Stemming from the analogous function of the human intestine, the term ‘peristalsis’ refers to the contraction and distension of the intestinal walls, which generates a wave motion, and this forms the basis of a Peristaltic Pump operation. Peristaltic pumps are widely used in any industry to transfer liquids from a feed container to adelivery point providing accurate dosage of chemicals, as part of a larger application. A typical application is the transfer of a detergent or a rinse aid from the chemical drum to the dishwasher. The dimensions and hydraulic characteristics of peristaltic pumps vary widely, ranging from a few ml per hour to thousands of l/h. Depending on these characteristics, the dimensions and materials used for these pumps can be very different, ranging from plastic to steel.
Two or more rollers or ‘shoes’ positioned at 180° (or less, depending on the number of rollers) pinch the tube (often referred to as the peristaltic tube) creating a vacuum that draws liquid from the feed container through an intake tube into the peristaltic tube. Here, it travels under pressure to the delivery tube and then to the delivery point. The first roller serves to squeeze the tube, creating the vacuum, while the successive one guarantees the seal between the suction and discharge sides, preventing any fluid return, As the compressed tube is released and resumes its normal shape, another vacuum draws fresh liquid into the tube ready for the next cycle. This is also known as ‘restitution’.
At the heart of the mechanism lies the peristaltic tube, which must be flexible enough to pinch without requiring excessive force but must simultaneously be elastic enough to recover its initial shape shortly after the rollers pass. Without these characteristics, the tube will not be able to create the necessary vacuum to aspirate the fluid at the desired amount.
The fluid flow that a peristaltic pump can generate depends on the following factors:
• The inner diameter of the peristaltic tube, or rather the section through which the fluid can flow
• The length of the ‘useful’ section of the tube on which the roller passes
• The rotation speed of the rollers by the motor that drives them
• The viscosity of the liquid being pumped and the back pressure of the system
The first two factors determine the volume of transportable fluid at each complete revolution of the roller. The higher the useful length of the pinched pipe, the greater the flow rate of the fluid at the same number of revolutions. Similarly, the greater the rotational speed, the greater the flow rate for the same volume of fluid contained in the useful portion of the tube.
The fundamental parts of a peristaltic pump are as follow:
• DC or AC motor, which has enough torque to compress the peristaltic tube at the required speed to obtain the required flow rate
• Peristaltic ‘head’, generally made of plastic, which provides housing and contrast for the peristaltic tube and rollers
• Roller bearings, generally plastic, which are driven by the motor and compress the peristaltic tube
• An electronic circuit that adjusts the speed of rotation and motor activation times
The main advantages of using peristaltic pumps are:
- No contact between the mechanical parts of the pump and the fluid. The fluid only has contact with the inner wall of the peristaltic tube and the suction and discharge tubes
- Possible to transfer abrasive or solid particles (within certain limits) without affecting the flow rate
- Can pump viscous liquids and liquids that are prone to gassing off
- Complete reversibility of the fluid direction by simply reversing the rotational direction
- Possible to work dry without damaging the pump itself
- Easy priming without the need for air purging
- Repeatability and consistency of dosage
- Simplicity of maintenance and quiet operation
Regarding the maintenance of the pumps themselves, no special action is required except for the replacement of the peristaltic tube either periodically or when they break.
The useful life of a peristaltic tube depends on many factors, such as the condition of use, the chemical to be dosed, its viscosity and its pH (not forgetting that tubes erode from the inside out), the speed of rotation of the motor, the length of the delivery tube and back pressure, the operating temperature and the tube material.
As we saw earlier, the tubing needs to be elastomeric to maintain the circular cross section after millions of cycles of compression and release. The most popular elastomers for pump tubing are nitrile (NBR), Hypalon, Viton, silicone, PVC, EPDM, EPDM + polypropylene (as in Santoprene), polyurethane and natural rubber. Of these materials, natural rubber has the best fatigue resistance, and EPDM and Hypalon have the best chemical compatibility.
In recent years, even peristaltic pumps have become increasingly complex, integrating control systems that can be regulated via electronics, with speeds and engine activation times managed to ensure accurate and repetitive dosages.
The future challenge, which is already underway, relates to the ‘digitalisation’ of the pumps, or the use of suitable integrated pump sensors, through which constant monitoring of key parameters can be achieved, even remotely. The information regarding parameters such as the amount of metered liquid, flow rate, pump activations, means end users are able to control the consumption of chemicals or to detect any deterioration in performance and to react, for example, by varying the dosing parameters or intervening with maintenance or moving to ‘predictive’ maintenance that will lower the cost of ownership of the equipment.
This is where the industry's major players are moving to meet the challenge of delivering products that fulfil the end user's needs.
About the Author:Andrea Lavarini is Product Manager at SEKO SpA. He has been Product Manager for two years now for both OEM peristaltic pumps and dosing systems designed for ware-washing at SEKO S.p.A., a worldwide leading company operating in more than 120 countries, that has been manufacturing metering pumps and dosing systems for over 40 years.