Wenzhou Unique Valve Co., Ltd.
Hard Sealed Valve Seats of Ball Valves
Jun 17, 2021
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1. Hard sealed valve seats of ball valves
The main function of a ball valve is to cut off or connect the fluid in the pipeline. The working principle of the ball valve is to apply a certain torque on the upper end of the valve stem with the help of a driving device to transmit to the ball so that it can rotate 90° to open or close the valve. There are hard sealed valve seats and soft sealed valve seats for ball valves. The structure of a hard sealed valve seat is only introduced in this article.
Four different kinds of hard sealed valve seats
(1) The surfacing type
The surfacing valve seat (Figure 1) is to surface hard alloys on a certain metal. The valve seat has an elastic tail seat, which helps to reduce friction on the sealing surface.
Figure 1 The surfacing valve seat
(2) The multi layer type
The multi layer valve seat (Figure 2) is made of alternately stacking multi-layer stainless steel sheets and multi-layer thin graphite sheets rolled and pressed by special processing methods, which improves the performance under high temperature and high pressure.
Figure 2 The multi layer valve seat
(3) The hollow tube type
The hollow tube valve seat (Figure 3) has a bushing inside and outside, and a stainless steel tube in the middle. The elastic force of hollow tubes makes the valve seat press against the sphere.
Figure 3 The hollow tube valve seat
(4) The rotary type
The main feature of the rotary valve seat (Figure 4) is that in the opening and closing process of the valve, the valve seat can rotate along the axis of the channel. The structure has a circle of teeth distributed on the circumference of the valve seat. Every time the sphere is closed, the valve seat rotates for one tooth. This makes the valve seat wear uniformly along the circumference, thereby improving the working condition of the valve seat and prolonging the service life of the valve seat.
Figure 4 The rotary valve seat
Four different kinds of hard sealed valve seats
(1) The surfacing type
The surfacing valve seat (Figure 1) is to surface hard alloys on a certain metal. The valve seat has an elastic tail seat, which helps to reduce friction on the sealing surface.
Figure 1 The surfacing valve seat
(2) The multi layer type
The multi layer valve seat (Figure 2) is made of alternately stacking multi-layer stainless steel sheets and multi-layer thin graphite sheets rolled and pressed by special processing methods, which improves the performance under high temperature and high pressure.
Figure 2 The multi layer valve seat
(3) The hollow tube type
The hollow tube valve seat (Figure 3) has a bushing inside and outside, and a stainless steel tube in the middle. The elastic force of hollow tubes makes the valve seat press against the sphere.
Figure 3 The hollow tube valve seat
(4) The rotary type
The main feature of the rotary valve seat (Figure 4) is that in the opening and closing process of the valve, the valve seat can rotate along the axis of the channel. The structure has a circle of teeth distributed on the circumference of the valve seat. Every time the sphere is closed, the valve seat rotates for one tooth. This makes the valve seat wear uniformly along the circumference, thereby improving the working condition of the valve seat and prolonging the service life of the valve seat.
Figure 4 The rotary valve seat
2. Sealing materials
Sealing materials of valve seats mainly include special polymer and metal.
Special polymer
Polymers are divided into two categories: general polymers and special polymers. General polymers mainly include polyvinyl chloride, polyethylene, polypropylene, epoxy resin, phenolic resin and furan resin. Special polymers mainly include polytetrachloroethylene, polychlorotrifluoroethylene, polyperfluoroethylene propylene, polyether ether ketone and polyimide.
(1) PTFE
Polytetrafluoroethylene (PTFE-F4) not only has excellent heat and cold resistance, but also has the characteristics of non stickiness, good self lubrication and low friction coefficient (0.05). However, it has a tendency to cold flow. It can work for a long time in temperatures between -250 and 260℃, and can bear a load of 5MPa. PTFE has a density of 2.2g/cm3, thermal conductivity of 0.25W/(mc), linear expansion coefficient of (10~15)×10-5/℃, tensile strength of 10 to 30MPa, elastic modulus of 400MPa, compressive strength of 710MPa, and bending strength of 14MPa. It can resist concentrated acid, concentrated alkali, strong oxidant (for example, aqua regia), any concentration of boiling hydrofluoric acid and other media.
(2) Polychlorotrifluoroethylene
Polychlorotrifluoroethylene (PCTFE-F3) has excellent cold resistance, high strength and good hardness, good dimensional stability, and no tendency to cold flow. The operating temperature is from -195 to 120℃. The density is 2.1g/cm3. The thermal conductivity is 0.058W/(mc). The linear expansion coefficient is (415-710)×10-5/℃. The tensile strength is between 35 and 40MPa. The modulus of elasticity is 1300MPa. The compressive strength is from 50 to 70MPa, and the bending strength is 70MPa. Polychlorotrifluoroethylene can resist inorganic acids at high temperatures, strong oxidants in salt solutions at lower temperatures, and most organic media at room temperatures.
