Plug Valve Specifications

These specifications are recommended for plug valves in the following typical services 100-pound air, 50-pound caustic, 15-pound condensate, 50-pound fuel, 150-pound hydrocarbon, and 40-pound exhaust. The ratings should be specified as 230 psi at 100°F, 150 psi at 500°F, and 100 psi at 750°F (maximum). 

For the 1-1/4-inch size, specify, 2,000-pound WOG, forged carbon-steel, screwed, wrench-operated, lubricated plug valve. 

For 2-inch to 6-inch size plug valves, specify 150-pound ANSI standard cast carbon-steel, flanged, 1/16-inch raised face, wrench operated, lubricated plug valve. 

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There are many configurations of plugs and seats ( trim ) that create different control responses. The design specification of control valves is beyond the scope of this book. 

More formal inspections also verily the operation of safety and other controls such as low water and fuel cutoffs, level controls, fusible plugs, pressure gauges, water glasses, gage cocks, stop valves, safety/relief valves, and BD valves and lines. Also, FW pumps, flue and damper arrangements, combustion safeguards, name plate specifications, set pressures, boiler connections, floats, mercury switches, bellows, and other components may be inspected. 

High Pressure Valves used for pressures nominally above 760 bar (11,000 psi, pressures above ANSI Class 4500) are often custom-designed for specific applications. Normally, these valves are of the plug type and use sped ally hardened plug and seat assemblies. Internal surfaces are polished, and internal corners and intersecting bores are smoothed to reduce high localized stresses in the valve body. Steam loops in the valve body are available to raise the body temperature to increase ductility and impact strength of the body material. 

The original project installed 3-inch (7.6 cm) Teflon-lined valve bodies and Teflon-lined plugs. Some of the supervisors seem to remember that the original valves were very stiff to operate. It is believed that well-meaning individuals requested easier operating valves, and no one worried about reviewing the plant specifications which were not always up-to-date. Even if piping specifications were consulted, this 3-inch (7.6 cm) branch fine on a 24-inch (61 cm) line was itself a plant modification that was not properly authorized. 

API Specification 6D Specification for Pipeline Valves (Gate, Plug, Ball,... 

K = constant which depends on the valve size p = specific gravity of the flowing liquid (relative to water) fix) = valve flow characteristic curve The valve flow characteristic curve, fix), depends on the geometrical shape of the plug s surface. Figure 13.10 shows the most common types of plugs while Figure 13.11 shows the flow capacity characteristics for the various valves. 

Other techniques sometimes used to prevent plugging at the relief valve base include (98a) using a liquid seal, steam tracing, and heating (from outside). The last two are particularly useful if the plugging can be caused by freezing. The reliability and effectiveness of any specific technique for a specific service must be carefully evaluated. 

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