Understanding P&IDs Drawings

In continuation of our lessons on how to read and interpret P&IDs, let us look at the piping and instrumentation diagram below:

At first this P&ID looks complicated, but on closer examination, it is actually a simple P&ID. For easier understanding the P&ID is broken into:

(a) Sensors/Measuring or Sensing elements:

TE 03 is a field mounted thermocouple that senses the change in the outlet temperature of the process liquid and converts the change in temperature to a milivolt signal. The milivolt signal(electric) then goes to TT 03, a temperature transmitter, which then converts the milivolt signal to a standard 4-20mA signal for transmission to TIC 03

LT 01 is a level transmitter which senses and measures changes in the level of the process liquid in the vessel(exchanger). The level measurement is converted into a standard 4-20mA signal for transmission to LIC 01. PT 02 is a pressure transmitter that measures the process pressure in the vessel. This measurement is then converted into a 4-20mA signal for transmission to PIC 02

(b) Controllers/Controlling Elements:

Also in the above P&ID are devices we call controllers. They receive the standard signals from the transmitters/sensing elements (TT 01, PT 02 and (LT 01)

TIC 03 is a control room mounted, Temperature Indicator and Controller. It receives the 4-20mA signal from TT 03 and compares it to a preset temperature set point and then initiates a control action by sending a corresponding electric signal to TCV 03 via TY 03

LIC 01 is a control room mounted Level Indicator and Controller. It receives a 4-20mA from LT 01 and compares it to a preset level set point within the controller. Based on this comparison, LIC 01 initiates a control action and sends a corresponding signal to the final control element LCV 01 via LY

Lastly, PIC 02 is a control room mounted, Pressure Indicator and Controller. It gets a 4-20mA signal from PT 02, compares it to a preset pressure set point and initiates a control action by sending a corresponding signal to final control element PCV 02 via PY

(c) Final Control Elements:

TCV 03, is a field mounted Temperature Control Valve which receives its control signal from TIC 03 (located in the control room) to either open or close to drain condensate in order to control the temperature of the process liquid

On TCV 03 is TY 03. TY 03 is an I/P converter which converts the electric signal it receives from TIC 03 to a pneumatic signal.

Similarly, LCV 01 (a level control valve) and PCV 02 (pressure control valve) get signals from LIC 01 and PIC 02 to either open or close, thereby controlling level and pressure respectively.

TY 03, LY 01 and PY 02 are called transducers. They convert electrical signals to pneumatic signals.

Having understood the P&ID above, it is important to note that:

There are three control loops in the P&ID namely temperature control (TE 03-TT 03- TIC 03 –TCV 03), level control (LT 01-LIC 01-LCV 01) and pressure control (PT 02-PIC 02-PCV 02)

The next time you see a P&ID, don’t get scared! The complexity of the P&ID is not important rather what is important is the individual control loops that make up the P&ID. Understand the P&ID, you understand the process!

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