What is a Valve Seat?
A valve seat is a critical component in a valve that provides a sealing surface for the valve disc or ball, ensuring proper shutoff and flow control. It plays a key role in preventing leakage and maintaining the efficiency of a system. Valve seats are found in various types of industrial valves, including control valves, ball valves, butterfly valves, and gate valves.
What Does a Valve Seat Do?
The primary function of a valve seat is to create a tight seal when the valve is closed, preventing process fluid from passing through the valve. The effectiveness of this seal depends on the material composition of the valve seat, which must withstand factors such as pressure, temperature, chemical exposure, and mechanical wear.
Categories of Valve Seats: Soft vs. Hard Seats
Valve seats generally fall into two main categories: soft seats and hard seats. The choice between these depends on factors such as operating conditions, leakage requirements, and media compatibility.
Soft Valve Seats: Soft valve seats are typically made from elastomers, polymers, or thermoplastics such as PTFE (Polytetrafluoroethylene), RTFE (Reinforced PTFE), TFM (Modified PTFE), Viton (Fluoroelastomer), and EPDM (Ethylene Propylene Diene Monomer). See the section below for further details about the most common soft seat materials.
Soft seats tend to be used when a bubble-tight seal is required (zero leakage), in applications handling gases, clean fluids, or low-pressure steam, when corrosion resistance is a priority, and for moderate-temperature applications (typically below 500°F / 260°C). They have a low coefficient of friction and have self-lubricating properties, and tend to be less expensive than hard seat materials.
Hard Valve Seats: Hard valve seats are made from metallic or ceramic materials, including: Stainless Steel, Stellite, Tungsten Carbide, Chromium Carbide and Ceramics. See the section below for further details about the most common hard seat materials.
Hard seats tend to be used when handling high-temperature, high-pressure, or abrasive media, or in severe service conditions such as those encountered in oil & gas, power plants, and chemical processing, or when fire-safe operation is required (soft seats can degrade in fire), and in high-cycling applications where wear resistance is crucial
How Valve Seat Material Selection Impacts Valve Leakage
The choice of valve seat material directly influences the leakage rate of a valve. Leakage classifications are defined by ANSI/FCI 70-2 and range from Class I (highest leakage) to Class VI (zero leakage). See our page on valve leakage classifications for further information on this topic.
Whereas soft seated valves are ideal for achieving Class VI (bubble-tight) shutoff, making them preferable for applications requiring absolute sealing, metal seated valves, generally provide greater durability at the expense of a lower leakage class, e.g. Class IV or V. This trade off allows minimal permissible leakage but offers better performance in extreme conditions.
Optimising Valve Seat Selection for Your Application
By choosing the appropriate soft or hard seat material for a valve, engineers can optimise valve performance, improve efficiency, and minimise downtime. Selecting the right valve seat material for a particular application requires evaluating:
- Process fluid operating pressure and temperature range
- The chemical compatibility of the valve seat material with the process media
- Leakage requirements as specified by the process engineer
- Maintenance and life cycle cost considerations. For example, corrosion, errosion and wear.
Soft Seat Materials In Detail
Polytetrafluoroethylene (PTFE): Polytetrafluoroethylene, popularly known as Teflon, is a synthetic fluoropolymer and is the most popular valve seat material. It has low friction, high resilience, and is highly resistant to chemicals and fire. Due to its non-toxicity, it is used in food and pharmaceutical manufacturing. PTFE seats have a temperature range of around -45°C to 230°C.Reinforced Polytetrafluoroethylene (RTFE): Reinforced Polytetrafluoroethylene is a modification of PTFE that contains 15% glass fibre. The glass fibre improves the wear and abrasion resistance of PTFE and increases the compressive strength and pressure rating of the seat. It is not compatible with hydrofluoric acid and caustic soda because of its degrading reaction to glass. RTFE seats have a temperature range of around -40°C to 230°C.
Ethylene-propylene Diene Monomer (EPDM): Ethylene-propylene Diene Monomer is a kind of synthetic rubber that has high resiliency and abrasion and wear resistance. EPDM valve seats can handle acids, alcohols, and alkaline solutions and are ideal for handling chlorinated water, bleach, and concentrated sulfuric acid. EPDM is also resistant to ozone. However, it is not compatible with petroleum, hydrocarbon solvents and oils, or turpentine. EPDM seats have a temperature range of around -40°C to 120°C.
Buna-N: Buna-N, also known as nitrile rubber, is a multi-purpose rubber that has high strength and abrasion resistance. Buna-N seats are used in the handling of gases, water, fuels, petroleum, and alcohol, but they are not compatible with acetones, ketones, chlorinated and nitro hydrocarbons, or ozone. Buna-N seats have a temperature range of around -0°C to 100°C.
Viton: Viton, or also known as FKM in its ASTM short form name is a fluorocarbon elastomer that is compatible with most chemicals, but it has poor performance in handling steam and water. Depending on the grade, Viton seats have a working temperature range around -26°C to +205°C.
Neoprene: Neoprene or chloroprene rubber is a synthetic rubber that possesses good flexibility. They are compatible with greases, oils, fats, and most chemicals, except for strong oxidizing agents. They are also flame resistant. Neoprene seats are used in handling refrigerants such as Freon and ammonia, and in pulp and paper manufacturing. Due to their black color, they are not recommended in handling utility water for bleached paper production lines. Temperature rating for Neoprene valve seats is around -29°C to 82°C.
Hard Seat Materials In Detail
PEEK: Polyetheretherketone (PEEK) is a high pressure, semi rigid elastomer. With a temperature rating of around -50°C to 280°C, PEEK valve seats are best suited to high temperature and pressure applications. PEEK also offers fairly good corrosion resistance.Inconel: Inconel is a nickel-chromium-molybdenum alloy that has excellent mechanical properties and good corrosion, oxidation, and scaling resistance that is maintained in extreme environments and high temperatures. Inconel seats can operate up to 700°C, depending on the grade of inconel. Inconel 625 and 600 are the commonly used Inconel grades in the construction of metal seats.
Stainless Steel: Stainless steel is a less expensive alternative to Inconel. See our page on Grades of stainless steel for more information on the types of stainless steels available, and their attributes. Stainless steel seats are commonly coated with TCC (Tungsten Carbide Coating) or CCC (Chromium Carbide Coating).
In Summary
Understanding valve seat materials is essential for both instrumentation engineers and piping engineers. The right selection enhances system reliability, ensures optimal flow control, and prevents costly leaks.
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