Expert advice on rotary PD pumps

In brief:

• The rotary positive displacement pump can outperform a centrifugal or vertical pump under certain operating conditions.
• The positive displacement pump can self-prime if the differential between the net positive inlet pressure available exceeds the net positive inlet pressure required.
• A consolidated range chart and a capability table are two important tools for selecting an efficient pump.

This is the third in a series of articles based on Hydraulic Institute’s comprehensive e-learning course, “Positive Displacement Pumps: Fundamentals, Design and Applications.”

Last month, we focused on positive displacement pump hydraulics. In this installment, we’ll focus on an overview of the common characteristics of rotary pumps, the pump types within the pump family and the application analysis and pump selection processes.

Rotary pumps are available to handle a broad range of flows and pressures, which explains why users recognize them as the solution for many pumping needs. Recent enhancements have increased rotary pump reliability and operating envelopes. They’re increasingly recognized for efficiency in an energy-conscious environment.

A rotary pump typically has a stationary pumping cavity containing rotating pumping elements powered by a drive shaft. This rotary motion is the distinguishing feature of this class of pumps, hence, the name rotary pump. The pumping elements are characterized by close-fitting running clearances. Rotary pumps have no need for separate inlet or outlet valves.

The rotating pump elements draw fluid through the suction port into the pumping cavity, transport it through the pumping elements and force it through the discharge port into the system. Pump element and pumping cavity geometry determine the volume of fluid pumped per shaft revolution. This volume is called the displacement. Most rotary pump types are configured for fixed displacement; however, they can produce variable flow rates by varying the shaft speed. Vane and piston rotary pumps produce variable volume by changing the internal geometry, that is, by varying the displacement of the pumping elements.

Rotary pump types

The most common types of rotary pumps and their subcategories are shown in the rotary pump tree (Figure 1).

Figure 1. The many varieties of rotary positive displacement pumps can be configured to handle a range of applications.

Vane-in-rotor pumps have movable vanes, or rigid blades, that are retained, but not necessarily fixed, by an eccentric rotor that turns within the pumping cavity. This action draws fluid in and forces fluid out of the chamber, thereby delivering flow. Vane pumps can have fixed or variable displacement.

Rotary piston pumps produce flow as fluid is drawn in and forced out by multiple pistons that reciprocate within cylinders in a cylinder block. The two types are axial and radial piston. Both are available as fixed and variable displacement pumps.

Flexible-member pumps transfer product from the inlet to outlet by making use of the elasticity of the flexible members. The flexible members might be a tube, vanes or a liner.

Lobe pumps carry the pumped fluid from the inlet to the outlet between rotor lobe surfaces and the pumping chamber. In this regard, they’re similar to gear pumps but don’t produce the shear effects. Timing gears coordinate the motion of the lobe surfaces, avoid surface contact and provide continuous sealing.

Gear pumps carry the pumped fluid between gear teeth and displace it as the gears mesh. The gear surfaces cooperate to provide continuous sealing, trap pockets of the pumped fluid and push it out the discharge port. One gear drives the idler or driven gear. There are two main variations — external and internal gear pumps.

Circumferential piston pumps have timed rotors and each rotor has one or more wing lobe elements, called pistons. There’s no sealing contact between the piston surfaces.

Single-screw pumps, commonly called progressing cavity pumps, have a rotor with external threads and a stator with internal threads. The geometry of the rotor and stator are such that cavities form and progress from inlet to outlet as the rotor spins to produce the pumping action.

Multiple-screw pumps are divided into two broad families — timed and untimed. Typically, multiple-screw pumps have two or more intermeshing screws and the flow is in the axial direction. The inlet fluid, which surrounds the rotors, is trapped as the screws rotate. With the rotors in tight-fitting bores and the casing acting as the stator, rotor action moves the fluid uniformly along the axis and forces it out at the other end.

Application analysis

The application analysis stage is a vital step in the selection process. Application of rotary pumps benefits from careful attention to the system’s performance requirements. Application analysis determines initial affordability, flexibility to cover the application’s entire operating range, reliability and energy requirements. It’s important that you understand, identify and communicate your requirements to your pump supplier. The recommended analysis sequence is (a) consider the characteristics of the fluids to be pumped, (b) consider the operating conditions the process requires and (c) determine the system’s hydraulic requirements (Figure 2).

Figure 2. These are the elements of the three-step process for identifying a suitable rotary positive displacement pump.