Basic Functions of Instruments in a P&ID ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Basic Functions of Instruments in a P&ID

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The primary functions of instruments and control components are monitoring, display, recording and control of process variables. Instrument and control symbols consist of an instrument bubble or circle with the instrument abbreviation lettered inside the bubble. The abbreviation completely describes the function of the instrument/control component.

Instruments/control elements can be grouped into different categories based on the process variable that the instrument or the control element is monitoring or controlling. The first letter in the instrument abbreviation indicates the process variable being monitored or controlled. The four common process variables are:
1)    Flow (F)
2)    Level (G)
3)    Pressure (P)
4)    Temperature (T)
Instruments can also be grouped according to the function they perform. The second letter in the instrument abbreviation commonly indicates the instrument function although sometimes it could be a readout or just a modifier of the first letter (usually the process variable). Again, a third letter could indicate either a device function or a modifier. You can get more information on P&ID symbols and lettering system at the ISA Web site  International Society for Automation

 Instrument Functions  in P&IDs
The common functions performed by instruments and control components are:
(a) Alarms (A): Alarms are devices responsible for alerting plant operators about an upset condition of the process variable. Alarms typically consist of sound and light outputs that attract the attention of the plant operators. On P&IDs, the alarm function is used to modify basic process variables such as pressure, temperature, level and flow

(b)  Controllers (C): A controller is a device that receives data from a measurement instrument, compares that data to a programmed set point, and, if necessary, signals a control element to take corrective action. Controllers are responsible for the control of the process variable. A typical controller receives input on the status of the process variable and compares the value with the “set point” and initiates the appropriate action. Actuators and control valves execute the control action. As explained in instrument abbreviations used in P&ID, the controller function is denoted by the letter (C).

(c) Indicators (I): An indicator is a human-readable device that displays information about the process. Indicators may be as simple as a pressure or temperature gauge or more complex, such as a digital read-out device. Some indicators simply display the measured variable, while others have control buttons that enable operators to change settings in the field. Indicators located at the process unit are also known as “Gauges”. A Level Gauge (LG) is an indicator used in the measurement of liquid level in process vessels. Again on P&IDs, the indicator function modifies basic instrumentation variables such as Level, flow, temperature and pressure

(d) Sensors: Sensors are the first element in a process control loop. They are often called the primary element. Sensors are devices that actually measure the value of the process variable. Examples of sensors are thermocouples and orifice meters used in temperature and flow measurements respectively. Transducers are used in converting the analog measurements into digital values. On P&IDs, sensors are represented by different letter combinations for example FE and TE represent a flow sensor(flow element) and a temperature sensor(temperature element) respectively

(e) Recorders (R): A recorder is a device that records the output of a measurement device. Different recorders display the data they collect differently. Some recorders list a set of readings and the times the readings were taken; others create a chart or graph of the readings. Recorders that create charts or graphs are called chart recorders. Their information is very useful in monitoring plant performance and in quality control of the products. On P&IDs, the recorder function is denoted by the letter (R). It signifies an instrument with a recording function

(f) Transmitters (T): A transmitter is a device that converts a reading from a sensor or transducer into a standard signal and transmits that signal to a monitor or controller. Transmitter types include:
1)    Pressure transmitters
2)    Flow transmitters
3)    Temperature transmitters
4)    Level transmitters
5)    Analytic (O2 [oxygen], CO [carbon monoxide], and pH) transmitters.

Transmitters are very common and popular in instrumentation system design. This is because, there is often the need to transmit data from sensors in the field to a central control room for monitoring or control purposes. On P&IDs, the transmitter function(T) modifies basic process variables such as pressure, temperature, level and flow. Typically, on a P&ID, you might have PT, TT, FT and LT. These are all transmitters. PT is a pressure transmitter; TT is a temperature transmitter; FT is a flow transmitter; and LT is a level transmitter.

I hope you have learnt something useful on P&IDs from this post. Please continue to:Instrument Abbreviations used in instrumentation diagrams for more information.