By Rick Balek
Peristaltic pumps have become the fastest growing positive displacement (PD) pumps in the chemical metering and sludge pumping market, now competing strongly with diaphragm metering and progressing cavity pumps. As recently as 10 years ago, peristaltic pumps, however, were seldom found in North American water and wastewater treatment applications. Hose and tubing design as well as pump capacity limited their use in both low and high flow applications.
Over the last decade, tens of thousands of peristaltic pumps have been installed in environmental pumping applications. This newly found popularity can be attributed to recent advancements in hose and tubing materials and the integration of sophisticated electronics including microprocessor controlled robotic grade drives. Now this most simple of pumping technologies has become a leading problem-solver for difficult pumping applications in municipal and industrial water and wastewater treatment.
Design Advancements
A century ago a peristaltic hose pump consisted of a steel "wheel" with three cylinders squeezing a woven fabric hose. The pump was mounted on a cast iron base, which was bolted to a wooden wagon drawn by a horse. The first commercially produced hose pumps were introduced in 1956.
Today's heavy-duty peristaltic pumps are designed to operate 24/7/365. They combine precise PD flow and low maintenance requirements with the ability to handle extremely abrasive and aggressive fluids, which quickly wear out more traditional PD pump technologies.
Integrating peristaltic pumps with SCADA (Supervisory Control and Data Acquisition) systems and PLCs (Programmable Logic Controllers) is now as simple as connecting wires to pump terminal strips.
Improvements in electronics allow the user to manually "trim" the pump speed while operating off of a remote input signal. And, enhancements in output signals eliminate the need for separate signal control boxes.
Improvements in direct coupled (not closed coupled) bearing design and heavier duty casing designs have led to the next generation of hose pumps. These pumps reduce the total number of parts by one-third, while providing even better performance and reliability.
In addition, capacities have increased to 350 gpm while the footprint of dual headed hose pumps have reduced in size by almost half, truly a small footprint for such high flow. Some tubing pump models also allow the "stacking" of multiple pump heads to pump the same or different chemicals to different locations using the same drive.
Tubes and Hoses
The latest hose design is triple-layered with an extruded inner layer surrounded by a four-layer reinforcement cord matrix and a smooth, machined/ground outer layer. Advancements in tubing materials are allowing pressures to reach 100 psi with continuous service. This is a perfect match for injecting small amounts of chemical into a pressurized water line.
Unlike tubing pumps which use rollers to occlude the tube, most hose pump technologies incorporate a sliding shoe with profiled or crescent shaped leading and trailing edges that give a gradual lead-in and termination to each hose occlusion. This arrangement prevents an abrupt imposition and release of pressure and significantly increases hose life.
A specially developed lubricant contained within the pump head eliminates external hose wear as a result of contact with the sliding shoes. Improved pump head design allows for "built in" leak detection upon a rare tubing failure while keeping the chemical safely contained inside a sealed head.
A hose and tubing pump rotor is not located in the pumped-product zone. The inside of the hose or tube is the only part of the pump that touches the pumped product. Unlike other PD pump technologies which have tight tolerances or interference fits of the wetted rotating parts, peristaltic pumps keep extremely corrosive chemicals such as ferric chloride and sodium hypochlorite, and also abrasive fluids such as various sludges, lime and powdered activated carbon (PAC) slurries from wearing the pump's rotating components.
In fact, a properly selected heavy-duty pump hose does not fail due to abrasive wear; it fails due to the total number of occlusions it sees in the course of its lifetime. Regardless of product, whether it is abrasive grit in a grit removal system, thickened sludge, lime slurry or nonabrasive polymer, the hose life is the same. This is mainly due to the hose pump design being a "truly" positive displacement pump that does not incur slip.
Hose replacement has never been easier or faster. Changing tubing takes only seconds.
Non-Contact Pumping
Peristaltic pumps are contamination-free. Since the liquid is completely contained within the tube/hose, there is no contamination of the pump or pumped fluid.
All peristaltic pumps are inherently "seal-less" pumps. There is no packing or mechanical seal that comes in contact with the pumped fluid. This is especially convenient from a housekeeping perspective by eliminating leaks. It is also an important safety feature when pumping dangerous chemicals because the design eliminates potential emissions leaking from a seal box.
Pumping mechanics
Most peristaltic pumps found in the environmental marketplace incorporate two rollers or shoes that occlude the hose. Twice per revolution of the pump's rotor, for a very brief moment, both the rollers/shoes squeeze the tube/hose at the 6 and 12 o'clock positions. At all times, at least one roller/shoe is completely occluding the tube/hose.
When the pump is turned off, the tube/hose is completely occluded such that the pump becomes a true in line check valve. Not even air in the line can pass through the occluded tube/hose.
All peristaltic pumps have the ability to run dry without damage. This prevents the pump from vapor locking and causing catastrophic failure to the pump itself. This is especially true when pumping liquids that contain air such as in dissolved air floatation sludge (DAF) or when a chemical emits an off gas such as with sodium hypochlorite.
Pumping Performance
There is no better self-priming pump design than a peristaltic pump, which when pumping water, can achieve up to a 31-foot suction lift.
A hose pump is a 100% volumetrically efficient pump technology in which flow is independent of pressure. Therefore, hose pump performance curves are not really "curves" at all, but are actually straight lines throughout the pressure range.
A major advantage of tubing pumps is the ability for up to seven different diameters of tubing to be used with the same pump head. This means the turn down capability of a single tubing pump can be as much as 27,500:1, achieved by using the largest diameter tube with the pump running at full speed and then switching to the smallest diameter tube while operating at minimum speed. It is literally like having seven different sized pumps in one small unit.
About the Author: Rick Balek has 17 years sales management experience with pump manufacturers and currently is national sales manager at Watson-Marlow Bredel Pumps.
