Study of Inherent Characteristic of Control Valve

To study inherent characteristic of control valve, equal % and linear experiment modes are conducted by a unique experimental set up:

With two control valves one operates based on air to close mode and other with air to open mode. when air is supplied to the diaphragm of control vale it results in closing then it has equal percentage characteristics and when in vice-verse it has linear characteristic. Pneumatic actuators are used to control the air supply to the control valves. In general tap water is circulated with a pump from bottom of the receiving tank to supply tank. By opening the manual gate valve water from supply tank is passed to receiving tank through the rotameter and control valve. Inlet pressure at the control valve can be measured in terms of water column. By air regulator stem of the control valve is moved and adjusted for the required flow rate. Stem opening in terms of mm can be observed by the scale fitted near the stem.

Theory of inherent characteristic determination: 

Fluid flowrate in a pipe line is controlled by the help of automated control value in modern industries. Extremely powered actuator and pneumatic singles by means of pressurized air, hydraulic etc allows the control room operator to open or partially open and close the valve. The amount of fluid passing through a valve at any time depends upon the opening between the plug and seat. Hence there is relationship between stem position, plug position and the rate of flow. The relation between the flow through the valve and the valve stem position (or lift) is called the valve characteristic.

In general, the flow through a control valve for a specific fluid at a given temperature can be expressed as: Q = f1(L,p0,p1)
Where, Q is volumetric flow rate, L is valve stem position or lift, and p0 and p1 are upstream and downstream pressures.

The Inherent flow characteristic of control valve is the relation developed between the flow of fluid and the valve movement at constant pressure drop across the valve ( fixed upstream and downstream pressures). Hence, inherent characteristic is , Q = f2(L)
It can also be written as m = Q/Qmax = f(L/Lmax )
m = f(x)
Where, Qmax is the maximum flow when the valve stem is at its max lift Lmax (valve is full open),
m is fraction of maximum flow, Q/Qmax and x is the fraction of maximum lift, L/Lmax

Procedure to determine inherent characteristic of control valve:

Valve Sizing Calculator 
  • Open the manual plug valve of equal percentage (air-to-close) control valve .
  • Open the valve up to 14 mm travel (full open).
  • Adjust the regulatory valve at the inlet of the control valve to maintain the flow at 400 LPH. Note down the pressure drop.
  • Slowly increase the air pressure by air regulator and close the control valve to travel the stem by 2 mm.
  • The pressure drop across the valve will increase. Maintain the pressure drop by adjusting the regulatory valve. Observe the flow rates.
  • Take the observations at each 2 mm stem travel till the valve is fully closed by repeating the above step.
  • Plot the graph of flow % of maximum versus valve lift % of full lift.
  • Repeat the experiment for linear valve (air to open).

Parameters observation:

Stem lift, mm                     Air to Close                                               Air to Open
                            Pressure in mm     H2O Flow in LPH        Pressure in mm       H2O Flow in LPH
14
12
10
8
6
4
2
0

Valve coefficient calculations:

G = Sp.g = 1 for water.
Q = m3/hr = LPH/1000.
Cv = 1.16 Q √(G/∆P)
∆P = ∆P in mm of H2O / (10.33 x 103)

Results in table form:
 
Stem lift, mm                   Air to Close                                   Air to Open
                         Flow in LPH        ∆P,mm H2O          Flow in LPH      ∆P,mm H2O


14
12
10
8
6
4
2
0
RESULT: The inherent characteristics for the air to open and air to close valves are verified