Introduction to Process Instrumentation

The physical variables which is measured and sometimes controlled are called process parameters.

• Pressure, level, flow, and temperature are the main parameters measured in the industry. 
• pH, Conductivity, humidity, density, turbidity, vibration, CO, CO₂, 0₂, and H₂ are also measured

Necessity of Measurement: It is necessary to measure different variables for better control of the process and to get the best product/output

Instruments & Control

Instrument: A device used directly or indirectly to measure or control a variable or both.

Instrumentation: When a machine or a process is furnished with instruments for measurement and control is called instrumentation. In fact, instrumentation is the application of instruments.

Classification or Types of Industrial Instruments

Measuring Instruments. Example: Temperature indicator, pressure gauge, etc.

• Measuring instruments with an indicating system
• Measuring instruments with a recording system
• Measuring instruments with a counting system

Controlling instruments. Example: electronic controller, pneumatic controller, etc.

Safety & interlock instruments. Example: pressure switch

Process Control: Keeping the process parameters or physical variables, or process variables, constant as per the desired value is called process control.

This is generally done by measuring the instantaneous value of a physical process variable, comparing it with the desired value, and using the difference to make to correction, which reduces this difference.

Control Technology: The knowledge, way, and system of controlling the parameters of a process are the control technology, and by control technology, we mean that the process would be controlled automatically or semi-automatically.

Manual & Automatic Control

Manual Control: When the corrective action is carried out by the operator to operate the process to a desired condition, then it is manual control.

Automatic Control: When the corrective action is carried out by the instruments, then it is automatic control.

Types of Control on the Basis of the Loop

• Open-loop control
• Close loop control

Advantages of Automatic Control

• Increased production
• Improved quality
• Greater product uniformity
• Saving on raw material
• Saving energy
• Saving in manpower
• Increased safety

Control Loop: The Control loop is the arrangement of instruments connected to control a physical variable to the desired value.

Related: What is a Control Valve and its Classifications

Elements of a Control Loop: 

• Process: a variable that has to be controlled
• Sensing/Measuring system; measures the physical variable
• Transmitter: transmits the measured signal to the controller
• The controller generates a control command according to the desired value
• The correcting element carries out the control command
• Connecting cables & tubes allows the signals to pass

Different Methods of Control Technique

Conventional Control System:

In this system, the control components are permanently connected with each other according to the requirements of the process. For continuous control of the process parameters, different instruments such as measuring, indicating, transmitting, and controlling instruments are physically connected with each other to control the process. On the other hand, ON-0FF  control sequential logic operation, process operation, and safety is done by permanently connecting the electrical/ electronic/pneumatic/hydraulic relays, timers, counters, switches, and actuators.

Advantages:

• All components can be checked physically
• All interconnections can be checked physically

Disadvantages:

• Difficult to change the control program of the process, because it is done by a physical cable
• High cost, low speed, low reliability

Computer Control System:

Computers are often used in both continuous control and sequential logic operation of the process. Except for field connections, the instruments and logic connections can be done by a software program. For continuous control of the process, a distributed control system (DCS) is used. On the other hand, automation, sequential logic operation, and safety of the process can be done by a programmable logic controller (PLC).

Advantages:

• Not difficult to check/calibrate the control components while it is in use
• Not difficult to change the control program, because the program is done by software
• Low cost, high speed & high reliability

Disadvantages:

• Control components & their interconnections cannot be checked
• For maintenance & programming, it needs trained manpower

Distributed Control System (DCS): DCS is the computer control process operation system in which the control is distributed in computer modules containing one or more microprocessor-based controllers, and each controller is capable of controlling several instrument loops.

Every controller or computer modules are connected by a single high-speed data link, called the data highway, and connected to the central console. The central console consists of a CRT display, keyboards, printers, memory, and communication interfaces.

Programmable Logic Controller (PLC): The programmable logic controller is the replacement of conventional sequence systems such as electromechanical relays, timers, counters, etc.

A PLC is a digital electronic system designed for use in an automatic control system. The system comprises programmable memory for the internal storage of instructions for implementing specific functions. The PLC can execute functions like logic and sequential control of processes, timing, counting, and mathematical functions. 

Modern PLCs with analog control features can provide a complete solution for operating a plant.

Indicating Instrument

A measuring instrument in which only the present value of the measured variable is visually indicated is called an indicating instrument.

Types of Indicating Instruments

• Pointer type indicator
• Digital type indicator

Types of Pointers

• Lance-type pointer
• Knife-edge pointer
• Actual measuring system

Types of Scale Plates

• Square plate
• Circular plate
• Rectangular plate
• Are plate

Recording Instrument

A measuring instrument in which the values of the measured variable are recorded continuously is called a recording instrument. A recording instrument continuously registers the measured parameter with respect to time. Recording may be in the form of hard copy (on chart paper) or soft copy (data on disk). As a result, more information is obtained about the actual production process and the nature and extent of deviations. This makes it possible to take appropriate action and keep the process well under control.

Types of Chart Papers

• Strip chart
• Circular chart

Recording Pen and Link: Instrument ink contains glycerin and attracts water so that it does not dry up in the pen, and the recorded lines remain wet for slightly longer than ordinary ink. Ordinary ink must not be used even in cases of emergency. This would only lead to clogging.