This post will begin a series of tutorials on P&ID to help many people seeking information on the subject to understand more about piping and instrumentation diagrams. Please read on and endeavour to go through all the posts on piping and instrumentation diagrams if you have the time. You will find the links to all my posts on P&IDs at the end of this post. Happy reading.
The P&ID above is that of a typical industrial heat exchanger. You look at the P&ID and you wonder: what is going on? Well the P&ID looks a little complicated if you are new to Piping and instrumentation diagrams. To understand what is actually going on, let us first get to understand what the process whose piping and instrumentation diagrams is depicted above is all about.
The heat exchanger is a process unit in which steam is used to heat up a liquid material. The material, called feedstock, is pumped at a specific flow rate with pump P-101 into the pipes passing through the heat exchanger chamber (called the tube) where heat is transferred from steam to the material in the pipe. It is usually desired to regulate the temperature of the outlet flow irrespective of the change in the demand (flow rate) of the feedstock or change in the inlet temperature of the feedstock. The regulation of the outlet temperature is achieved by automatic control of the steam flow rate to the heat exchanger (E-101). The P&ID diagram utilizes certain standard symbols to represent the process units, the instrumentation, and the process flow.
The Piping and Instrumentation Diagram:
Instruments on the P&ID
Recall that instruments are represented in P&IDs by bubbles defined by ISA standard 5.1. In this P&ID, there are two sets of instrument bubbles used: plain circle bubble and a circle bubble with a solid line across it. As indicated on the P&ID, the plain circle bubbles represent field mounted instruments while circle bubbles with a solid line across represent control room mounted instruments.
Signals on the P&ID
Two kinds of signals are represented on the P&ID. They are:
- Electrical signals
- Pneumatic signals
Electrical signals are represented by the dashed lines with red colour on the P&ID. The pneumatic signals are represented by solid lines with double strip across. They are colored blue on this P&ID
Detailed description of P&ID
Flow Indicator and Controller.This control room mounted instrument controls the flow of cold feedstock entering the tube side of the heat exchanger by accurately positioning a control valve (FCV 101) on the cold feedstock flow path. A Flow transmitter, FT 101, in conjunction with a flow sensor (orifice plate) measures the flow of cold feedstock and sends a corresponding electrical signal to controller, FIC 101, in the control room. The controller then compares the measured flow with its set point and sends an electrical signal to a I/P(current to pneumatic) converter, FY 101, which converts the electrical signal to a corresponding pneumatic signal used to accurately position the control valve FCV 101. Similarly, FT 103 measures the flow of steam into the exchanger using a flow sensor (orifice plate) and sends a corresponding electrical signal to Flow Recorder, FR 103 to indicate the measured flow.
Flow Recorder. This control room mounted instrument records the steam flow rate. It measures the steam flow rate in conjunction with a flow transmitter, FT 103 and a flow sensor (orifice plate).
Hand Switch, ON/OFF. This hand switch is mounted in the control room .This switch turns on/off cold feedstock pump P-101. When the switch is in the ON condition, the pump is running. When the switch is in the OFF condition, the pump is not running.
Hand Valve, OPEN/CLOSED.This valve opens/closes the steam block valve through which steam is routed from the header to the shell side of the heat exchanger.
Pressure Alarm Low,
This alarm fires should the steam header pressure be less than the pressure required for the heat exchanger to work accurately. Note that the alarm module is mounted in the control room.
Pressure Indicator, This control room mounted instrument displays the steam pressure at the shell side of the heat exchanger. This pressure measurement is done using pressure transmitter, PT 100.
Pressure Indicator, This instrument displays the steam header pressure. Pressure measurement is also done using pressure transmitter, PT 103
Temperature Alarm High/Low,
This alarm fires should the temperature of the feedstock at the exchanger outlet goes beyond or falls below stipulated temperatures for high or low temperature of the feedstock coming out of the exchanger.
This control room mounted instrument displays the temperature of the steam entering the shell side of the heat exchanger.
This is a field mounted instrument that measures the temperature of the outlet feedstock from the heat exchanger. This measured temperature is converted to electrical signal that is sent to TAH/L 102 for alarming purposes and TIRC 102 for indication, recording and controlling purposes.
Temperature Indicator, Recorder, and Controller,
This control room mounted instrument controls the temperature of the feedstock at the exchanger outlet by accurately positioning the valve TCV 102 that regulates the steam flow to the exchanger. TT 102 measures the temperature of the feedstock at the exchanger outlet. This measured temperature is sent in the form of electrical signals to TIRC 102. This controller then sends a corresponding electrical signal to an I/P (current to pneumatic) converter, TY 102 which converts the electrical signal to pneumatic signal that is then used to accurately position the temperature control valve, TCV 102. Note also, the electrical signal from TT 102 is also used for alarming purposes (TAH/L 102)
This control room mounted instrument displays the temperature of the feedstock entering the exchanger. This is done by using temperature transmitter TT 101, which measures the temperature of the cold feedstock entering the exchanger in the form of electrical signals and sends it to TR 101.