This post contains a detailed look at a typical compressor P&ID and the design considerations that go into it. The design of a compressor system is first conceptualized in a process flow diagram or compressor PFD. Subsequently, a more detailed design is created using a piping & instrumentation diagram or compressor P&ID.
Table of content:
Typical compressor P&ID
Procedure to prepare P&ID diagram of Centrifugal compressor system
Typical compressor P&ID
Figure 1 - Typical centrifugal compressor P&ID
Procedure to prepare P&ID diagram of Centrifugal compressor system
Selection of compressor symbol
The proper centrifugal compressor symbol should be selected first, as shown in figure-1. Normally, a centrifugal compressor is accompanied by a Knock Out Drum (KOD) at the compressor suction and an aftercooler at the compressor discharge, as per a typical compressor PFD. Symbols for these equipments should also be placed on the P&ID before proceeding ahead. All the equipment symbols should be selected from the legend sheets of a particular project.{adinserter 1}
Representation of nozzles
All the nozzles on the compressor, suction drums, and aftercooler should then be correctly represented with size and flanges. This includes inlet and outlet nozzles and equipment drains/vents as shown in the typical P&ID in figure-1.
Representation of Knock Out Drum (KOD)
Centrifugal compressor suction KOD is intended for removing the entrained liquids before sending gas to the compressor. A demister, mesh pad, etc. are used in the knock-out drum to efficiently remove the liquid droplets.
Representation of aftercoolers
Compressor aftercoolers are generally air coolers and the related fan, motor, etc. It should be indicated on the P&ID. A proper symbol for a heat exchanger should be used if cooling water is used.
Drawing lines and connections
Inlet, outlet lines for each piece of equipment, anti-surge line, drain/vent lines, the line to the relief valve, etc. are the next to be drawn up. Line number, material class, size, etc. are to be correctly assigned to each of the lines.
Anti-surge line: An anti-surge line from the aftercooler discharge to the suction KOD inlet should be provided for anti-surge control. When the compressor approaches the surge condition (low flow, high differential head), the anti-surge valves open up to lower the pressure differential and circulate higher gas flow.
Drawing isolation valves, spectacle blinds, spacers
Isolation valves, spectacle blinds, spacers, etc. to be used for maintenance should be drawn up on various lines between the equipment. The requirement for isolation valves, spectacle blinds, spaces, etc., depends on the project standards, which should be followed while indicating these on the compressor P&ID.
Sometimes, to minimize the number of isolation valves between the equipment, they can be placed only at the suction KOD inlet which is an inlet of the centrifugal compressor system, and discharge of the aftercooler which turns out to be the outlet of the centrifugal compressor system.
Spectacle blinds or spacers can be used for isolation between individual equipments for quick maintenance. This is simply a guideline and project standards need to be followed when indicating the isolation requirements.{adinserter 2}
Representation of valves
A check valve should normally be provided on the compressor discharge to avoid reverse flow when the pump is not in operation.
Pressure relief valves can be provided on the compressor discharge line, downstream to the check valve, to protect the equipment downstream of the compressor.
Emergency Shutdown (ESD) valves can be provided on inlet/outlet lines of the compressor system to isolate the whole system in case of a shutdown. The inlet line of the suction KOD corresponds to the inlet of the compressor system. Aftercooler discharge and liquid outlet of the suction knock-out drum correspond to the outlet lines of the compressor system. Shutdown valves can be located on these lines as shown in figure-1.
Check this post to appropriately select the control valve and safety valve p&id symbol with the proper actuator.
Representation of gauges and transmitters
Pressure gauges should be provided on the suction and discharge of the compressor. Level gauges need to be located on the compressor suction knock-out drum, temperature gauges on the inlet, and outlet lines for the aftercooler.
Pressure transmitters should be provided on the compressor suction and discharge line. A flow transmitter should be provided on the compressor suction line. Signals from these transmitters are sent to an ‘Anti-Surge Controller’. Based on the gas flow and differential pressure head developed by the compressor, the anti-surge controller operates the anti-surge valves to prevent compressor surge conditions.
Level transmitters provided on the suction knock-out drum are responsible for liquid level control in the drum. Alarms are usually provided for high and high liquid level conditions.
A temperature transmitter can be provided on the aftercooler for temperature control by sending a signal to adjust the fan speed of the air cooler.{adinserter 1}
Add performance controller
Sometimes, a performance controller can be included in the centrifugal compressor system to control the rotating speed (RPM) of the compressor based on inlet pressure, flow, etc. to achieve optimum performance. The performance controller will typically adjust the motor/turbine speed.
Provide Drains & Vents
Drains and vents are to be provided on the suction/discharge lines, compressor casing, suction Knock Out Drum, air cooler body, etc. for completely draining/venting the compressor and associated piping, for maintenance.
For purging the compressor system, a nitrogen connection can be provided right after the first isolation valve on the suction KOD inlet line.
All the guidelines given here are very general and may be modified as per the specific requirements of any particular project.
