The important common to all processes is that a process is never in a state of static equilibrium except for a very short period of time and process is a dynamicentity subject to continual upsets or disturbances which tend to drive it away from the desired state of equilibrium. If a process is to operate efficiency, disturbances and the process must be controlled.
A process designed for a particular object and is then found. Some time by trial and error and some time by reffering from the previous experience that control of a particular variable associated with some stages of the process are necessary to achieve the desired efficiency. Process may be controlled more precisely to give more uniform and high quality products by the application of automatic control, often leading to high profits. Additionally, process which response too rapidly to be controlled by human operator can be controlled automatically. Automatic control is also beneficial in certain remote, hazards or routine operations. After a period experimentation, computer are being used to operate automatically control. Processing systems, which may too large and too complex for effective direct human control.
Feed back control
When a controller corrects a variable, it does so in response to a signal send from the measuring device. This signal device is called called feedback.
Nagative feed back control
Negative feed back ensures that the difference between Tr (set point) and Tm (measured value) is used to adjust the control element in order to reduce the error.
Positive feed back control
If the signal to the comparator will be obtained by adding Tr and Tm, we would have a positive feed back system, which is unstable. Error signal (E) is obtained as,
E = Tr + Tm
Components of the control system
Any operation or series of operation that produces desired final results is a process. In this discussion the process is the purification of natural gas.
As all he parts of the control system, measuring elements is perhaps the most important. If the measurements are not made properly the remainder of the system can not operate satisfactorily. The measured variable is chosen to represent the desired conditions in the process.
Analysis of the measurement variables to be measured
Variables to be measured
- Pressure measurement
- Temperature measurement
- Flow rate measurement
- Level measurement
Variables to be recorded
Indicated temperature, composition , pressure ,etc.
The controller is the mechanism that responds to any error indicated by the error deducting mechanism,.
Output of the controller is the some predetermined function of the error . There are three types of the controller. Proportional action , which moves the control valve indirect proportion to the magnitude of the error .
Integral action which moves the control valve based on the time integral of the error and the purpose of the integral action is to drive the process that to its set point when it has been disturbed.
Ideal derivative action (pre acts) and its purpose is to anticipate where the process is heading by cooking at the time a rate of change of error.
Final control element
The final control element receives the signal from the controller and by some pre determined relationship changes the energy input to the process.
Characteristics of the controller
In general the process controller can be classified as
- Pneumatic controllers
- Electronic controllers
- Hydraulic controllers
In the purifications of gas condensate the controller and final control element may be pneumatically operated due to the following reasons..
The pneumatic controller is very rugged and almost free of maintenance .
The maintenance men need no sufficient training.
Pneumatic control appears to be safer in a potentially explosives atmosphere which is often present in the petrochemical industry.
Modes of the control
The various types of controls are called ‘modes’ and then determine the type response obtained. In other words these describe the action if controller that is the relationship of output signal to the input or error signal.
It must be noted that it is error that activates the controller. The four basic mode of control are:
- Off-on control.
- Integral control.
- Proportional control.
- Rate of derivative control.
In industry purely integral, proportional or derivative modes seldom occur alone in the control system.
The off-on control, with the very high gain. In this case the error signal at once off the valve or any other parameter upon which it sits or completely sets the systems.
These are used to control the feed rate into a process unit. Orifice plates are the most used as the flow rate sensor. Normally orifice plates are designed to give a pressure drop in the range of 20 to 200 inch of water. Ventury tubes, and turbine are also used.
Thermocouples are the most used temperature sensing device. The two dissimilar wires provide a milli volt emf that varies with the ‘hot-function’ temperature. Iron constrict any thermocouple are commonly used over the 1300 F temperature range.
Borden tubes, bellows and diaphragms are used to sense pressure and differential pressure. e.g. in a mechanical system the process forse is balance by the movement of spring. The spring position can be related to process pressure.
Liquid levels are indicated in the variety of ways. The three most common are ;
- The following position of the float that is lighter than the fluid.
- Measuring are apparent weight of a heavy cylinder as it is buoyed up more or less by the liquid(they are called displacement meters).
- Measuring the difference in static pressure between two fixed elevations, one is the vapour above the liquid and the other under the liquid surface. The differential pressure between the two level taps is directly related to the liquid level in the vessel.
The transmitter is the interface between the process and its control system. The job of the transmitter is to convert the sensor signal (mill volts, mechanical movement, pressure difference, etc.) into a control signal 3 to 15 psig air pressure signal, 1 to 5 or 10 to 500 milli ampere electrical signal etc.
The interface in the process are the other end of the control loop is made by the final control element in an automatic control valves. The flow of steam that open or closes orifice opening as the steam is raised or lowered. The steam is attached to the diaphragm that is driven by changing air pressure above the diaphragms. The fore for the air pressure is opposed by a spring.
Instrumentation and control objectives
The main objectives are as follows:
- Safe plant operation.
- To keep the process variable within known safe operating limit.
- To detect dangerous situations as they develop and to provide an alarm and automatic shut down systems.
Production Rate: To achieve the design product output.
Product Quality: To maintained the product composition with in the specific quality standards.
Cost: To operate at the lowest production cost.
In a typical chemical processing plant these objectives are achieved by combination of automatic control, manual monitoring and laboratory analysis.
Programmable logic control (PLCS)
These were developed in the early 1970s and represent the earliest and most successful application of computers in process control.
Today, PLCS are one of the most highly developed and one of the most rapidly evolving examples of computers in the process control.
The term programmable means that equipments behaviour is defined by instructions (software), not by its manufacturing specification. PLC was the name given to the original equipment, which performed only Logic operations. Today’s units are called simply programmable controllers (PCs) and many be part of integrated control.
The first PLC was small computers designed primarily to serve as pilot relay replacements.
PLC’s are widely used today as input/output (I/O) counts of 3000 or more are common. The I/O modules are serviced over a network by a separate processor unit located for the convenience of the user. The large I/O counts also made the PLC a rich source of data.
Integrated control system
Many development are occurring simultaneously. PLC’s have acquired loop control capabilities in addition, the logic capabilities of control systen are being enhanced, and DCS is entering the traditionally discrete control field of the PLC’s.
Instrumentation & control process
- Instrumentation & process control
- Feed back control
- Negative feed back control
- Positive feed back control
- Components of the control system
- Measuring elements
- Analysis of the measurement variables to be measured
- Variable to be recorded
- Final control element
- Characteristics of the controller
- Modes of the controller
- Flow controller
- Temperature controller
- Pressure controller
- Level controller
- Control valves
- Instrumentation & control objectives
- Programmable logic control
Integral control system