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| This lined ball valve is an example of a quarter turn valve. Image courtesy Flowserve - Atomac |
All valves operate by providing control of the position of an internal structure that impedes fluid passage to some degree. Generally, fluid flow at the valve can be characterized as one of three conditions, unrestricted (valve fully open), stopped (valve fully closed), and throttled (valve partially open). Process operational requirements will dictate whether just two (fully open and fully closed) or all three of those conditions will be needed. Many aspects of the fluid, the process, and the surrounding environment come into play when making an appropriate valve selection. Not always an easy task.
Linear valves are generally characterized by their straight line motion that is used to position the valve plug, disc, diaphragm or other flow controlling element. The shape, size, and arrangement of the linear valve trim is generally intended to empower the operator with a range of flow through the valve. Through its positioning, the linear valve is able to regulate fluid flow at a slower, but more accurate rate. The valves can move a disk or a plug into an orifice, or push a flexible material, such as a diaphragm, into the flow passage. Gate valves and fixed cone valves are common examples of linear motion valves. Linear valves are best applied as flow controllers, and are often suited for frequent operation and repositioning.
Quarter turn valves traverse from fully open to fully closed by a 90 degree rotation of a shaft connected to the controlling element. Their comparatively simple operation allows for a design that is rugged and compact. One distinction of the quarter turn valves is their ability to quickly reposition from open to closed positions. Torque requirements to operate the valves are generally low to moderate. Ball and butterfly valves are examples of quarter turn valves.
Depending on the specific scenario, linear valves and quarter-turn valves are optimal choices for particular process environments. The accuracy of the linear valve and its ability to move in a linear fashion as opposed to a quarter-turn comes coupled with easy maintenance and decreased likelihood of cavitation. Both valve types enjoy widespread use and should generally not be viewed as competing designs for the same application. Each has a range of applications where it excels.
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