Linear vs equal-percentage trims
A linear trim adds the same Cv per percent of travel, which suits level loops with constant drop. An equal-percentage trim adds Cv geometrically, which compensates for a system whose available drop shrinks as flow rises. The datasheet rangeability R, typically 25 to 50, fixes the exact curve; without it any opening-to-flow number is a guess.Why the pressure drop moves with opening
The valve sees only part of the system head. As it closes, line friction falls and the valve takes a larger share of the total drop. That shifting drop is why installed characteristics differ from inherent bench curves, and why flow must be recomputed at each opening instead of scaled.What you must enter instead of a percentage
Use the rated Cv at full open, the opening fraction h, the valve dP at that opening, and the fluid SG. For equal-percentage trims add R. Control below roughly 10% open is unreliable on most trims, so resize rather than throttle there.Worked check: 60% open, linear trim
Rated Cv 100 at h = 0.6 gives Cv(h) = 60. At dP 1 bar and SG 1 the flow is 60 x 1.156 x sqrt(1) = 69.36 m3/h. Run the same inputs through the opening estimator to confirm, then size the valve with the required-Cv tool.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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