Basic Control Theory
Contents
Choice and Selection of Controls
This tutorial will concentrate on available automatic control choices (such as self-acting, pneumatic or electric) and the decisions which must be made before selection. Guidance is offered on the basis of the three most important considerations of safety, stability and accuracy.
This Module will concentrate on available automatic control choices and the decisions which must be made before selection. Guidance is offered here rather than a set of rules, because actual decisions will depend upon varying factors; some of which, such as cost, personal preferences and current fashions, cannot be included here.
Application
It is important to reflect on the three basic parameters discussed at the beginning of Module 5.1: Safety, Stability and Accuracy.
In order to select the correct control valve, details of the application and the process itself are required. For example:
- Are any safety features involved? For instance, should the valve fail-open or fail-closed in the event of power failure? Is separate control required for high and low limit?
- What property is to be controlled? For instance, temperature, pressure, level, flow?
- What is the medium and its physical properties. What is the flowrate?
- What is the differential pressure across a control valve across the load range?
- What are the valve materials and end connections?
- What type of process is being controlled? For instance, a heat exchanger used for heating or process purposes?
- For temperature control, is the set point temperature fixed or variable?
- Is the load steady or variable and, if it is variable, what is the time scale for change, fast or slow?
- How critical is the temperature to be maintained?
- Is a single loop or multi-loop control required?
- What other functions (if any) are to be carried out by the control? For instance, normal temperature control of a heating system, but with added frost protection during ‘off’ periods?
- Is the plant or process in a hazardous area?
- Is the atmosphere or environment corrosive by nature or is the valve to be fitted externally or in a ‘dirty’ area?
- What motive power is available, such as electricity or compressed air, and at what voltage and pressure?
Motive power
This is the power source to operate the control and drive the valve or other controlled device. This will usually be electricity, or compressed air for a pneumatic system, or a mixture of both for an electropneumatic system. Self-acting control systems require no external form of power to operate; they generate their own power from an enclosed hydraulic or vapour pressure system.
To some extent, the details of the application itself may determine the choice of control power. For example, if the control is in a hazardous area, pneumatic or self-acting controls may be preferable to expensive intrinsically safe or explosion-proof electric/electronic controls.
The following features are listed as a general comment on the various power source options:
Self-acting controls
Advantages:
- Robust, simple, tolerant of ‘unfriendly’ environments.
- Easy to install and commission.
- Provide proportional control with very high rangeability.
- Controls can be obtained which fail-open or fail-closed in the event of an unacceptable overrun in temperature.
- They are safe in hazardous areas.
- Relatively maintenance free.
Disadvantages:
- Self-acting temperature controls can be relatively slow to react, and Integral and Derivative control functions cannot be provided
- Data cannot be re-transmitted.
Pneumatic controls
Advantages:
- Robust.
- They operate very quickly, making them suitable for processes where the process variables change rapidly.
- The actuators can provide a high closing or opening force to operate valves against high differential pressures.
- The use of valve positioners will ensure accurate, repeatable control.
- Pure pneumatic controls are inherently safe and actuators provide smooth operation.
- Can be arranged to provide fail-open or fail-closed operation without additional cost or difficulty.
Disadvantages:
- The necessary compressed air system can be expensive to install, if no supply already exists.
- Regular maintenance of the compressed air system may be required.
- Basic control mode is on/off or proportional although combinations of P+I and P+ I +D are available, but usually at greater cost than an equivalent electronic control system.
- Installation and commissioning is straightforward and of a mechanical nature.
Electric controls
Advantages:
- Highly accurate positioning.
- Controllers are available to provide high versatility with on-off or P+I+D combinations of control mode, and multi-function outputs.
Disadvantages:
- Electric valves operate relatively slowly, meaning they are not always suitable for rapidly changing process parameters such as pressure control on loads that change quickly.
- Installation and commissioning involves both electrical and mechanical trades and the cost of wiring and installation of a separate power supply must be taken into account.
- Electric actuators tend to be less smooth than their pneumatic counterparts. Spring return actuators are required for fail open or fail closed functions: This can substantially reduce the closing force available and they usually cost more.
