Peristaltic pumps deliver accurate and contamination-free dosing

4th March 2013

Whether you work in a laboratory, on a pilot plant or in full-scale production units, there is a strong possibility that at some time you will need to dispense materials accurately from one point to another.

Applications as varied as dispensing nutrients to fermentation vessels, adding flavourings and other additives to foodstuffs, or dosing municipal water supplies with chemicals, all demand levels of accuracy that can only best be met by positive displacement delivery methods. Unlike conventional centrifugal pumps, where output volumes have to be controlled by flow meters and valves, positive displacement systems deliver a fixed volume for each cycle of the device, irrespective of input or output flow conditions.

This ability to dispense fixed volumes at controllable flow rates is the common feature behind positive displacement devices such as syringe mechanisms, piston pumps, diaphragm pumps, gear and lobe pumps, and peristaltic pumps. But it is only the lowest common denominator.

At first sight, a peristaltic pump is simplicity itself. Fluid flows through a flexible tube, which a number of rollers pass over, repeatedly squeezing and releasing the tube. The tube's restitution or recovery to its normal diameter after its 'squeeze' creates a vacuum, which draws more fluid into the tube to be positively displaced by the rollers' squeezing action. This simple operation owes much to the physiological phenomenon of peristalsis, in which muscles alternately contract and relax vessels, such as intestines, to move material through the body. Behind the simplicity, however, lies a sophisticated metering and dosing technology that is proving its worth in a wide variety of applications (Fig.1).

According to Colin Paine, for example, his company's decision to switch to peristaltic pumps in the production of its Dipslide bacterial contamination test kits has proved to be "a self-funding investment - a finance director's dream". Paine is managing director of Casp Products, which introduced peristaltic pumps from Watson-Marlow Bredel to dispense viscous nutrient media at 50°C on to the Dipslides in a process requiring more than 5000 individual 3.5ml high-precision doses per shift, at a rate of under a second each.

"Since the pumps were installed, the resulting improved dosing accuracy has increased production yield by 5-8 per cent," Paine says of the dispensing pumps selected following successful trials on a 500 series unit with twin 313 pumpheads. In many ways this one application highlights many of the individual attributes of the company's wide range of dispensing solutions, all based on its proven peristaltic pump technology.

Take hygienic processing, for example. Casp Products' manufacturing takes place in a cleanroom environment and it's vital that the nutrient media is not contaminated in any way.

In a peristaltic pump, the fluid being pumped only comes into contact with the inside of the tube. As the roller mechanism is external to the tube, there are no shaft seals to worry about that could leak - in either direction - causing contamination of the product or the surrounding environment. And, because the tube is always occluded by at least one roller, there is no need for additional check-valves in the line, which can wear and need cleaning or replacing.

In some applications, particularly those involving different batches of material being dispensed sequentially through the same pump, residues of product have been known to cause contamination, but peristaltic pumps such as the new 520Di model are entirely crevice-free and it's a very easy job to change the tube between batches.

This last point was in fact one of the major selling points behind the Hutchison/MRC cancer research centre at Cambridge University becoming one of the first users of another new pump - the Sci-Q 323 model. Again chosen after an extensive trial, this pump excelled in its ease of tube replacement. "An important requirement," says Karen Hébert, Hutchison's media unit supervisor, "was the ability to swap tubes quickly and simply. Because we often deal with media that needs to be kept within specific temperature ranges under aseptic conditions, we sometimes need to change tubes in seconds - to start dispensing before the media cools."

The 323 at Hutchison/MRC is used on tasks involving dispensing volumes of between a few millilitres up to 20litres - a range of operations catered for by the wide range of tube diameters available, and the choice of five drives and five pumpheads in the 323 range.

The importance of the tube in peristaltic pumping is, of course, paramount.

Watson-Marlow Bredel's new £1.4m tubing extrusion plant at its hq in Cornwall, England, features a class J10,000 cleanroom which is used in the manufacture of special silicon tube ranges developed to meet growing demand for very accurate dispensing.

One such example is Pumpsil-D. This is post-cured after extrusion to remove residual linear and cyclic siloxanes that can leach out of other platinum-cured silicone tubing.

The post-curing process allows cross-linking between the silicone molecules, resulting in tighter bonds that maximise the tube's resistance to flexing stress. This improves the tube's restitution, its ability to revert to its original shape after compression during the pumping action - a vitally important feature in maintaining dispensing accuracy.

Product manager Martin Johnston says: "Pumpsil-D is the result of over two years' research and development, including a six-month testing period of trialling the product both internally and at customer sites. Over 80 per cent of those involved in the trial are now using it as a selected product."

Another important aspect of the Pumpsil-D tubing, particularly to highly regulated industries like pharmaceuticals, is that each piece of tubing is permanently laser-etched, end-to-end, with part number, lot number and use-by date. This "LaserTraceability" feature meets all the necessary requirements for validated processes - requirements, such as cGMP, that often also demand full traceability of the material being handled by the pump.

These demands are also met by the company's dispensing pumps, which connect to a PC for display, storage and printing of batch records, complete with all dispensing parameters such as dates, times and operator ID. With or without a PC, the pumps all feature their own in-built software to control the dispensing action.

At Aurora Scientific, manufacturer of water quality testing systems, the handling of chemicals such as acids is coupled with the need for accurate dosing. Having selected to use a 520Di dispensing pump, the company is able to operate the pump using process signals fed back from the production equipment, signalling when the pump is to begin dosing.

"It was immediately apparent that peristaltic technology suited our production applications," explains Brian Hepburn, proprietor at Aurora Scientific. "Moreover the pump also had no problem meeting the high dosing accuracy and repeatability requirements we had".

Ramp settings are particularly useful when the pumps are being used in filling operations. Basically, programmable ramping allows filling to start and stop without splashing or foaming - the flow being gradually 'ramped' up to a constant speed, and then decelerated towards the end of filling.

Similarly, end-of-dose dripping is prevented by the programmable drip control, which momentarily reverses the roller direction at the end of each dose.

- Ashley Shepherd is UK sales manager at Watson-Marlow Bredel.





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