Figure 1 - Typical PFD for a centrifugal compressor system

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Typical PFD for Centrifugal Compressor SystemsThe following figure-1 represents a typical process flow diagram (PFD) for a compressor system.Common equipments included in such systems are compressors, driver motors or turbines, suctionknock out drums (KOD) to remove traces of liquid from the gas going into the compressor andaftercoolers which help lower the temperature of the discharge gas from compressor. Often anti-surge controllers along with anti-surge valves are also part of this system to avoid operating thecompressor at surge conditions.Centrifugalcompressorsare typicallydriven by anelectric motoror a steamturbine. Thesample PFD infigure-1indicates use ofan electricmotor. Thedriver motor orturbine isconnected tothe compressorby a shaft whichcan rotate atdifferentspeeds forwhich the motoror turbine isdesigned.Compressor manufacturer creates a compressor map which is essentially a graph of compressorcurves plotted at different rotational speed (RPM) values. For a given value of gas flow though thecompressor, discharge pressure of can be controlled by controlling the rotational speed of thedriver. Hence a pressure controller installed on the compressor discharge stream, sends a signal tothe driver motor or turbine to control the rotational speed. Sometimes a compressor performancecontroller can be used which takes into account a lot of other parameters than just the dischargepressure, to effectively maintain the rotational speed.Presence of small liquid droplets in the gas compressor can be damaging to the compressor.Hence Knock Out Drums (KOD) at compressor suction are desirable to remove even small tracesof liquid droplets from gas going to the compressor. These compressor suction drums may beequipped with demister pads and wire mesh to improve the efficiency of liquid droplet removal.When gas is compressed the gas temperature also rises along with its pressure, since there is notenough time to vent the heat to atmosphere. Often high temperatures of the compressed gas arenot desirable hence air coolers can be installed at the compressor discharge to enable temperaturecontrol of the discharge gas. These are known as compressor aftercoolers.Compressor surge is seen as a very dangerous and detrimental phenomenon for compressorsystems, because it causes the compressor to vibrate and damages the compressor parts.Compressor surge occurs due to high back pressure on the compressor discharge so that very

small gas flow can be pushed through. This is indicated by a surge line on any compressor map. Toavoid compressor surge the back pressure at compressor has to be lowered so that more gas flowcan be circulated through the compressors. This can be accomplished by an anti-surge controllerwhich opens the anti-surge valves so that excess pressure from the compressor discharge isvented to the compressor suction KOD. The anti-surge valves also enable more gas flow to becirculated through the system, avoiding prolonged compressor operation in the surge conditions.

Typical PFD for Centrifugal Compressor Systems