Calculation of Pressure Regulators
Designing and selecting pressure regulating valves is not a secret science mastered only by a handful of experts. The procedure described below allows the user to select a suitable valve for a particular application with relatively little effort. The calculations based on the so-called Kv value method have been considerably simplified compared with the very accurate calculations given in IEC 534; they produce, however, results which for our purposes are sufficiently accurate.
The Kv value is the flow coefficient which corresponds to a water flow rate - given in m³/h - at a differential pressure of 1 bar and a water temperature between 5 and 30 °C.
The American system uses the flow coefficient Cv which corresponds to a water flow rate - given in USgal/min - at a pressure difference of 1 psi and a water temperature of 60 °F. The relationship between Kv and Cv is:
Kv = 0,86 x Cv
The Kvs value quoted in technical documentation is the Kv value at nominal valve lift for a specific series of valves. The Kvs value allows the maximum throughput to be calculated for a valve.
The methods of calculating the Kv value described here have been, as mentioned above, considerably simplified. Many factors have been excluded from the calculation. By treating steam as an ideal gas and excluding the specific volume, a maximum error of 5% may result which, however, in view of the allowances used, is acceptable.
The calculations are simple; a knowledge of basic arithmetic and finding roots is sufficient. Tables or diagrams are not absolutely necessary but can be helpful if available.
The operating pressures and setting ranges specified in our design examples are given as pressures above atmospheric [barg = bar], as is customary. The calculations, on the other hand, are based on absolute pressures [bara]. For instance, if an outlet pressure of 7 bar is specified an absolute pressure of 7 + 1 = 8 bara must be used in the calculation. Flow rate and density should be specified for liquids in their operating state and for gases in their standard state (0°C, 1013 mbar).
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