SURGE OF A CENTRIFUGAL COMPRESSOR

 

Surge

Surge is defined as a sudden upward movement or a sudden large increase.

Compressor Surge

For compressor surge, the sudden upward movement is the increase in discharge pressure and

Compressor surge can be defined as

ü  The point at which the compressor cannot add enough energy to overcome the system resistance or back pressure.

ü  The unstable phenomenon that occurs at low discharge flow and high discharge pressure.

ü  The phenomenon of momentary flow reversal.

ü  The Surge Point represents the minimum inlet volumetric flow where a compressor can maintain performance.

Compressor surge line is the locus of all the surge points for a compressor operating on different RPMs or different IGV positions and can be shown as follows.

Here Valve Sizing line is also known as surge margin line which is calculated as the safety factor for anti surge operation and discussed in detail under the topic of "Surge Test"

                                    

 Compressor surge cycle

1.       Compressor is working at point D; decreased inlet flow shifts the operating point operating point near to surge point A.  Continued reductions in flow will eventually trigger surge at point A.  

2.       When surge occurs, the flow direction reverses to point B.

3.       The flow reversal causes the inlet and discharge pressures to start equalizing. As pressures equalize, the reversed flow rate decreases to point C.

4.       Now the compressor regains its ability to produce head and returns to its performance curve at point D.

                 

 

Surge cycle features

• Flow reverses in 20 to 50 milliseconds. (From A to B in above graph)
• Flow regains to produce normal head in 20 to 120 milliseconds. (From C to D in above graph)
• Surge cycles (A-B-C-D-A) at a rate of 0.3 s to 3 s per cycle.

         

 Surge results in

• Compressor discharge piping vibration 

•  Rise in compressor discharge temperature 

•  Damage to seals, bearings, impeller and shaft. 

•  "Whooshing noise or "Clanking" noise 

•  Unstable Flow and Pressure & Trips may occur.