Is a Metal-seated Ball Valve for You?Andrew Sleasman | February 17, 2015
Metal-seated ball valves were first introduced to the market to address many of the shortcomings of their soft-seat counterparts. The ability to withstand severe service conditions such as high temperatures, cryogenic conditions, abrasive mediums, slurries, and flow control make them ideal candidates for the use in 0il refinery, petrochemical, mining, pulp and paper, iron and steel mill, electric power generation and water treatment industries.
Ball valves are a low-torque, quarter-turn valve with low resistance to flow that are suitable for on-off utility and process services. They have a straight-through configuration and good control characteristics but are not generally used for throttling applications. The majorities of ball valves have soft-seat inserts and use a thermoplastic material such as polytetrafluoroethylene (PTFE) or nitrile butadiene rubber (NBR). Metal-seated ball valves use metal such as 316 stainless steel or Monel as the seat material.
Thermoplastic seals are recommended for clean service only. They are not suitable for dirty, abrasive or high-or extremely low-temperature service. For more severe service applications, metal-seated ball valves are commonly the choice. Each ball valve seat type has its own characteristic and its own advantages and disadvantages.
Uses and Applications
Metal-seated ball valves incorporate a metal-to-metal seal between the seats and ball of the valve assembly. They are made for the severe service of abrasives, corrosives, high-temperature or high-pressure applications. Most metal-seated ball valves are typically used for isolation, but many are also used for uni- or bi-directional control as they offer tight shutoff in severe service applications. They adhere to ANSI Class IV leakage standards which allows for no more than 0.01% leakage of full open valve capacity. The test pressure for this classification is fixed between 45 psig and 60 psig. If the application pressure is significantly higher, the leakage rate may be greater than 0.01%.
For metal-seated valves, the design of the seats and coatings are key factors in determining if the valve is used with abrasives or high temperatures. Some seats have elastomer insert elements that may degrade and cause leakage under high temperatures. Most of the elastomer seals have a maximum temperature of around 220° C (450° F). Other options such as metal powder-filled Teflon seats or similar materials allow for the temperature to be stretched to 315° C (600° F). If greater temperature resistance is needed, especially with any significant pressure, full metal seats with hard coatings can be used to achieve temperature resistance up to 815° C (1500° F).
Coatings for Severe Service
In severe-service applications where ether high temperatures, severe flashing or hydraulic shock is expected, or where corrosion resistance or abrasion resistance is needed, hard coatings are required. Seat and ball corrosion prevention and temperature resistance is critical. If operating temperatures are below 150° C (302° F), the coating would normally be tungsten carbide (HVOF). For higher temperatures and greater hardness, other carbides such as chrome carbide and cobalt-chromium alloys can be used to achieve hardness values of 70+ HRC and temperature values up to 815° C (1500°F). These coatings have extremely good wear and abrasion resistance at high temperature to continuously provide tight shut-off.
Floating or Trunnion?
Metal-seated ball valves are available in two-piece or three-piece designs and, depending on size, it can have either a floating or trunnion-mounted ball, which differ in terms of how the ball is mounted and how the seats seal against the ball.
The ball in a trunnion valve is mechanically anchored at the top and bottom and uses a spring mechanism and line pressure to push the upstream seat against the ball. The spring in a trunnion-mounted valve seal holds the seat to the face of the valve during a rotation when there is no, or low differential pressure. The sealing force comes from the fluid forcing the seat against the ball face. This design is usually for larger and higher pressure valves.
By contrast, a floating ball valve has a simpler structure, as the valve has a floating ball that is not anchored by a trunnion inside the valve body. Under line pressure it drifts toward the downstream side and pushes tightly against the seat to form a seal. Floating ball valves offer good sealing performance but are mainly used in middle- or low-pressure applications. The load from a floating ball onto the seat can be high, thus explaining why floating ball valve diameters rarely are greater than six inches.
Manufacturers offer many different options including valve materials, seat materials, coatings and mechanical design to suit specific application needs. Metal-seated ball valves can be initially expensive to purchase, but when the cost of downtime is factored in, they can be well worth the investment.
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