Explain the function and use of final control elements (e.g., motor operated valves, solenoid actuated valves, dampers, and pumps, etc). Explain the importance of final control elements and considerations used during the selection process (e.g., reliability, response time, and other essential features) ?
Final control elements are devices that complete the control loop. They link the output of the controlling elements with their processes. Some final control elements are designed for specific applications.
For example, neutron-absorbing control rods of a reactor are specifically designed to regulate neutron-power level; however, the majority of final control elements are general application devices such as valves, dampers, pumps, and electric heaters. Valves and dampers have similar functions. Valves regulate flow rate of a liquid while dampers regulate flow of air and gases. Pumps, like valves, can be used to control flow of a fluid. Heaters are used to control temperature.
These devices can be arranged to provide a type of on-off control to maintain a variable between maximum and minimum values. This is accomplished by opening and shutting valves or dampers or energizing and de-energizing pumps or heaters. These devices can also be modulated over a given operating band to provide a proportional control. This is accomplished by positioning valves or dampers, varying the speed of a pump, or regulating the current through an electric heater. There are many options to a process control. Of the final control elements discussed, the most widely used in power plants are valves. Valves can be easily adapted to control liquid level in a tank, temperature of a heat exchanger, or flow rate.
Final Control Elements–Control Valves. Process control engineers tend to treat final control elements in the same way they treat measurement devices–with absolute indifference. To most of them, a valve is a valve is a valve. Its job is to open and close according to what the controller tells it and it does just that.
The problem is, in a shocking number of cases, it does not. Control valves and dampers, being mechanical devices, are subjected to a lot of mechanical issues like wear and tear – deterioration with time.
Present measurement devices are very robust and most do not deteriorate drastically with time. The controller running inside the modern distributed control system (DCS) is even more reliable with a hot standby ready to take over in case of failure.
The same cannot be said about final control elements. Therefore the weakest link in the process control loop is frequently the final control element.
The problems of control valves usually manifest as deadband, stiction, and hysteresis.
Deadband is a general phenomenon where a range or band of controller output values fails to produce a change in the measured process variable. This is bad for process control. Present process control systems execute at a rate of about 3 times per second. Each time they execute, the output changes in the magnitude of (usually) less than 1 percent. But most relevant in the case of deadband, the changes can occur in either direction.
If a control valve is suffering from a deadband problem, when the controller output reverses direction, the control valve does not respond; therefore, the process variable also does not respond to the command of the controller. The controller does not receive the command and so issues another (sometimes more drastic) command.
When the control valve finally comes out of its deadband, the controller command has caused it to overshoot. The controller then tries to go back the other direction only to be faced with the same situation; the process will be driven to overshoot in either direction and cycles continuously, forming what is called a limit cycle.
This is somewhat similar to deadband except that it does not only happen when the controller changes direction. Again stiction (also known as sticky valve) can be due to a variety of reasons; a common one is packing friction.
In terms of process control, the effect of stiction is also like deadband whereby the valve fails to respond when required and when it does respond, it overshoots the set point. The controller then tries to bring it back the other way.
Hysteresis occurs when the same change in the controller output in both directions results in a different change in the process value.
For example, when the controller output is 20 percent, the process variable is 30°C. When the controller output increases to 25 percent, the temperature increases to 35°C. However, when the controller goes back down to 20 percent, the temperature only goes down to 33°C. This results in different process gains in both directions and will confuse the controller, which has been tuned for only one process gain.