Application News: Selecting pumps for food processing
May 2008: Many fluid handling problems arise because the pumps are neither designed nor suitable for applications and/or conditions for which they have been selected. It is the application that will always determine the type of pump to be used.
Once the nature of the medium has been established, it is necessary to consider other parameters including, delivery volume and pressure, temperature, flow characteristics, vapour pressure, solids content, corrosiveness, toxicity and hygiene. The correct materials are selected according to the properties of the pumped medium. In the food and beverage industries the choice of materials is particularly important, as compliance with EHEDG, FDA and 3A regulations is a basic requirement.
Pumping media containing substantially large solids is a common problem that has to be faced at various stages of the food processing cycle. Either the solids need to be pumped without bruising, or transferred without damaging the pump itself. There is also the issue of high accuracy adding or dosing of ingredients into processes to be considered.
The pump technologies that have high profiles within food processing are rotary lobe, rotary piston, reciprocating positive displacement, progressing cavity, peristaltic hose and air-operated double diaphragm designs. In addition, there is also the presence of a unique oscillating piston pump. The common factor in all these types is their gentle pumping action and ability to handle media containing solids.
Rotary lobe pumps employ timing gears which eliminate contact between the rotors and enables them to handle non-lubricating fluids. There are various designs of rotor, including bi-wing and multi-lobe options. Providing low shear and gentle handling of the liquid, thereby minimising product degradation, rotary lobe pumps are also easy to clean in place (CIP) or strip clean between operations for batching applications.
The rotary piston pump, whilst offering many of the benefits as the rotary lobe pump is the external circumferential piston (ECP). In this design the arc-shaped rotary pistons, or rotor wings, travel in annular-shaped cylinders machined in the pump body. The resulting long sealing path reduces slippage and produces a smooth product flow without destructive pulses or pressure peaks, and without the need for valves or complex parts.
Because the rotors produce a scooping action, they do not squeeze and compact the medium being pumped. This pump type combines a very gentle, pulse-free pumping action with the high suction capacity necessary for allowing thick mixes to be drawn into the pump without any separation of the ingredients.
Reciprocating positive displacement process pump is a very different technology and is a proven method for the in-line proportioning and dosing of liquids and provides the highest levels of accuracy and repeatability available. The factors affecting the choice of a metering or dosing pump as opposed to other types of pump are normally associated with precision over a range of flows and the fact that the capacity is not influenced by pressure fluctuations within the process.
The metering pump is very adaptable and due to its design is ideal for the continuous, in-phase metering of single or multiple liquid systems at varying rates. It is the only pump with two flow adjustment methods, speed and stroke length, that can be used independently or simultaneously to provide optimum performance. For instance in a process where an ingredient has to be added accurately to a varying main flow, the proportion can be maintained by speed adjustment, whilst any corrections necessary to maintain product quality can be made by adjustment of stroke length. This means that the metering pump can be finely tuned to the requirements of the system.
The peristaltic hose pump can also be considered for applications where media needs to be added to a processing system with high levels of accuracy and repeatability. Using the principle of moving the media through a hose by squeezing the hose between rollers or shoes at specific intervals to create a positive displacement action, any possibility of contamination of the pumped media is wholly eliminated. Applications for this pump are diverse and many, including the addition of sensitive solids, dosing of flavours and syrups, and even removal of waste. Capable of dry-running and self-priming, the pump provides that all-important requirement for low shear.
For duties where media needs to be moved at low velocities, progressing cavity pumps also provide low shear characteristics. Employing a single rotor to move fluids in a continuous flow, the performance of progressing cavity pumps is largely down to the geometry of the rotor and the stator in which it turns. The materials from which these two elements are manufactured need to be matched to the pumped media in which they come into contact. In order to achieve life cycle costing goals the rotor and stator need to offer high levels resistance to aggressive fluids, which brings into play rotor geometry and material specification.
In any review of pumps for the food processing industries, the air operated double diaphragm (AODD) positive displacement pump cannot be overlooked as the number of applications for it is simply enormous. This is because it can transfer almost anything. The absence of close fitting parts allows solids to pass through the pump without harm, whilst abrasive and viscous products can be pumped without any damage to the fluid characteristics.
Air-operated double diaphragm pumps (AODD) use compressed air to act upon the diaphragm and draw the liquid media into the pumping chamber and then eject it, so there are no rotary parts involved. When running, the dynamic components that come into contact with the fluid are the two diaphragms which are connected by a common shaft, the two inlet valve balls and the two discharge valve balls. The diaphragms act as a separation membrane between the compressed air supply and the fluid. Driving the diaphragms with compressed air, rather than the shaft balances the load on the diaphragm which removes mechanical stress and therefore extends diaphragm life. The valve balls open and close on the valve seats to direct the flow.
The absence of close fitting parts allows solids to pass through the pump without harm, whilst abrasive and viscous products can be pumped without any damage to the fluid characteristics.
The consequences of wrong pump specification can go beyond frequent downtime, maintenance costs and high demand for spares. There can also be penalties resulting from unnecessarily expensive and complex plant installations, occasioned by a need for control, monitoring and safety devices, which, in turn attract additional maintenance costs. There is a risk of producing substandard product, but worst of all may be the prospect of health and safety risks.
Pumps that have been designed for specific duties, although sometimes more expensive at the outset, can often repay their extra cost in more simple/lower cost installations. This is achieved by contributing to lower maintenance costs and higher plant availability, while performing duties that would be considered arduous applications for more general purpose designs. From the perspective of the customer and/or end user, any assistance that pump suppliers can give towards establishing a greater understanding of what is required when it comes to pump selection has to be welcomed.