Piping & Instrumentation Diagrams Tutorials on Pressure Control

In this article, we will study pressure control through P&ID Diagrams

Generally, we all know about the terms P&ID and pressure.

Pressure Monitoring and Controlling in P&ID

The monitoring and controlling of pressure in every process industry is the most crucial and important task for stable and efficient plant operation. The project engineers should be aware of the (P&ID) piping and instrumentation diagram during the design and development of the system. Generally, the operation or working pressure will be defined according to customer requirements. And process efficiency is defined by the supplier´s technology.

Working Pressure at battery limit

An Air separation unit produces nitrogen gas at high purity and very low pressure below 1 Bar. If the requirement or demand raises to 15 Bar then a compressor unit must be included downstream to build the pressure to the required demand valve of 15 Bar. The discharge setting of the compressor depends on the pressure drop between the battery limits and equipment.

Process Efficiency

The efficiency of the processing unit must mark for unity which means both supply and demand must be close to each other. To reach the best efficiency rate the compressor unit must be added on the upstream side to build a gas pressure.

Pressure in the P&ID.

The process engineer should be well known about the thermodynamic properties of the fluid according to the operating pressure of the working fluid. In some cases, the decrease in fluid pressure leads to gas liquefaction or liquid vaporization. The Heat and material balance of the process flow diagram (PFD) illustrates this kind of information.

During the designing and development phase of the P & ID, some special care must be taken to design the pressure range because it depends on the thickness of the material and piping pressure ratings. But the best way in identifying the design pressure is to observe the set point value of the pressure relief valve and pipe name. Because

❖ The set point value of the pressure relief valve (PRV) depends on the maximum available working pressure of the system or equipment at its defined temperature.

❖ The pipe name or pipe code usually has information about the pressure rating.

To monitor, and control for safety purposes we need to represent the pressure transmitters, pressure indicators, high-pressure alarms, and low-pressure switches in the P & ID.


According to a signal from a pressure transmitter the pressure indication to a SCADA or HMI system can only be local (PI) or with a transmitter (PIT). But the operation in any case needs to be able to read pressure measurements.

Alarms and Shutdown

Depending on company and industry code representation standards of Alarm and Shutdown may vary.


PAH and PAL refer to Pressure Alarm High or Low action (H = high, L=low). Pressure Alarm High-High (PAHH) refers to Shutdown action.

• If Pressure Alarm High (PAH) reaches the set point the plant will continue to run even if a process disturbance was identified beyond operation range.

• If Pressure Alarm High-High (PAHH) reaches the set point the plant will shut down to prevent unsafe and hazardous conditions that may lead to damage and accident.

Identification of Pipe Line Number in P&ID


To control the pressure remotely, the field instrument PIC must be introduced in the P&ID. PIC indicates the pressure during control. The pressure measured by PT or PIT is sent to the controller. The controller sends an electrical signal to the valve actuator or equipment in order that pressures to increase or decrease to reach the set point.

Considering a natural gas vessel to develop piping and instrumentation diagrams.

  1. The natural gas enters the vessel through a manually controlled inlet valve at the header side and exits through the outlet valve.
  2. The inlet gas pressure is indicated and controlled by various pressure sensing and controlling devices such as pressure indicator and controller (PIC), and pressure transmitter (PT).
  3. The flow rate of gas entering the vessel is measured by the flow meter and the measured value is displayed for the visibility of operators using indicators.
  4. The alarm systems are provided to notify the higher and lower flow rates and pressure of the gas inside the vessel.
  5. And excess pressure is liberated or vented out to a flare system through a pressure safety valve (PSV).
  6. The pressure of the gas vessel must be monitored and controlled by using some field instruments like pressure transmitter (PT), pressure indicator, and controller (PIC).
  7. A check valve and manual hand valve are fixed on the outlet header side.
  8. The outlet pressure is measured and indicated using a Pressure Transmitter (PT), Pressure Indicator, and Controller (PIC).

Now let us study in a detailed view the piping and instrumentation diagram developed for a gas vessel with appropriate Tag numbers assigned for various instruments.

The Tag numbers on the Piping & Instrumentation Diagram have been assigned randomly just to clarify and to give the P&ID an orderly look.

To develop a Piping & Instrumentation Diagram the following points must be noted.

  1. An empty bubble without any solid line indicates the instrument is field mounted.
  2. The bubble with a solid line indicates that the instrument is control room mounted.
  3. The first alphabet of tag numbers represents the process variable.
  4. The second and third alphabet followed by the first alphabet of the tag number represents the functions performed by that instrument for a particular process variable.
  5. The number followed by the alphabet represents the logical numerator.

Development of P & ID for Gas vessels.

  1. The inlet valve is a manual hand valve HV 107 regulates the flow of gas into the vessel.
  2. The pressure transmitter PT 101 measures inlet pressure and sends proportional electrical signals to pressure indicator PI 101.
  3. A flow transmitter FT 102 measures the inlet flow and is sent to flow indicator FI 102.
  4. The alarm systems are used to indicate the flow rate.
  5. Flow Alarm High /Low FAH 102 and FAL 102 is Alarm switch to indicate higher and lower flow rates.
  6. A pressure safety valve PSV 104 to vent excess pressure to a flare system.
  7. The outlet valve is a manual hand valve HV 105regulates the flow of gas from the vessel.
  8. The pressure transmitter PT 103 measures vessel pressure and sends proportional electrical signals to the pressure indicator PIC 103.
  9. The pressure indicator and controller PIC 103 send a pneumatic signal to actuate the Pressure Control Valve PCV 103.
  10. On the developed P & ID an Instrumentation alarm system called pressure alarm switch PAH 103 is provided for indicating alarm signal for the high vessel.
  11. A check valve CV 106 on the outlet header prevents the reverse flow of gas into the vessel.
  12. The pressure transmitter PT 108 and pressure indicator PI 108 are to be provided to measure and indicate the pressure of the gas at the outlet side.

What is a Flaring system?

Flare Systems known as gas flares are important pieces of process plant equipment such as petroleum and oil refineries. The combustible gases that are released by Pressure relief valves by different types of plant equipment are burned in flaring systems.