Peristaltic Pumps Offer Custom Fluid Solutions
Peristaltic pump selection tips help solve fluid transfer problems
Previously in Pumps and Systems magazine
Introduction
The most important advantage of peristaltic pumps is their use of tubing as the pump chamber. The fluid is inside the tubing and does not contaminate the pump, nor does the tubing contaminate the fluid. Cleanup requires only a change of tubing. This saves costly breakdown and maintenance time.
Figure 1
Peristaltic pumps transfer fluids successfully in industries such as food processing, pharmaceutical manufacturing, and chemical processing, as well as in laboratory research, agriculture, and water treatment. Specific applications include dispensing, sampling, metering, filtering, fermentation (Figure 1), and general fluid transfer. In general, if the fluid flows, a peristaltic pump can pump it. In many situations, peristaltic pumps are ideal for pumping abrasive as well as aggressive fluids.
The key to a successful peristaltic pump application is to optimize the selection of pump head, tubing, and drive. Application requirements thus dictate the features of pump products being selected for purchase. Many different types and shapes of pump heads, tubing materials, and tubing sizes are available to meet specialized needs. Each different combination provides a special fluid transfer capability.
Peristaltic Principle
Peristaltic pumps operate on a simple principle. The alternating pattern of squeezing and releasing the tubing moves the fluid through the pump.
As a roller passes over the tubing, it is first occluded (squeezed), then released. The progression of this squeezed area forces fluid to move in front of the roller. The tubing behind the rollers recovers its shape, creates a vacuum, and drawing fluid in behind it. As the roller moves faster, vacuum pockets are created more quickly and the fluid moving through the system picks up speed. The rollers act as check valves to prevent siphoning or loss of prime.
The distance between the rollers creates a "pillow" of fluid. This volume is specific to the ID of the tubing and the geometry of the rotor. Flow rate is determined by multiplying pump head speed by the size of the pillow by the number of pillows per revolution. This pillow volume stays very constant except with highly viscous fluids. Among pumps with the same diameter of rotor, pumps with large pillows will deliver higher volumes of fluid per revolution. A greater degree of pulsation exists with these higher flow rates, not unlike the pumping profile of a diaphragm pump. Pumps with small pillows deliver small volumes of fluid per revolution. With many of these small pillows developing in rapid succession, the motion of the fluid appears smoother, similar to that seen in gear pumps.
Advantages and Limitations
Isolation of the fluid within the tubing, flexibility in quick tubing changes, reversibility of flow, and ability to run dry are the primary competitive features of the peristaltic pump. See Table 1 for a complete listing of feature benefits.
Some potential limitations are service life, chemical compatibility, and pressure. Consider these limits when evaluating tubing options.
Table 1:
Feature Benefits of Peristaltic Pumps
- Fluid does not contact any part of the pump except the tubing.
- No Seals to leak
- No valves to clog or wear
- Self priming (up to 30 ft. (8.8 m) in some models)
- Pumps liquids, gases, solids, or mixed phases
- Can use one continuous piece of tubing from inlet to outlet
- Some tubing materials can be easily sterilized.
- Easily cleaned at the end of the day, saves time, no corners or fluid holes to collect material or bacteria
- Easy and fast product change—simply change tubing—no cross-contamination
- Operates in any position (orientation insensitive).
- Wide range of flow rates
- Many types of tubing are available.
- Wide selection of drives/motors
- Easily repaired
- Fewer parts to inventory
- Multiple channel applications
Major Markets/Applications
Laboratory Research and Development
Tubing pumps offer excellent repeatability in low volume dispensing and metering applications. The valveless design eliminates clogging and siphoning of fluid under most conditions. The modular design of the system allows one pump drive to be used for many applications. Common laboratory research and development applications include cell tissue transfer, destaining, perfusion, liquid chromatography, and transfer of acid and base solutions.
Pharmaceutical Manufacturing
The non-contaminating and non-invasive design of tubing pumps makes them very popular. Sterilizing the USP class VI tubing is easy. A wide variety of drive options fit the pump into many different applications. Pumping nutrients or pH adjusters for fermentation, filtration of media, and dispensing of cosmetics are just a few of the many applications for this market.
