Positive Displacement Pumps

Sections 3.1 to 3.4 discuss centrifugal pumps, where energy is imparted to the fluid through centrifugal forces generated by the rotating impeller. Positive displacement (PD) pumps rely on a different physical process to impart energy into the fluid. Fluid enters the pump, and is trapped in a cavity between moving and stationary elements. The moving element pushes the fluid out through the discharge side of the pump. This process can be thought of as a batch process when it comes to pumping. Energy is not imparted through centrifugal motion, but by direct application of forces to an individual cavity of fluid. With each rotation, only a specific volume is discharged from the pump, regardless of the system pressure. As a result of this feature, positive displacement pump should include a pressure relief valve on the

discharge piping arrangement.

PD pumps are typically used in the water and wastewater fields for pumping chemical solutions and viscous liquids at relatively small capacities and/or high pressures. Applications include sludge conveyance, either inter-plant, intra-plant or for long distance pumping to a sludge disposal site and chemical conveyance and feed.

Positive displacement pumps are usually of the rotary or reciprocating type.

3.5.1. Rotary Pumps

Rotary pumps are classified as vane, progressive cavity, flexible membrane (diaphragm) and lobe types, with numerous variations of these classifications. Each type of Rotary Pump isolates an individual pocket of fluid, and pushes it out the pump discharge. Table 3-1 from the Hydraulic Institute describes the various types of rotary Pumps. Table 3-2 lists the advantages and disadvantages of a few of the common types of rotary pumps.

For specific details on individual pump construction, see the Pump Summary Sheets.

Table 3-1: Various Types of Positive Displacement Rotary Pumps Internal Gear. Internal gear pumps (Figure 2) carry fluid between the gear teeth from the inlet to outlet ports. The outer gear (rotor) drives the inner or idler gear on a stationary pin. The gears create voids as they come out of mesh and liquid flows into the cavities. As the gears come back into mesh, the volume is reduced and the liquid is forced out of the discharge port. The crescent prevents liquid from flowing backwards from the outlet to the inlet port.

Figure 2 External Gear. External gear pumps (Figure 3) also use gears which come in and

out of mesh. As the teeth come out of mesh, liquid flows into the pump and is carried between the teeth and the casing to the discharge side of the pump. The teeth come back into mesh and the liquid is forced out the discharge port. External gear pumps rotate two identical gears against each other. Both gears are on a shaft

with bearings on either side of the gears. Figure 3

Vane. The vanes - blades, buckets, rollers, or slippers - work with a cam to draw fluid into and force it out of the pump chamber. The vanes may be in either the rotor or stator. The vane-in rotor pumps may be made with constant or variable

displacement pumping elements. Figure 4 shows a sliding vane pump.

Figure 4 Flexible Member. This principle is similar to the Vane principle except the vanes

flex rather than slide. The fluid pumping and sealing action depends on the elasticity of the flexible members. The flexible members may be a tube, a vane, or a liner.

Figure 5 shows a flexible vane pump.

Figure 5 Lobe. Fluid is carried between the rotor teeth and the pumping chamber. The rotor

surfaces create continuous sealing. Both gears are driven and are synchronized by timing gears. Rotors include bi-wing, tri-lobe, and multi-lobe configurations. Figure 6 is a tri-lobe pump.

Figure 6 Circumferential Piston. Fluid is carried from inlet to outlet in spaces between

piston surfaces. Rotors must be timed by separate means, and each rotor may have one or more piston elements. See Figure 7.

Figure 7 Screw. Screw pumps carry fluid in the spaces between the screw threads. The fluid

is displaced axially as the screws mesh.

Single screw pumps (Figure 8) are commonly called progressive cavity pumps.

They have a rotor with external threads and a stator with internal threads. The rotor

threads are eccentric to the axis of rotation. Figure 8

Multiple screw pumps have multiple external screw threads. These pumps may be timed or untimed. Figure 9 shows a three-screw pump.

