Control Valve – Valve Selection And Rangeability

Sliding-Stem – For sliding-stem valves, the ratio of the maximum to the minimum usable Cv is highly variable, but is generally between 20:1 and 100:1. The minimum usable flow coefficient is determined by the size and shape of the cage openings when the valve plug is near the closed position. Refer to Figure 47 and note that the equal-percentage characteristic will result in the smallest controllable Cv.

Rotary-Shaft Valves – Rotary-shaft control valves typically provide wider rangeability than globe style valves because of their full bore design. Rangeability from 100:1 to 300:1 is common. V-notch ball valves provide exceptionally wide rangeability because of the shape of the V-notch when the ball is near the closed position. Refer to Figure 48.

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