Dimensioning of control valve - Part 1 LIQUIDS
Correctly dimension the flow rate through a valve and therefore its DN is of fundamental importance. A valve undersized, nullifies the adjustment as the valve is intended to remain permanently open; a valve oversized, besides causing overload and increased wear on the components, determines abnormal fluctuations of the controlled variable as it is subjected to a continuous regrettable "oscillation".
This first part, examines the dimensioning of valves for fluids in situations of liquid with a density equal to or different from 1 kg / dm3 and liquids in temperature.
In general, the diameter DN of the regulating valve, is achieved by determining, with appropriate diagrams or formulas, the value of the flow coefficient or efflux Kv, according to the actual operating conditions of the plant. For the Kv valve determination, and then the DN of the same are necessary:
- Q = flow rate through the valve
- P1 = absolute pressure valve upstream in bar
- P2 = absolute pressure valve downstream in bar
- DP = pressure drop across the valve in bar
A) liquids with density r = 1 kg / dm3 (water)
with the following formula:
where:
- Q = flow in m3 / h
- DP = pressure drop across the valve in bar absolute
- r = density in kg / dm3
- Kv = flow coefficient in m3 / h
It is obtained:
B) liquids with density r other than 1 kg / dm3
Apply formulas (1) or (2), where r will assume the specific value of the operating fluid.
C) Liquid at temperature
The flow rate Q is function of the utilized thermal power and the temperature difference between of flow and return circuit.
From (1), Q is:
where:
W = thermal requirement in Kcal / h per liter
t1 = flow temperature in °C
t2 = return temperature in °C
and DP pressure drop is:
- for circuits with variable flow, equal to or greater than the DP of control circuit;
- for circuits with constant flow rate equal to or greater than the DP of supply circuit.
The proper valve will be so choice with a kvs near to that calculated.
follows
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