Yes the pump was running as per truck unloading procedure, oonce a compartment is empty operator is disconnecting the hose from that compartment and connecting to next one, while doing this there was gravity back flow from higher pressure side ( 15 meter product head in the tank) toward lower pressure. My question is how liquid can pass through a running impeller and find its way to the suction hose and cause a spill. As i said this is the procedure used for more than 30 years. The only difference is the said pump is much smaller in size comparing to others. I am thinking of increased clearances between wear ring and eyelet possibly. Centrifugal pumps have what is called Net Positive Suction Head.  Means pressure needs to be applied to the inlet of the pump for the pump to work.  If this pressure is not met, then no matter how fast the impeller is spinning, fluid will not flow, and will flow back to balance the pressure in the pipe. Magnetic pumps