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Saturday, March 17, 2012

General concepts of process control systems

A successful operation of chemical plants depends on the following requirements - safety, production specifications, environmental regulations, operational constraints, and economical. There are many types of processes and processing equipment (absorbers, heat exchangers, dryers, separators, reactors, distillation columns, pumps, etc). They are connected together to reach the objectives of the plant - to produce a desired product from the stock using the most safety and economical procedures. Therefore, we must be able at all times control all process parameters and all operations of a plant. We can solve this problem by using a control system, which consists of measuring devices, controllers, valves, computers, transmission lines and intervention of plant personnel.

Objectives of every control system are as follows:

1)      to suppress the influence of external disturbances, i.e. the effect of surroundings on the process;

2)      to ensure the stability of a chemical process, i.e. to keep process parameters (variables) as close as possible to their desired values;

3)      to optimise the performance of a chemical process, i.e. to meet the requirements of safety, satisfaction of production specification and maximising of economic objectives.

Let's consider two systems for the control of temperature and level of liquid in a stirred tank shown in Fig. 1.1. These two control systems have all elements inherent to every control system, namely:

1)      a process - the stirred tank;
2)      controlled variables - temperature of the effluent liquid (or temperature of the liquid in the tank) and level of the liquid in the tank;
3)      a transducer, a measuring instrument, a transmitter;
4)      a transmission line (electrical, pneumatic or hydraulic) that carries a measurement signal from a measuring instrument (or transducer) to a controller;
5)      a control signal from a controller to a final control element (for example, a control valve);
6)      a controlling device or a controller;
7)      a final control element - a control valve.

In Fig. 1.1 we used the following symbols and abbreviations:

CV                - a control valve.
Fin, Fout,Fst- flowrates of inlet and outlet streams of the liquid, and steam, respectively;
Hs                       - a desired value (set point) of the liquid level in the stirred tank;
LT                 - a level transmitter;
TE                 - a thermocouple;
Tin, Tout, Tst   - temperatures of inlet and outlet streams of the liquid, and steam,
Ts                   - a desired value (set point) of the temperature in the stirred tank.

Figure 1.1. Process control systems for a stirred tank.
A process is an equipment or apparatus for which supply of energy, material, etc., and demand must be balanced. We don't include in this concept control hardware. There are many types of processes: from simple, such as our example or a tank for storage of liquid hydrocarbons, where liquid level should be kept within an acceptable range, to complex processes such as distillation columns and reactors.

A process variable is a physical or chemical property, quantity or other condition which can be changed. Here are several examples of process variables: a flow rate of a feed stream into a distillation column; a pressure of gas in a storage tank; a level in a condenser; a temperature of a liquid stream (feed to a reactor) exiting from a furnace; pH of a solution in a stirred tank; density and viscosity of liquid products of oil refining (these parameters are usually used as an indicator of desirable quality of products); a resistance of an electrical circuit; a speed of rotor revolution, etc.

In general, process variables can be classified as shown in Fig. 1.2. Now we can give explanations of types of process variables with the examples from Fig.1.1.

Figure 1.2. Classification of process variables.

Input variables reflect the effect of the surroundings on the process (Fin, Tin, Fst, Tst
Output variables denotes the effect of the process on the surroundings (Fout, Tout, H)
Manipulated variables can be adjusted freely by the human operator or a controller (Fst, Fout)
Disturbances are not the result of an adjustment by an operator or a controller(Fin, Tin, Tst)
Measured variables - their values can be directly measured by a measuring device (Fin, Tin, Tout, H)
Unmeasured variables - their values are not or cannot be measured directly (in chemical processes - the feed composition for a distillation column).

A transducer is a device which can convert information of one physical form to another type of its output (for example, resistance temperature detector converts the change of the measured temperature into the change of the electrical resistance of a metal conductor).

A measuring instrument is an element which senses, detects or converts the above mentioned physical parameter or condition into a form or language that a person (an operator) or controller can understand.

a manometer converts the change of pressure into the movement of an arrow along a scale of an instrument;
a mercury-in-glass thermometer converts the change of temperature into the change of the length of a mercury column.

A transmitter is a device that converts a process variable into a form of a signal suitable for transmission to another location. Temperature is detected by a temperature transmitter, then it is converted to an analog electrical (4-20 mA, 0-5 mA, 0-20 mA, 0-10 V, 0-5 V, -10 to +10 V, -5 to +5 V dc), or pneumatic (20-100 kPa), or digital signal which is proportional to the temperature under measurement. This signal is sent to a controller. A measuring instrument or transducer must be capable faithfully and accurately detect any changes that occur with the measured process parameter.

Transmission lines are used for carrying a measurement signal from a measuring instrument or transmitter to a controller, and from a controller to a final control element (control valve, for example). Very often these lines are equipped with amplifiers to increase the measurement signal, since it is very weak (for example, a thermal electromotive force of a thermocouple has the magnitude of several dosens of millivolts).

A controller is an element that compares a current value of a controlled variable (the input variable for a controller) with a desired value (the set point) and takes appropriate control actions to adjust values of manipulated variables in the way to reach the desired value of process variable. In our example (see Fig.1.1) the controller changes flowrates of steam entering the heat exchanger and effluent liquid.

As the result of this action, the process variable (the temperature of the liquid in the outlet of the stirred tank in Fig. 1.1) changes in such a way, that it approaches closer to the desired value (set-point). This is a closed-loop or a feed-back control system. Feedback is an information about the status (or the magnitude) of the controlled variable which can be compared with its desired value. A control system without feed-back is called an open-loop control system. In other words, if we remove the temperature transducer and level transmitter, controllers and control valves, or simply brake links between thermocouple and level transmitter and controllers, or between controllers and control valves (see Fig.1.1) and leave the tank not under control we will get an open-loop or unregulated system. In this case information about controlled variables (the temperature and level of the liquid in the tank) is not used for control of these process variables.

It is interesting to note that an unregulated system can be selfregulated. Assume that the flowrate of the inlet liquid suddenly has increased to another value. Since Fin>Fout, the liquid will be accumulated in the tank (the level will increase). Thus increased liquid level will inevitably increase the head of the liquid, which in its turn will increase the value of Fout. This will continue until that moment when Fin=Fout. After that, the level will be maintained on another value.

A final control element is a device which receives the control signal from a controller and changes the amount of matter or energy entering the process in a way to bring the controlled variable (process variable) to its set point. As examples, we can mention control valves, relay switches providing on-off control, variable-speed pumps, etc.

Article Source: Dr. Alexander Badalyan, University of South Australia


Basics of Instrumentation & Control

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