If you’re an engineer working in a process with centrifugal or axial compressors, you’re probably well-versed in the topic of compressor surge. If not, I came across a nice post from several years ago on the Mechanical Engineering Site, Centrifugal Compressor Surge Basics, Mechanism. Here is how the author defines surge.

Surge is a characteristic behaviour of a centrifugal compressor that can occur when inlet flow reduced such that the head developed by the compressor is insufficient to overcome the pressure at the discharge of the compressor. Once surge occurs, the output pressure of the compressor drastically reduced, resulting in flow reversal within the compressor.

Several conditions can induce these compressors to enter a surge condition, including suction throttling, discharge valve throttling, changes in speed, inlet valve choking, driver input speed, and changes in the properties of the gas being compressed.

The author highlights the adverse consequences that can result from compressor surge.

• During the surge, a significant mass gas will flow in the reverse direction. As a result of a large dynamic force act on the impeller or blading within the compressor. Due to this the components of the compressor (such as thrust bearings, bearing, casing) exposed to large changes in axial force on the rotor. If the surge is not controlled it may result in fatigue damage to compressor or piping components.
• During the surge, the reversal of flow within the compressor results in hot compressed gas returning to the compressor inlet. If the surge is not controlled, as a result, the temperature at compressor inlet will increase and leads to a potential rubbing of close clearance components. Due to the differential thermal expansion of components within the compressor.
• The vibration level of the compressor is very high during the surging.
• A reversal of flow may lead to process-related problems that could shut down a plant.

I mention all this because we’ve written many posts about Compressor Surge Control here on the blog. I’ll specifically call out the most recent post, Centrifugal and Axial Compressor Control – Freely Available Educational Resource.

Process automation hall-of-fame member Greg McMillan made his book about centrifugal and axial compressor control freely available for download in PDF form. If you have these types of compressors and aren’t already an expert, it’s well worth your time to download a copy and become one.