23 Mar, 2018

Surge in Centrifugal Compressors

Surge is without a doubt the most important boundary in natural gas compression. Surge represents the point at which the compressor can no longer impart enough energy to the gas to push it out of the compressor and through the check valve in the discharge piping. This usually happens somewhere between 50-80% of the design flow, depending on the number of impellers in the case and the impeller design.

When surge takes place inside the compressor the flow of gas actually reverses for an instant.

If the flow drops for any reason (eg blockage in the intercooler piping) the impellers cannot add as much energy into the gas, so the developed pressure drops.  If this carries on the compressor reaches a point where the developed pressure is the same as the discharge pressure – at this point flow STALLS.  At stalled flow condition the gas flow is zero momentarily so there is zero pressure developed and the compressor pressure collapses.  Now the pressure in the discharge piping is much higher than the pressure inside the compressor so the gas flow SURGES back through the compressor.  Now the suction pressure is increased as there is more gas in the suction (meaning the overall head across the compressor is less) so the compressor grabs the gas from suction and flow restarts so pressure is generated again.  If the suction pressure drops far enough the cycle starts all over again.

The cycle frequency is dependent on the system, i.e. piping geometries, distance to discharge check, differential pressures and the volume of the system. Surge cycle frequencies may, therefore, vary from 0.1 sec to 2 sec or more per cycle. Surge is an unstable operating condition. It must be avoided, as operation of the compressor under surge conditions can damage the compressor and the auxiliary equipment. Anti-surge control is accomplished by recycling gas from the discharge side of the compressor back into the suction in order to keep the inlet flow above surge conditions and reduce the head on the compressor.

It should be noted that Surge can occur in both Centrifugal and Axial flow compressors, as a result, turbochargers, gas turbine engines may also approach surge during high head conditions…. (usually at high load conditions – take off & acceleration)

► Gas Turbine Surge results in an explosion that drives gas flow both out of the exhaust and the inlet

► Usually prevented by reducing adjusting inlet guide vanes on the AFC, to reduce air mass flow – outlet power drops

► Turbocharger Surge results in a “fluttering” as gas flow reverses momentarily through the compressor.

► Usually prevented by opening the “blow off valve”, bypassing exhaust gases around the turbocharger reducing speed and mass flow through the compressor.

For more information on surge troubleshooting and prevention, we recommend attending Fundamentals of Pump and Compressor Systems(ME-44) and Compressor Systems - Mechanical Design and Specification (ME-46). To read more by Frend, view his published articles on Linkedin

 

FULL LIST OF COURSES TAUGHT BY INSTRUCTOR RON FREND

 Piping Systems - Mechanical Design and Specification - ME-41

 Fundamentals of Pump and Compressor Systems - ME-44

 Compressor Systems - Mechanical Design and Specification - ME-46

 Gas Turbines - ME-48

 Heat Transfer Equipment - PF-43

 Offshore Pipeline Design and Construction - PL-43

 Process Plant Reliability and Maintenance Strategies - REL-5

 Risk Based Inspection - REL-61