This part of IEC 60534 includes equations for predicting the flow of compressible and incompressible fluids through control valves.
The equations for incompressible flow are based on standard hydrodynamic equations for Newtonian incompressible fluids. They are not intended for use when non-Newtonian fluids, fluid mixtures, slurries, or liquid-solid conveyance systems are encountered.
At very low ratios of pressure differential to absolute inlet pressure (△p/p1), compressible fluids behave similarly to incompressible fluids. Under such conditions, the sizing equations for compressible flow can be traced to the standard hydrodynamic equations for Newtonian incompressible fluids. However, increasing values of △p/p1 result in compressibility effects which require that the basic equations be modified by appropriate correction factors. The equations for compressible fluids are for use with gas or vapour and are not intended for use with multiphase streams such as gas-liquid, vapour-liquid or gas-solid mixtures.
For compressible fluid applications, this part of IEC 60534 is valid for valves with xT ≤ 0,84 (see table 2). For valves with xT > 0,84 (e.g. some multistage valves), greater inaccuracy of flow prediction can be expected.
Reasonable accuracy can only be maintained for control valves if Kv/d < 0,04 (Cv/d < 0,047).