Piping and Instrumentation Diagrams Tutorials II: Pressure Control

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In continuation of my series on piping and instrumentation diagrams tutorials, we shall look at how to develop and construct a simple piping and instrumentation diagrams (P&ID).

Before we start, I will advise you to go through Tutorials I . If you are completely new to P&ID, I will advise that you go through my various posts on piping and instrumentation diagrams to ensure that we are on the same page when we use the information provided to develop our P&ID.

Tutorial Question

Develop the piping and instrumentation diagram (P&ID) of a gas vessel which has natural gas entering it through an inlet header and leaving it through an outlet gas header. The gas vessel will show the following instrumentation:

(a) An inlet valve that is manually controlled on the gas inlet header

(b) A pressure transmitter and pressure indicator to indicate inlet gas pressure

(c) A flow transmitter and flow indicator to measure and indicate inlet flow. Also show instrumentation for high and low flow rates alarms

(d) A pressure safety valve to relieve excess pressure to a flare system

(e) A hand valve on the gas outlet header

(f) A pressure transmitter, a pressure indicator and controller and control valve for monitoring and controlling gas vessel pressure

(g) An alarm to indicate high vessel pressure

(h) A check valve on the outlet header

(i) A pressure transmitter and a pressure indicator on the outlet header to indicate outlet pressure

Developing the Piping and instrumentation diagram (P&ID)

To develop this P&ID, the following tips were relied on: 

• Know the symbols for all kinds of valves 
• Know how to represent various instruments on P&IDs using ISA 5.1 standard 
• Know how to construct the various abbreviations for instruments on P&IDs 
• Know the common signals and their representation on P&IDs 

Based on the above tips I have developed the piping and instrumentation diagram (P&ID) below for the gas vessel instrumentation:

The P&ID of the Gas Vessel

Let us now take a detailed look at the piping and instrumentation diagram developed from the information given above. Please note that the tag numbers on the P&ID have been arbitrarily added to give the P&ID an orderly look.

On the developed P&ID, please note that:

• The plain instrument bubble without a solid line across represents a field mounted instrument.
• The instrument bubble with a solid line across represents an instrument that is mounted in a central control room.

Developing the P&ID

(a) we were told the gas vessel instrumentation should have an inlet valve. On the P&ID, inlet valve HV 107 is provided to manually regulate gas flow into the vessel.

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(b) A pressure transmitter and a pressure indicator is to be provided to indicate inlet pressure. On the developed piping and instrumentation diagram, PT 101 and PI 101 are provided for this purpose. PT 101 measures the inlet pressure and sends the measured value via electrical signals to the pressure indicator, PI 101

(c) A flow transmitter and a flow indicator are to be provided to measure and indicate flow. Also, instrument for alarming high and low flow rates are to be included. On the developed P&ID, FT 102 measures the flow and FI 102 indicates the flow. Also FAH 102 is a flow alarm switch for indicating high flow rates while FAL 102 is for indicating low flow rates

(d) A pressure safety valve is to be provided in the instrumentation to relieve and vent excess pressure to a flare system. On the developed P&ID, PSV 104 is provided to relieve excess pressure and protect the gas vessel.

(e) A manually controlled hand valve is to be provided on the gas vessel outlet header. On the developed P&ID, hand valve, HV 105 is provided.

(f) A pressure transmitter, a pressure indicator and controller and a control valve are to be provided. On the developed P&ID, PT 103 is the pressure transmitter that measures the gas vessel pressure and sends the signal (electrical) to PIC 103, a pressure indicator and controller to indicate the measured pressure and at the same time send a command pneumatic signal depending on its set point to actuate the pressure control valve, PCV 103

(g) Instrumentation for alarm to indicate high vessel pressure is to be provided. On the developed P&ID, pressure alarm switch PAH 103 is used to signal an alarm for high vessel pressure

(h) A check valve on the outlet header is to be provided in the vessel instrumentation. On the developed P&ID, CV 106 is the check valve provided to prevent back flow into the gas vessel

(i) Lastly, a pressure transmitter and pressure indicator is to be provided to measure and indicate gas outlet pressure. On the developed P&ID, PT 108 and PI 108 are performing this function.

If after going through this piece you still have some difficulty, I will advise you go through all the introductory posts on piping and instrumentation diagrams before coming back to study Tutorials II