One of the things to consider when designing control valves is the valve coefficient.
As you know, the coefficient of the valve ( C v ) is: If one American gallon per minute passes through the control valve at 60 degrees Fahrenheit (ISA standard conditions), one pound per square inch will create a pressure difference between the two ends of the valve. .
With the help of the following equation, the valve coefficient can be easily calculated.
C v = Q (SG / Δ P) 1/2
In the above relation, Q is the flow rate, SG is the specific gravity of the fluid and ΔP is the pressure difference between the two ends of the control valve.
It can be concluded from the above relation that the larger the coefficient of the valve, the practically the capacity of the valve (Capacity) increases.
Also, a smaller valve coefficient will mean a greater pressure drop at both ends of the valve. Since the valve coefficient is directly related to the flow, the valve should be selected in such a way that the maximum possible flow in a line is considered to be a maximum of 80% of the valve coefficient .
In other words, after performing the calculations and finding the valve coefficient in the maximum value (Calculated C v ), we multiply the calculation rate by about 1.2 and we will reach the value of the valve coefficient (Selected C v ) for selection and design.
Selected C v = 1.2 x Calculated C v
Note that according to the geometric shape of the flow as well as the construction topology of the control valves (which differ in different manufacturers), the coefficient of the valve alone can not be understood from the above relation.
Coefficients such as Kv, Kc, Xt, FL, Fd each play a role in the calculations. We take a brief look at these coefficients below.
C v = 1.156 x K v
K c : Cavitation factor
(X t : ΔP / P 1 (P 1 : Inlet Pressure
F L : Pressure Recovery Factor
F d : Valve Style Modifier
Manufacturers’ handbooks should be used to know these coefficients.
For example, the F L coefficient for Globe valves in a manufacturer such as Fisher is about 0.8 and for Ball valves is about 0.55.
Engineering software is used for ease of calculation.
General software such as InstruCalc and Conval are comprehensive applications that are widely used.You can also use Fisher’s first vue software, which was very popular in the last decade. This software is now known as FSM. Samson sizing software is also one of the other things that can be mentioned.
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