Principles of flow calculations
07 April 2009
Understanding basic principles of flow calculations lets personnel estimate flow quickly and determine valve size. John Baxter, market manager and Ulrich Koch, senior engineer at Swagelok Company explain.
Fig 1: The flow rate through a fixed orifice can be calculated from the meter dimensions of pipe diameter and orifice size and shape
Valve size is often described by the nominal size of the end connections but a more important measure is the flow that the valve can provide. And determining flow through a valve can be simple. Using the principles of flow calculations, some basic formulas, and the effects of specific gravity and temperature, flow can be estimate well enough to select a valve size – easily, and without complicated calculations.
Flow Calculation Principles
Fig 2: Calculating the flow rate through a valve is much more complex. The valve flow coefficient (Cv) takes into account all the dimensions and the factors – including size and direction changes – that affect fluid flow
The principles of flow calculations are illustrated by the common orifice flow meter (Fig 1). We need to know only the size and shape of the orifice, the diameter of the pipe, and the fluid density. With that information, we can calculate the flow rate for any value of pressure drop across the orifice (the difference between inlet and outlet pressures).
For a valve, we also need to know the pressure drop and the fluid density. But in addition to the dimensions of pipe diameter and orifice size, we need to know all the valve passage dimensions and all the changes in size and direction of flow through the valve.
Fig 3: Valve manufacturers determine flow coefficients by testing the valve with water using a standard ISA test method
However, rather than doing complex calculations, we use the valve flow coefficient, which combines the effects of all the flow restrictions in the valve into a single number (Fig. 2).
Valve manufacturers determine the valve flow coefficient by testing the valve with water at several flow rates, using a standard test method2 developed by the Instrument Society of America for control valves and now used widely for all valves.
Flow tests are done in a straight piping system of the same size as the valve, so that the effects of fittings and piping size changes are not included (Fig. 3).
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