Most engineers are more familiar with centrifugal pumps than positive displacement pumps. In many cases, however, positive displacement pumps, particularly rotary variants, can provide the most cost-effective and efficient fluid handling. That's because, unlike centrifugal pumps, which create pressure, positive displacement pumps create flow.
In a centrifugal pump, an impeller rotates to move liquid through the process. The impeller's velocity imparts energy on the fluid. The resulting rise in pressure, or head, is proportional to the velocity of the liquid.
In contrast, a positive displacement pump moves a set volume of liquid. Pressure is created as the liquid is forced through the pump discharge into the system. The pump converts energy into pressure. This is achieved as an increasing volume within the pumping chamber is opened to suction and then is filled, closed, moved to discharge and displaced. The delivered capacity is nearly constant throughout the discharge pressure range. This constant capacity or flow will intersect a system curve at a defined point, allowing a high degree of control (Figure 1).
Some flow variation may occur due to internal slip or pump wear. Slip stems from the fluid's viscosity and the system pressure — with lower viscosity or higher discharge pressure creating more internal slip. Pump wear also results in increased slip. Many factors contribute to wear, including the nature and abrasiveness of the liquid pumped, pressure and age.
It's important to note that a rotary unit will continue to pump if there's a downstream blockage. So, rotary systems require some type of safety pressure-relief valve at or immediately downstream of the pump to protect against over-pressure.
Positive displacement pumps come in many different types. Here, we'll focus on rotary pumps. The Hydraulic Institute categorizes rotary units in seven primary segments — vane, piston, flexible member, lobe, gear, circumferential piston and screw; this breakdown helps in understanding the various nuances of pump design and operation. (The Hydraulic Institute also provides family trees for kinetic (such as centrifugal), vertical (submersible), sealless centrifugal (canned motor), reciprocating power (horizontal or vertical) and direct acting (horizontal or vertical) pumps.)
Rotary pump types differ in internal components but all operate on similar principles to create flow, as typified by the single- and doubled-ended three-screw pumps shown in Figure 2: liquid enters at suction and moves axially through the pump to discharge; the volume of each pumping chamber determines the amount of liquid delivered.
Table 1 summarizes the method of operation, advantages and limitations of each type. Treat the comments in the table as a rough guide; direct specific questions or comments to vendors being considered.