Fluoroelastomer Specialists
According to Pollution Equipment News:
“The simplicity of a peristaltic metering pump makes it a very appealing method for injecting a wide variety of chemicals into water treatment applications. Understanding the variables that result in wear on the pump tube can assist the reader in specifying the most appropriate tubing for the application.
“Peristaltic pump overview
“Commonly called ‘squeeze tube pumps,’ the new generation of peristaltic metering pump is quite different from the low pressure, non-industrial peristaltic pumps most people are familiar with seeing in a hospital setting. These industrial workhorses are now capable of long tube life and high output pressures. Some models also include such features as tube failure detection systems, flow verification sensors, and sophisticated control electronics for connection to SCADA systems. But one of the greatest benefits of a peristaltic pump is its simplicity. Peristaltic pumps utilize a circular pump ‘head’ and simple rotating roller designed to pinch the tubing and gently squeeze the fluid through specially designed tubing. Without valves to clog, metal springs to corrode or ball seats to fail they can effectively pump both fluids and gasses, eliminating the possibility of vapor locking and loss of prime.
Variables to tubing wear
- “Tubing materials – The tubing material must withstand the chemical being injected, return to its original shape after many thousands of occlusions (compressions), and operate at the required system pressure. Specifying the optimum tubing material is critical for a successful application.
“Chemical resistance – Chemical incompatibility will result in a breakdown of the tubing material properties, often manifested as a change in the stiffness of the material, either softening or hardening. In most cases, chemical resistance problems will be apparent within the first few days of use. However, in some cases, the chemical will attack the tubing material slowly over a long period of time, reducing the life of the tube.
Dimensions – The general rule of thumb here is the larger the tube diameter, and the thinner the wall thickness, the shorter the tube life expectancy.
“Material properties – The physical properties of the tubing material will greatly influence not only its suitability for general use in a peristaltic pump, but also the amount of time the tube will last in a particular application. Tubing manufacturers offer a variety of tubing formulations, many of which are suitable for use in peristaltic pumps and many which are not. For example, the peristaltic pump tube must be capable of precisely returning to its original shape many thousands of times after being squeezed by the roller. Many tubing materials lack this memory making them unsatisfactory for peristaltic pump applications. The end user must be cautious when selecting the tubing material for the application. Most pump suppliers will either offer assistance with the tubing selection or offer pre-assembled ‘tube assemblies’ designed specifically for their peristaltic pumps, greatly reducing the possibility of miss-application.
- “System pressure – The pressures acting on the tubing will directly affect the tube’s life. Both the inlet and outlet pressures should be considered and particular attention should be paid to “hidden” variables that can add to the system pressure such as piping system components and fluid viscosity.
“System pressure – The most obvious (and perhaps most influential) variable affecting tube life is the piping system pressure. But often, system components and installation factors that can increase the pressure at the pump tube are overlooked. For example, most manufacturers recommend installing a check valve in the discharge piping directly after the pump tube to prevent the system fluid from flowing back through the pump during routine pump maintenance or pump tube rupture. A spring loaded check valve will increase the pressure at the pump tube by a value equal to the cracking pressure of the valve, and therefore, valves with high cracking pressures should be avoided. Another often overlooked variable that can increase the pressure at the pump tube is the physical distance from the pump to the point where the chemical is injected into the system, especially important to consider when injecting viscous fluids such a polymers. The pressure at the pump tube will increase as the distance from the injection point increases, the chemical viscosity increases, and the discharge piping diameter decreases. Small diameter orifices in fittings should also be avoided when pumping viscous chemicals. - “Number of occlusions – The tube life is affected by the number of times the tubing is pinched (number of occlusions) per day. Reducing the number of occlusions will increase the life of the tube. Three variables affect the number of occlusions per day; the RPM of the roller assembly, the number of rollers on the roller assembly (occlusions per revolution), and the operation time load on the pump.
“Roller RPM – Unlike many types of pumps, peristaltic pumps are capable of operating at very low revolutions per minute (RPM) while maintaining very high accuracy, repeatability and priming capability. Therefore, to increase tube life, specify the pump so that the typical operation of the pump is at the low end of the operating range when possible. The maximum possible RPM of a specific pump model will vary from manufacturer to manufacturer with maximum RPMs of 600 or more being not uncommon, though at this high RPM, tube life will be greatly diminished. With effective turndown ratios of 1000:1 or more, minimum effective RPMs of 0.1 can be achieved.
“Number of rollers – A given peristaltic pump model may have anywhere from one (offset cam type roller) to six or more individual rollers which squeeze the tube, pinching off the captured fluid and delivering it to the discharge end of the pump tube. Multiple rollers per assembly result in smaller volumes of chemical injection per revolution, resulting in less pulsation and a reduced likelihood that an individual roller will wear out resulting in lost pumping capability. However, since tube life is directly proportional to the number of times the tube is pinched per revolution, the cost associated with the higher number of rollers is tube life.
“Pump run time – Most users rate their pump tube’s life in terms of days, weeks, or months of operation time. It is therefore important to note the amount of time per day the pump has run when comparing tube life. For this reason, many manufacturers use the total number of occlusions rather than time when rating their tubes. - “Amount of tubing squeeze – The rollers must over squeeze the tubing to ensure that the fluid being pumped is effectively trapped in the tubing and delivered to the injection point. Factors such as system pressure, suction lift, fluid viscosity, tube material, and others will affect the amount of squeeze required for a particular application. If the tube is under-squeezed, the fluid can escape or flow backward toward the suction side of the pump tube when the roller rotates in the head. This can occur when the pump is operated against a higher system pressure than recommended. If the tube is over-squeezed, it is being subjected to more force than is necessary and tube life will be diminished. Properly matching the roller design with the type of tube being used will result in the most efficient pump design and longest tube life for a particular application.
- “Roller/pump head design – The roller diameter, roller materials, type of bearing surfaces, and pump head design can also affect the life of the pump tube as well as the life of the roller assembly. Roller diameter – A large diameter roller will pinch off a greater surface area of the tube while rotating, resulting in lower tube life; however, large rollers will rotate fewer revolutions per roller assembly revolution, potentially resulting in longer roller life.
“Roller bearings – The roller must rotate on a shaft, therefore the type and design of the bearing surfaces can increase or decrease the life of the roller. The design of the bearing surface can also assist in preventing chemicals and debris (from tubing surface wear) from entering the roller axle area causing drag on the roller.
“Roller material – The roller assembly materials of construction should be of sufficient strength to withstand the repeated compressions of the pump tube while offering resistance to the chemicals that may potentially be spilled in the pump head area. Manufacturers offer a number of different methods for protecting the roller assembly from chemicals including drain ports to remove chemical, float switches to shut down the pump when a spill occurs and a cup fills, and electronic sensors to shut down the pump when chemical is detected in the pump head area. Based on the effectiveness of the method, the roller assembly may incur damage resulting in drag on the roller assembly and reduced roller and tube life.
“Conclusion Because of the many variables that affect the life of the tube in a peristaltic metering pump, careful examination of the specific application and properly specifying the pump and tubing for the application will result in the longest possible tube life.”
