JAXAshby wrote:
genee/rickie claim that oil pumps have to spin for some time to draw a prime.

I am trying to visualize how an oil pump draws a prime, particularly on oil at
say 15 degrees. In order to draw a prime the pump would have to evacuate the
air above the oil and below the pump,

correct

the difference in air pressure on the
evacuated side vs ambient air pressure leaves no more than a few pounds of
pressure total (can't be more than 14.7# total, for that is atmospheric
pressure).

correct

Then the oil would have to vaporize and then be drawn into the
pump, then to be compressed back to liquid to then be pumped to the bearings
needing pressure oil.

The pumped fluid doesn't vaporize. At this point, the ambient air
pressure through the vent (e.g. the PCV on an engine crankcase) is
higher than the pressure in the line evacuated by the pump. The
differental pressure (even if only a couple of psi) pushes oil up to the
impeller or positive displacement device in the pump. The differential
pressure must be greater than the weight of the oil in the line to prime
the pump (ambient air pressure inlet pressure plus weight of pumped
fluid). The greater the difference, the faster the prime and the greater
the available suction head (distance pump can be above steady state
fluid level). I believe in most conditions that the effect of viscosity
is limited to "how long" and not "if" priming takes place. This assumes
the pump can run dry indefinitely and ignores second-order effects like
friction of the fluid against the line.

dave

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