Thursday 3 January 2019

Process Control

A process is broadly defined as an operation that uses resources to transform inputs into outputs. It is the resource that provides the energy into the process for the transformation to occur. Below figure shows a hot water generation process commonly found in plants. The input to this process is cold water and the output of the process is hot water. Steam is the resource that provides energy for the transformation to occur within the heat exchanger plates.

Process control is the act of controlling a final control element to change the manipulated variable to maintain the process variable at a desired Set Point.

Below figure shows a block diagram of a process with a final control element and sensors to measure the manipulated variable and process variable. In single loop control systems, the actual value of the manipulated variable is often not measured, the value of the process variable is the only concern.

Process control example

We see that the manipulated variable (MV) is steam pressure. The final control element is the valve, by changing the valve opening we are changing the flow of steam which we can measure by its pressure. The process variable (PV) is the temperature of the water exiting the heat exchanger; this is the measure of the process output that responds to changes in the flow of steam.

This is a controllable process because opening the valve will always lead to an increase in temperature, conversely closing the valve will always lead to a decrease in temperature. If this were not true, if sometimes on closing the valve we had an increase in temperature, the process would not be controllable.

Following are definitions of some terms used above:

  • The manipulated variable (MV) is a measure of resource being fed into the process, for example, how much thermal energy.

  • A final control element (FCE) is the device that changes the value of the manipulated variable.

  • The controller output (CO) is the signal from the controller to the final control element.

  • The process variable (PV) is a measure of the process output that changes in response to changes in the manipulated variable.

  • The Set Point (SP) is the value at which we wish to maintain the process variable at.

In order to process always required stable installation tools control. Control tools are install with the purpose of:

Maintaining the safety and security of employment

Security in the operation of a chemical plant is the primary requirement for people. Who work in factories and for the continuity of the company. To maintain security, provided the various operating conditions operating pressure is like a plant, temperature, concentration of chemicals, and so on should be maintained at a certain boundary is allowed.

Specification of the desired product

Factory should produce a product with a certain amount (according to capacity design) and with certain qualities according to specifications. For that it needs a control system to maintain production levels and product quality that you want.

Process Equipment & Function

Keep process equipment can function as desire in the design of equipment use in the operation of the production process have certain operational constraints that must be met. The pump must be maintain at the column distillation, NPSH must be guard. To prevent DOS flooding, temperature and pressure in the reactor should be maintain in order. To secure and operate still conversion into optimum product, fill the tank should not be dry or overflowing, and still many other obstacle constraints to be aware of.

Keep the operation of the factory remain economical.

Factory operation aim at producing products from raw materials that give the maximum profits. So that the factory should be run on the conditions. Which cause the operating costs be the minimum and maximum profit being earn.

Meet the environmental requirements

Of operating the factory must meet various environmental regulations. That provide specific terms for various chemical plant effluents.

To meet the above requirements needed oversight (monitoring). That continuously against the chemical plant operating and intervention from the outside (external intervention). To achieve the purpose of the operation. This can be complete through a series of equipment. (measuring instrument, controller, shellfish, and computers). Even and human intervention (plant managers, plants operators) that together form a control system.

In operation the factory needed various prerequisites and specific operating conditions. So that the necessary efforts towards monitoring the operating condition of the plant and process control. So that the conditions of stable operations.

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