- Positive displacement pumps require a particular minimum absolute pressure at the input port.
- PD pump throughput is a function of viscosity, differential pressure and speed.
- PD pumps aren’t self-priming.
This is the second article based on Hydraulic Institute’s new “Positive Displacement (PD) Pumps: Fundamentals, Design and Applications” e-learning course. The Hydraulic Institute (HI) is continuing its mission to drive industry innovation by delivering to pump professionals an indispensable resource that supports their continuing education needs.
Because fluid properties affect positive displacement pumps differently than centrifugal pumps, it’s critical to understand the interaction between the pump and the fluid and how the operation of positive displacement pumps differs from centrifugal pumps. Without this foundation of fundamental principles, effective understanding of the myriad positive displacement pumps is unnecessarily difficult.
On the most basic level, pumps provide pressure and flow that accomplish a specified task. This basic premise helps explain that positive displacement pumps produce flow, not pressure. The pressure the pump needs to produce is a function of the system’s frictional losses, restrictions and elevation changes.
Another basic concept is the difference between gauge pressure and absolute pressure (Figure 1), as these two reference points have caused more than their fair share of confusion over the years.
Figure 1. Use these conversion factors to move between gauge pressure and absolute pressure.
Positive displacement pumps theoretically produce flow that’s independent of discharge pressure, and the pump’s flow characteristics affect the system design and pressure. Figure 2 and Figure 3 show how differential pressure and speed affect the pump’s flow rate. The mechanism is a fundamental difference between positive displacement pumps and centrifugal pumps.
Figure 2. This chart depicts pump throughput as a function of speed with viscosity as a parameter.
Figure 3. This chart depicts pump throughput as a function of differential pressure with viscosity as a parameter.
Of course, nothing comes for free, so pumps require a certain input power to perform work.
The fluid counts
Fluid characteristics play a major role with positive displacement pumps because most handle fluids other than water. The most important property is viscosity, the fluid’s resistance to a shearing force, which quantifies how easily the fluid will flow.
Along with viscosity, it’s essential to understand how a fluid reacts to being pumped. A fluid can have a constant viscosity, at a particular temperature and pressure, regardless of the rate of shear. This is called a Newtonian fluid. If the viscosity varies with shear rate, the fluid is non-Newtonian.