Control valve flow characteristics
|Formula||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 chosing it for a new valve is
of prime importance in order to ensure a good control of the
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 :
|Nomenclature||ΔPc=Pressure drop of the circuit - excluding the
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
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
|Usual values||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. The valve must also be designed to work at flows corresponding to 50 to 80 percent of the Cv of the valve, this is translated by q0 coefficients typically in the range 1.2 to 2.|
|Other relations||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 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
Linear characteristic valve installed in series of the 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.
Figure 1 : Change of characteristics of a linear valve as a function of the pressure drop it generates in the line
Exponential characteristic valve installed in series of the 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.
Figure 2 : Change of characteristics of an exponential valve as a function of the pressure drop it generates in the line
|Source||You can access the Excel files having been used to generate the graphs here : Link|