Process Control – Essentials

Many of the processes carried out in industry require control. You can’t have a quality product without control. Manufacturing, oil and gas, agriculture, and other industries require chemical or mechanical operations that need to be controlled to reach the finished product or desired results. Even in our homes, process control takes place in many of the devices or machines that we use. Here are some examples of home appliances that have process control integrated:

• Refrigerator: It includes a process control system that makes it cycle on and off to keep your food and drinks cold at a certain temperature.
• Air conditioner: This is another device that includes an integrated process control. You set a certain temperature using your remote and it ensures that the temperature of the room is maintained at that setpoint.
• Pressing iron: It includes a thermostat or temperature switch that automatically turns off when the temperature has...

Introducing process control

In a very simple way, we can define process control as the technique of monitoring and adjusting a process to yield the desired result.

In industry today, raw materials undergo a series of procedures (processes) before they become a finished product. These processes need to be carefully monitored and adjusted to ensure quality products are produced efficiently, economically, and safely. The monitoring of processes and the adjustments necessary are usually automated using control systems. If the monitoring and adjustments were to be done manually, there would be a need for more workers and we cannot achieve the best possible results this way due to human errors.

Types of process control systems

Automatic process control can be categorized as one of the following:

• Open-loop
• Closed-loop (feedback)
• Feedforward
• Feedforward-feedback

Let’s have a look at each of the types.

Open-loop control

In open-loop process control...

Exploring process control terms

Here, we are going to look into some of the common terminology related to process control:

• Process: A process is any operation or event (or sequence of operations or events) that causes a physical or chemical change to an input. Raw materials in industry undergo some kind of series of operations, whether being heated, ground, or mixed, for example, before they resemble a finished product.
• Sensors: A sensor is a device that senses or detects something – just as we have ears that hear, eyes that see, a nose that smells, and a tongue that tastes. We have sensors that sense or detect various physical properties such as temperature, pressure, level, or flow. Sensors basically convert a physical property into an electrical quantity.

Hence, the four common sensors used in process control are a temperature sensor, a pressure sensor, a level sensor, and a flow sensor – some others include a pH sensor, speed sensor, or a position...

ISA symbology

The International Society of Automation (ISA) is a non-profit professional association of engineers, technicians, and managers engaged in industrial automation. ISA is one of the leading standard organizations in the process control trade. They developed a set of symbols for use in the design of process control loops and other engineering diagrams. This set of symbols is referred to as ISA symbology and is detailed in one of their publications (ISA5.1, Instrumentation Symbols and Identification), which you can purchase at their website (https://www.isa.org/).

ISA symbology is used when creating Process (or Piping) and Instrumentation Diagrams (P&IDs).

A P&ID is a diagram that depicts the details of the piping and instruments used in a processing plant. Figure 12.14 shows a simple P&ID of a process control loop for a basic level of control. It is usually developed from a Process Flow Diagram (PFD) at the design stage of an industrial process plant. The...

Temperature measurement and transmitters

Temperature is the measure of the degree of hotness or coldness of something. Temperature measurement is an important task in the process industry and can be achieved with the aid of a sensor and transmitter. Common sensors are resistant temperature detectors (RTDs) and thermocouples.

An RTD is a device whose resistance changes as the temperature changes. It has a temperature range of -260 to 850 degrees Celsius. It has better sensitivity than a thermocouple but it is more expensive. The types of RTD sensors, when categorized by the resistant element material, are Pt100, Pt1000, Ni120, and Cu100. The letters represent the material of the element while the number represents the resistance at 0 degrees Celsius. Pt100 is the most common RTD. The resistance element is Platinum and it has a resistance of 100 Ohms at 0 degrees Celsius.

An RTD temperature sensor requires an RTD transmitter to produce a standard signal output (for example, 4 to...

Pressure measurement and transmitters

Pressure can be defined as the amount of force applied per unit area (F/A). The processes in industry require accurate and reliable pressure measurements for safe operations and the production of quality products. The effects of pressure include position movements, changes in resistance, or other physical changes that can be measured. Common pressure sensors employ a Bourdon tube, diaphragm, bellows, force balance or variable, and a capacitance arrangement.

A pressure transmitter is required to produce a standard signal (4 to 20 mA or 0 to 10 V) from whatever pressure effect the sensor is based on. Mostly, a pressure transmitter comes with an inbuilt sensor and is sold as a single package, as shown in the following figure:

Figure 12.8 – A front view of a pressure transmitter (Credit: Showlight Technologies Ltd.: www.showlight.com.ng)

The following figure shows a simple connection to a pressure transmitter...

