Saturday, 6 June 2015

SIMPLE CONTROL SYSTEMS



Introduction

In general, process control systems are concerned with the automatic control of one of the four main process variables (pressure, level, flow and temperature).  There are, of course, other process variables which we may wish to control but in this Unit we will concentrate on the main ones.
Let us start by taking a look at some typical control loops.  The control loops we shall look at are all closed loop systems.

Pressure Control Systems

There are two ways in which the pressure of fluid inside vessels can be controlled.  These are illustrated in Figure 1-1.  Study these before you read on.

 

Figure 1-1   Pressure Control Systems
Figure 1-1(a) shows a pressure comparator/controller (working in conjunction with a pressure sensor) sending a correcting signal to a pressure control valve positioned in the upstream air supply side of the vessel.
If the pressure inside the vessel falls below the desired value then the controller will send a correcting signal to the valve which will then open.  This will allow the pressure to build up to the desired value.
If, on the other hand, the pressure goes above the desired value then the controller will send a signal to close the valve until the pressure has fallen back to the desired value.
Figure 1-1(b) illustrates an alternative way of controlling the pressure inside the vessel.  In this case the control valve is on the demand or downstream side of the vessel.
In consequence, it will need to close if the pressure in the vessel falls below the desired value and open if it goes above.
This process of comparing and correcting goes on continuously, thus the desired pressure can be maintained at all times.
Level Control Systems
Figure 1-2   Level Control Systems
Figure 1-2(a) shows a liquid level comparator / controller (working in conjunction with a level measuring sensor) sending correcting signals to a level control valve fitted to the liquid supply line.
If the liquid level rises above the desired value then the controller will close the valve until the level is back to normal.
If, however, the level falls below the desired value then the controller will open the valve to allow the tank to fill. In this way the liquid level in the tank can be maintained at the desired value.
Figure 1-2(b), the liquid level in the tank is controlled in the opposite way. When the liquid level rises above the desired valve then the valve is instructed to open and vice versa.

Flow Control Systems

Figure 1-3 shows a flow comparator / controller (working in conjunction with a flow sensor) sending correcting signals to a flow control valve.  Study it before you read on
Figure 1-3   Flow Control System
If the flow of fluid in the pipeline goes above the desired value then the controller will send a signal to the flow control valve to close.  This would bring the flow rate down to the desired value.
However, if the flow rate falls below the desired value then the controller opens the valve to allow the flow rate to rise to the desired value.  In this way the desired flow rate could be maintained.

Temperature Control Systems

Figure 1-4 illustrates one way in which the temperature of a process fluid can be controlled.  Study this figure before you read on.
Figure 1-4   Process Fluid Temperature Control
In this case, if the process fluid outlet temperature goes above the desired value then the comparator/controller will send a signal to the control valve on the steam line to close.  This would reduce the steam flow to the heating coil and allow the temperature of the process fluid to fall back down to the desired value as indicated on the set point.
Conversely, if the temperature of the process fluid falls below the desired value then the comparator/controller will send a signal to the control valve to open.  This would allow the temperature of the process fluid to rise back up to the desired value.
In this way the desired temperature of the process fluid can be maintained at all times.
As an alternative to supplying steam we could have a cold water supply system to remove heat.  The process fluid temperature control loop and its components would be similar to Figure 1-4.

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