sealed materials
- Copper alloys used for low-pressure valves
- Chrome stainless steel for ordinary high and medium pressure valves
- Stellite alloys for high-temperature and high-pressure valves as well as strong corrosive valves
- Nickel-based alloys for corrosive media
Materials for valve stems
In the valve's opening and closing process, the valve stem is subjected to tensile, compressive and torsional forces, and is in direct contact with the medium. At the same time, there is a relative frictional movement between the valve stem and the packing. Therefore, the valve stem's material must ensure that it has sufficient strength, impact toughness, certain corrosion resistance, scratch resistance, and good processability. Commonly used materials for valve stems are as follows:
Carbon steel
When it is used in water and steam with low pressure and medium temperature not exceeding 300℃, A5 ordinary carbon steel is generally used. 35 high-quality carbon steel is often used for medium pressure and water and steam with medium temperatures not exceeding 450°C.
Alloy steel
When it is used for medium pressure and high pressure, and the medium temperature does not exceed 450℃, such as water, steam, petroleum and other media; 40Cr (chrome steel) is generally used. 38CrMoALA nitrided steel can be selected for high pressure and medium temperature not exceeding 540℃ such as water and steam, while 25Cr2MoVA chrome molybdenum vanadium steel is generally used in high-pressure steam whose temperature does not exceed 570℃.
Stainless and acid-resistant steel
For medium and high pressure, non-corrosive medium and mildly corrosive medium with medium temperature not exceeding 450℃, 1Cr13, 2Cr13, 3Cr13 chromium stainless steel can be selected. Stainless acid-resistant steel such as Cr17Ni2, 1Cr18Ni9Ti, Cr18Ni12Mo2Ti, Cr18Ni12Mo3Ti and PH15-7Mo precipitation hardening steel can be selected for corrosive media.
Heat-resistant steel
4Cr10Si2Mo martensitic heat-resistant steel and 4Cr14Ni14W2Mo austenitic heat-resistant steel can be selected for high-temperature valves with temperatures not exceeding 600℃.
Materials used for stem nuts
The valve stem nut directly bears the axial force of the valve stem in the opening and closing process of the valve, so it must have a certain strength. Meanwhile, it is driven by a thread with the valve stem, which requires a small friction coefficient, no rust and no seizing.
Copper alloys
Copper alloys have a small friction coefficient and do not rust, which is one of the commonly used materials at present. For low-pressure valves less than 1.6Mpa, ZHMn58-2-2 cast brass can be used. For high-pressure valves, ZHAL66-6-3-2 cast brass can be used.
Steel
When the working conditions do not allow copper alloys to be used, high-quality carbon steel such as 35 and 40, stainless acid-resistant steel such as 2Cr13, 1Cr18Ni9, and Cr17Ni2 can be selected. The following situation should be paid attention to:
In the valve's opening and closing process, the valve stem is subjected to tensile, compressive and torsional forces, and is in direct contact with the medium. At the same time, there is a relative frictional movement between the valve stem and the packing. Therefore, the valve stem's material must ensure that it has sufficient strength, impact toughness, certain corrosion resistance, scratch resistance, and good processability. Commonly used materials for valve stems are as follows:
Carbon steel
When it is used in water and steam with low pressure and medium temperature not exceeding 300℃, A5 ordinary carbon steel is generally used. 35 high-quality carbon steel is often used for medium pressure and water and steam with medium temperatures not exceeding 450°C.
Alloy steel
When it is used for medium pressure and high pressure, and the medium temperature does not exceed 450℃, such as water, steam, petroleum and other media; 40Cr (chrome steel) is generally used. 38CrMoALA nitrided steel can be selected for high pressure and medium temperature not exceeding 540℃ such as water and steam, while 25Cr2MoVA chrome molybdenum vanadium steel is generally used in high-pressure steam whose temperature does not exceed 570℃.
Stainless and acid-resistant steel
For medium and high pressure, non-corrosive medium and mildly corrosive medium with medium temperature not exceeding 450℃, 1Cr13, 2Cr13, 3Cr13 chromium stainless steel can be selected. Stainless acid-resistant steel such as Cr17Ni2, 1Cr18Ni9Ti, Cr18Ni12Mo2Ti, Cr18Ni12Mo3Ti and PH15-7Mo precipitation hardening steel can be selected for corrosive media.
Heat-resistant steel
4Cr10Si2Mo martensitic heat-resistant steel and 4Cr14Ni14W2Mo austenitic heat-resistant steel can be selected for high-temperature valves with temperatures not exceeding 600℃.
Materials used for stem nuts
The valve stem nut directly bears the axial force of the valve stem in the opening and closing process of the valve, so it must have a certain strength. Meanwhile, it is driven by a thread with the valve stem, which requires a small friction coefficient, no rust and no seizing.
Copper alloys
Copper alloys have a small friction coefficient and do not rust, which is one of the commonly used materials at present. For low-pressure valves less than 1.6Mpa, ZHMn58-2-2 cast brass can be used. For high-pressure valves, ZHAL66-6-3-2 cast brass can be used.
Steel
When the working conditions do not allow copper alloys to be used, high-quality carbon steel such as 35 and 40, stainless acid-resistant steel such as 2Cr13, 1Cr18Ni9, and Cr17Ni2 can be selected. The following situation should be paid attention to:
- For electric valves, stem nuts with melon-shaped clutches require heat treatment to obtain good hardness or surface hardness.
- Copper alloys are not suitable for the working medium or the surrounding environment, such as ammonia, which is corrosive to copper.
- When choosing a steel stem nut, special attention should be paid to the seizure of the thread.
Materials for fasteners, packings and gaskets
Materials for fasteners
Fasteners mainly include bolts, studs and nuts. Fasteners directly bear pressure on the valve and play an important role in preventing the outflow of the medium. Therefore, the selected materials must ensure sufficient strength and impact toughness at the operating temperature. According to the medium pressure and temperature, the fastener's material can be selected according to the table.
Materials for fasteners
Fasteners mainly include bolts, studs and nuts. Fasteners directly bear pressure on the valve and play an important role in preventing the outflow of the medium. Therefore, the selected materials must ensure sufficient strength and impact toughness at the operating temperature. According to the medium pressure and temperature, the fastener's material can be selected according to the table.
| Names | Nominal pressure (Mpa) | Medium temperatures (℃) | |||||
| 300 | 350 | 400 | 425 | 450 | 530 | ||
| Bolts and studs | 1.6 to 2.5 | Q235（A3） | 35 | 30CrMoA | - | ||
| 4.0 to 10.0 | 35 | 35CrMoA | 25Cr2MoVA | ||||
| 16.0 to 20.0 | 30CrMoA | 35CrMoA | 25Cr2MoVA | ||||
| Nuts | 1.6 to 2.5 | Q235（A3） | 30 | 35 | - | ||
| 4.0 to 10.0 | 30 | 35 | 35CrMoA | ||||
| 16.0 to 20.2 | 35 | 35CrMoA | |||||
When alloy steel is selected, it must be heat treated. When there are special requirements for corrosion resistance for fasteners, stainless acid-resistant steels such as Cr17Ni2, 2Cr13 and 1Cr18Ni9 can be selected.
Materials for packings
For valves, packing is used to fill the space of the packing chamber of the valve bonnet to prevent the medium from leaking through the valve stem and the packing chamber of the valve bonnet.
Requirements for packings are as follows:
- They should have good corrosion resistance. They must be able to resist the corrosion of the medium when being in contact with the medium.
- They should have good sealing performance, and the packing does not leak under the action of the medium and working temperature.
- The friction coefficient should be small to reduce the friction torque between the valve stem and the packing.