Level measurement and transmitters

The accurate measurement of levels is required in industry to maintain the proper fluid levels for safe operation, quality production, or the smooth running of equipment. Various sensor technologies are available for measuring levels in liquid and also in solids. Capacitance level sensors measure the change in capacitance to determine liquid levels. An ultrasonic level sensor emits a sound wave. The liquid level is directly proportional to the time delay between the wave emission and its reflection. Differential pressure sensor uses the difference in pressure between two points in a tank to determine the level. Level sensors mostly have a transmitter integrated that enables them to produce a standard signal of 4 to 20 mA or 0 to 10 V as a level varies.

There are various level transmitters. The following figure shows a hydrostatic level transmitter that is usually immersed in water or some liquid whose level is to be measured:

...

Flow measurement and transmitters

Flow measurement is an operation carried out to measure the flow of fluids in a pipe using an instrument. Several flow-measuring instruments exist, which include the following:

• A differential pressure flow meter: This uses a pressure difference created by a sensing element (for example, an orifice plate or a Venturi tube) to determine the rate of a flow. A differential pressure transmitter has a high- and low-pressure port to which the high- and low-pressure side of the sensing element will be connected and it produces a standard signal of 4 to 20 mA that corresponds to the flow rate.

The following figure shows a Rosemount differential pressure transmitter:

Figure 12.12 – A Rosemount differential pressure transmitter (this file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license)

• A velocity flow meter: This determines a flow rate by measuring the flow velocity...

Understanding the process control loop

The process control loop is a set of devices and tools designed to maintain the desired output from a process variable. Control loop components and instruments measure the variable, respond to it, and control it to maintain a setpoint.

A control loop system can be either an open-loop or closed-loop system, which we explained in an earlier section of this chapter. It usually consists of a sensor or transmitter, a controller, and an actuator. The following figure shows a process control loop for level control:

Figure 12.16 – The process control loop for level control

The process control loop in the preceding figure is explained briefly as follows: the pump supplies water to the tank and a control valve (the final control element) controls the flow rate depending on what it learns about the tank level through the signal it receives from the level controller, which has a setpoint. The level transmitter provides...

A practical example of single-loop process control

Here, we will use a temperature sensor, a temperature controller, and a contactor (actuator) to keep the temperature of a heater at the desired level (or setpoint).

Materials/devices

The required materials include the following:

• A thermocouple temperature sensor (Type K)
• A temperature controller (JTC903)
• A contactor
• A heating element

Wiring

Make the connection as shown in the following diagram (Figure 12.17):

Figure 12.17 – Pictorial diagram of a temperature control system (Credit: Showlight Technologies Ltd.: www.showlight.com.ng)

Operation:

1. Set the dial on the front of the temperature controller to the setpoint of your choice, that is, the temperature you want to maintain – for example, 100 degrees Celsius.
2. Supply power to the circuit by switching on the breaker.
3. When the temperature of the heater is below the setpoint, the common terminal...

Wiring and programming the analog input of a PLC (Siemens S7 1200) for process control (hands-on)

In this section, we are going to dig deeper into our PLC wiring and programming by learning how to wire and program the analog input of PLC to control a process. Analog programming is often seen as the complicated aspect of using a PLC. I have made it simple and easy to understand here. The practical part (hands-on) will give you a better understanding. While every part of this book is important, in particular, Chapter 7, Understanding PLC Hardware and Wiring, Chapter 8, Understanding PLC Software and Programming with TIA Portal, and Chapter 9, Deep Dive into PLC Programming with TIA Portal, are all required to easily understand this section.

There are specific controllers designed for temperature control – for example, the JTC903 controller used in the previous section for temperature control. We also have controllers designed specifically to measure pressure or level. However...

Summary

Well done! You have successfully completed this chapter, which is an important stage of industrial automation. You should now understand process control and be able to explain various process control terms and understand ISA symbols according to ISA5.1. You have also learned about the sensors and transmitters required for temperature measurement, level measurement, pressure measurement, and flow measurement, which are common in industry. You should now be able to wire a temperature sensor and transmitter, level transmitter, and pressure transmitter to generate a 4- to 20-mA analog output, which is very important for process control. We have also learned how to keep the temperature of a heater at the desired level by using a temperature controller.

The last section covered more advanced topics within the field of industrial automation, where we discussed the basic theory you need to know about analog input signal processing in a PLC (S7-1200), which includes reading analog...

Questions

The following are some questions to test your understanding of this chapter. Ensure you have read and understood the topics in this chapter before attempting the questions:

1. In a/an ____________ process control, the output does not change the control action.
2. In a/an ___________ process control, the output has effect on the control action.
3. In a/an_____________ control, disruption is measured and corrective action is taken to correct any errors before they reach the system.
4. A _______________ is a device that converts a signal produced by a sensor into a standard signal (4 to 20 mA, 0 to 10 V, or 3 to 15 psi) applicable in process control.
5. ________________ can be referred to as the target value of the process variable.
6. ________________ is the difference between the process variable or measured value and the set point.
7. The devices that the controller operates to keep the process variable at a setpoint are called _________________.
8. ISA is an...