"This standard is intended for valves used in pressure or flow control of process liquids and is not intended to apply to fluid power components as defined in the National Fluid Power Association (NFPA) Proposed Recommended Standard T.3.5.7. The equations presented in this standard are based on the Bernoulli equation for Newtonian incompressible fluids. Use of these equations for non-Newtonian fluids@ fluid mixtures@ slurries@ or liquid-solid conveyance systems may result in inaccuracies. Factors have been introduced to account for piping geometry and fluid property characteristics that influence the flow capacity of a valve. The flow rate through a valve operating with fixed inlet conditions will increase as the downstream pressure is decreased until vapor formation within the valve limits the flow rate. This limiting or maximum flow condition will be referred to as ""choked flow"
ISA S39.1-1972 history
1972ISA S39.1-1972 Control Valve Sizing Equations for Incompressible Fluids