The information below is from Positive displacement Pumps: fundamentals, Design and Applications, a recent Hydraulic Institute self study course.
Listed below are 12 reason why the Hydraulic Institute suggests using positive displacement pumps (PD Pumps).
Selected rotary technologies and air operated piston pumps easily handle highly viscous fluids. Due to high friction losses in centrifugal pumps, their flow rate and efficiency start to drop above 500 SSU. Flow and efficiency in a rotary pump, however, typically increase with viscosity. Positive displacement pumps can handle fluids with viscosities of several million SSU.
Positive displacement pumps, such as vane or air operated double diaphragm (AODD), are often applied on very thin fluids. Other liquids, such as oil, have viscosities that vary with temperature. With variable viscosity liquids, a moderately small change in viscosity may have a large effect on centrifugal efficiency but little effect on positive displacement pump efficiency.
In many fluid applications, liquid shear is not a problem; however, it is critical in some applications. Positive displacement pumps excel in the handling of shear sensitive fluids.
Progressing cavity pumps handling high solids content sludge in a waste treatment plant and reciprocating pumps are applied on coal slurry pipeline with solids contents as high as 40 percent by weight. This is sometimes a surprising positive displacement pump characteristic, but widely varied applications serve as examples.
A constant source of liquid is a centrifugal pump requirement, but unfortunately all processes do not provide such constant sources. If there is insufficient liquid, a gas bubble forms in the suction and causes loss of prime (the pump stops pumping). Positive displacement pumps, on the other hand, are capable of handling a high percentage of air or gas entrainment.
Beyond the range of centrifugal pumps are many chemical, sandblasting and high-pressure water-cutting applications where positive displacement pump technology dominates.
Flow below 100-gpm and above 200-psi provides excellent application opportunities for postive displacement pump technology.
For viscous fluids where both positive displacement and centrifugal pumps can operate, Positive displacement pumps can often be 10 to 40 points more efficient than centrifugal pumps.
Any of the previous three characteristics individually are a reason to use positive displacement pumps; however, in applications where all of these conditions occur simultaneously, a positive displacement pump solution is ideal.
Magnetic drives and canned motor pumps are available in positive displacement pump designs. The requirement is also met by designs where the pumping environment does not have a shaft penetration, such as peristaltic or diaphragm pumps.
The ability to self-prime is a useful feature for Positive displacement pumps as it allows substantial flexibility in system layout and eliminates the need for suction priming systems. Positive displacement pumps are self-priming, have excellent suction lift capabilities (raising liquids on the suction side) and are capable of drawing down to near vacuum.
At a constant speed, Positive displacement pumps deliver practically constant flow. Flow is constant even if the system pressure varies, which is a desirable condition in certain systems.
Since a positive displacement pump is a constant flow device, certain designs that limit slip are ideal for metering fluids in or out of systems. This application, of course, requires accuracy and repeatability. It also may need flow variation, which is typically obtained mechanically or electronically by speed variation.