General Industry
Peristaltic pumps offer predictable service in continuous-duty applications while solving many tough pumping problems. These pumps will handle waste water, suspended solids, harsh chemicals, and other challenging fluids up to 45 L/min (12 GPM). Many drive designs handle rugged plant environments. The self-priming, run dry capabilities of the pump prevent catastrophic failures in many process systems. A consistent service schedule prevents pump downtime. Some popular applications include the pumping of dyes, etching chemicals, printing inks, laundry chemicals, lapping fluids, and lubricating oils.
Commercial Food Processing
A wide range of tubing materials is available to meet USP, FDA, NSF, and 3A requirements. Tubing pumps easily handle viscous fluids and small or soft particulates without clogging, and fast pump cleanups make them a particularly attractive labor saving tool. Popular applications include dispensing juices, yogurts, condiments, molasses, and other food products on production lines or into individual servings.
Customizing for a Specific Application
Peristaltic pumps can be adapted to specific applications and marketplaces by considering the primary features and benefits required by the user. Tubing pumps consist of three major components (pump head, tubing, drive), all of which are interchangeable in the most popular models. To make the best choices, evaluate each component individually.
When evaluating pump components, it is important to know the critical limits of the fluid transfer application. Understand the flow rate, type of fluid, service life requirements, pressure, viscosity of the fluid, size of any particulates present, shear rate of fluid, and degree of pulsation acceptable.
Pump Head Selection
The selection of a pump head for any application is a critical step. The use of the proper pump head eliminates unnecessary problems that can develop during setup and operation. The following are features of the pump head to consider:
Flow Range
Flow range requirements dictate the size of tubing and ultimately the type of pump head for a specific application.
Materials of Construction
Chemical-resistant materials of construction and shielded bearings help pumps withstand exposure to aggressive fluids or rugged environments. Pump heads with high-performance plastic bodies provide a lightweight, chemically resistant product at an attractive price.
Number of Rollers
Fewer rollers on a given size rotor allow quicker fluid transfer, but at greater pulsation. More rollers reduce pulsation and improve dispensing accuracy but decrease flow rate and tubing life. Vacuum and pressure performance of a pump improve as the number of rollers occluding the tubing at one time increases.
Ease of Loading
Figure 2
The method of tube loading greatly affects user satisfaction with the pump. Pharmaceutical, food service, and printing applications frequently require several tubing changes over the course of a shift or day.
In view of time saved during cleanup or changeovers, easy-loading pumps translate into hundreds of dollars in labor savings (Figure 2).
Number of Tubing Sizes Accepted
A pump head design that accepts only one tubing size can maximize the performance (e.g., pressure, vacuum, and flex life) for that one tubing size. A pump head designed to handle a range of tubing sizes has some averaged design features. This gives the user greater flexibility in the range of flow rates with one pump head.
Fixed or Variable Occlusion
How much control of occlusion is enough to adapt the pump? Fixed occlusion pumps optimize the performance for repeated uses and reduce the chance of operator error. When operated with precision extruded tubing, they deliver excellent repeatable service.
Adjustable occlusion pumps allow the tubing to be over occluded to facilitate priming or reduced to improve tubing service life in low pressure applications. Minor adjustments in flow rates are possible. This is helpful when synchronizing flow rates in multichannel pump heads.
Stackability
Design a multichannel pump with one drive system. The number of flow channels possible depends upon the additional torque required for each tube channel up to the limits of the drive.
Select from a stack of individual pump heads or a more compact cartridge pump head. Individual pump heads offer a wider range of performance options.
Cartridge pump heads have become increasingly popular for their small overall package size. Several individual cartridges mount on one pump body. They are ideal for applications requiring very low flow rates and/or synchronous flow.
Specialty Pump Heads
Unique pump head designs meet specific market needs. Pump heads are now available for smooth fluid transfer, fast dispensing volumes, long pump operation, and extraordinary chemical resistance.
These pump heads have special features that enable them to maximize a particular benefit. Smoother fluid transfer is accomplished by combining two flow channels that have offset pulsation. Extraordinary chemical resistance is possible by using PTFE tubing. A special tube set is matched with a special pump head to provide chemical-resistant or high-purity fluid transfer.
Tubing Selection
Figure 3
Proper tubing selection is as important as selecting the optimal pump head. General-purpose pump heads accept a wide range of nominal tubing sizes. Specialty pumps usually require special tubing profiles, tubing sets with collars or special fittings.
