Components
Strategy
Although the measurement is before the manipulation this NOT feedforward control. Like all examples of useful control systems in this course it is standard feedback control. The position of the measurement relative to the adjustment does not matter in this case because the flow must be the same on both sides of the valve.
Operation
If the flow increases then the valve is closed and if the flow decreases then the valve is opened.
It is usual to measure flows upstream of valves since, especially with gases, valve position can affect the calibration of certain types of flowmeter.
This is the standard way of regulating flows. There are virtually no situations in which the flow though the control valve is not the one which is measured.
Components
Strategy
The temperature of the fluid downstream of the valve is regulated using feedback control.
Operation
If the temperature of the stream increases then the valve is closed. If it decreases the valve is opened.
This control system will work regardless of the cause of a change in temperature, which can be brought about by a change water or steam temperature or of water flowrate.
Unlike the previous example it is of course necessary to measure the temperature downstream as this is what is to be regulated!
With any proposed control system simply ask yourself: "Does moving the valve change the measured variable?" If the answer is "No" then the control system cannot work.
Components
Strategy
This is a example of attempted feedforward control. The level in the tank is never measured. Changing the valve position does not affect the measured variable. The level is supposed to be kept constant by matching the outlet flow to the inlet flow.
There is no point in doing this usless for some reason the level in the tank cannot be measured.
Operation
If the inlet flowrate increases then the outlet flowrate is increases by the same amount. If the inlet flowrate decreases then the outlet flowrate is decreased by the same amount.
This will not work! This is a bad control system. Feedforward should only be used to augment the performance of a feedback system. A correct control system is shown below.
Components
Strategy
This is a typical split range feedback control loop
Operation
The pressure in the tank is measured. If it is too low, due to a decrease in the flowrate of nitrogen, then the vent line is closed and the N2 valve is opened and the pressure builds back up. If it is too high, due to an increase in N2 flowrate, then the N2 valve is closed and the vent opened and the pressure allowed to fall.
Shown above is a simple flash vessel. The aim of this is to separate a stream into vapour and liquid by altering the pressure (and hence temperature). There are three control loops.
On a flash vessel it is necessary to have the flow of one stream controlled. This is to ensure that the throughput is known. This flow control operates the same as that shown above i.e if the flow decreased then the valve is opened and vice versa.
The composition of the resulting streams depends on the pressure in the vessel. If the pressure increases then the valve is opened to allow more vapour through. If the pressure decreases then the valve is closed to allow the pressure to build back up again.
Notice, however, that both these actions will have an effect on the level of liquid in the tank. If the pressure decreases then more liquid will vapourise and the level will decrease. If the pressure increases then the level will rise.
The shaded valve denotes an inventory loop. This is to ensure that the mass balance around the vessel balances i.e. what goes in must come out. In this case it is simply a feedback liquid level control loop. If the level increases then the valve is opened and if the level decreases then the valve is closed. Thus the vessel should not overflow or run dry.
Components
Strategy
This shows a typical cascade control scheme of a heat exchanger. The aim is to keep T2 as close to the setpoint as possible.
Operation
An increase in T2 can be caused by
To compensate the flow through the tubes should be increased. However, there is a second loop which compensates for changes in the flowrate.
The TC (Primary Loop) detects changes in the temperature brought about by changes in T1, F or the temperature in the tubes. The FC (Secondary Loop) detects changes in the cooling water flow and hence eliminates anticipated effects on the temperature T2.
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