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Is a Metal-seated Ball Valve for You? Andrew Sleasman

作者:互联网    浏览:546    发布时间:2015-8-7 9:26:22

Metal-seated ball valves were firstintroduced to the market to address many of the shortcomings of their soft-seatcounterparts. The ability to withstand severe service conditions such as hightemperatures, cryogenic conditions, abrasive mediums, slurries, and flowcontrol make them ideal candidates for the use in 0il refinery, petrochemical,mining, pulp and paper, iron and steel mill, electric power generation andwater treatment industries.

Ball valves are a low-torque, quarter-turnvalve with low resistance to flow that are suitable for on-off utility andprocess services. They have a straight-through configuration and good controlcharacteristics but are not generally used for throttling applications. Themajorities of ball valves have soft-seat inserts and use a thermoplasticmaterial such as polytetrafluoroethylene (PTFE) or nitrile butadiene rubber(NBR). Metal-seated ball valves use metal such as 316 stainless steel or Monelas the seat material.

Thermoplastic seals are recommended for cleanservice only. They are not suitable for dirty, abrasive or high-or extremelylow-temperature service. For more severe service applications, metal-seatedball valves are commonly the choice. Each ball valve seat type has its owncharacteristic and its own advantages and disadvantages.

 

Uses and Applications

Metal-seated ball valves incorporate ametal-to-metal seal between the seats and ball of the valve assembly. They aremade for the severe service of abrasives, corrosives, high-temperature orhigh-pressure applications. Most metal-seated ball valves are typically usedfor isolation, but many are also used for uni- or bi-directional control asthey offer tight shutoff in severe service applications. They adhere to ANSIClass IV leakage standards which allows for no more than 0.01% leakage of fullopen valve capacity. The test pressure for this classification is fixed between45 psig and 60 psig. If the application pressure is significantly higher, theleakage rate may be greater than 0.01%.

For metal-seated valves, the design of theseats and coatings are key factors in determining if the valve is used withabrasives or high temperatures. Some seats have elastomer insert elements thatmay degrade and cause leakage under high temperatures. Most of the elastomerseals have a maximum temperature of around 220° C (450° F). Other options suchas metal powder-filled Teflon seats or similar materials allow for thetemperature to be stretched to 315° C (600° F). If greater temperatureresistance is needed, especially with any significant pressure, full metalseats with hard coatings can be used to achieve temperature resistance up to815° C (1500° F).

 

Coatings for Severe Service

In severe-service applications where etherhigh temperatures, severe flashing or hydraulic shock is expected, or wherecorrosion resistance or abrasion resistance is needed, hard coatings arerequired. Seat and ball corrosion prevention and temperature resistance iscritical. If operating temperatures are below 150° C (302° F), the coatingwould normally be tungsten carbide (HVOF). For higher temperatures and greaterhardness, other carbides such as chrome carbide and cobalt-chromium alloys canbe 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 athigh temperature to continuously provide tight shut-off.

 

Floating or Trunnion?

Metal-seated ball valves are available intwo-piece or three-piece designs and, depending on size, it can have either afloating or trunnion-mounted ball, which differ in terms of how the ball ismounted and how the seats seal against the ball.

The ball in a trunnion valve is mechanicallyanchored at the top and bottom and uses a spring mechanism and line pressure topush the upstream seat against the ball. The spring in a trunnion-mounted valveseal holds the seat to the face of the valve during a rotation when there isno, or low differential pressure. The sealing force comes from the fluidforcing the seat against the ball face. This design is usually for larger andhigher pressure valves.

By contrast, a floating ball valve has asimpler structure, as the valve has a floating ball that is not anchored by atrunnion inside the valve body. Under line pressure it drifts toward thedownstream side and pushes tightly against the seat to form a seal. Floatingball valves offer good sealing performance but are mainly used in middle- orlow-pressure applications. The load from a floating ball onto the seat can behigh, thus explaining why floating ball valve diameters rarely are greater thansix inches.

Manufacturers offer many different optionsincluding valve materials, seat materials, coatings and mechanical design tosuit specific application needs. Metal-seated ball valves can be initiallyexpensive to purchase, but when the cost of downtime is factored in, they canbe well worth the investment.