A control valve converts stem travel into flow through its trim characteristic and the pressure drop the system leaves for it. A linear trim delivers Cv(h) = Cv x h; an equal-percentage trim delivers Cv(h) = Cv x R^(h-1) with rangeability R from the datasheet. Because system pressure drop redistributes as the valve strokes, 50% open essentially never means 50% flow. Enter the rated Cv, the opening fraction, the actual valve dP, and SG — or do not estimate at all.
Formulas
Opening to flow
Q = 1.156 x Cv(h) x sqrt(dP / SG)
Cv(h) is the trim characteristic at opening h; SG is relative to water.
Equal-percentage trim
Cv(h) = Cv x R^(h-1)
R is datasheet rangeability; h is the 0-1 opening fraction.
Worked example
60% open on a linear trim
Inputs: Cv 100; h 0.6; dP 1 bar; SG 1.
Calculation: Cv(h) = 60. Q = 60 x 1.156 x 1 = 69.36 m3/h.
Lesson:
Frequently asked questions
Can I convert opening % to flow %?
Only with the manufacturer Cv(h) table or rangeability R. Otherwise the result is a guess.
Linear or equal-percentage?
Equal-percentage passes less than linear at the same mid opening; the exact ratio needs R.
Why is control poor below 10% open?
Most trims hunt and cavitate near the seat. Resize or use a smaller trim.
Sources and further verification
- Control valve Wikipedia
- Flow coefficient Wikipedia
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