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Control valves flow characteristics

Control valve flow characteristics and sizing of valves according to the circuit they are in

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1. Definition of valve flow characteristics
2. Usual value
3. Characteristics inside a circuit
4. Linear valve installed in series a circuit
5. Exponential valve installed in series in a circuit
6. Nomenclature used in the page

1. Definition of valve flow characteristics

What is the characteristic of a valve ?

The flow characteristics of a control valve relates the opening of the valve to the flowrate that can cross the valve. Not all the valves have the same flow characteristics and knowing for an existing valve or choosing it for a new valve is of prime importance in order to ensure a good control of the line.
There are basically 4 key types of valves characteristics :

- Linear valve
- Exponential valve - also called equal percentage
- Semi quick opening
- Quick opening

Those typical characteristics are shown on the graph below :
Control Valves flow characteristics


2. Usual values

How to size a control valve ?

In order to have a good control, the design of the control valve should be such that it represents around 1/3 of the total pressure drop of the line at maximum flow. The valve must also be designed to work at flows corresponding to 50 to 80 percent of the valve coefficient Cv of the valve fully opened, this is translated by q0 coefficients typically in the range 1.2 to 2.

3. Characteristics inside a circuit

How is changing the characteristic of a valve in a circuit ?

One must be very careful when interpreting valve behavior on the field or designing a new installation. Indeed, the characteristics presented above are ideal and not the actual one. Once in the circuit, the characteristic will actually vary depending on the pressure drop through the valve compared to the total pressure drop of the line.

This actually shows that there is only 2 basic types of valve : linear or exponential, with each type that can evolve to one of the standard characteristics when the valve is mounted in series of the circuit - be careful the characteristics is different if the valve is mounted in parallel

4. Linear valve installed in series in a circuit

A linear valve is in the field, once installed on the line, almost never linear. It will be linear only if the totality of the pressure drop of the line is created by the valve which is not practically often true. It must however be noted that, when the valve represent only few percent of the total pressure drop, the characteristic will actually be the one of a quick opening valve.
Change of characteristics of a linear valve as a function of the pressure drop it generates in the line

Figure 1 : Change of characteristics of a linear valve as a function of alpha = pressure drop through the valve / total pressure drop in the pipe network considered, at maximum flow.

For the limit condition alpha = 1, which can be considered as the valve alone, we find the linear charactertistic presented above. If one wants to keep a linear characteritics once the valve is in series in a pipe with other element, the pressure drop represented by the valve will have to be at least 2/3 of the total pressure drop, which may not be practical. As mentionned, the practice will be more to have a valve representing around 1/3 of the total pressure drop, which means that its behavior will not be ideal.

5. Exponential valve installed in series in a circuit

An exponential valve will show a behavior of equal percentage only if it creates the majority of the pressure drop in the line. If it creates few pressure drop, the characteristics will flatten and approach a linear characteristics. Change of characteristics of an exponential valve as a function of the pressure drop it generates in the line

Figure 2 : Change of characteristics of an exponential valve - or equal percentage - as a function of alpha = pressure drop through the valve / total pressure drop in the pipe network considered, at maximum flow.

For the limit condition alpha = 1, which can be considered as the valve alone, we find the exponential charactertistic presented above.

You can access the Excel files having been used to generate the graphs here : Link

6. Nomenclature used in the page

ΔPc=Pressure drop of the circuit - excluding the valve
ΔPv=Pressure drop of the valve only
ΔPl=total pressure drop of the line, valve + circuit
ΔPcm=Pressure drop of the circuit - excluding the valve - at maximum flow Qvm to be regulated
ΔPvm=Pressure drop of the valve only at maximum flow Qvm to be regulated
d=ΔPvm/ΔPl

Cvgm=Cv of the valve opened for the maximum flow Qvm to be regulated
Cvgo=Cv of the valve full opened
q0=Cvgo/Cvm=margin to make sure the valve is not fully opened at the maximum flow to be regulated, which means the capacity of the valve full open must be higher than the flow to be regulated

Qv=flow through the valve opened at h
Qv0=flow through the valve wide opened

h=relative travel, h=0 when valve closed and h=1 when valve full opened