Control valve Cv (flow coefficient) is a standardized number that describes how much liquid a valve will pass at a given opening for a given pressure drop, and it is the starting point for sizing almost every control valve in a plant. Get Cv wrong and the valve either starves the process of flow or spends its life barely cracked open, hunting and wearing out the trim.
Cv is defined as the number of US gallons per minute of 60 degree F water that will pass through a valve with a pressure drop of 1 psi across it. A valve with a Cv of 10 passes 10 gpm of water at a 1 psi drop, and flow scales with the square root of pressure drop from there. This definition was standardized by the former Instrument Society of America and now lives in ISA-75.01.01, a modified national adoption of the international standard IEC 60534-2-1. Because every manufacturer tests and publishes Cv the same way, a Cv value on one vendor's datasheet is directly comparable to another's.
Outside North America, valves are usually rated in Kv, defined as the flow in cubic meters per hour of water that passes through the valve at a 1 bar pressure drop. Kv and Cv describe the same physical property in different unit systems, and the conversion is straightforward:
When comparing a US datasheet to a European one, convert first. Mixing the two without converting is a common and completely avoidable sizing error.
For non-flashing, non-choked liquid flow, the simplified sizing relationship is:
Cv = Q x square root of (SG / delta-P)
where Q is the required flow rate in gpm, SG is the specific gravity of the fluid relative to water, and delta-P is the pressure drop across the valve in psi at that flow. You calculate the required Cv at minimum, normal, and maximum expected flow, then pick a valve whose Cv curve covers that range with the plug positioned in the useful part of its travel, generally not wide open and not barely cracked. Full sizing per ISA-75.01.01 / IEC 60534-2-1 adds correction factors for piping geometry, choked flow, and viscosity, but the Cv equation above is the core relationship every one of those corrections builds on. Because valve losses translate into throttling that has to be made up elsewhere in the system, valve sizing is worth doing alongside a check of pump curves and affinity laws so the pump and valve are sized as a pair, not in isolation.
Rangeability is the ratio of a valve's maximum usable Cv to its minimum controllable Cv, measured under laboratory conditions with a constant pressure drop. It is a property of the valve and trim design, not of the installed system. Globe valves are commonly cited in the 30:1 to 50:1 range, while butterfly valves typically run lower, often cited around 20:1 to 30:1, though actual figures vary by manufacturer and trim style, so always check the specific product data.
Turndown is the practical, installed version of that same idea: the ratio of maximum to minimum flow the valve can actually control once it is in the piping system, subject to real pressure drop variation, noise, and process dynamics. Turndown is almost always lower than the lab-rated rangeability. A valve rated 50:1 on paper might only deliver a usable 20:1 to 25:1 turndown once installed. Undersizing rangeability is one of the most common causes of a valve that "hunts" near minimum flow, a symptom that often also shows up as abnormal vibration at the actuator or piping, worth checking against ISO 10816-3 vibration severity guidance if the valve assembly is shaking more than expected.
The inherent flow characteristic is the relationship between valve travel (percent open) and Cv, plotted at a constant pressure drop, exactly as the manufacturer tests it in the lab. There are three common inherent characteristics.
| Characteristic | Behavior | Typical use |
|---|---|---|
| Quick opening | Most of the flow capacity is delivered in the first 20 to 30 percent of travel, then flow gain flattens | On-off service, some relief and safety applications |
| Linear | Cv increases proportionally with travel | Systems where the pressure drop across the valve stays roughly constant across the flow range |
| Equal percentage | Equal increments of travel produce equal percentage changes in the existing Cv, so the curve is logarithmic | Systems where the valve's share of total system pressure drop changes a lot between low and high flow, such as most pressure and temperature control loops |
The inherent characteristic assumes constant pressure drop across the valve, which almost never holds true in a real piping system. As flow increases, friction losses in the pipe, fittings, and heat exchangers increase too, so the valve's share of the total available pressure drop shrinks. The result is the installed flow characteristic, the curve you actually get in service.
The practical consequence is a shift in apparent behavior: a linear valve installed in a system with significant variable pressure drop tends to behave more like a quick-opening valve, gaining most of its flow early in the stroke, while an equal-percentage valve tends to flatten out and behave more like a linear valve. This is exactly why equal-percentage trim is the default choice for most control loops. It is deliberately over-corrected in the lab so that, once installed, it settles into something closer to linear and controllable across the working range.
Correct Cv sizing keeps a valve working in its controllable range for years, but trim wears, actuators drift, and pressure drops shift as strainers foul and pumps age, so the installed characteristic you sized for today is not guaranteed tomorrow. Fabrico reads machine condition and OEE straight from the line, using computer vision to catch drift and wear that sensors alone miss, and it auto-routes a work order the moment a real loss shows up so a sticking or oversized valve gets fixed before it costs a batch. It is built and hosted in the EU with EU data residency, and it runs under ISO 27001, ISO 20000-1, and ISO 9001. Book a Fabrico demo.
There is no universal "good" Cv, it depends entirely on the required flow rate, fluid specific gravity, and available pressure drop at your operating conditions. Calculate the required Cv at minimum, normal, and maximum flow using the sizing equation, then select a valve whose rated Cv curve covers that range with the plug in a controllable part of its travel.
Yes. Kv is approximately 0.865 times Cv, and Cv is approximately 1.156 times Kv. Always convert before comparing datasheets from manufacturers that use different unit conventions.
That is the intended behavior. Equal-percentage trim delivers small flow changes early in the stroke and larger changes later, on a logarithmic curve, specifically to counteract the fact that the valve's share of system pressure drop is highest at low flow and lowest at high flow. Once installed, this typically flattens toward something closer to a linear response.
Rangeability is a lab-rated property of the valve and trim, measured at constant pressure drop. Turndown is the actual usable ratio of maximum to minimum flow once the valve is installed in a real system with variable pressure drop and process noise, and it is almost always lower than the rated rangeability.
Zakažite sastanak KSNUMKS-to-KSNUMKS sa našim stručnjacima ili se direktno upišite u naš besplatni plan.
Nije potrebna kreditna kartica!
Login
Blog
