MARK ATKINSON
Technical support engineer
Watson-Marlow Inc.
Wilmington, Mass.
Peristaltic pumps move fluids through tubes by squeezing the tubes with moving sets of rollers. Pump components include tubing and the pumphead, which has a stationary track and a rotating spring-loaded roller assembly known as the rotor. Tubing is held stationary in the track while rollers squeeze the tubing closed, forcing fluid ahead of the rollers. When a closed tube reopens, a partial vacuum draws fluid down its length. As fluid moves through the tubing, the next roller traps more fluid and the process repeats.
One or more rollers squeeze the tubing closed at all times. This prevents back flow and siphoning and eliminates the need for a check valve. Peristaltic pumping is useful when sterility and low shear are required or when the fluid contains solids. Maintenance costs are low because the tubing is inexpensive and easily changed. Applications for peristaltic pumps come from pharmaceutics, pollution control, food processing, and beverage dispensing.
Because tubing is the main pumping element and the sole pump component in contact with the fluid, proper tube selection is important. For example, tube elasticity determines its pressure range, while flex characteristics affect tube-pumping life. Similarly, tube dimensions affect flow properties, with bore size determining flow rate, and wall thickness affecting pump efficiency.
Each application may have several factors that drive tubing selection. Among these are chemical compatibility, flow rate, pressure, and duty cycle. The first category to check is chemical compatibility. Charts of this information are available, but when data doesn’t exist, simple immersion tests may determine if a material is compatible. Immerse a short length of tubing in a closed container of fluid for 48 hr then examine for signs of attack, swelling, embrittlement, or other deterioration. Tubing materials can handle most acids and caustics and other aggressive chemicals such as sodium hypochlorite.
Tube life span is also important when selecting a material. Although silicone and PVC last hundreds of hours, thermoplastic vulcanizate (TPV) lasts thousands of hours. Other important material-selection factors are permeability of the material, whether it may be autoclaved or sterilized, and whether it meets medical and food-quality standards.
TPV tubing is the most common material for chemical and industrial applications. It has the longest life span, resists a broad range of chemicals, has good mechanical properties, and is relatively inexpensive. When TPV is not an option, common alternatives are silicone and PVC. Silicone is popular in pharmaceutical and sanitary applications. Neoprene and fluoroelastomer tubing materials are another choice. They resist some chemicals that TPV doesn’t. However, neither lasts as long as TPV.
After selecting a material, tube size must be determined based on flow rate and pressure requirements. Size affects tube pumping life, so the duty cycle must be taken into account. Bore size and pumphead determine displacement per rotor revolution and rotor rpm. Flow rate is generally proportional to rotor rpm. Rotor rpm also affects tube life because it governs how often the rollers contact the tubes. For continuous-duty applications and when long tube life is the goal, choose a bore that lets you run the pumphead at a lower rotor rpm. Tube life, however, is not important in many applications, such as sanitary environments, where contamination must be avoided. In these cases tubing is discarded after each use, so it’s fine to use smaller-bore tubing with rotors running at high speeds.
When maximum flow is the goal, choose large-bore tubing with rotors running at high speeds. Tubes with a bore-to-wall thickness ratio of 3:1 or lower usually provide the best suction and pressure performance.
Fluid viscosity must also be considered. As viscosity increases frictional losses also increase, which increases suction requirements. This means that viscous fluids cause flow rates to drop. Bear in mind the 3:1 maximum bore-to-wall-thickness ratio and choose a pump that can use the largest bore tubing possible. When pumping viscous fluids, lower rotor rpms are necessary to allow time for the tubing to rebound after being squeezed. This often means choosing a larger pump for viscous fluids.
When using new tubing, it’s important to realize that it requires a break-in period before flow rates stabilize. Flow rates decrease during the first few days of use before reaching steady state. When applications require precise flow, calibrate pumps after tubes undergo at least a one-hour running- in period.
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