EXPERIMENT
Hysteresis
of control valve
1. EXCLUSIVE SUMMARY
The objective of this experiment is to
study the hysteresis
of control valve. To achieve this objective, the apparatus shown in the
figure 1 was used. Valve coefficient in equal%, quick opening and linear
control valve for both cases (~decreasing and increasing pressure) is in the
range of 0-1.67, 3.89-0 and 1.86-0 with the pressure 18-0 psig respectively.
The pressure drop of water for equal %, quick opening and linear control valve
is in the range of 0.139-0.077 bar, 0.139-0.033 bar and 0.139-0.064 bar. Losses percentage (~hysteresis %) for equal
%, quick opening and linear control valve is in the range 0.0324-0.94, 0.022-
0.87 and 0.0191- 0.36 with increasing pressure.
2. INTRODUCTION
The control valve is
essentially a variable resistance to the flow of a fluid, in which the
resistance and therefore the flow, can be changed by a signal from a process
controller.
Relation exists between the
pressure along the pipe and the flow rate so that if pressure is changed, then
the flow rate is also changed.
Hysteresis is a predictable
error resulting from the differences in the transfer functions when a reading
is taken from above and below the value to be measured. In case of control
valves for same actuator signal different stem travel (hence valve
coefficients) are obtained depending upon the direction of change in the
signal. The maximum error in stem travel
(or valve coefficient) expressed in % for same actuator pressure while opening
and closing the valve is indicated as hysteresis.
In order to specify the size of a valve in terms of its capacity to
provide flow when fully open, the following equation is used,
Where,
q = flow rate, gpm
∆pv, = pressure drop across
the wide-open valve, psi
G = specific gravity of
fluid at stream temperature relative to water; for water G = 1.
Cv = factor associated with
capacity of valve
Above equation applies to
the flow of an incompressible fluid through a fully open valve. Manufacturers
rate the size of a valve in terms of the factor Cv. Cv (valve coeffient) is
defined as the flow (gpm) of a fluid of unit specific gravity through a fully
open valve.
Where,
q = flow rate, m3/hr
∆pv, = pressure drop across
valve, Kgf/m2
G = specific gravity
relative to water
The relation between Kv and
Cv is:
For gases and steam,
equation 1 is used in which Cv is still used as a factor. In general, as the
physical size of a valve body (i.e., size of pipe connectors) increases, the
value of Cv increases. For a sliding stem and plug type of control valve, the
value of Cv is roughly equal to the square of the pipe size multiplied by ten.
Using this rule, a three-inch control valve should have a Cv of about 90.
3. OBJECTIVE
To study the inherent characteristics of control valve.
4. Experimental Setup
The setup is designed
to understand the control valve operation and its flow characteristics. It
consists of pneumatic control valves of linear, equal% (& quick opening)
type, stainless steel water tank with pump for continuous water circulation and
rotameter for flow measurement. An arrangement is made to measure pressure at the
valve inlet in terms of mm of water. An air regulator and pressure gauge is
provided for the control valve actuation. In case of additional optional
requirement a valve positioner is fitted on linear valve.
 |
| Control valve set-up |
1. Start up the set up. Open the flow
regulating valve of the control valve to be studied (Linear/Equal%/quick
opening). Open the respective hose cock for pressure indication. (Close the
flow regulating valves and hose cocks of other control valves.)
2.
Ensure that pressure regulator
outlet is connected to the valve actuator of the control valve under study.
Keep the control valve fully open by adjusting air regulator.
3.
Adjust the regulating valve and set
the flow rate. (Set 400 LPH flow for linear/equal% valve or 600 LPH for quick
opening valve).
4.
Note for measuring flow rates below
rotameter minimum range use measuring jar.
6. Results and Discussions
 |
Figure 2 Hysteresis of equal% control valve
|
The above figure show the Hysteresis of
equal% control valve. Valve coefficient is represent on y-axis and Pressure in
psig is represent on the x-axis. Valve coefficient in equal% control valve for
both cases (~decreasing and increasing pressure) is in the range of 0-1.67 with
the pressure 18-0 psig respectively. The pressure drop of water is in the range
of 0.139-0.077 bar. Losses percentage (~hysteresis %) is in the range 3.24-94%
with increasing pressure.
 |
| Hysteresis of quick opening control valve |
The above figure show the Hysteresis of quick
opening control valve. Valve coefficient in quick opening control valve for
both cases (~decreasing and increasing pressure) is in the range of 0-3.89 with
the pressure 0-18 psig respectively. The pressure drop of water is in the range
of 0.139-0.033 bar and Losses percentage (~hysteresis %) is in the range
2.2-87% with increasing pressure.
 |
| Hysteresis of linear control valve |
The above figure show the Hysteresis of
linear control valve. Valve coefficient in linear control valve for both cases
(~decreasing and increasing pressure) is in the range of 0-1.86 with the
pressure 0-18 psig respectively. The pressure drop of water is in the range of
0.139-0.064 bar and Losses percentage (~hysteresis %) is in the range 1.91-36%
with increasing pressure.
7. CONCLUSIONS
Valve coefficient in equal%, quick opening and linear control
valve for both cases (~decreasing and increasing pressure) is in the range of
0-1.67, 3.89-0 and 1.86-0 with the pressure 18-0 psig respectively. The
pressure drop of water for equal %, quick opening and linear control valve is
in the range of 0.139-0.077 bar, 0.139-0.033 bar and 0.139-0.064 bar. Losses percentage (~hysteresis %) for equal
%, quick opening and linear control valve is in the range 0.0324-0.94, 0.022-
0.87 and 0.0191- 0.36 with increasing pressure.
8.
References
1. Coughanowr D., LeBlanc S., ‘Process Systems
Analysis and Control’, Mc-Graw Hill Science Engineering Math, 2nd
Edition, 2008, P-300-309.
