The basic principle of the peristaltic pump traces back to human “peristalsis” (a term referring to the alternating contraction and relaxation of muscles around a tube to force the contents through it).
An elastomeric tube is squeezed along its length by rollers or shoes that pushes the fluid contained within. The tube’s restitution after squeezing produces a vacuum that draws fluid continuously into the tube. This creates a gentle pumping action that doesn’t cause any damage to the product.
Contamination is avoided because the product is contained in the tube (the only pump part that comes in contact with the pumped fluid).
The peristaltic pump is a very versatile pump. It is a self-priming, positive displacement pump, able to deliver relatively high pressure, pass solids and transport corrosive and abrasive media. But selecting the right one for the job is important.
There are basically two types of peristaltic pumps: low-pressure low-flow pumps are commonly used in laboratories or for low-flow dosing or metering applications; and larger industrial-class pumps handle higher flows and can generate higher pressures. This article will concentrate on industrial pumps.
There are a number of parameters that need to be considered when selection the right peristaltic pump for a particular application. These are: duty cycle; flow; pressure; suction conditions; the correct hose (tube) for the pumped media; temperature limitations; cleaning requirements (CIP or SIP); the effect of possible contamination; and maintenance aspects.
Duty cycle: It is important to consider the duty cycle when selecting a pump. If the pump has to operate on a continuous basis (for hours on end), the speed must be reduced to prevent a shortened hose life. Good manufacturers will have performance curves that detail what part of the pump curve can be treated as continuous duty and what part should only be used intermittently.
Flow: Most performance curves are based on water-like substances and flooded suction. When dealing with more viscous fluids or high suction lifts, this will need to be factored into sizing the pump for the correct flow inclusive of de-rating speeds for these aspects. Good manufacturers will have de-rating charts for increased viscosities.
Pressure: Not all peristaltic pumps are created equal. The better designs will enable pumps to be applied on applications with pressures to 15bar. This requires hoses rated to these pressures and rollers, rotors and other components designed for these high pressures. Manufacturers will detail which models are rated to these pressures and what the flow and speed limitations are.
Suction conditions: Peristaltic pumps are capable of high suction lifts, but any peristaltic pump selected for a duty requiring a suction lift should have a pulsation dampener fitted. Also, if the viscosity exceeds 2000cPs, the suction line diameter should be increased. Another good “rule of thumb” is to keep the suction line as short as possible.
Correct hose: Selecting the right hose material for an application is critical in achieving extended hose life, and also critical if hygiene is required for transporting food or pharmaceutical products.
Typical hose materials include:
• Natural isoprene for abrasive fluids or moderately aggressive fluids: This is the most common hose material and is often available with various pressure ratings.
• Natural isoprene with substrate for food products: These hoses will have FDA approval numbers and be suitable for the transportation of food grade products. Should be able to sterilise with hot water and a diluted cleaning agent and also steam cleaning.
• EPDM hoses are suitable for a wide range of chemicals.
• NBR hoses are suitable for oils, greases, hydrocarbons and various solvents.
• NBR for food products: These hoses will also have FDA approval numbers and are suitable for oils and fats. Should be able to sterilise with hot water and a diluted cleaning agent and also steam cleaning.
• Hypalon hose is suitable for a range of concentrated acids and alkaline liquids.
• Pharmed thermoplastic elastomer base polypropylene is a limited-use hose for liquid pharmaceutical products. Will have FDA approval numbers and should be capable of sterilisation with hot water.
Temperature limitations: Good manufacturers will have temperature limitations in their hose selection guides. Ensure that operating temperatures do not exceed the temperature rating of the hose.
Cleaning requirements: Where hoses need to be cleaned in place (CIP) or sterilised in place (SIP), the hose selection needs to allow for this. Also, it is useful if the pump rollers (or shoes) are easy to retract to allow full flushing of the hose. Automated retraction makes this process even simpler.
Contamination: If product contamination is an issue, selecting a peristaltic pump with rollers instead of shoes is best. The pumps with shoes will need their casings to be full of lubricating fluid. When the hose starts to fail, the lubricating fluid may contaminate the product being pumped.
Maintenance: A peristaltic pump does not require a lot of maintenance apart from hose changes. It is in this area that pumps with certain designs and certain features can save time and money. A pump with rollers does not have a casing full of lubricant, so does not need this oil drained and then refilled again after the hose change, and no clean-up of the mess.
A pump with a roller design does not need casing lubricant
Early leak detection is another feature that reduces maintenance costs. This is only possible for pumps with the roller-on-bearing design. Because casings are not filled with fluid, a simple “plunger-in-cup” designed leak detector can pick up hose failures early, minimising product loss and mess.
A remotely operated retractable roller will also reduce maintenance costs by extending hose life. Between pumping cycles (over night, weekends, holiday periods), the roller can be retracted to remove pressure on the hose during these periods, greatly extending hose life.
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