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A versatile pump for liquids and solids

03 June 2017

The Progressive Cavity (PC) pump can offer a versatile tool in the food processing environment, providing a pumping solution that can handle a wide range of both liquid and solid food products. Food Processing finds out more. 

PC pumps are self-priming and can pump liquids and highly viscous pastes and solids, with auger feeds as part of the pump. They can also handle abrasive products and those that contain soft solids. 

In practice these capabilities mean that ingredient dosing, product transfer, accurate metering of product directly into production processes, and filling of finished products can be achieved using CIP- able pumps which comply with hygiene standards demanded by food applications. 

By-products and solid and liquid waste from food and beverage production can also be removed in hygienic enclosed systems using PC pumps.  They are capable of transporting products over long distances to waste reception areas and enhancing the value of by products and compliance with waste regulations. For example, when handling animal by-products.  Chemicals for water treatment can be dosed accurately where needed, even those which are highly corrosive.

How do they work?
PC pumps belong to the rotary positive displacement class where a defined amount of product is dispensed for each pump revolution. The pumping elements comprise a single helical rotor that rotates within a double helical stator. The action of rotation produces cavities which are progressed through the stator creating a continuous virtually pulsation free flow. The stator is typically made of an elastomer; this has an interference fit which creates a sealing line with the rotor.

The interference fit is important and it explains why the PC pump has possibly the lowest shear action of any rotary pumps. It prevents slip from the discharge back through the pump, maintaining the integrity of delicate products. 

Only one drive shaft and one seal is necessary. In order to produce greater pressure more helical stages are added, producing standard discharge pressures of 48 bar.

Some of the benefits produced by this action include:
• The interference fit makes pumping against a near full vacuum a possibility. For applications such as degassing, efficiency is achieved by enabling the degasser to work at its maximum performance while enhancing the quality of the product.  PC pumps provide a steady flow and optimise the performance of the degassing equipment. 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, preserving the product quality of shear sensitive products. Even whipped products and those containing entrained air can be pumped without damage. 

• High suction lifts of up to 9m can be achieved, making PC pumps suitable for lifting products from IBCs, barrels or mixing tanks.  On the waste side of processing, surface mounted PC pumps can lift from waste water sumps, removing the need for submersible equipment.

• The cavity has a given chamber volume which means the PC pump is suitable for batch dosing applications where specific amounts of product must be dosed accurately. A typical example is, batch dosing tomato sauce onto pizza dough.

• Because the product moves through the pump in a cavity there is no squashing action so very soft solids can be handled with little or no damage, again preserving product quality.
• They can run both clockwise and counterclockwise, enabling a single pump to fill and empty a vessel.

• Low pulsation flow, with no ball valves needed and no backflow, provides a very reliable method of dosing, also solid particles can be handled without the danger of sticking ball valves, often a problem with other positive displacement pump types.

• Calibration is simple, the flow volume is constant for each cycle of operation and is proportional to the rotational speed. It is not affected by the viscosity of the product, unlike some pumps which stop when viscosity is high. This linear accuracy means that only one variable – pump speed – needs to be changed to vary the flow volume. 

• High pressures can be generated due to the interference fit and increasing the number of pump stages which means pumping viscous products through heat exchangers can easily be achieved.  Also where necessary long distance transfer can be undertaken, often to remove viscous by products and waste from production areas in enclosed pipework.

The correct PC pump can be selected for most tasks as materials of construction and the product feed mechanism can be adapted to meet application requirements. For hygienic use, for example, a fully stainless-steel pump with CIP capability would be used, while one used to dose corrosive or abrasive chemicals would be manufactured from chemically resistant materials. The right pump for each task is almost always available.

Installation and lifetime costs
Where pumps are specified correctly, taking into account the pH, temperature and product to be handled, whole life costs can be optimised by balancing initial investment with projected ownership costs, taking into account spare parts, labour and energy costs. Because there are relatively few moving parts in the PC pump they can be refurbished easily.
PC pumps can be installed in many different positions – horizontally or vertically where space is tight. Installations are often lower cost than other alternatives. Pulsation dampeners are not required, neither are flow control valves or check valves and pipework can be of smaller diameter due to the lack of pulsation. They are quiet and efficient, only running when needed rather than constantly regardless of product demand.

One PC pump manufacturer, SEEPEX, has made some significant developments to the standard PC pump offering – most notably with designs such as its ‘Smart Conveying Technology’ which allows inspection of the pumping elements, removal of blockages and enables planned maintenance to be carried out without the need to remove pipework. This reduces downtime, maintenance costs and the space needed for installation. Operation can be optimised using integrated level controls, protection against dry running and against over pressurisation.

Although PC pumps have good suction characteristics there are some cases where they may need a helping hand, specifically when the product doesn't free flow. This has resulted in the development of auger mechanisms to constantly feed viscous media into the pumping elements of rotor and stator. 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. Media with dry solids content up to 45% and viscosity in excess of 1,000,000cps have all been successfully transferred with open hopper pumps.  However, in addition to transfer of high viscosity products these have additional functions. For mixing and back-mixing, for example, liquid can be added to solids in the feed auger, or solids can be added to a paste. The action of the auger mixes the two together before feeding them into the pumping elements.

For cutting and chopping rotating knives on the auger coupling rod, together with static knives on the pump body provide a chopping action.  This enables whole fruit and vegetables, chicken, fish and other mixed waste products to be chopped and pumped, keeping production areas clean and free from waste and debris
Overcoming problems
Progressive cavity pumps have many uses within the food and beverage industry, providing versatility and flexibility in use. Many have been developed to overcome some of the most complex problems associated with challenging and variable raw materials and changing production processes. They maintain product quality together with high hygiene standards, the main demands of two of customers in the food and beverage industry.   

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