EEI MASS Practice Test 9

Valves and Their Jobs

¹A power plant contains thousands of valves, and nearly every one of them was chosen for a reason. Some valves are meant to stop flow completely so that a pump or heat exchanger can be removed for repair. Others are meant to regulate flow, holding a tank level or a line pressure steady. Still others act on their own to keep fluid from running backward. A maintenance worker who understands which job a valve was designed for is less likely to misuse it and more likely to notice when it is failing.

²The gate valve is the usual choice for isolation. Turning its handwheel raises or lowers a flat or wedge-shaped gate across the flow path. When the gate is fully raised, the passage through the valve is nearly as open as the pipe itself, so very little pressure is lost. A gate valve should be operated either fully open or fully closed. If it is left partly open, fast-moving fluid rushes under the edge of the gate, causing vibration and cutting grooves in the gate and its seats. Once the seats are damaged, the valve may no longer shut off tightly when it is needed most.

³When flow must be adjusted, a globe valve is usually installed instead. Inside a globe valve, the fluid is forced to change direction and pass through an opening, where a disc moves toward or away from a seat. Because the disc travels straight toward the seat, small turns of the handwheel produce small, predictable changes in flow. The price of this control is a higher pressure drop, since the winding path through the body resists flow even when the valve is wide open.

⁴Ball valves and butterfly valves are known as quarter-turn valves. A ball valve contains a sphere with a hole bored through it; a butterfly valve contains a disc that pivots on a shaft in the middle of the pipe. Turning the handle ninety degrees moves either valve from fully closed to fully open. Their speed makes them useful wherever a line must be shut quickly, and butterfly valves are light and compact enough to serve on very large pipes such as cooling water lines.

⁵A check valve has no handle at all. Flow in the intended direction pushes its disc or flapper open, and if the flow tries to reverse, the disc is pressed against its seat and closes the line. Check valves are often placed at a pump’s discharge so that water in the piping cannot spin the pump backward after it stops. Because a check valve works without anyone touching it, a worn or stuck one may go unnoticed until backflow causes trouble.