Axial Piston Pumps are typically used to pump fluids at high pressure
with high efficiency. My application is to use such a pump as a sump
pump to pump water at about 700 psig.  I would like to get information
as to their principles of opertation, hydraulic efficiency, history,
suppliers, and other general information concerning this class of
pump.  I am less interested in suppliers than in learning the details
of their various designs. Patent status of various designs would be
helpful. I basically want a primer on this topic, with a primary focus
on their submerged use in water.    A good answer will have a good
cross section of
information, focusing on water applications.