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Process Control Basics

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What is a Process?
The word ‘Process’ used popularly in process control and the process industry refers to the ways and methods applied in changing or refining raw materials into end products suitable for mankind. The raw materials which can either be in a liquid, gaseous or a mixture of solid and liquid(slurry) are during processing transferred, measured, mixed, heated or cooled, filtered, stored, or handled in some other way to produce the end product.
Process industries include the chemical industry, the oil and gas industry, the food and beverage industry, the pharmaceutical industry, the water treatment industry, and the power industry.

What is Process Control?
Process Control refers to the methods used to control process variables during the manufacture
of a product. The manufacturing process for a product is controlled for the following reasons: 
(i) To reduce product variability – Reducing product variability leads to increase in product quality.
(ii) Increase efficiency – Increase in the efficiency of a manufacturing process will ensure that minimum resources are required to produce a product which consequently saves money for the manufacturers.
(iii) Ensure process safety – Every process must be controlled to operate safely. If this does not happen, the consequences of a run-away process can be catastrophic as witnessed in the 1986 nuclear plant disaster in Chernobyl near pripyat, about 130 km (about 80 mi) north of Kyiv (now in Ukraine) in the then USSR which exploded and burned.
Common Terms and Concepts in Process Control

Process Variable
In industrial process control, the PROCESS VARIABLE or PV is measured by a sensor or instrument in the field and acts as an input to a controller, which takes action based on the value of it. Common process variables include – level, flow, temperature, density, PH(acidity or alkalinity), mass, conductivity etc.

Set Point
The SETPOINT is the target value of the process variable that is desired to be maintained. For example, if a process temperature needs to be kept within 5 °C of 100 °C, then the SETPOINT is 100 °C.

It is an automatic device responding to changes in the physical quantity most representative of controlled process, and controlling a manipulated variable to maintain it at its setpoint value or to change it to a preset program.

Manipulated Variable
The variable to be manipulated, in order to have control over the process variable (PV), is called the MANIPULATED VARIABLE or MV. If we control a particular flow for instance, we manipulate a valve to control the flow. Here, the valve position is called the MANIPULATED VARIABLE and the measured flow becomes the PROCESS VARIABLE. The diagram below illustrates the relationship between setpoint, measured variable and a controller:

Error is the difference between the measured variable and the setpoint and can be either positive or negative. The objective of any control scheme is to minimize or eliminate error. Therefore, it is imperative that the concept of error be well understood. Any error can be seen as having three major components. These include – the magnitude, duration and rate of change of the error:
(i) Error magnitude - The magnitude of the error is simply the deviation between the values of the setpoint and the process variable. The magnitude of error at any
point in time compared to the previous error provides the basis for determining the change in error. The change in error is also an important value.
(ii) Duration of error - Duration refers to the length of time that an error condition has existed.
(iii) Rate of change of error - The rate of change of error is shown by the slope of the error plot shown below:

Offset is a sustained deviation of the process variable from the setpoint. For example, if in a pressure control loop, the control system held the process pressure at 10.5 bar consistently, even though the setpoint is 10 bar, then an offset of 0.5 bar exists.

Load Disturbance
A load disturbance is an undesired change in one of the factors that can affect the process variable. For example, in a temperature control loop adding cold process fluid to a vessel we are maintaining the temperature at a constant value would be a load disturbance because it would lower the temperature of the process fluid.