Gut reaction

Colin Carter looks at peristaltic pumps, which mimic the mechanisms used in the human body to move delicate and soft materials simply, reliably and cheaply.

The human body is a highly-tuned, complex set of processes which has evolved over the ages, developing the best techniques to perform a variety of functions in the most effective manner.

Researchers and development engineers are increasingly finding it a great source of inspiration for tapping into to help with real-life problems. The proliferation of research into, and latterly products coming to market using, neural networks as a way to make control decisions, is one example. This methodology works in the same way as the human brain has been doing for millennia.

The word ‘peristaltic’ is derived from ‘peristalsis’ — a medical phenomenon pertaining to the transportation of matter along the gut by a succession of waves of involuntary muscular contractions. In other words, the muscles squeeze material from one end of the ‘process’ to the other.

With that image in mind, think of how this process of squeezing a tube (as we used to with toothpaste before the advent of ‘pump action’ dispensers) can be harnessed to move materials.

Peristaltic pumps — the latest example of ‘borrowing from nature’ — use a flexible hose or tube in a circle or arc, with an array of ‘squeezers’ (either rollers or shoes) circulating in such a way as to push material from one end of the tube to the other. This moves fluids smoothly and without the kind of mechanical battering other pumps usually give the pumped material.

By their very nature, peristaltic pumps are most suited to applications where a delicate substance is to be pumped, where there are hygiene issues, or where materials are ‘dosed’ as part of a process.

The advantages of a squeezing motion on delicate materials are obvious. If you don’t want your products mashed completely ‘go peristaltic’ rather than risk damage by the shear forces imparted by ‘conventional’ pumps.

Hygiene is another benefit. As the only part of the pump in contact with the liquid is the tube, cleaning is simple — you just change the tube. And you can be sure there is no product lurking in corners causing contamination problems; cleaning in place is relatively simple.

As the liquid is pumped through a squashable tube there are no corners or ‘traps’ where material can collect — a process which can block pumps fast if liquids containing solid materials are involved.

Another feature which makes peristaltic pumps attractive to users is that of reduced maintenance costs and associated downtime. As the only moving part touching the fluid being pumped is the hose, this is the only component that is liable to corrosion/erosion. And, unlike most other pumps, which generally need to be taken off-site for maintenance, they are relatively simple to change in situ.

The cost implications are obvious in terms of parts and downtime.

According to Richard Green of peristaltic pump manufacturer Watson-Marlow: ‘Peristaltic pumps are ideal for moving fluids containing particulate material or even lumps of matter. For example, if you are filling pies with a mixture of liquid and fruit chunks, the end product is much more appealing tothe user if those chunks of fruit remain intact.’

Stephan Menzer of Verderflex, another manufacturer, added: ‘And lumps of material can be quite large — our pumps can move liquids containing solids up to a quarter of the bore size with little problem; something a diaphragm or screw pump, for example, would be unable to do.’

This relatively gentle treatment also means peristaltic pumps are finding applications moving fluids that are sensitive to shear forces in the pharmaceutical industry — liquids containing delicate cells can be pumped without cell damage.

By the same token, the pumps are also suited for pumping abrasive mixtures, such as those found in mining and mineral extraction. Finnish company Larox Flowsys’ LPP pump uses a mechanism which only compresses the hose once per revolution, and incorporates a sensor to detect hose wear. This, the company said, reduces the risk of over-compression, which can lead to hose splitting.

The company claims that LPP can pump virtually anything, being an ideal replacement for more expensive centrifugal units in mining and chemical industries, where seals and glands can cause dilution or leakage problems. A single LPP pump, set for high-pressure capability, can even replace a series of staged centrifugal pumps, said the company.

Peristaltic pumps also have advantages for dosing purposes due to the linear relationship between the pump speed and liquid flow rate. For dosing this is a big advantage — the control scheme is simplified greatly if you don’t need to apply an algorithm to correct for flow rate/pump speed nonlinearities.

These features make peristaltic pumps attractive for many processes, and their use has grown tremendously over the last few years.

Mike Heap of Verder UKclaimed the company’s turnover figures for peristaltic pumps have doubled over the last four years. He said: ‘I see that trend continuing, especially in dosing applications, as users are realising peristaltic pumps are simple, reliable and cheap compared to other pumps, especially for high viscosity, two-phase and abrasive materials.’

As if to validate Heap’s viewpoint, a recent report on the European positive displacement pumps market showed peristaltic pumps were responsible for 6.8 per cent of revenues in this market for 2004.

The report, by market analyst Frost & Sullivan, was more cautious as to the future growth of the market however, predicting slow but steady growth over the next six years; it says ‘greater demand for higher performance and specification, specifically relating to hygiene, is anticipated to be the major contributing factor to market growth’.

So there you have it — from the deepest depths of your bowels comes the model for these pumps, which are ideal for applications with tricky fluids or fluids with particles.