Process Control

Process Control

Process control term is one of the most frequent thing in our busy life. Before knowing  about Process control we will just see at basic thing what is process.

Process: Process is a sequence of actions to convert one form of material or energy in to the other form of material or energy. For example, Converting heat energy in to electrical energy in all power plants is a continuous process. In chemical industries, two are three products of different or same quantities will be added to produce an end product of some other material.

Process of Heat to Electrical Energy

Conversion Process of Heat energy to electrical Energy

Process Control: Each process should be controlled by some means to provide an accurate  end product. Process control is a method to control the process parameters when manufacturing a product or transforming one form of energy to the other.  Generally this is not a new topic, everything in this world is a controlled process, without this control it will leads to catastrophic thing.

         For example, in human body Mind is a controller which controls all the parts in human body at any point of time.  Let us see with an example of man driving a car/bike, the human eye and ear sends the signal to mind about the road and traffic then the mind process it and sends it to hand and leg to increase the acceleration or to put break.

Process control in the process industries refers to control the sequence of activities according to requirements of end product. For example of process control in manufacturing industries, the factors like how much proportion of ingredients should be added, at what temperature these should be added and the pressure at which the materials processed should be precisely controlled for better quality product. Same for the power plants the proper control of process variables increases the efficiency of conversion process from heat energy to electrical energy.

Advantages/Importance of process control in industries:

Precise control of process is needed in process industries for three main reasons.

  1. Reduce in the mismatch in the end product.
  2. Increase the efficiency and reduce the cost.
  3. Increase the safety of personal and machines.

Reduce the mismatch in the end product

Process Control can reduce the mismatches/variability of the end product with the required product which ensures the high quality product. For example consider manufacturing an iron rods which are used to insert in a vessel with exact size of holes. if the end product iron rods are similar as the design the it will be easy to insert in the holes. this can be done proper controlling of iron cutting machine with design sizes. This proper controlling reduces the padding of iron rods to fit in to the holes after manufacturing which in turn reduces the manufacturing cost.

Efficiency : Process efficiency can be maximized by controlling the process component near a specific set point. For example, consider that the process component is the material temperature which is used for chemical processing, if this temperature maintained at near the set point the efficiency of the process will increase.

In practical, if u consider a power plant designed efficiency is 50% for designed steam conditions like pressure and temperature. the controller designed to control the steam pressure and temperature should be designed such a way that the plant operates in the design limits for providing better efficiency. With this best control and operation we can minimize the maintenance cost also.

Personal and component safety 

In any process industry or plant, safety of the equipment and personal is primary goal rather than production. A run away process with out any control leads to catastrophic behavior in industries. 

           Consider a Steam Generator (SG) which converts the water in to high quality steam by heat energy received from the heat source like coal, nuclear, gas etc. The pressure of the SG should be controller precisely to design value which otherwise leads to a greater damage to the SG and personal working in that area. we can imagine  200 bars pressure cylinder is busting how much it can impact. so to reduce this type of accidents in process industries a proper control system is necessary for all the process. 

Parts/ Tasks of Process control:

process control tasks

process control tasks

The are four important parts of any control system

  1. Measurement
  2. Comparing
  3. Processing/Controlling
  4. Adjusting

 

Measurement of process parameter: This is the first in process controller which involves the measurement of process variable to be controlled. They are several controlling parameters in process industry like pressure, temperature, flow, level, speed, voltage,power etc.The measurement of each parameter will be different according to their nature and availability of the instruments in market. The various measurement techniques are explained here.

The measured signal should be converted in to proper form before giving it to controller for processing. the form can be Pneumatic signal (pressure of a fluid in kg/cm2) for mechanical/hydraulic controller, Analog signal (4-20 MA) for electronic controller and digital signal (0 or 5 Volts) for Digital electronic controller. The transducer will do this task of conversion of one form of signal in to another form.

Examples:

a) In hydraulic turbine speed controller the turbine speed signal is converted in to primary oil pressure signal, which is supplied to controlling part. This pressure signal will acts as a repulsive force for hydraulic speed set point spring tension. Further the axillary control oil pressure will according to this spring tension.

b) Normally all the  electronic measurement instruments converts the process parameter in to corresponding electrical signal of 4-20(MA). Thermocouple, LVDT etc are the examples.

c)   Now a days increase in demand digital systems and simplicity made the users to thing to implement the controlling part in computer, PLC’s with digital processing techniques. For this operation the analog signal should be converted to digital signal of 0-5 V. This will be done by I/O cards placed near the controller.

the converted signal should be amplified if necessary to transmit to the longer distances from the measuring unit normally field to control room. The entire task of signal amplifying/ shaping and cropping will be done by signal conditioning unit.

Comparing and processing: The feed back process parameter is compared with set point given by the operator and the resultant error signal is applied to the controlling algorithm to process it. The controlling algorithm process the error signal and generates the new output signal to adjust the final control element to minimize the error.

The various control algorithms are:

  • Proportional control algorithms
  • Integral control algorithms
  • Derivative control algorithms
  • Proportional-Integral (PI) control algorithms
  • Proportional- Derivative(PD) control algorithms
  • Proportional-Integral- Derivative (PID) control algorithms

Advanced controllers:

  • Genetic algorithm based controller
  • Fuzzy logic controller

Adjustment:

This is the final step in process controller as it is the final controlling element of process variable.Normally this will be the valve to control the fluid or to vary the resistance and reactance to control the voltage.

If it is a valve, the output signal will be given to actuator to position the valve according to the signal. The actuator can be pneumatic actuator which operates on compressed air pressure or electrical actuator which operates on electric motor.  

 

 

Schematic of electronic actuator with valve

Schematic of electronic actuator with valve

 

Schematic of pneumatic actuator with valve

Schematic of pneumatic actuator with valve

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