Centrifugal Compressors

Centrifugal Compressor Surge - Centrifugal compressor surge is seen as a very dangerous and detrimental phenomenon in compressed air systems, dangerous because it causes the compressor to vibrate and detrimental because it causes damage to the compressor parts. Compressor surge only occurs in dynamic compressors (Axial and Centrifugal) due to their nature. To understand compressor surge, a solid understanding of how a compressor actually works is needed.

Compressor Choke or Stonewall - Compressor choke is an abnormal operating condition for centrifugal compressor. Choking of centrifugal compressor occurs when the compressor is operating at low discharge pressure and very high flowrates. These high flowrates at compressor choke point are actually the maximum that the compressor can push through. Any further decrease in the outlet resistance will not lead to increase in compressor output.

Centrifugal Compressor Maps - Compressor maps are developed by the manufacturer of dynamic compressors. They are compressor equivalents of the pump performance curves.  It is the performance chart of a specific compressor which manufacturer calculates and draws up for the unique design characteristics of that compressor.

Typical PFD for centrifugal compressor systems- Common equipments included in centrifugal compressor systems are compressors, driver motors or turbines, suction knock out drums (KOD) to remove traces of liquid from the gas going into the compressor and aftercoolers 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 the compressor at surge conditions.

Typical P&ID arranement for centrifugal compressor systems - Typical P&ID arrangement for centrifugal compressors is shown including the requirements for isolating, draining/venting and bypassing the compressors used in a process plant. Requirements for flow, temperature and pressure monitoring is indicated along with requirement of emergency anti-surge valves, driver motor/turbine, knock out drum, aftercoolers etc.

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