The term P&ID is elongated as Piping and Instrumentation Diagram or Drawing.
In other words, it is known as Process and Instrumentation Diagram or simply a P&I diagram or drawing.
Piping and Instrumentation Diagrams are also known as Engineering Flow Diagrams or Mechanical Flow Diagrams.
P&IDs are mostly used in process industries to represent the process flow and other installed equipment and instruments.
These P&IDs show an interconnection between process equipment and the instrumentation used to control the process.
The Piping and Instrumentation diagram is explained by considering Industrial Heat Exchanger as an example.
The industrial heat exchanger is equipment or a process unit in which the process fluid or feedstock is heated by using steam as a heating source.
The process fluid may be water or any other industrial fluid such as cane juice or massecuite.
This fluid is lifted or transferred to the heat exchanger chamber called tube at a required flow rate using a pump and piping arrangement.
The process fluid is made to flow in the tube or climber and the steam has flowed in the calendria of the exchanger device.
In the heat exchanger, the heat from the steam is transferred to the process fluid inside the pipe.
This heat exchanger is designed to regulate the required temperature of the process fluid at its outlet irrespective of changes in the flow rate or demand of the feedstock.
This temperature control is done by automatic control of steam flow rate to the heat exchanger.
The below P&ID diagram with a certain set of standard symbols represents process equipment and required instrumentation with control of process flow.
The standard set of instrument symbols in the P&ID is defined by ISA standard 5.1
In this tutorial, we are using only two sets of symbols
- The circle without any lines indicated the instrument is field mounted.
- The circle with a horizontal solid line represents the given instrument is control room mounted.
The P&ID diagram and description of the Industrial Heat Exchangers are illustrated below.
Flow Transmitter FT 101
This instrument is installed in confluence or attached to the orifice plate that measures the actual flow rate of the process fluid.
The Flow Transmitter sends the corresponding electrical signal to the electronic controller.
Flow Indicator and Controller FIC 101
This instrument is control room mounted.
This instrument indicates and controls the flow rate of the process fluid (cane juice) entering to heat exchanger vessel with the accurate position of the flow control valve.
The controller will compare the signal received from FT101 with the set point and sends an electrical signal to the current to pneumatic [I-P] converter FY 101.
Current To Pneumatic [I-P] Converter FY 101
This unit converts the received electrical signal from Flow Transmitter FT 101 to a proportional pneumatic signal for valve opening with reference to the table shown below.
| Electrical Signal | Pneumatic Signal | Valve Position Opened % | Valve Position Closed |
|---|---|---|---|
| 4 mA | 3 PSI | 0 % | 100 % |
| 8 mA | 6 PSI | 25 % | 75 % |
| 12 mA | 9 PSI | 50 % | 50 % |
| 16 mA | 12 PSI | 75 % | 25 % |
| 20 mA | 15 PSI | 100 % | 0 % |
Flow Transmitter FT 103
The amount of steam flowing into the calendria of the heat exchanger is measured by this Flow Transmitter FT 103 mounted with a flow sensor called an orifice plate.
This transmitter sends the measured value to flow recorder FR 103.
Flow Recorder FR 103
- The flow recorder is a control room-mounted instrument.
- This is a recording device to store or save the exact value of the steam flow rate.
Hand Switch HS 101
This is an ON/OFF switch it is a control room-mounted instrument.
This will turn on or turn off the cold feedstock pump as required.
The ON state indicates that the pump is running, and the OFF state indicates the pump is turned off.
Hand Valve HV 102
This is hand operated valve called a manual valve this valve will open or close the steam stop valve at the steam header side to allow the steam to flow to the calendria or shell of the heat exchanger.
Temperature Alarm High TAH 100 & Temperature Alarm Low TAL 100
This is a control room-mounted annunciator device to alert the operator when the temperature of the steam header or temperature at the exchanger outlet (coming out of the exchanger) rises above or falls below the set point or working temperature value for the heat exchanger.
Pressure Alarm High PAH 103 & Pressure Alarm Low PAL 103
This is a control room-mounted annunciator device to alert the operator when the pressure at the steam header or exchanger outlet (coming out of the exchanger) rises above or falls below the set point working pressure value for the heat exchanger.
