Compressor pumping

Pumping is a fundamental phenomenon of dynamic compressors. These machines are characterized by the fact that compression is achieved by energy exchange in the gas set in motion in rows of vanes. Axial and centrifugal compressors belong to this category of turbomachinery.

It's well known that an aircraft wing loses lift at high incidence and "stalls". A similar phenomenon occurs with turbomachinery. This high incidence is achieved at low flow rates. As the compressor is connected to two networks at different pressures, the high-pressure capacity of the discharge side empties into the low-pressure capacity of the suction side, in a counter-current flow through the compressor.

Surge is the overall aerodynamic instability of the compressor in its environment, during low-flow operation. When the discharge network has emptied sufficiently into the suction, the compressor finds new operating conditions enabling it to re-establish the flow in the right direction, until a new cycle of instability begins.

These large fluctuations in flow are known as pumping, due to the nature of this phenomenon of relaxation oscillations, which, seen from the outside, resembles the instability of a control loop (speed pumping...).

Pumping is a phenomenon feared by operators and manufacturers alike. The sudden reversal of flow is like a shock, and its mechanical consequences are disastrous:

• fin failures on axial compressors ;

• very high levels of radial vibration, with destruction of internal sealing devices on centrifugal compressors.

It is therefore obvious that specific means of protection will have to be installed on the machines to preserve their integrity and maintain their performance over time. These means are all the more complex in that the widest possible range of flow rates is always required for machine operation, so that production can be adapted permanently, economically and flexibly to consumption requirements.

Anti-pumping devices prevent the compressor from dropping too low in flow. They therefore limit the machine's operating range somewhat. Their sizing and adjustment take into account not only the compressor's aerodynamic performance, the characteristics of the upstream and downstream networks and the nature of the gas, but also the fact that they must protect the machine during rapid transients.

We need to be aware that different services to be provided between summer and winter, pressure levels that vary over time (depletion of natural gas fields, storage units...), or the handling of gases whose nature changes (increase in molar mass), require specific...