13 common factors affecting the lifespan of centrifugal pumps

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1. Radial forces Industry statistics show that the main cause of unexpected shutdowns in centrifugal pumps is the failure of bearings and/or mechanical seals. Bearings and seals are the “canaries in the coal mine” – they serve as early indicators of a pump’s health and precursors to any malfunctions within the pumping system. Anyone who has worked in the pumping industry for a long time probably knows that the first rule of best practices is to operate the pump at or near its Best Efficiency Point (BEP). At BEP, the pump design will be subjected to minimal radial forces. When operating far from the BEP, the resultant vector of all radial forces forms a 90° angle with the rotor and tries to deflect and bend the pump shaft. High radial forces and the resulting shaft deflection are killers of mechanical seals, and also a factor contributing to shortened bearing life. If the radial force is large enough, it will cause the shaft to deflect or bend. If the pump is stopped and the shaft runout is measured, no problems are found, because this is a dynamic condition rather than a static one. The bent shaft running at 3,600 rpm deflects twice per revolution; therefore, it actually bends 7,200 times per minute. This high-cycle deflection makes it difficult for the sealing surfaces to remain in contact and maintain the fluid layer (liquid film) required for the proper operation of the seal. 2. Lubricant contamination: For ball bearings, over 85% of bearing failures are caused by contamination. This contamination can be dust and foreign particles, or water. Even a water content of 250 parts per million (ppm) can reduce the bearing life by four times. The service life of lubricating oil is crucial. The operation of the pump is similar to a car driving continuously at 60 miles per hour. Working 24 hours a day, 7 days a week, the odometer would quickly register 1,440 miles per day, 10,080 miles per week, and 524,160 miles per year. 3. Suction pressure Other key factors affecting bearing life include suction pressure, driver alignment, and to a certain extent, pipe strain. For ANSI B 73.1 type single-stage horizontal cantilevered process pumps, the axial force generated on the rotor is directed toward the suction inlet. Therefore, to a certain extent and within certain limits, the counteracting suction pressure actually reduces this axial force, thereby lowering the load on the thrust bearing and extending the service life of the pump. For example, the bearing life of a standard Type S ANSI pump operating at a suction pressure of 10 psig is typically 6 to 7 years; whereas, at a suction pressure of 200 psig, the bearing life can be extended to over 50 years. 4. Driver misalignment: Misalignment between the pump and the driver can cause overloading of the radial bearings. The lifespan of radial bearings has an exponential relationship with the degree of misalignment. For example, with a mere 0.060-inch deviation (misalignment), end users may experience bearing or coupling problems after three to five months of operation. But if the deviation is 0.001 inches, the operating time of the same pump could exceed 90 months. 5. Pipe strain Pipe strain is caused by misalignment between the suction and/or discharge pipes and the pump flanges. Even in well-designed pumps, pipe strain can easily transmit these potential high stresses to the bearings and their corresponding bearing housing interfaces. Force (strain) can cause the bearing fit to become non-circular and/or inconsistent with other bearings, thereby placing the centerlines on different planes. 6. Fluid properties: Fluid properties such as pH, viscosity, and specific gravity are key factors. If the fluid is acidic or corrosive, the wetted parts of the pump (such as the casing and impeller materials) must remain intact (corrosion-resistant) during operation. The solid content in the fluid, as well as its size, shape, and abrasiveness, are all influencing factors. 7. Frequency of use (of the service) The frequency of use (of the service) is another important factor: How often does the pump get activated within a certain period of time? I’ve seen with my own eyes a pump that starts and stops every few seconds. Compared to pumps operating continuously under the same conditions, the wear rate of pumps in these services is much higher. In this situation, a change in the system design is urgently needed. Under the same conditions, pumps with submerged suction operate more reliably than those operating under suction lift conditions. Suction conditions require more work and increase the chances of more air being drawn in or, worse, dry running. 8. Net Positive Suction Head margin: The greater the difference between the available Net Positive Suction Head (NPSHA, i.e., the system’s cavitation margin) and the required Net Positive Suction Head (NPSHR, i.e., the necessary cavitation margin), the lower the likelihood of cavitation occurring in the pump. Cavitation can damage the pump impeller, and the resulting vibration can affect the service life of seals and bearings. 9. Pump speed: The operating speed of the pump is another key factor. For example, a pump operating at 3,550 rpm wears out 4 to 8 times faster than a pump operating at 1,750 rpm. 10. Impeller balance: An unbalanced impeller in cantilevered pumps or certain vertical designs can cause the shaft to oscillate. This oscillation causes the shaft to deflect, similar to the radial forces that occur when a pump operates away from its BEP. Radial deflection and axial swing may occur simultaneously. I always recommend balancing the impeller at least according to the ISO 1940 G 6.3 standard. If the impeller is cut for any reason, it must be rebalanced. 11. Piping layout and inlet flow velocity Another important consideration for prolonging the pump’s lifespan is the way in which the piping is laid out, that is, how the fluid is “fed” into the pump. For example, compared to horizontal elbows, elbows on the vertical plane on the pump suction side have fewer adverse effects – the hydraulic load on the impeller is more uniform, and thus the load on the bearings is also more uniform. Additionally, the fluid velocity on the suction side should be kept below 10 feet per second. Personally, I recommend keeping the velocity below 8 ft/s; 6 ft/s is even better (assuming it’s a non-slurry fluid). Replacing turbulent flow with laminar flow affects how the impeller is loaded and changes the dynamic characteristics of the rotor. 12. Pump operating temperature: Whether it’s high or low temperatures, the pump’s operating temperature—and especially the rate of temperature change—has a significant impact on the pump’s lifespan and reliability. The operating temperature of the pump is very important; the pump’s design must be compatible with this operating temperature. But what’s more important is the rate of temperature change. I recommend (I’m rather conservative) keeping the rate of change below 2 degrees Fahrenheit per minute. Different qualities and materials expand and contract at different rates, which affects gaps and stresses. 13. Shell penetrations: Although not often considered, shell penetrations represent an option rather than a standard feature for ANSI pumps. This is because the number of such penetrations on the pump shell has a certain impact on the pump’s lifespan; these locations are primary sites for corrosion and the development of stress gradients (increases). Many end-users wish to drill and tap the housing for drainage, venting, and instrument ports. Each time drilling and tapping is performed on the housing, a stress gradient is created in the material; this stress gradient becomes a source of stress cracks and a site where corrosion begins.