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Versatile, low-shear pumping

01 May 2008

After the electric motor, pumps are perhaps the second most widely used machine in the world and centrifugal pumps are the most common type. However, as Gerald Muldoon explains, the robust design and novel capabilities of positive displacement pumps are also making them increasingly popular

There are many different types of pump design, which fall broadly into two main classes - centrifugal and positive displacement. Centrifugal pumps are most widely used and, although there are many varieties of centrifugal pump, they have a common design principle. Conversely, positive displacement pumps come in a much wider variety of designs. The progressing cavity (PC) pump, which has applications ranging from water to virtually non-flowing media, is proving to be a particularly cost and energy efficient method of pumping across many different sectors.

A centrifugal pump comprises two, usually metal, pump elements - a disc shaped hollow pumping chamber and a fan shaped impeller, which spins at high speeds within the chamber, with a narrow clearance at its inner surface.

The inlet to the pump is set on the axis of the spinning impeller, and the discharge (outlet) is located on the circumference of the chamber so that liquid enters the pump on the axis and is forced by the spin of the impeller (centrifugal force) out of the discharge.

The very differently designed PC pump is based on a helical metal rotor. The metal rotor rotates within the hollow core of a highly durable synthetic rubber stator, which is encased in a metal cylinder. This hollow core is a double helix shape, which means it is twice the helix of the rotor. The combination of the single helix rotor and the double helix stator core leaves a succession of cavities along the length of the pump, with sealing provided by the precise fit of the rotor and stator.

As the rotor turns, these cavities progress along the stator, carrying the pumped medium, plus anything entrained (solids or gases) within it. It is this distinct design that enables the PC pump to transport a wide range of materials in many different sectors, from water and wastewater to food and beverage.

The centrifugal pump is compact for its flow handling capacity and simple in design and build. Maintenance is usually straightforward and, under ideal conditions (usually water at a constant speed) lifetime cost of ownership can be relatively low. It can be connected to high speed motors and engines, but it operates most effectively with water-like viscosities, over a relatively narrow band of speed. As such, sizing the pump to the duty is very important.

Centrifugal pumps offer a very wide capacity range, from a few litres per hour to 40,000m3/h plus. They are suitable for clean-in-place procedures, thanks to their all-metal wetted interior surfaces, and corrosion resistant coatings can be specified to cope with aggressive fluids. However, the suction pulling power of centrifugal pumps is low and the pumping chamber must be kept full (primed) if it is to lift liquid and discharge it. As such, the pump has to be kept primed using valves and kept below the liquid level for gravity fill or even submersed.

Although a centrifugal pump will develop good head on open flow, it does not pump well against pressure, such as through a half-closed valve or through small bore pipework, especially with liquid viscosities higher than that of water. This can lead to significantly higher pipework costs in some applications. For example, the minimum trenching required for burying a 100mm pipe for a centrifugal system is significantly greater than the 50mm pipe usually required for a PC pump.

Pumping slurries, entrained solids and high viscosities is very difficult with a centrifugal pump. Unless the design is specially modified, the impeller can become blocked, requiring a strip-down to restore pumping operation. The relatively high operating speed of the pump can create additional problems with liquid cavitations and entrained gases, which cause wear and even damage to the pump. High speed also produces high shear rates in the liquid, which is often undesirable, particularly if the integrity of entrained solids needs to be maintained.

PC pumps offer very different capabilities. For example, flow rates can be reduced to nearly zero with relatively little loss of efficiency over a range of speeds. They are therefore more easily adapted to variable speed drive control, which helps ensure greater process efficiency by matching energy use and flow rate to instantaneous requirements. They are also highly accurate – between 1 and 2% for dosing and batching duties – and they have no need for additional valves, unlike centrifugal alternatives.

Significantly, PC pumps can pull suction and vacuum without being primed, the interface between rotor and stator simply needs to be wetted before use. They have a good discharge pressure of up to 24bar on standard pumps and up to 72 bar on purpose built units, and will pump over long distances through a small bore pipeline, reducing installation costs.

The cavities of a PC pump enable a wide range of media to be pumped, including slurries, entrained large and small solids or gases, and fluids ranging from those with water-like viscosity up to highly viscous and non-flowing pastes, all with a gentle, low shear pumping action. The rotor and resilient stator combination also resists abrasion, such as might be caused by sand entrained in water.

While the different designs of centrifugal and PC pumps have historically defined distinct applications for each, increasingly, PC pumps are being applied across the board. A project for Lancashire-based Ruttle Plant Hire is a good example of a PC pump being specified for an application traditionally satisfied by centrifugal pumps.

The company required a unit that would be capable of discharging diesel fuel from a new custom-built, fuel oil delivery tanker to a number of different recipients at different speeds, but with high efficiency.

A PC pump was installed and driven by a power take-off from the vehicle at a typical operating speed of 700rpm, to deliver diesel fuel oil at up to 100 litres/hour and 3bar pressure. Ruttle now has a highly versatile fuel discharge system that works reliably and has proved to be so efficient, that it has reduced the duration of the daily fuel delivery round by over two hours. The pump can deliver fuel in appropriate quantities to various sizes of machinery ranging from large earthmovers to small mechanical diggers.

The centrifugal pump may be the most widely used type, but application examples like the foregoing would seem to indicate a growing demand for alternative pump types. The unique action and capabilities of PC pumps are bringing new cost and energy efficiencies to the processing of a broad range of materials, from food and beverage to wastewater and sludge.

- Gerald Muldoon is senior sales engineer at Mono NOV

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