Why is it said that the pressure difference between the outlet and inlet of the turbine increases when a centrifugal compressor operates at reduced load?

When the flow rate of the incoming gas drops instantaneously and falls below the allowable minimum level, there is a significant deviation in the direction of airflow within the compressor as well as in the installation angle of the blades at their inlets. This results in severe \"rotational separation\" of the airflow within the blade channels, causing the gas to stagnate there. As a consequence, the compressor pressure drops suddenly; however, the pressure in the outlet system does not drop instantly. This allows the high-pressure gas in the exhaust pipe to flow back into the compressor, thereby replenishing the flow rate within the blade channels and allowing the compressor to resume normal operation. When the flow rate inside the compressor decreases again, gas flow in the system reverses direction, and this process repeats itself. As a result, periodic oscillations occur in the airflow within the system, accompanied by intense noise – this is what constitutes compressor surge. 　　

For this, you can refer to the compressor’s performance curve chart. At a constant rotational speed, as the flow rate increases, the pressure difference between the inlet and outlet of the compressor decreases. That means that as the flow rate decreases (i.e., as the load is reduced), the pressure difference across the compressor’s inlet and outlet increases.

According to the performance characteristics of centrifugal compressors, the inlet flow rate decreases from normal levels to lower values as the load is reduced; meanwhile, the compression ratio of the compressor increases. Therefore, the statement that \"the pressure difference between the outlet and inlet of the impeller increases when a centrifugal compressor operates at reduced load\" is in line with the natural laws governing centrifugal compressors. Of course, this increase has certain limits. One can view the performance curve of the compressor.

When the flow rate is low, the pressure increases, so the pressure difference becomes larger

If the volume flow increases while the speed remains constant, why does the pressure difference decrease?