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# What Exactly Is Flow Rate?

The valve flow coefficient, also known as the flow rate, is the rate at which media flows through the valve and the amount by which the flow pressure changes (drops) across the valve. The flow rate is affected by the state and viscosity of the media, as well as the valve size, type, and diameter of the pipe connections. A formula can be used to calculate the flow rate value; the larger the value, the higher the flow rate through the valve at a given pressure drop.

Flow rate is expressed as Cv or Kv depending on the measurement system used (imperial or metric, respectively). The minimum required Cv value is frequently listed in a valve’s product description or elsewhere in its specifications.

Flow coefficient in imperial units = Cv value

As follows:

The flow rate of water in gallons per minute (gpm) in the United States.

When the temperature is 60 degrees Fahrenheit

With a 1 psi pressure drop across the valve

The minimum required Cv value can be calculated if the desired flow rate, pressure drop, and media characteristics are known. Fluids and gases have different formulas.
Calculate the Cv Value of Gases

Because gases are compressible fluids that are affected by temperature, the flow coefficient formula is more complicated for them. To accurately estimate flow, two formulas are used: the critical flow formula is used when the upstream pressure equals or exceeds two times the downstream pressure; and the critical flow formula is used when the upstream pressure equals or exceeds two times the downstream pressure (also known as choked flow). If the upstream pressure is less than twice the downstream pressure, the sub-critical flow is used.

We’re working on a gas calculator and will post it here as soon as it’s finished. In the meantime, please use the following formula to calculate gas fl