This alarm device has an audible siren with indicating lamp with toggling indication.
Pressure Indicator P I 100
- This is a control room-mounted instrument to display the existing pressure of steam at the calendria side of the heat exchanger.
- This indicator receives the measured value from the pressure transmitter PT100
Pressure Indicator PI 103
This instrument PI 103 displays the pressure inside the steam header.
Pressure Gauge PG101
- It is a field-mounted device to indicate the actual steam pressure of the system (Heat Exchanger).
- This is an analog type indicating device consisting of a pointer and scale arrangement.
Pressure Transmitter PT 100
This is a pressure measuring and transmitting instrument.
This pressure transmitter consists of impulse tubing connected to a pressurized vessel or equipment.
This will directly sense the existing pressure valve and sends the measured value in terms of electrical signal (4 to 20 mA) to the control room.
Note that while calibrating the pressure transmitter an appropriate scaling must be done with zero and range pressure of the vessel in accordance with 4 to 20 mA electrical signal.
Temperature Transmitter TT 102
- This is a field-mounted instrument to measure the temperature of process fluid at the outlet side of the heat exchanger.
- The sensor may be RTD or TC measures the temperature valve of the process fluid and this electrical signal is sent to TAH/L 102 for alarming purposes
Temperature Indicator TI 103
This is a control room-mounted instrument for displaying the temperature of the steam entering the calendria of the heat exchanger.
Temperature Indicator, Recorder, and Controller TIRC 102
- These are control room-mounted instruments provided for indicating, controlling, and recording the temperature of the process fluid at the outlet section of the heat exchanger by accurately positioning the temperature control valve TCV 102.
- This controller sends a corresponding electrical signal to the current to pneumatic [I-P] converter TY 102.
Temperature Transmitter TT 102
This is a field-mounted device that receives the measured value from the sensor (RTD & T/C) and transmits the temperature value of the process fluid at the outlet section of the heat exchanger in the form of electrical signals to TIRC 102.
The electrical signal transmitted by TT 102 is also used for alarming purposes (TAH 102 /TAL 102)
Current to Pneumatic [I-P] converter TY 102
This converts the electrical signal received from TT 102 to the corresponding pneumatic signal for positioning the temperature control valve TCV 102.
Temperature Indicator, Recorder, and Controller TR 101
These are control room-mounted instruments provided for indicating, controlling, and recording the temperature of the process fluid at the inlet side of the heat exchanger.
|Pneumatic signal||
|Electrical signal|| 
|Process Pipeline|| 
|Process Safety valve|| 
|Spectacle blind|| 
|Control room mounted instrument or in panel|| 
|Instrument mounted on the front side of the panel in control room|| 
|Field mounted instrument|| 
|Field mounted control valve|| 
|Control valve with actuator mounted in field|| 
|Field mounted manual valve|| 
| Instrument | Meaning | Function |
|---|---|---|
| FT | Flow Transmitter | To measure and transmit the flow |
| TI | Temperature Indicator | To indicate the temperature of process fluid |
| FIC | Flow Indicator and Controller | To Indicate (Display) and control the flow rate of process fluid |
| PI | Pressure Indicator | To indicate the pressure of the process vessel |
| LT | Level | Transmitter |
| LI | Level Indicator | To indicate the level of process fluid in the vessel |
| LIC | Level Indicator and Controller | To Indicate (Display) and control the level of process liquid in the vessel |
| LAL | Level Alarm Low | Level measured by LT is configured for Low alarm such that when the liquid level in the container reaches to desired set value. Low signal LAL is indicated in the control room. |
| LAH | Level Alarm High | Level measured by LT is configured for High alarm such that when the liquid level in the container reaches to desired set value. High signal LAH is indicated in the control room. |
| SL | Solenoid | This receives an electrical signal and allows the control of the manual valve with actuator through pneumatic signals |
| TT | Temperature Transmitter | To measure and transmit the temperature value in the form of electrical quantity to the Control room |
| TIC | Temperature Indicator and Controller | To indicate and to control the temperature of process fluid |