- Intrinsically safe or explosion-proof electric controls are needed for use in hazardous areas; they are an expensive proposition and, as such, a pneumatic or electropneumatic solution may be required, as described below. Special installation techniques are required for these types of hazardous areas
Electropneumatic controls
Advantages:
- Electropneumatic controls can combine the best features of electronic and pneumatic controls. Such systems can consist of pneumatically actuated valves, electric/electronic controllers, sensors and control systems, plus electropneumatic positioners or converters
The combination provides the force and smooth operation of a pneumatic actuator/valve with the speed and accuracy of an electronic control system. Fail-open or fail-closed operation can be provided without cost penalty and, by using suitable barriers and/or confining the electric/electronic part of the control system to ‘safe’ (non-hazardous) areas, they can be used where intrinsic safety is required.
Disadvantages:
- Electrical and compressed air supplies are required, although this is not normally a problem in industrial processing environments.
There are three important factors to take into account when considering the application and the required power source:
- Changes in load.
- Whether the set value is critical or non-critical.
- Whether the set value has to be varied
The diagrams in Figure 5.4.1 and 5.4.2 help to explain.
What type of controls should be installed?
Different applications may require different types of control systems. Self-acting and pneumatic controls can be used if load variations are fairly slow and if offset can be accepted, otherwise electropneumatic or electric controls should be used. Figure 5.4.3 shows some different applications and suggestions on which method of control may be acceptable.
Types of valves and actuators
The actuator type is determined by the motive power which has been selected: self-acting, electrical, pneumatic or electropneumatic, together with the accuracy of control and actuator speed required.
As far as valve selection is concerned, with steam as the flowing medium, choice is restricted to a two port valve. However, if the medium is water or another liquid, there is a choice of two port or three port valves. Their basic effects on the dynamics of the piping system have already been discussed.
A water application will usually determine whether a three port valve is used to mix or divert liquid flow. If changes in system pressure with two port valves are acceptable, their advantages compared with three port valves include lower cost, simplicity and a less expensive installation. The choice of two port valves may also allow the inherent system pressure change to be used to switch on sequential pumps, or to reduce or increase the pumping rate of a variable speed pump according to the load demand.
When selecting the actual valve, all the factors considered earlier must be taken into account which include; body material, body pressure/temperature limits, connections required and the use of the correct sizing method. It is also necessary to ensure that the selection of valve/actuator combination can operate against the differential pressure experienced at all load states. (Differential pressure in steam systems is generally considered to be the maximum upstream steam absolute pressure. This allows for the possibility of steam at sub-atmospheric pressure on the downstream side of the valve).
Controllers
Safety is always of great importance. In the event of a power failure, should the valve fail-safe in the open or closed position?
Is the control to be direct-acting (controller output signal rises with increase in measured variable) or reverse-acting (controller output signal falls with increase in measured variable)?
If the application only requires on/off control, a controller may not be needed at all. A two-position actuator may be operated from a switching device such as a relay or a thermostat. Where an application requires versatility, the multi-function ability of an electronic controller is required; perhaps with temperature and time control, multi-loop, multi-input/output.
Having determined that a controller is required, it is necessary to determine which control action is necessary, for instance on/off, P, P I, or P I D.
The choice made depends on the dynamics of the process and the types of response considered earlier, plus the accuracy of control required.
Before going any further, it is useful to define what is meant by ‘good control’. There is no simple answer to this question. Consider the different responses to changes in load as shown in Figure 5.4.4.
Self-acting control is normally suitable for applications where there is a very large ‘secondary-side’ thermal capacity compared to the ‘primary-side’ capacity.
Consider a hot water storage calorifier as shown in Figure 5.4.5 where the large volume of stored water is heated by a steam coil.
When the water in the vessel is cold, the valve will be wide open, allowing steam to enter the coil, until the stored water is heated to the desired temperature. When hot water is drawn from the vessel, the cold water which enters the vessel to take its place will reduce the water temperature in the vessel. Self-acting controls will have a relatively large proportional band and as soon as the temperature drops, the valve will start to open. The colder the water, the more open the steam valve.
Figure 5.4.6 shows a non-storage plate type heat exchanger with little thermal storage capacity on either the primary or the secondary side, and with a fast reaction time. If the load changes rapidly, it may not be possible for a self-acting control system to operate successfully. A better solution would be to use a control system that will react quickly to load changes, and provide accuracy at the same time.
