- gate,
- globe,
- check,
- butterfly,
- ball,
- plug, or
- diaphragm.
In the process of performing its control function, a valve must satisfy two conditions.
- First, the fluid cannot be allowed to leak into the environment.
- Second, there can be no internal leakage.
That is, when the valve is closed there can be no flow, either along the normal fluid path or between valve parts, where flow is never intended. These two conditions are not always met absolutely. Valve testing standards, such as the American Petroleum Institute Standard API 598, permit a very small amount of leakage at the seating surfaces for some types and sizes of valves.
All valve types are manufactured with ends that mate with the common piping connection methods:
- threaded (also called screwed),
- flanged,
- butt-weld
- socket-weld.
- solder, and
- grooved.
The appropriate connection method to be used on the valves in a specific pipeline is decided by the piping designer.
The designers decision is based on factors such as line size, fluid pressure, materials of construction, and ease of assembly. Small valves are usually manufactured with threaded or socket-weld ends; whereas large valves are usually manufactured with flanged or butt-weld prepared ends.
Some valves are made with no “ends.” There are two different styles, wafer valves and lug valves, both of which are designed to be used with flanges.
The wafer-style valve is used between mating flanges. Its circular body fits just inside the circular bolting pattern of the flanges. When tightened, the extended-lenqth flange stud bolts cause the flanges to seal the ends of the valve and hold the valve in position.
The lug-style valve is also circular but has projections (called lugs) with threaded holes spaced around its perimeter. The locations of the holes match those of the mating flanges, and the threads match the flange stud bolts used in assembly. The lug-style valve can be situated between flanges by using short stud bolts with each flange, with the valve body acting to hold the pipeline together, or the lug valve can be bolted to a single flange at the end of a pipeline.
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