quote:
Originally posted by satterfi:
Automotive oil pumps are not perfect PD pumps.
The flow rate is a function of rpm, viscosity and system pressure. That’s why pump manufacturers publish performance curves for PD pumps with different viscosities and pressures.
You folks who make simplified statements regarding PD pumps should either get educated or get some real world experience.
Ignatz’s observations are consistent with the PD pump curves I’ve got.
Hey guys, PDP curves in head versus GPM are essentially a straight line until the head gets high. The reason for this is explained below.
Guys, you will not find any engineering reference in any book or on the web that says that a PDP is not designed to pump a given volume of substance per turn of it's impellers regardless of what that substance is. Whenever they do not follow this basic rule, it is because of two things:
1) A bypass valve is operating to relieve pressure
2) As the discharge pressure of the pump increases, some amount of liquid will leak from the discharge of the pump back to the pump suction, reducing the effective flow rate of the pump. The rate at which liquid leaks from the pump discharge to its suction is called slippage.
Other than the operation of the bypass valve and slippage, the pump's output is not pressure sensitive at all. Note the following quotes from various Mech E sites: "Positive displacement pumps deliver a definite volume of liquid for each cycle of pump operation. Therefore, the only factor that effects flow rate in an ideal positive displacement pump is the speed at which it operates. The flow resistance of the system in which the pump is operating will not effect the flow rate through the pump"
Another: "The flow delivered by a centrifugal pump during one revolution of the impeller depends upon the head against which the pump is operating. The positive displacement pump delivers a definite volume of fluid for each cycle of pump operation regardless of the head against which the pump is operating"
Another: "All are constant flow machines whose pressure rises to whatever value is necessary to put out the flow appropriate to the pump speed."
Now for you guys insisting that high viscosity fluids pump slower or less at a fixed RPM, go to this site:
http://www.vikingpump.com/documents/When_to_Use_a_PD_Pump.pdf
Please note that at a fixed RPM, a PDP will actually pump more as the viscosity goes up (compare to a centrifugal pump) because of increased volumetric efficiency.
Now another quote from Pumpworld.com "A PDP will generate the same flow at the same RPM regardless of discharge pressure"
Now from PipingDesign.com: "Positive Displacement pumps are knows as "flow generators", while centrifugal pumps can be thought as pressure (or head) generators. PD pump curves show that they produce almost (depending on viscosity and internal clearances) the same flow, regardless of the differential pressure. All are constant flow machines whose pressure rises to whatever value is necessary to put out the flow appropriate to the pump speed. "
Therefore, if the bypass ain't open boys......then Ignatz's pump is spinning slower with the high vis stuff!!!! Very simple.
All I can say is thank God they don't give out engineering degrees to anybody. A PDP is the mechanical equivalent of a syringe with a continual stroke where the piston speed is analagous to the pump RPM. If I'm wrong, then so are all the mechanical engineering text books and internet sights.........not very likely.