For interest, take a standard centrifugal pump, disconnected from the pipework and run it - air will be discharged from the discharge flange - admittedly very low pressure - but there is flow taking place.
A Swedish submersible pump company used on their pumps a spring loaded discharge port which would close when pumping and open during times there wasn't any water flow - it opens to allow air to be pumped as an aid to motor cooling - it was reasonably effective and backed up by thermal sensor in the windings.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