(3) Polyperfluoroethylene propylene
Polyperfluoroethylene propylene (PFEP-F46) has high temperature and low temperature resistance, which can work for a long time in temperatures between -85 and 205℃. Its density is2.1g/cm3, thermal conductivity 0.25W/(mc), linear expansion coefficient (9.5~19.6)×10-5/℃, tensile strength from 19 to 20MPa, compressive strength 710MPa, and bending strength 14MPa. It has chemical stability equivalent to that of polytetrafluoroethylene, and can resist various concentrations of inorganic acids, alkalis, alcohols, ketones, hydrocarbons, halogenated hydrocarbons, fenhydrocarbons, grease, detergents, etc.
(4) Eurelon
Eurelon (N1010) has good tensile strength, good impact toughness, good hardness and fatigue strength at room temperatures, especially excellent wear resistance, self-lubricating properties, and low friction coefficient (0.03). Its density is 1.5g/cm3, thermal conductivity 0.25W/(m#c), linear expansion coefficient about 10.5×10-5/℃, tensile strength 196MPa, elastic modulus 1300MPa, compressive strength 310MPa and bending strength 310MPa. Polyamide has good resistance to chemicals, weak acid, weak alkali, general solutions and oil.
(5) Para-Polyphenylene
Para-polyphenylene (PPL) has the characteristics of high-temperature resistance, chemical resistance, solvent resistance, self-lubrication, radiation resistance, and low friction coefficient (0.13). Its density is 2g/cm3, linear expansion coefficient about 5.1×10-5/℃, hardness 80HS, tensile strength 16MPa, compressive strength 48MPa, and bending strength 28MPa. It can resist the corrosion of strong acid and alkali such as sulfuric acid and hydrofluoric acid.
(6) Polyetheretherketone
Polyetheretherketone (PEEK) has the heat resistance of thermosetting plastic, chemical stability and molding processability of thermoplastic plastics. The continuous operating temperature can reach above 240℃. The glass fiber reinforcement can reach above 300℃, and the density is 1.3g/cm3. The tensile strength is 162MPa, and the bending strength is 233MPa. Polyetheretherketone can withstand the erosion of most inorganic solvents, except for concentrated sulfuric acid and nitric acid, which can be used for a long time in a steam of 200 to 260℃.
(7) Polyimide
Polyimide (PI) is a new type of high-temperature resistant engineering plastic, which can be used for a long time in the air from -240 to 260℃. It has excellent electrical insulation, abrasive resistance, high-temperature radiation resistance and physical and mechanical properties. Its tensile strength is 100MPa, and the elastic modulus is 4MPa. It can resist dilute acids and most organic solvents at room temperatures, but it is not resistant to strong oxidizing acids and strong acids; it is not suitable for long-term immersion in water.
Special polymer
Polymers are divided into two categories: general polymers and special polymers. General polymers mainly include polyvinyl chloride, polyethylene, polypropylene, epoxy resin, phenolic resin and furan resin. Special polymers mainly include polytetrachloroethylene, polychlorotrifluoroethylene, polyperfluoroethylene propylene, polyether ether ketone and polyimide.
(1) PTFE
Polytetrafluoroethylene (PTFE-F4) not only has excellent heat and cold resistance, but also has the characteristics of non stickiness, good self lubrication and low friction coefficient (0.05). However, it has a tendency to cold flow. It can work for a long time in temperatures between -250 and 260℃, and can bear a load of 5MPa. PTFE has a density of 2.2g/cm3, thermal conductivity of 0.25W/(mc), linear expansion coefficient of (10~15)×10-5/℃, tensile strength of 10 to 30MPa, elastic modulus of 400MPa, compressive strength of 710MPa, and bending strength of 14MPa. It can resist concentrated acid, concentrated alkali, strong oxidant (for example, aqua regia), any concentration of boiling hydrofluoric acid and other media.
(2) Polychlorotrifluoroethylene
Polychlorotrifluoroethylene (PCTFE-F3) has excellent cold resistance, high strength and good hardness, good dimensional stability, and no tendency to cold flow. The operating temperature is from -195 to 120℃. The density is 2.1g/cm3. The thermal conductivity is 0.058W/(mc). The linear expansion coefficient is (415-710)×10-5/℃. The tensile strength is between 35 and 40MPa. The modulus of elasticity is 1300MPa. The compressive strength is from 50 to 70MPa, and the bending strength is 70MPa. Polychlorotrifluoroethylene can resist inorganic acids at high temperatures, strong oxidants in salt solutions at lower temperatures, and most organic media at room temperatures.
