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How to Calculate the Valve Flow Coefficient

The valve flow coefficient, or Cv rating, of a valve is the volumetric flow capacity of the valve at a fixed pressure drop. The conventional method for calculating the Cv value in the U.S. is to measure the volumetric flow rate in gallons per minute (gpm) when the pressure drop across the valve is at exactly one pound per square inch (psi). The metric units are liters per minute flow and kilograms per square centimeter pressure. The valve flow coefficient helps determine how much fluid can flow through the pipe given the valve's design. This in turn helps determine which pipe can carry the necessary amount of liquid in a structure.

Calculation

The pipe with the lowest valve flow coefficient requires the least energy through which to pump liquids.

Determine the diameter of the pipe if it is not already known. Measure the pipe with calipers or a tape measure if necessary.

Determine the resistance coefficient of the pipe, which is denoted as K. According to "Pump Characteristics and Applications" by Michael Volk, the value for K is available in tables based on the valve or fitting. The resistance coefficient K is also available from vendor as part of the valve's performance specifications.

Find the square root of K, the resistance coefficient of the pipe.

Square the value of the pipe diameter. Then multiply that value by 29.9.

Divide the result of Step 4 by the square root of K found in Step 3. The result is the valve flow coefficient Cv.

Things You Will Need

  • The type of valve design or the valve coefficient of resistance
  • Diameter of the pipe or a means to measure the diameter

Tips

  • Larger diameter valves have a larger volumetric capacity and thus a larger Cv rating.
  • According to "Practical Fluid Mechanics for Engineering Applications" by John J. Bloomer, the resistance coefficient for a sharp edged inlet is 0.5 while an inward projecting pipe has K = 1.0.

Warning

  • When comparing Cv values of different valves, only the valves of the same size can be compared directly. However, valves of the same diameter but different types can be compared by this method.

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