Valve Flow and Control Performance Testing: Cv/Kv, PRVs and TMVs

The usual failure on flow is treating Cv/Kv — capacity at one fixed opening — and control performance — how steadily a valve holds when throttled, and how fast it recovers — as the same thing. The first is a number. The second is a curve plus a set of dynamic criteria. Pressure reducing valves, PICVs, thermostatic mixing valves and radiator valves are all judged on the second.

This cluster groups flow coefficient, PRVs, control valves and thermostatic valves together, because they often share one flow system with different sample fixtures.

1. Cv/Kv is a number; performance is a curve

Metric	Valve type	What is measured	Typical mistake
Cv / Kv	Ball, butterfly, gate, control	Flow at a stated opening and differential pressure	Omitting opening and Δp, so two numbers cannot be compared
Characteristic curve	Control and balancing valves	Cv at each point from 10% to 100% opening	Full-open only: equal-percentage and linear look identical
Outlet pressure vs flow	PRVs	Flatness of outlet pressure as flow rises	Static only: holds at no flow, collapses at a tap
Flow constancy, rangeability	PICV, dynamic balancing	Whether flow holds as Δp moves	Single point, no Δp sweep
Temperature stability	TMVs, TRVs	Outlet temperature deviation under disturbance	Steady state only, no response time

Cv and Kv differ only in units: Cv is US gpm of 60 °F water at 1 psi; Kv is m³/h of 20 °C water at 1 bar. The two differ by about 15% (Kv ≈ 0.865 Cv). A quotation stating “Cv = 100” against a drawing stating “Kv = 100” is a 15% capacity gap — visible on a large line.

2. Find the article by question

Cv/Kv: definition, test and conversion

PRVs and PICVs

Thermostatic mixing and radiator valves

3. Three misjudgements

Reporting full-open flow as Cv. Cv/Kv is defined at a stated opening and Δp. Full-open flow is maximum capacity, not the coefficient — and using it for control valve sizing turns an equal-percentage characteristic into an assumed linear one.

Judging a PRV on static pressure alone. Holding 0.3 MPa at no flow and dropping to 0.2 MPa when a tap opens is the classic spring PRV failure. EN 1567 looks at the flatness of the outlet pressure–flow curve plus creep at zero flow.

Judging a TMV on steady-state temperature. The value of a thermostatic mixing valve is how fast it returns to set point after a disturbance — cold supply interruption, hot water step, flow change. Without those cases there is no evidence of stability.

4. Configuring the bench

The common requirement is accurate flow, stable Δp and controllable temperature. The measurement chain sets the repeatability of Cv: a 0.5% flowmeter paired with a 1.6% Δp transmitter will scatter more than the bench itself can resolve.

Valve flow test system — flow and Δp per opening to derive Cv/Kv, including balancing and differential valves
Thermostatic mixer test bench — independent hot and cold control
TRV performance tester — temperature-flow characteristic and set-point response

Continue by test item

These clusters are grouped by test item rather than by standard number. The other four:

Pressure and tightness — shell, seat and air tightness
Life, torque and fatigue — fix the conditions first
Special and safety tests — fire, cryogenic, emissions, type test
Certification and market access — EU, US and China

Not sure about the test pressure yet? Use the valve test scope configurator — pick valve, size, class and standard, and it returns shell and seat pressures, hold time and a bench list you can email.

Related test equipment

Each test above maps to a bench. Pick by valve type and by the item you need to prove:

Valve flow test system — flow and differential pressure per opening to derive Cv/Kv
Thermostatic mixer test bench — independent hot and cold temperature and pressure control
Thermostatic radiator valve tester — temperature-flow characteristic and set-point response
Valve testing equipment range — browse all models by valve type and test item