Plant & Works Engineering
Choosing the correct pump
Published:  08 October, 2018

Lesley Eaton, business development and marketing manager, SEEPEX, looks at how progressive cavity pumps can overcome the toughest processing challenges.

Process pumps are the world’s most popular piece of rotating equipment, outnumbering all other process equipment such as compressors, mixers, blowers – in some plants they can represent as much as 85% of rotating equipment, and they use 90% of all industrial electric motors. Industries as diverse as oil and gas, food and beverage, environmental water and waste water treatment, pharmaceutical and chemical, all rely on pumps to move products around their processes.

Despite pumps being essential to many processing operations, there remain challenges associated with choosing the correct pump for the job. However, developments in technology mean that the progressive cavity (PC) pump can deliver superior pumping ability while overcoming some of the common top pumping headaches found across all industries.

Challenge 1: Maintaining product integrity

The design of the PC pump moves the product through the pump in a series of cavities, which prevents slip from the discharge back through the pump. Slip creates shear, so compared to other pump designs, which have clearances between the lobes, screws or gears, PC pumps have a very low shear action. This preserves the quality of shear-sensitive products, such as multiphase products in the oil industry, polymers used in waste water treatment, even whipped products in food and beverage production, all of which are transferred without loss of integrity and functionality.

Challenge 2: Lifting products

High suction lifts of up to nine metres can be achieved, making PC pumps ideal for lifting products from IBC, barrels or mixing tanks. The ability to pump against almost a full vacuum ensures the efficient use of degassing equipment and enhances the quality of the final product in many food products as well. Furthermore, surface-mounted PC pumps can lift from wastewater sumps, removing the need for submersible equipment.

Challenge 3: Batch and continuous dosing

The cavities have a given chamber volume which means the PC pump is ideal for batch dosing applications where specific amounts of product must be dosed extremely accurately; for example, dosing tomato sauce directly onto pizza dough or addition of polymers or chemicals to make up tanks. Furthermore, since the flow has very low pulsation, dosing into mixers or pipelines delivers the right amount in a smooth, continuous action, meaning that the accurate addition of ingredients or chemicals into a liquid flow is possible without complicated controls or lengthy calibration methods.

Challenge 4: Handling solid particles

The PC pump is able to handle solid particles without compromising either the product or the pump efficiency - ideal in applications as diverse as wastewater treatment, to sauces containing vegetables and spices. They are used in any area where solid particles prove problematical for other pump designs, often causing check valves to stick which result in breakdowns and downtime issues.

Challenge 5: Variable flow rates

PC pumps are available with flow rates from 100ml/hr to 500m³/hr, and the ability to handle temperatures ranging from -20 C to 220 C. As the flow volume is constant for each cycle of operation, and is proportional to the rotational speed, calibration is simple and is unaffected by the viscosity of the product, unlike some pumps which slow when viscosity is high. This linear accuracy means that only one variable – pump speed – needs to be changed to vary the flow volume.

Challenge 6: Viscosity

PC pumps have excellent suction characteristics and can handle a wide range of viscosities; from water to products with viscosity over 1,000,000 cps. The pumping action means that variations in viscosity do not affect the flow rate. This has practical advantages in applications where ds% and viscosity can vary such as wastewater treatment, or when viscosity is affected by temperature, for example molasses transfer into pharmaceutical processes. The ability to keep a steady flow rate despite product viscosity variation offers accuracy, process stability and reduced downtime unlike some pump designs, which stall when viscosity exceeds their operating parameters.

Challenge 7: Solids pumping

There are some instances where products don’t free flow hence the development of auger mechanisms to constantly feed non-flowable media into the pumping elements. The auger is fabricated as part of the integral coupling rod between the drive and rotor, using the rotation produced by the drive to push forward viscous and even solid products. As a result, products that do not flow are pushed into the pumping elements and can then be moved through pipes (the alternative would have been to use a conveyor). This technique makes long distance transfer possible, plus waste products from production areas can be moved to the factory exterior. In addition, the development of sophisticated auger mechanisms in some PC pumps means that they have the ability to handle and even mix solids – liquid can be added to solids in the feed auger, or solids can be added to a paste, with the action of the auger mixing the two before feeding them into the pump. These large pumps are used extensively to mix sludges in waste water treatment. Furthermore, products with dry solids content up to 45% and a viscosity in excess of 1,000,000cps can be successfully transferred using these open hopper designs.

Challenge 8: Design compatibility

Corrosive and abrasive products, water industry standards, strict hygiene requirements, low/high pH, or high temperature, are all factors that need to be considered when configuring or choosing a pump to ensure that the build materials are suitable for the product and processing conditions. PC pumps can be constructed – and the product feed mechanism adapted – in line with the application requirements. For example, a fully stainless steel pump with CIP capability can be used for hygienic applications, whilst one used to dose corrosive or abrasive chemicals can be manufactured from chemically resistant materials.

Challenge 9: Operating pressures

Pumps have to overcome pressure when transferring products. These can be generated by several factors including pipework and valve configuration, length of pipework, as well as the product characteristics which can cause high friction losses in the pipework if they are very sticky. The need to overcome operating pressures of downstream equipment such as heat exchangers also puts demands on the pump. PC pumps are able to generate pressures up to 48bar as standard, due to the interference fit between the rotor and stator plus the absence of slip between the pumping elements, meaning that they can pump long distances - many km in some cases.

Challenge 10: Installation and operation

PC pumps are easy to operate and install. They have both forward and backward pumping action in the same pump, so one PC pump can often do the job of two alternative designs. System costs are reduced due to the fact that calibration pots, check valves and pulsation dampeners are not needed, plus they can be installed horizontally or vertically, making them ideal for factories with restricted floor space.

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