Figure 9 The above table was obtained from the Hydraulic Institute Pump Type and Nomenclature, 1994  

Table 3-2: Advantages and Disadvantages of Various Rotary Pumps

 Excellent for high-viscosity liquids

 Constant and even discharge flow regardless of pressure conditions

 Operates well in either direction

 Can be made to operate with one direction of flow with either rotation

 Low NPSH required

 Single adjustable end clearance

 Easy to maintain

 Flexible design offers application customization

 Usually requires moderate speeds

 Medium pressure limitations

 One bearing runs in the product pumped

 Overhung load on shaft bearing

External Gear

 High speed

 High pressure

 No overhung bearing loads

 Relatively quiet operation

 Design accommodates wide variety of materials

 Four bushings in liquid area

 No metal-to-metal contact

 Superior Clean-in-Place and Service-in-Place capabilities

 Non-pulsating discharge

 Requires timing gears

 Requires two seals

 Reduced lift with thin liquids

 Dry running may damage lobes

Vane  Handles thin liquids at relatively higher pressures

 Compensates for wear through vane extension

 Sometimes preferred for solvents, LPG

 Can run dry for short periods

 Can have one seal or stuffing box

 Develops good vacuum

 Can have two stuffing boxes

 Complex housing and many parts

 Not suitable for high pressures

 Not suitable for high viscosity

 Not good with abrasives

Table of advantages and disadvantages was obtained from Viking Pumps Progressive Cavity Pumps

The most common type of rotary positive displacement pump is the progressive cavity pump. Progressive cavity pumps are characterized by an external-helical rotor turning inside of an internal-helical stator tube.

The mating surfaces of the rotor against the stator create a series of cavities along its length. As the rotor turns inside of the stator, these cavities close as adjacent cavities open thereby creating a “progressing”

cavity moving axially towards the discharge end of the stator. The fluid being pumped travels within these cavities from the suction side of the pump to its discharge. For more information regarding the

progressive cavity pump, see the pump summary sheet. Progressive cavity pumps are typically used to convey primary, thickened, de-watered sludge and polymers.

Note to Design Engineer: Progressive Cavity Pumps require dry running protection or else the stator may be damaged.

Inclined Screw “Archimedes Screw Pump”

The inclined screw pump is a continuous spiral vane attached to a central shaft, mounted in a trough or pipe. When the screw is rotated, the spiral vane scoop water from the free water surface at the entrance of the pump and discharges it at a higher elevation. It is a continuous propeller pump and flows are axial, with no centrifugal action. The primary advantage of an inclined screw pump is that it is a natural variable flow pump which operates at a constant speed. As the free water surface at the suction rises, the

submergence of the inlet increases and the pump is able to scoop more liquid. The Archimedes screw pump is usually large capacity low head, non-clogging and therefore advantageous in raw sewage and wastewater applications. MWH Largest installation is 96-inch diameter rated for 100 mgd each at the Helix Water Treatment plant, CA. For more information on the inclined screw pump, see the pump summary sheet.

3.5.2. Reciprocating Pumps

Reciprocating pumps are classified as a positive displacement pump because they also impart energy into a single cavity of fluid. Unlike a rotary pump, which imparts energy through a rotating element, reciprocating pumps impart energy linearly through a piston arrangement. With each stroke, the pump cylinder first fills with fresh liquid and then discharges through a check valve. A constant volume of water is displaced with each stroke, regardless of pressure. As a result, the head capacity curve of a

reciprocating pump at constant speed is a straight vertical line. The actual flow rate is only a function of the speed of the pump, not the system pressure.

Reciprocating pumps have the capability to generate extremely high pressures. As a result of this characteristic, reciprocating pumps typically require protection to avoid damaging the pump or piping. A pressure relief valve is affixed to the discharge piping of the pump. A situation where this over

pressurizing of the discharge piping might occur involves a downstream valve. If for example, a downstream valve is closed while the pump is running; the pressure in the discharge piping rapidly increases. The pressure relief valve will protect the pump and piping by opening and diverting the flow to a drain or storage tank until the pump is turned off.

Hydraulic Diaphragm Pumps (JESCO, Milton Roy, Wallace & Tiernan)

Diaphragm pumps are reciprocating positive displacement pumps that employ a flexible membrane instead of a piston plunger to displace the pumped liquid. High pressure hydraulic diaphragm pumps have a pressure chamber on one side of a diaphragm and a fluid delivery chamber on the other side of the diaphragm. The pumping forces are generated by a reciprocating piston assembly. The assembly includes a piston, a hydraulic fluid chamber and a pressure regulator which maintains the desired discharge pressure. Note, these types of pumps are self priming and may be run dry without damage.