AFT Fathom allows the user to enter valve coefficient data in terms of K_v or C_v. Conversions are handled seamlessly in the background. K_v is the metric equivalent to C_v.

Manufacturer Testing in Standard Units

C_v (Coefficient of flow for Valves) is a single parameter which relates fluid density (given in units of specific gravity), pressure drop (in psi), and flow rate (in gallons per minute).

One can get a better understanding of the role of C_v by setting the pressure drop (dP) and SG to 1, in which case C_v is equal to flow in gallons per minute. C_v could be considered the flow capacity (in gpm) of a given fluid (i.e. a given SG) for a given pressure drop. Or, one may wish to calculate the pressure drop through a valve, with a given C_v at a given flow rate.

Manufacturer Testing in Metric Units

If the engineer is provided with valve data in metric units, they are also likely to be provided a value for K_v and not C_v.

The only difference is the units used for Q and dP. K_v uses flow units of m³/hr and pressure drop units of bar.

If one wishes to calculate pressure drop through a valve, with a given K_v, at a given flow rate, the same relationship applies.