Introduction to Process Control & Control Valve

on
Dears,
This article is still under development. I will complete it ASAP with various example of a process control loop. Hope you all have a good day :)

Beside of the proper design of the static equipment, rotating equipment, and piping, one of the keys to ensure that the facility could be operated smoothly is the process control system. As a process engineer, typically, we are not go to deep about the controller tuning parameter and controller type as this activity will be done with special service company like Honeywell (But it is of course great if you like to deep dive for this subject). Majority of controller system in an oil and gas facility is the Proportional-Integral-Derivative (PID) type controller. To be noted that each proportional, integral, and derivative will lead to specific action that will effect controller performance and contain distinctive tuning parameter. In this article, we are not going to discuss in detail about the mathematical function for this PID controller as we will focus to the control philosophy and the general aspect of a process control. 

Control Loop

In order to control a process variable in the processing plant, control loops are installed at the chemical processing plant. With this loop, we will able to control required parameter such as pressure, temperature, liquid level, flowrate, and heating value.
Typical control loop of the controller will include following components:
  1. Process System / Plan : Interconnected system of unit operations (vessel, pump, compressor, column, reactor, etc.). The main aim for the process control is to keep to operating condition of this process system at the required/stable value.
  2. Measurement device / sensor / transmitter : device to measure the operating condition of the process system. Some typical measurement devices are thermowell for temperature measurement, pressure transmitter, D/P level transmitter, radar level transmitter, orifice flowmeter, and venturi flowmeter.
  3. Controller : Main aim of this controller is to reduce or minimize the error value (Difference between set point and measured process value)
  4. Final Control Element : Convert the controller output into an action to manipulate a variable to change the operating condition. Typically a control valve will act as the final control element and the manipulated variable is the control valve percentage opening.

Typical Components in a Process Control Loop
Source : https://controlguru.com/the-components-of-a-control-loop/

Meanwhile, some variable and constant involved in this control loop are:
  1. Actual Process Variable : actual process value that is tried to be measured by the measurement sensor/device with some acceptable value of measurement error tolerance.
  2. Measured Process Variable (PV) : actual process value plus minus measurement error
  3. Controller Set Point (SP) : desired process value to keep the process system
  4. Controller error (e) : the difference between set point value and measured process variable value. A controller will try to reduce this value into 0.
  5. Controller Output Signal : Output of the controller that will give an instruction to final control element to manipulate a process variable in order to reduce the controller error.
  6. Manipulated Control Variable : process variable that has to be changed in order to reduce controller error e.g. fan speed, control valve opening

Comments

Post a Comment