Valve Flow Testing: Cv Measurement, Flow Curves, and Test Standards

Accurate knowledge of valve flow capacity is essential for proper valve sizing, control system design, and process performance prediction. The flow coefficient Cv (or its metric equivalent Kv) characterizes the relationship between pressure drop across the valve, flow rate, and fluid density. Flow testing is performed by valve manufacturers to measure the actual Cv of production valves and to generate the flow characteristic curves (percent Cv versus percent travel) used in control valve sizing calculations. Understanding how flow testing is performed and what the resulting data means enables engineers to use published valve flow data correctly.

Definition and Measurement of Cv

The valve flow coefficient Cv is defined as the flow rate of water in US gallons per minute that will pass through a fully open valve with a pressure drop of one pound per square inch across the valve. The metric equivalent Kv is defined as the flow rate in cubic meters per hour of water at a pressure drop of one bar. The relationship between Cv and Kv is: Kv equals 0.865 times Cv. Flow testing to measure Cv is standardized in ISA-75.02 (ANSI/ISA Flow Measurement), which specifies the test piping arrangement, upstream and downstream straight-pipe lengths, measurement instrumentation, and data reduction procedure required to obtain consistent and reproducible Cv measurements.

• Cv test fluid: water at 60 to 100 degrees Fahrenheit per ISA-75.02

• Upstream straight pipe: minimum 18 pipe diameters upstream of valve inlet

• Downstream straight pipe: minimum 6 pipe diameters downstream of valve outlet

• Pressure taps: 2 pipe diameters upstream and 6 pipe diameters downstream of valve face

• Multiple openings tested: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 percent of rated travel

Flow Characteristic Curves

The inherent flow characteristic of a control valve is defined as the relationship between the valve opening (expressed as percent of rated travel) and the flow coefficient (expressed as percent of maximum Cv) under constant pressure drop conditions. The three standard inherent characteristics are linear, in which Cv is directly proportional to valve opening; equal percentage, in which equal increments of opening produce equal percentage increases in Cv; and quick-opening, in which most of the Cv increase occurs in the first portion of valve travel. The inherent characteristic is determined by the trim geometry and is measured during flow testing.

Installed Characteristic and Sizing Implications

The installed flow characteristic of a control valve differs from the inherent characteristic because the pressure drop across the valve changes as flow changes. When a control valve is installed in a piping system with significant friction losses, the available pressure drop decreases as flow increases, distorting the relationship between valve opening and actual flow. Equal percentage valves are typically specified for process control applications because their installed characteristic approximates a linear relationship between valve opening and flow when they are installed in systems where the valve pressure drop is a moderate fraction of the total system pressure drop.