Many types of flexible tubing materials are available on the market at a wide range of prices (Figure 3). Only a few of these materials are suitable for pump tubing. Similar looking materials can deliver vastly different pump performance characteristics. A good pump tubing possesses good tensile and compression capabilities. Most pump manufacturers offer a range of prequalified tubing for use in their pumps. These tubing formulations offer consistent flex life and flow rate.
Select tubing materials based on the requirements of the application and the preferences of the operator. Pay special attention to the following criteria:
Chemical Compatibility
When considering tubing pumps for pumping aggressive fluids, it is critical to select the correct tubing material. The wrong tubing can lead to a hazardous situation with potential to damage equipment and harm people. Consult the pump and tubing manufacturer's chemical compatibility charts for every new application. With new or unrated chemicals, test the tubing in the fluid before testing in the pump. Immerse a short section of tubing in the fluid. Check for changes in tubing size, color, weight, and strength.
If possible, test the tubing in the pump before extended use. The flexing of the tubing during the occlusion process works the chemical into the tubing wall and accelerates any decomposition that may take place due to weakness in the tubing material.
Non-contamination
Isolation of the fluid is essential for many applications in the laboratory, pharmaceutical, and biotechnology marketplaces. Countless studies are available on the inertness of silicone tubing and basic silica material with various biological materials. Several other tubing materials are available as well that meet USP Class VI criteria. Some new pumps use special tube sets constructed from inert PTFE tubing.
NOTE: Small particles of pump tubing materials will break off into the fluid after extended pump operation. Industry experts refer to this as "spallation."
In some applications these particles are visible to the naked eye. To minimize such degradation, one should change the tubing more frequently.
Flex Life in the Pump Head
Different tubing materials have differing abilities to withstand the repeated squeezing action of the rollers. In general, each tube size, tube material, pump head style, and operating speed in combination has its own life characteristics. Service life, or flex life in the pump, is the primary concern in a new application. Today greater than 5000 hours of operation is commonplace in systems employing thermoplastic elastomer (TPE) materials.
Pump performance is very consistent in a specific application. Maximize the life of a pump system by selecting a tubing material that offers long flex life, using thicker wall tubing, and/or by operating a larger pump at slower speed.
Clarity
Visibility of fluid movement in the tubing is valuable for many applications to confirm pump priming or a run dry condition. Opaque tubing materials can, on the other hand, limit the exposure of light-sensitive fluids.
Durometer
The stiffness of the tubing wall is important in determining the pumpability of a specific tubing. If the tubing is too soft, it collapses. If it is too hard, the pump cannot operate. Durometer (e.g., Shore A, etc.) is the measurement of this physical characteristic for a particular tubing material.
Pressure
Pressure performance is usually a limitation of the pressure handling capabilities of the tubing. Most peristaltic pumps use unsupported tubing, which has pressure limits of 2-3 bar (30-45 psi). Some larger pumps on the market use a supported tubing (braid reinforced) tubing immersed in a lubricating fluid to generate pressures up to 15-20 bar (300 psi).
Drive Selection
Choose a drive motor based on the necessary type of control. Most pump manufacturers offer some degree of interchange between pump heads and drives. This can be very important in the initial stages of pump testing and various types of research.
Fixed Speed Drives.
These represent the simplest method of fluid transfer. The pump system operates at a single constant speed for the entire time of operation. Flow rates can sometimes be changed by changing the tubing size.
Variable Speed Drives.
When flow rate flexibility is crucial, consider a variable speed drive. Pumping the right amount of fluid in the right amount of time can be tricky. Variable speed drives usually provide infinitely variable flow from minimum to maximum speed. Motor speed, turn-down ratios, and percentage speed regulation are key factors to consider in selecting a drive. Reversibility facilitates purging of the suction lines before changing tubing.
Digital Display.
Microprocessor-based control improves the level of pump operation. Tight control of line and load regulation provides stable fluid transfer. Simple calibration of your pump system allows digital display of flow rate and other important setup guidelines.
Remote Control.
Integration of pumps into automated systems for research and process control has become increasingly important. Controllable functions include start or stop, reverse, prime, dispense or copy, and speed. Select from analog (e.g., DC voltage, current, or contact closure) or digital (e.g., RS-232) interfaces.
Summary
Peristaltic pumps offer a high degree of customization to meet the specific requirements of many applications. New developments are constantly expanding the capabilities of this versatile pump design.
For more information, contact Amy Ebelhack, Masterflex Product Manager.