The primary objective of the designer when specifying instrumentation and control schemes are:
6.1.1: Safe Plant Operation
- To help the process variables within know safe operating limits.
- To detect dangerous situations as they develop and to provide alarms and automatic shutdown systems.
- To provide inter locks and alarms to prevent dangerous operating procedures.
6.1.2: Production Rate
To achieve the design product output.
6.1.3: Product Quality
To maintain the product composition within specified quality standard.
To operate at the lowest production cost, commensurate with the other objective. These are not separate objectives and must be considered together. Measurement is a fundamental requisite of process control either the control will be affected automatically, semi-automatically or manually. The quality of the control obtainable is a function of the accuracy, repeatable and reliability of the measuring devices employed.
The objective of an automatic process control is to use the manipulated variable to maintain the controlled variable at its set point in spite of disturbances. Instruments are provided to monitor the key process variables during plant operations. Instruments monitoring critical process variables will be fitted with automatic alarms to alert, the operations to critical and hazardous situations.
Pneumatic instruments are used in this plant. The main process parameters are all indicated in the control room where automatic or remote control is carried out centrally.
The process parameters e.g. temperatures, pressure flow, liquid level etc. are converted to signals with transducers and then indicated, recorded and controlled with secondary instruments.
6.2: TEMPERATURE MEASUREMENT AND CONTROL
This is used to control the temperature of outlet and inlet streams in heat exchanger, condenser, heater and column, Most temperature measurement in the industrial processes is made by means of thermocouples to facilitate bringing measurements to a centralized location. Usually where high measurement accuracy is required resistance thermometer are used. For the control of temperature, pneumatic feedback control is used. Outlet temperature of the system is measured, with the help of thermocouple. This temperature is fed to a comparator. The difference between the set point and the input signal is called error, is fed to the controller, which controls diaphragm valve to adjust the flow of hot or cooling medium.
6.3: PRESSURE MEASUREMENT AND CONTROL
The temperature and pressure are valuable indications of material state and composition. In fact, these two measurements considered together are the evaluating devices of industrial materials. In compressed gas systems etc., pressure is of primary importance; pumping equipments are furnished with pressure measuring devices. Pressure measurement becomes an indication of the increase or decrease. Most pressure measurement in the industry is made through elastic devices, either directly connected for local use or transmission type to a controlised location.
6.4:FLOW MEASUREMENT AND CONTROL
Most flow measurements in the industry are done by variable head devices. To lesser extent variable area and displacement types are used.
For instrumentation and control of different sections and equipments of plants, following control loops are most often used.
1. Feed back control loop
2. Feed forward control loop
3. Ratio control loop
4. Auctioneering control loop
5. Split range control loop
6. Cascade control loop
Here is given a short outline of these control schemes, so that to justify out selection of a control loop for specified equipment.
6.5.1:FEED BACK CONTROL LOOP
A method of control in which a measured value of a process variable is compared with the desired value of the process variable and any necessary action is taken. Feed back control is considered as the basic control loops system. Its disadvantage lies in its operational procedure. For example if a certain quantity is entering in a process, then a monitor will be there at the process to note its value. Any changes from the set point will be sent to the final control element through the controller so that to adjust the incoming quantity according to desired value (set point). But in fact changes have already occurred and only corrective action can be taken while using feed back control system.
6.5.2:FEED FORWARD CONTROL LOOP
A method of control in which the value of disturbance is measured than action is taken to prevent the disturbance by changing the value of a process variable. This is a control method designed to prevent errors from occurring in a process variable. This control system is better than feed back control because it anticipates the change in the process variable before it enters the process and takes the preventive action. While in feed back control system action is taken after the change has occurred.
A control loop in which, the controlling element maintains a predetermined ratio of one variable to another. Usually this control loop is attached to such as system where two different systems enter a vessel for reaction that may be of any kind. To maintain the stoichiometric quantities of different streams, this loop is used so that to ensure proper process going on in the process vessel.
6.5.4:AUCTIONEERING CONTROL LOOP
This kind of control loop is normally used for a huge vessel where readings of a single variable may be different at different locations. This type of control loop ensures safe operation because it employs all the readings of different locations simultaneously, and compares them with the set point, then the controller sends appropriate signal to final control element.
6.5.5:SPLIT RANGE LOOP
In this loop controller is preset with different values corresponding to different actions to be taken at different conditions. The advantage of this loop is to maintain the proper conditions and avoid abnormalities at very differential levels.
6.5.6:CASCADE CONTROL LOOP
This is a control in which two or more control loops are arranged so that the output of one controlling element adjusts the set point of another controlling element. This control loop is used where proper and quick control is difficult by simple feed forward or feed backward control. Normally first loop is a feed back control loop.
Thus, we can see that PID control is the optimal technique for temperature control of a distillation column.