Exercise 2: Simple Processes - Answers

The aim of this section is to devise suitable control schemes for safe operation of the plant systems described in the tutorial. You have to think about


Process 1

In this first example the parameter being measured is the composition of the material in a reactor. Thus the diagram shows an open tank as described with a composition transducer analysing the contents.

On the basis of this analysis, the amount of material sent to main distillation and the amount sent for reprocessing is adjusted. Therefore the outflow from the tank is split into two streams and the flowrates of these streams adjusted accordingly.

Now we can think about the safe operation of this system i.e what could go wrong. The two things that could happen are that the tank could run dry or overflow. Thus a level indicator/alarm is shown. This will alert the operator when the tank is at a low or high level. Also as an extra precaution the overflow is controlled i.e if the tank does overflow then the material is not lost.

The diagram below shows the final system.


Process 2

In this second example there are two parameters of interest, the flowrate and the pressure.

The flowrate entering the compressor is measured. We are told in the description that if this is not high enough then gas is bypassed from the compressor discharge back to inlet suction. Therefore the manipulated variable for this loop is the flowrate of this bypass.

For the control of the pressure it is a simple matter to manipulate the flowrate of the discharge since it is a gas.

The diagram below shows the final system.


Process 3

The third example is slightly more complicated. The first thing to draw is the furnace showing the two feeds - air and natural gas - and the one exit stream - the flue gas.

The two important parameters to be regulated are the temperature of the flame and the composition of the flue gas. Remember at this stage that only one control valve is allowed on each line so the two manipulated variables are the flows of the two feeds.

The composition of the flue gas is dependent on the flowrate of air and the temperature of the flame is dependent on the flowrate of natural gas.

To ensure that an explosive mixure cannot be formed in the furnace we must monitor the composition of the feed entering. This can be done by placing a flow indicator (an orifice meter) on the natural gas line and then adjusting the flow of air accordingly.

To ensure that full gas main pressure shall never be applied to the burners we can simply split the stream in two to half the pressure and by the use of single action valves have only one part at a time.

Finally a flame monitor is included that shuts off both feed streams in the event of the flame going out.

The diagram below shows the final system


Please move Back to previous page