# Calculation Methods - Check Valves

The pressure loss across a valve is a function of flow. In FluidFlow this loss relationship can be expressed as either: K, Idelchik, or Kf. Many literature sources contain these values as generic constants. This means that for a given type of valve a specific K (or Kf) value exists, and this is usable over all valve sizes for that specific type of valve. For each type of valve available in FluidFlow there are at least two generic definitions usually obtained from Miller [4], Idelchik [5] or Crane [13]. The pressure loss coefficient, K is defined by the equation: K = 2 ΔP / (ρv2) The pressure loss coefficient expressed as equivalent length in terms of pipe diameters, Kf is defined by the equation: Kf = 2 ΔP / (ρfTv2) where fT is the fully turbulent flow friction factor which is simply a function of size. Idelchik provides K values as a function of size and these values are used directly by FluidFlow if you select this type of relationship in the check valve database. For maximum accuracy you should obtain valve data directly from the valve manufacturer. If this is not possible then the next best approach is to use the generic relationships.