COMPETITIVE AND BUSINESS INTELLIGENCE

PERFORMANCE & INNOVATION - NEWSLETTER, Issue 17

25-APR-2007

Ongoing update of what's news from Regulatory Boards
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Understanding the mechanism of Peristaltic Pumps

Peristaltic pumps use rotating rollers pressed against special flexible tubing to create a pressurized flow. The media is moved through the tube with each rotating motion. A rotor with a number of 'rollers', 'shoes' or 'wipers' attached to the external circumference compresses the flexible tube. As the rotor turns, the part of tube under compression closes (or 'occludes') thus forcing the fluid to be pumped to move through the tube. Additionally, as the tube opens to its natural state after the passing of the cam ('restitution') fluid flow is induced to the pump. This process is called peristaltism and is used in many biological systems such as the gastrointestinal tract.

The tubing in peristaltic pumps is often replaceable or disposable. Some manufacturers will give standard choices of materials for tubing or hose including Autoprene, Silicon, and other polymer materials. Silicone rubber is often used in laboratory applications when abrasive and caustic fluids or gases are used. The inner diameter of the tubing is a consideration when working with highly viscous fluids since it will directly affect the flow rate.

Tube life will be affected by factors such as temperature, back pressure, pump speed, and chemical compatibility of the tube carrying a pumped medium.

The individual components of peristaltic pumps include a pump head, drive, and tubing. Peristaltic pumps are also referred to as flexible member pumps, flexible tube pumps, dispensing pumps, or dosing pumps.

Flow control for Peristaltic Pump

The principal limitation of the peristaltic pump is that its fluid delivery is not absolutely uniform. There is a trade-off with the tubing diameter between uniform delivery and lifetime and convenience. For some applications, this is not a problem but for others it is.

Operating a peristaltic pump without any type of flow monitoring or flow control typically has the following disadvantages:

Pumps require frequent calibration to correlate flow rate and pump speed
Flow can vary as much as 10-30 percent due to variations in operating pressures
Process optimization is difficult since flow for each process step is uncontrolled
Low- or no-low-flow conditions may occur due to pump or other failures leading to damage or scrapped wafers

One of the possible solutions consists of implementing a closed-loop flow control system that may increase overall tool uptime. Tools operators will no longer need to calibrate the peristaltic pumps, because the controller automatically increases or decreases the pump speed to meet the flow rate requirements.

A closed-loop flow control system will maintain a constant flow rate at a desired flow set-point by increasing or decreasing the pumping speed of the peristaltic pump.

For closed-loop flow control application , the flowmeter must respond quickly to flow changes and must not be affected by the presence of bubbles in the fluid stream.

Implementing a closed-loop flow control system can have the following advantages:

Eliminates the need for frequent pump calibrations and increases tool uptime
Maintaining a constant flow of fluid at the desired flow rate allows for process optimization during each step by controlling fluid flow
Provide process alarms for low or no-low flow conditions
Minimizes fluid waste

Advantage

The advantages of peristaltic pumps are that the components of the pump may be chosen when the integrity of the media is a requirement of the application since the fluid type does not contact any internal parts. Seals and valves are not needed as in other pumps. Many peristaltic pumps come with wash down capabilities.

Cleanness: One of the main advantages of the Peristaltic Pump is cleanliness. Because the only part of the pump in contact with the fluid being pumped is the interior of the tube, it is easy to sterilise and clean the inside surfaces of the pump.

Cost-effective: Furthermore, since there are no moving parts in contact with the fluid, peristaltic pumps are inexpensive to manufacture. Their lack of valves, seals and glands makes them comparatively inexpensive to maintain, and the use of a hose or tube makes for a relatively low-cost maintenance item compared to other pump types.

Peristaltic pumps are also reversible and can be flushed to clean out the tubing or hose.

The chief advantage of the peristaltic pump is that it is reliable, rugged and inexpensive by comparison to the alternatives. The peristaltic pump has a completely disposable fluid path and is self-priming.

Disadvantage

The principal disadvantage is that there is a pause in the flow as one roller lifts off the tube and another begins pushing on the tube. The practical effect of the pause (often called a “pulse” for when the liquid does flow) can be minimized by using very small diameter tubing or by increasing the number of roller. Each roller push then moves only a small volume of liquid.

Application

Peristaltic pumps are typically used to pump clean or sterile fluids because the pump cannot contaminate the fluid, or to pump aggressive fluids because the fluid cannot contaminate the pump. Some common applications include pumping aggressive chemicals, high solids slurries and other materials where isolation of the product from the environment, and the environment from the product, are critical.

Higher pressure peristaltic pumps which can typically operate against up to 16 bar, typically use shoes and have casings filled with lubricant to prevent abrasion of the exterior of the pump tube and to aid in the dissipation of heat. High pressure peristaltic pumps typically use reinforced tubes, often called 'hoses', and the class of pump is often called a 'hose pump'.

Lower pressure peristaltic pumps typically have dry casings and use rollers. They typically use non-reinforced tubing, and the class of pump is sometimes called a 'tube pump' or 'tubing pump'.

Peristaltic pumps are used in pharmaceutical , chemical, and food and beverage applications. They have a variety of medical applications. They can be used to add nutrients to blood, to force blood through filters to clean it, or to move blood through the body and lungs during open heart surgery.

Case study: Flexicon PF6 – Single Unit Filler Utilities PF6 is fully programmable, peristaltic filler (dispenser) which as a single unit can dispense liquids of volumes from 0.2 ml up to 9999 ml.
	PF6 has been specially developed for laboratory applications where extreme flexibility, frequent product changes and absolute accuracy are key parameters. PF6 is programmable for a number of different applications, and full production documentation can be printed, if a printer is connected.
	Peristaltic Principle in PF6 PF6 operates with a peristaltic dispenser head (tube pump), where the liquid only comes into contact with the flexible tube, the tube connections and the filling needle. The tubes are usually made of silicone, but other materials can also be used.
The dispenser head is designed in such a way that sterilized tubes can be assembled in the head without affecting the sterility. Flexicon tubes are made of raw materials medically approved by the FDA. The tubes are delivered in sealed packages and are provided with a batch number which makes it possible to trace the tubes all the way back to the raw material source. For this reason PF6 is specially suited for aseptic applications and for preventing cross contamination. The dispenser head is self-priming, and the dispenser head itself can stand to be run dry. It is recommended not to let the dispenser head be run dry for a long period with connected tubes, since this will lead to particle release. In this cas a peristaltic dispenser head is not suitable for viscous products.
References First Ten Anstromg , Comparison of External Pumps, 2004-04-12, p.1 Entegris , Flow Control for Peristaltic Pumps on CMP process Tools, p.1, 2 Spectrum Chromatography , Peristaltic Pump Tubing, URL : http://www.lplc.com/tubing/pumptube.html, consulted on 18 Nov 2006 Introduction to Perilstatic Pump , 20 October 2004, www.firsttenangstroms.com http://www.clarksol.com/html/tubing.htm PF6 Oprator’s Manual , http://www.flexiconamerica.com/
About the author Kossi Molley is graduated in Biology and in Chemical Engineering. He is Principal Consultant for MIVADO.BIZ, a Competitive and Business Intelligence Branch of MIVADO Global Performance, inc.
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