Consider Positive Displacement Pumps

Such units offer advantages over centrifugal pumps in some services.

By Sean McCandless and Richard Meighan, Colfax Fluid Handling

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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.

 

 

Figure 3 depicts the pressure-versus-flow capabilities of types of rotary pumps. The ranges and the maximum values shown aren't absolute; custom designs with alternate speeds, clearances or special materials may exceed these values. To learn more, visit the Hydraulic Institute at www.pumps.org.

 

SCREW PUMPS

These devices are further divided into two segments: single-screw (progressing cavity) and multiple-screw. Multiple-screw pumps, in turn, are classified as timed or untimed. Each of these variants offers distinct advantages and limitations (Table 2).

 

Let's look at each type in a bit more detail:

 

Single-screw (progressing cavity). This design most commonly is fitted with a rigid threaded rotor that rotates within an elastomeric stator with internal threads (Figure 4). Although capable of pumping water, it's most effective in handling contaminated and viscous fluids. So, this pump finds wide use in wastewater service and typically is the unit of choice for sludge, as well as abrasive or stringy fluids where the constant flow characteristics of a positive displacement pump are preferred. Sanitary 3A-rated stainless steel versions handle a wide range of food products from meat to dairy, molasses to concentrated juices.

 

A unique feature of this pump to keep in mind is the option of an auger feeder fit into a hopper-style inlet. This allows the pump to handle up to 45% solids' content. Not many types of pumps can deal with non-fluids such as recycled tires — but a progressing cavity pump can.

 

 

The benefits of the progressing cavity pump include:

 

• suitability for abrasive fluids;

• ability to handle solids and stringy material;

• metering capability;

• availability of sanitary-service designs; and

• low shear.

 

Timed multiple-screw. The two- or twin-screw pump (Figure 5) typifies this design. Unlike in a single-screw unit, the twin-screw pump's rotor doesn't contact the casing. In fact, there's no metal-to-metal contact between the rotors themselves or the rotors and casing. Rotors are precision machined and supported by bearings synchronized by oil-lubricated timing gears on one end of the rotors. This sophisticated design means the twin-screw pump isn't a low cost option but enables it to handle difficult applications when other designs fail. It truly can be the pump of last resort.

 

 

Advantages of the timed twin-screw pump include:

 

• ability to handle extremely high to water-thin viscosities (such as required for some flushing cycles);

• capability (of some designs) to cope with multiphase feeds (fluid, gas and contaminants);

• ability to provide high flow rates;

• extremely low NPSHR (net positive suction head required), well suiting it for difficult vapor pressure fluid applications;

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