Chapter 2 Control System & DCS Description

CHAPTER 2

CONTROL SYSTEM AND DCS

2.9 INTRODUCTION ABOUT CONTROL LOOPS

A few common control systems are

i. Ratio Controllers
ii. Cascade controllers
iii. Split range controllers
2.9.1.1 Ratio Control

Ratio control system is a technique where variable is manipulated to keep it as a ratio

In this figure the ratio control system consists

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flow rate of the first pipe and second flow control. The flow controller controls the flow
of the second pipe with respect to the flow in the first pipe.
2.9.1.2 Example of ratio control

A common example is when the ratio of two reactants must be controlled is shown in the figure.
One of the flow rates is measured but allowed to float, that is , not regulated. The outer flow rate
is both measured and adjusted. The outer flow rate is both measured and adjusted to provide the
specified control ratio. The flow rate of reactant A is measured and added with appropriate
scaling, to the measurement of flow rate B. The controller reacts to the resulting input signal by
adjustment of the control valve in the reactant B input line.

2.9.1.3 Application of ratio control:

➢ Blending operations

➢ For holding the fuel-air ratio of reactants of the optimum.

➢ Maintaining a stoichiometric ratio of reactance of a reactor.

➢ Keeping a specified reflux ratio for a distillation column etc.

2.9.1.4 Cascade control

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In a cascade control arrangement, there are two (or more) controllers of which one controller's
output drives the set point of another controller. For example: a level controller driving the set
point of a flow controller to keep the level at its set point

Cascade control can improve control system performance over single-loop control whenever
either: (1) Disturbances affect a measurable intermediate or secondary process output that
directly affects the primary process output that we wish to control; or (2) the gain of the
secondary process, including the actuator, is nonlinear. In the first case, a cascade control system
can limit the effect of the disturbances entering the secondary variable on the primary output. In
the second case, a cascade control system can limit the effect of actuator or secondary process
gain variations on the control system performance. Such gain variations usually arise from
changes in operating point due to setpoint changes or sustained disturbances. A typical candidate
for cascade control is the shell and tube heat exchanger of Figure 1.

Fig 1 A sheel and Tube heat exchange

The primary process output is the temperature of the tube side effluent stream. There are two
possible secondary variables, the flow rate of steam into the exchanger and the steam pressure in
the exchanger. The steam flow rate affects the effluent temperature through its effect on the
steam pressure in the exchanger. The steam pressure in the exchanger affects the effluent
temperature by its effect on the condensation temperature of the steam. Therefore, either the
steam flow rate or the steam pressure in the exchanger can be used as the secondary output in a

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cascade control system. The choice of which to use depends on the disturbances that affect the
effluent temperature.

If the main disturbance is variations in the steam supply pressure, due to possibly variable steam
demands of other process units, then controlling the steam flow with the control valve is most
likely to be the best choice. Such a controller can greatly diminish the effect of steam supply
pressure variations on the effluent temperature. However, it is still necessary to have positive
control of the effluent temperature to be able to track effluent temperature set point changes and
to reject changes in effluent temperature due to feed temperature and flow variation. Since there
is only one control effort, the steam valve stems position; traditional cascade control uses the
effluent temperature controller to adjust the set point of the steam flow controller, as shown in
Figure 2.

Fig.2 Casecad control of effluent temperature via steam flow

If feed flow and temperature variations are significant, then these disturbances can be at least
partially compensated by using the exchanger pressure rather than the steam flow as the
secondary variable in a cascade loop, as shown in Figure 3.

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Fig.3 Casecad control of effluent temperature via shell side pressure

The trade-off in using the configuration of Figure 3 rather than that of Figure 2is that the inner
control loop from the steam pressure to the valve stem position may not suppress variations in
valve gain as well as with an inner loop that uses the valve to control the steam flow rate. This
consideration relates to using a cascade control system to suppress the effect of process
uncertainty, in this case the valve gain, on the control of the primary process variable, the
effluent temperature. We will have a lot more to say about using cascade control systems to
suppress process uncertainty in the following sections.

To repeat, cascade control has two objectives. The first is to suppress the effect of disturbances
on the primary process output via the action of a secondary or inner control loop around a
secondary process measurement. The second is to reduce the sensitivity of the primary process
variable to gain variations of the part of the process in the inner control loop.

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2.9.1.5 Split Range Control

A very common control scheme is split range control in which the output of a controller is split
to two or more control valves. For example:

➢ Controller output 0% Valve A is fully open and Valve B fully closed.

➢ Controller output 25% Valve A is 75% open and Valve B 25% open.

➢ Controller output 50% Both valves are 50% open.

➢ Controller output 75% Valve A is 25% open and Valve B 75% open.

➢ Controller output 100% Valve A is fully closed and Valve B fully open.

Different arrangements are possible. For example, figure 1 shows a split range pressure
controller on a separator with two valves, one to the flare and one to the compressor suction. In
this case, the ‘split’ is configured as follows:

➢ Controller output 0% Both valves are closed.

➢ Controller output 25% Valve A is 50% open and Valve B still closed.

➢ Controller output 50% Valve A is fully open and Valve B closed.

➢ Controller output 75% Valve A is fully open and Valve B 50% open.

➢ Controller output 100% Both valves are fully open.

The idea is that the suction valve is used for normal pressure control while the flare valve only
opens to disperse high pressures.

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In this application, the flare valve will need to open quickly in response to high pressures, but
the compressor suction valve will need to move much more slowly to prevent instability in the
compressors. The main problem with split range control is that the controller only has one set of
tuning parameters. If the controller is tuned to be fast acting to optimize the performance of the
flare valve, the suction valve will also move rapidly to produce unstable gas flows to the
compressors. If the controller is tuned slower to stabilize the compressors, then the flare valve
will not open fast enough as the pressure rises. A further issue is that the process response of the
route to flare generally differs to the process response of the route to the compressors, so both
routes will anyway require very different tuning for optimal control.

The solution is to replace the split range controller with two independent controllers, both
reading the same pressure transmitter, but one controlling the flare valve and the other the
suction valve. Not only can each controller be tuned correctly for its dedicated service, but
different set points can also be used to prevent the flare valve from ‘popping’ unnecessarily.

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The ease with which a split range controller can be replaced with two ordinary controllers
depends on a number of factors. If the ‘split’ is calculated in the DCS or PLC so that each valve
has its own output from the control system, then the addition of a new controller is simply a
matter of software configuration. However, occasionally, the control system only has one output
wired to both controllers and the ‘split’ produced by configuration of the valve positioned. A new
output will then be required from the control system to one of the valves and the valve
positioners must be reconfigured to operate over the full 0-100% output range.

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