(3) Polyperfluoroethylene propylene
Polyperfluoroethylene propylene (PFEP-F46) has high temperature and low temperature resistance, which can work for a long time in temperatures between -85 and 205℃. Its density is2.1g/cm3, thermal conductivity 0.25W/(mc), linear expansion coefficient (9.5~19.6)×10-5/℃, tensile strength from 19 to 20MPa, compressive strength 710MPa, and bending strength 14MPa. It has chemical stability equivalent to that of polytetrafluoroethylene, and can resist various concentrations of inorganic acids, alkalis, alcohols, ketones, hydrocarbons, halogenated hydrocarbons, fenhydrocarbons, grease, detergents, etc.
(4) Eurelon
Eurelon (N1010) has good tensile strength, good impact toughness, good hardness and fatigue strength at room temperatures, especially excellent wear resistance, self-lubricating properties, and low friction coefficient (0.03). Its density is 1.5g/cm3, thermal conductivity 0.25W/(m#c), linear expansion coefficient about 10.5×10-5/℃, tensile strength 196MPa, elastic modulus 1300MPa, compressive strength 310MPa and bending strength 310MPa. Polyamide has good resistance to chemicals, weak acid, weak alkali, general solutions and oil.
(5) Para-Polyphenylene
Para-polyphenylene (PPL) has the characteristics of high-temperature resistance, chemical resistance, solvent resistance, self-lubrication, radiation resistance, and low friction coefficient (0.13). Its density is 2g/cm3, linear expansion coefficient about 5.1×10-5/℃, hardness 80HS, tensile strength 16MPa, compressive strength 48MPa, and bending strength 28MPa. It can resist the corrosion of strong acid and alkali such as sulfuric acid and hydrofluoric acid.
(6) Polyetheretherketone
Polyetheretherketone (PEEK) has the heat resistance of thermosetting plastic, chemical stability and molding processability of thermoplastic plastics. The continuous operating temperature can reach above 240℃. The glass fiber reinforcement can reach above 300℃, and the density is 1.3g/cm3. The tensile strength is 162MPa, and the bending strength is 233MPa. Polyetheretherketone can withstand the erosion of most inorganic solvents, except for concentrated sulfuric acid and nitric acid, which can be used for a long time in a steam of 200 to 260℃.
(7) Polyimide
Polyimide (PI) is a new type of high-temperature resistant engineering plastic, which can be used for a long time in the air from -240 to 260℃. It has excellent electrical insulation, abrasive resistance, high-temperature radiation resistance and physical and mechanical properties. Its tensile strength is 100MPa, and the elastic modulus is 4MPa. It can resist dilute acids and most organic solvents at room temperatures, but it is not resistant to strong oxidizing acids and strong acids; it is not suitable for long-term immersion in water.
3. Metal sealing materials
The metal sealing surface has good hardness and a suitable hardness difference. Surface hardening can increase hardness and enhance wear resistance. Metal sealing materials mainly include chromium-based cemented carbide, cobalt-based cemented carbide, nickel alloys, tungsten carbide and stainless steel (Table 1).
Table 1 Metal sealing materials
Table 1 Metal sealing materials
| Materials | Working conditions | Hardness HRC |
| Chromium-based cemented carbide | ||
| D507 | For valves used below 450℃ | Higher than or equal to 40 |
| D507M o | For valves used below 510℃ | Higher than or equal to 38 |
| D507M o | For valves used below 600℃ | Higher than or equal to 37 |
| D577 | For valves used below 510℃ | Higher than or equal to 28 |
| Cobalt-based cemented carbide | ||
| STL.6 | For valves used below 650℃ | Higher than or equal to 40 |
| STL.12 | For valves used below 650℃ | Higher than or equal to 42 |
| D802 | For valves used below 650℃ | Higher than or equal to 40 |
| D812 | For valves used below 650℃ | Higher than or equal to 42 |
| Nickel alloys | ||
| Dni347 | Used for sealing surfaces of the nuclear power plant's valve | |
| Dni357 | Used for the valve seat's sealing surface of heat-resistant and corrosion-resistant media | |
| Tungsten carbide | ||
| D 707 | For valves used below 650℃ Higher than or equal to 60 |
Higher than or equal to 60 |
| D 717 | For valves used below 650℃ Higher than or equal to 60 |
Higher than or equal to 60 |
| Stainless steel | ||
| A 102 | For corrosion-resistant valves used below 300℃ | |
| A 202 | For non-oxidizing acids | |
| A 212 | Used in urea and synthetic fiber media | |
| A 902 | Used for sulfur, nitrate, phosphoric acid and oxidizing acid, etc. | |
4. Conclusion
The valve seat and sealing ring are the key components for a ball valve. The sealing ring should have excellent stability, a small friction coefficient, good sealing performance, and not easily age. The valve seat should have a certain degree of strength and rigidity, a moderate clearance and not easily deforme under pressure. The metal sealed valve seat should meet the functions of high temperature, high pressure, strong corrosion, ultra-low temperature, zero leakage, long service life, fire safety, pressure and wear resistance.
