Control system & automation HNC 4th assignment
julian kay
assessment_4.pdf
1. A control valve is to be used to control water flow at a maximum rate of
120 US gallons per minute. If the maximum pressure drop across the
valve is 100 psi, determine the valve size required for this particular
application. The table below relates CV and valve size.
TABLE 1
Valve size (inches)
1
1
2
3
4
6
8
CV
0.3
3
14
35
55
108
174
400
725
1 2
1 4
1 2
2
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2. FIGURE 1 shows the valve characteristics of three different types of
valve trim.
FIG. 1
(i) Identify each of the three characteristics.
(ii) State, with reasons, a typical type of application for each of the three
types of valve trim.
0 20
20
40
% Valve opening
40 60 80 100
% F
lo w
r a te
( co
n st
a n t
p d )
80
100
60
(a)
(b)
(c)
0
3
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3. Write brief notes on the following topics:
(a) flashing in control valves
(b) cavitation in control valves
(c) control valve noise
(d) reasons for the use of valve positioners.
4. The level of liquid in an open tank (FIGURE 2) is to be controlled by
regulating the flow of liquid into the vessel.
(a) Complete the drawing to show how the level can be controlled by
linking the liquid inflow and liquid level controls in cascade.
(b) Draw a control block diagram of the cascaded system.
(c) Describe the response of the cascaded system to a change in the
measured level of liquid.
FIG. 2
F1 FCV 1
F1C 1
L1C 1
4
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5. FIGURE 3 shows the block diagram of the control of an electric heating
system. The heater is driven from a voltage-controlled power supply, the
voltage V1 being derived from a potientiometer. The output temperature,
θO, is subject to disturbances, θD, because of changes in the ambient temperature. It is proposed to apply 'disturbance feedback control' to the
system by the inclusion of a transducer that measures the external
temperature and feeds a signal back to the input via a proportional
controller of gain H.
Determine the required value of H to eliminate the effect of the disturbance.
FIG. 3
2 amps/volt
θO
θD
V1
0.05 V/ °CH
0.4 °C/amp
Proportional controller
Proportional controller
Proportional controller Transducer
Power supply
Heater
+ –
+ +
5
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6. The purpose of the arrangement shown in FIGURE 4 is to mix the two
liquid products A and B in a fixed mass ratio. Product A, which is itself a
mixture, is a 'wild' flow, whilst product B, a pure compound, is controlled.
As the mixture leaves the tank the transmitter ρTX measures its density.
(a) Complete the diagram to show how the arrangement could be
controlled by the method of 'variable ratio control'.
(b) Identify which transmitter provides 'feedforward'.
(c) Describe how the control system responds to a disturbance caused by
a variation in the density of product A.
FIG. 4
Product B
Mixing tank
FTXFTX
Mixture
ρTX
Product A
6
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7. FIGURE 5 shows a partially completed diagram of a flow control system.
The flow controller is reverse acting and has a 0.2 to 1.0 bar pneumatic
output signal which will supply both control valves V1 and V2.
The small range control valve, V2, only needs to operate on the first 25%
output change of the controller output signal. For larger flow rates the
small range valve will remain fully open and control will be achieved by
operation of the large range valve. Note the differing air failure action of
the two valves.
FIG. 5
Design a system utilising valve positioners which will meet the prestated
specifications.
Large range
Small range
CAF
V1
OAF
V2
FC RA
7
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© Teesside University 2011
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setdistillerparams << /HWResolution [2400 2400] /PageSize [612.000 792.000] >> setpagedevice
