Basics of Valves and Selection


Gate Valves are designed to operate fully open or fully closed. Because they operate slowly they prevent fluid hammer, which is detrimental to piping systems. There is very little pressure loss through a gate valve. In the fully closed position, gate valves provide a positive seal under pressure. However, under very low pressure, i.e. 5 psi, light seepage would not be considered abnormal with this kind of valve.


Ball Valves are also designed to be operated fully open or fully closed with any liquid containing particles that could scratch the ball. Many people use them successfully for throttling clear water. Ball valves have low pressure drops, open and close quickly, are simple, and are trouble free. With the development of Teflon seals, ball valves have grown in popularity. Opening or closin a ball valve too quickly can cause fluid hammer.


Butterfly valves, like ball valves, operarte with a 1/4 turn. They are generally used for handling large flows of gases or liquids, including slurries, but should not be used for throttling for extended periods of time. They are also very compact relative to flanged gate and ball valves.


Globe valves, as is the case with all valve designs, have both advantages and disadvantages. Like a gate, they close slowly to prevent fluid hammer. You can throttle the flow and they will not leak under low pressure when they are shut off. Flow and pressure control valves as well as hose bibs generally use the globe pattern. The disadvantage of this design is that the ā€œZā€ pattern restricts flow more than the gate, ball, or butterfly valves. All psi ratings apply to water, oil, or gas. Steam is usually derated by one half. (250 psi water or 125 psi steam).


Like the gate valve, a plug valve has an unobstructed flow, yet requires only a 90 degree turn to open it. It also requires very little headroom. Stem corrosion is minimal because there are no screw threads. Almost all plug valves now are furnished with an elastomer-coated plug and will seal off driptight. However, plug valves are available in much larger sizes than ball valves and are highly suitable for use in wastewater plants.