For designers who need to displace a fluid or substance from one location to another, it’s easy to specify pump types that have been favoured historically, such as centrifugal, gear or piston pumps. But there’s a growing trend that’s difficult to ignore – rapidly increasing uptake of positive displacement (PD) pumps. But why is this? Andrew Ashton, UK sales manager at Watson-Marlow Pumps Group, comments

On the most basic level, a pump provides pressure and flow to accomplish a specified task, and this basic premise helps explain that PD pumps produce flow, not pressure, i.e. flow from the pump is directly proportional to pump speed. This is the fundamental difference that needs to be understood – a centrifugal pump (non-PD) has varying flow depending on pressure or head, whereas a PD pump has more or less constant flow regardless of pressure. Mechanical efficiency is equally unaffected. Another major difference is the effect viscosity has on pump capacity. A centrifugal pump loses flow as viscosity rises, but a PD pump’s flow actually increases. This is because higher viscosity liquids fill the clearances of the pump causing higher volumetric efficiency.

A centrifugal pump also performs best in the centre of its performance curve. When moving left or right on the curve, additional considerations come into play. For instance, moving far enough to the left or right, pump life is reduced due to either shaft deflection or increased cavitation. With a PD pump it is possible to operate the pump on any point of the curve. In fact the volumetric efficiency as a percentage actually improves at the high speed part of the curve. This is due to the fact that volumetric efficiency is affected by slip, which is essentially constant. At low speed the percentage of slip is higher than at high speed.

Think peristaltic

As a form of PD pump, peristaltic models have become particularly popular because nothing but the hose or tube touches the fluid, eliminating the risk of the pump contaminating the fluid, or the fluid contaminating the pump. The peristaltic pump action is created by compressing the tube between rotating rollers and a track. In between each roller pass, the tube recovers to create a vacuum and draws in fluid. Complete closure of the tube provides the pump with its PD action, preventing backflow and siphoning, and eliminating the need for check-valves when the pump is not running.

Peristaltic pumps are easy to install, simple to operate and inexpensive to maintain as there are no valves, stators, diaphragms, seals or glands. They are self-priming up to 9.5m and dry-running, and offer reversible flow direction. They are also well suited to a host of applications including slurries, viscous, shear-sensitive and aggressive fluids, and can be combined with tubing types to suit industrial, food-grade, printing and biopharm processes.

Every year, thousands of peristaltic pumps are fitted as original equipment in systems ranging from fermenters and bioreactors to orange juice and milkshake dispensers, from road-cutting machines to medical and diagnostic devices, and from seed-coating machines to printing presses.

Peristaltic pumps from the Watson-Marlow range are available for OEM use at flow rates from microlitres a minute to hundreds of litres per minute, and from one to 32 channels of flow. Of additional benefit, peristaltic pumps are lightweight when compared with conventional metallic equivalents and take up considerably less space, while many manufacturers’ OEM pumpheads can be fitted to existing cased drives, enabling the advantages of digital control – RS232, RS485, Firewire – and 4-20mA or 0-5V to be available in a single package.

‘Sine’ of the times

Sinusoidal pumps are also growing in popularity as designers increasingly come to appreciate the benefits of these highly reliable and economical PD pumps for sanitary and industrial applications. Here, an innovative sinusoidal rotor overcomes the limitations of conventional rotary lobe pumps to produce powerful suction with low shear, low pulsation and gentle handling. With a single shaft and rotor, there is no need for the complex timing gears and multiple seals associated with conventional rotary lobe pumps. One rotor, one shaft and one seal equate to simple and economic maintenance.

OEMs in the food sector are demonstrating particularly high take-up of sine pump technology. For example, food processing specialist DC Norris of Sandy, Bedfordshire, has replaced rotary lobe pumps with MasoSine SPS 2.5 models on a challenging mashed potato transfer task after existing pumps could not provide enough suction to cope with the high viscosity levels. Elsewhere, Leamington Spa-based customised food manufacturing machinery specialist, Delta Food Systems, recently changed design from an integral tri-lobe pump to a MasoSine SPS series model on a pumping production station designed to transfer viscous meat mixes for savoury products such as sausage rolls. Benefits experienced include gentler handling that avoids break-up of the material – even with solids – and far more even flow.