The subject of automatic controls is enormous, covering the control of variables such as temperature, pressure, flow, level, and speed.
The objective of this Block is to provide an introduction to automatic controls. This too can be
divided into two parts:
• The control of Heating, Ventilating and Air Conditioning systems (commonly known as HVAC);
• Process control.
Both are immense subjects, the latter ranging from the control of a simple domestic cooker to a complete production system or process, as may be found in a large petrochemical complex.
The Controls Engineer needs to have various skills at his command - knowledge of mechanical engineering, electrical engineering, electronics and pneumatic systems, a working understanding of HVAC design and process applications and, increasingly today, an understanding of computers and digital communications.
The intention of this Block is to provide a basic insight into the practical and theoretical facets of automatic control, to which other skills can be added in the future, not to transform an individual into a Controls Engineer.
This Block is confined to the control of processes that utilise the following fluids: steam, water, compressed air and hot oils.
Control is generally achieved by varying fluid flow using actuated valves. For the fluids mentioned above, the usual requirement is to measure and respond to changes in temperature, pressure, level, humidity and flowrate. Almost always, the response to changes in these physical properties must be within a given time. The combined manipulation of the valve and its actuator with time, and the close control of the measured variable, will be explained later in this Block.
The control of fluids is not confined to valves. Some process streams are manipulated by the action of variable speed pumps or fans.
The need for automatic controls
There are three major reasons why process plant or buildings require automatic controls:
• Safety - The plant or process must be safe to operate.
The more complex or dangerous the plant or process, the greater is the need for automatic controls and safeguard protocol.
• Stability - The plant or processes should work steadily, predictably and repeatably, without fluctuations or unplanned shutdowns.
• Accuracy - This is a primary requirement in factories and buildings to prevent spoilage,increase quality and production rates, and maintain comfort. These are the fundamentals of economic efficiency.
Other desirable benefits such as economy, speed, and reliability are also important, but it is against the three major parameters of safety, stability and accuracy that each control application will be measured.
Automatic control terminology
Specific terms are used within the controls industry, primarily to avoid confusion. The same words and phrases come together in all aspects of controls, and when used correctly, their meaning is universal.
The simple manual system described in Example 5.1.1 and illustrated in Figure 5.1.1 is used to introduce some standard terms used in control engineering.
Example 5.1.1 A simple analogy of a control system
In the process example shown (Figure 5.1.1), the operator manually varies the flow of water by opening or closing an inlet valve to ensure that:
• The water level is not too high; or it will run to waste via the overflow.
• The water level is not too low; or it will not cover the bottom of the tank.
The outcome of this is that the water runs out of the tank at a rate within a required range. If the water runs out at too high or too low a rate, the process it is feeding cannot operate properly.
At an initial stage, the outlet valve in the discharge pipe is fixed at a certain position.
The operator has marked three lines on the side of the tank to enable him to manipulate the water supply via the inlet valve. The 3 levels represent:
1. The lowest allowable water level to ensure the bottom of the tank is covered.
2. The highest allowable water level to ensure there is no discharge through the overflow.
3. The ideal level between 1 and 2.