Improving pump reliability
Over the past ten years, pump users have made progress toward improving pump reliability. Much of it came from total quality (TQ) initiatives where tools such as root-cause failure analysis, statistical analysis techniques, and a heightened awareness were used.
Over the past ten years, pump users have made progress toward improving pump reliability. Much of it came from total quality (TQ) initiatives where tools such as root-cause failure analysis, statistical analysis techniques, and a heightened awareness were used. Education through "best practice" forums has allowed users to identify the issues that account for a significant portion of pump failures and downtime.
Since that time most users have or are implementing reliability improvement programs. These involve activities that allow them to identify and execute corrective actions long before equipment failure occurs. The result is increased mean-time-between-failure (MTBF) intervals.
The results of these initiatives have been impressive. Surveys of pump users in the North American chemical industry have shown typical improvements in MTBF between 15 and 24 months.
While these improvements have helped many companies improve profitability and maintain competitiveness, the ever-increasing pressures of today's marketplace have many users looking for additional measures to further increase MTBF and maintain their competitive edge.
Analyzing the actions taken to date shows the focus has been primarily on improving the mechanical aspects of equipment reliability. A holistic approach is needed to gain a better understanding of how a pump functions within a system.
Robust pump selections
During the design process of a pumping system designers and engineers consider many variables. One area to consider, which can improve pump reliability, is the criteria used to select and hydraulically size pumps.
Over the years there have been many guidelines and unwritten rules established to help engineers specify pumps. Basic selection criteria often included objectives such as selecting a pump to operate at its best efficiency point (BEP), running at slower speeds, and providing adequate net positive suction head (NPSH). While these all represent sound engineering practices, often, in real world applications, it is not practical or possible to meet all of these criteria. The engineer is left to make a subjective judgment as to which pump is best for a given service.
Three common factors, which are pertinent to most pump selections, are operating speed, impeller diameter, and operating point. In situations where traditional ideal selection criteria cannot be satisfied, one method proposed to help engineers predict and compare the projected reliability of one pump to another is the concept of reliability factors.
These factors are nondimensional numbers used to provide a relative index ranging from 0 to 1 for an attribute as compared to the ideal for that attribute. The factors have been confirmed by laboratory testing.
Operating speed factor (F r )
Operating speed affects pump reliability through rubbing contact, primarily at the faces of mechanical seals. It also has a significant impact on reducing bearing life though increased cycling, lubricant degradation, and reduced lubricant viscosity due to increased temperature.
Operating speed also has a negative impact on pump component wear (impellers, casings, etc), especially in services where the pumpage is abrasive. Speed has an inherent impact on a pump's suction performance, which can ultimately lead to decreased reliability brought on by cavitation problems due to high NPSHr requirements (Fig. 1).
Impeller diameter factor (F d )
Many people assume that selection of an impeller trim at the maximum diameter is the best choice because it is at this point the geometry of the impeller and casing are the best match. This is true if the pump is operating at its BEP or design point. However, off-BEP operation with a maximum trim impeller can result in some undesirable effects that are detrimental to pump reliability.
When a pump is operating at an off-BEP condition, the exit angle of the fluid leaving the impeller is mismatched with the angle of the tongue of the casing. This results in a pressure pulsation that ultimately leads to increased shaft deflection, decreasing the life of the mechanical seal. It is important to note that at reduced speeds the effects of impeller diameter trim is somewhat reduced but still present.
To alleviate this phenomenon, the clearance between the OD of the impeller and the tongue of the casing should be increased so the magnitude of the pressure pulsation is decreased, resulting in increased pump reliability (Fig. 2).
Operating point factor (F q )
Centrifugal pumps are typically designed to achieve a single flow and head at a given speed. This point is identified as the BEP on a pump curve. At this point all the geometry of the pump's hydraulic design is matched and the pump operates at its highest level of performance and efficiency and has the lowest hydraulic loading.
When this pump is applied to service above or below the BEP point, hydraulic loading and other performance aspects are not optimized and pump reliability is adversely affected. In practice this condition has been found to be less detrimental for smaller pumps. F q is contingent on pump design size or output (Fig. 3).
The use of each of these factors gives a better understanding of the ramification of a given pump selection as compared to another for a given aspect. Most real-world pump applications involve a compromise of all these factors. An overall assessment can be made simply by multiplying all of these factors together to create a reliability index (RI).
RI = F r x F d x F q
The result of this calculation provides a quantifiable ratio that can be used to help decide the merits of one pump selection to another and help determine which one might offer the best performance with regard to reliability.
RI = 1.0 does not imply infinite reliability and RI = 0.0 does not imply zero reliability. They simply are an indication that one selection might be better suited than the other. It is also important to note that this methodology must not be used to compare one pump design to another. The mechanical designs of any two pumps are likely to be different, which can also influence pump reliability.
Robust pump designs
In addition to hydraulic selection, further improvements in pump reliability can be obtained by selecting pumps that have robust mechanical designs. One of the benefits of the TQ movement was that much data was collected and analyzed, from which it was found that the majority of pump failures were actually the result of seal failures. By using root cause analysis techniques, it has been found that these failures were not the result of poor seal designs, but the result of poor seal environments.
Mechanical seal designs have face contact where a supply of clean, cool sealing fluid with adequate lubrication for the seal faces is critical to extending seal reliability. Based on testing, it has been found that seal chambers designed with enlarged and tapered cavities provide the best seal environment to promote extended seal life.
As a further improvement to these designs, some pump manufacturers have made seal chamber designs even more robust by incorporating devices into them that better control the flow pattern within the seal chamber cavity. These devices (Fig. 4) function to keep solids and grits, often present in process streams, away from the seal faces and spring mechanism of the mechanical seal to eliminate premature wear and failure.
Bearing failure is the second most common cause of pump failures. Bearings, like seals, are a wearing component where a proper operating environment must be maintained for extended life. The proper operating environment is one that provides adequate cooling to dissipate frictional heat and maintains an environment free of contaminants. Research has shown that the use of a large capacity oil sump significantly extends pump bearing life and reliability (Fig. 5).
One other robust pump design feature to consider is labyrinth-style oil seals. The primary benefit of a labyrinth-style seal is retention of lubricating oil within the oil sump without contacting the rotating pump shaft. Compared to lip seal designs, which rely on the physical contact of an elastomer with the rotating shaft, significant reliability improvements are gained.
Smart pumping solutions
Even with the adoption of all of the recommendations and best practices mentioned here, it is still difficult to achieve maximum reliability. Many failures are the result of random system upsets or operator errors. Failures such as valves not opening and causing pumps to dead head and run off the operating curve are difficult to predict and prevent.
In recent years some pump manufacturers have begun to offer solutions that involve a robust pump design packaged with self-contained monitoring and control systems which can continuously monitor pump performance versus system demand (Fig. 6).
These intelligent pumping solutions include a pump, an array of pressure, temperature, and flow sensors, a variable speed drive, a chip containing the pump's performance capabilities, DCS communication, and control software. In operation, these systems act continuously to provide diagnostic monitoring of the pump's operation to detect and prevent failures, and constantly regulate pump performance to match system demand, resulting in maximized performance and often leading to significant energy savings.
Mr. Knecht is available to answer questions on improving pump reliability. He can be reached at 315-568-7370. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, firstname.lastname@example.org
Typical Reliability Improvement Initiatives
Vibration monitoring and trending
Lubrication oil sampling and analysis
Pump alignment and proper installation
Why large oil sumps extend bearing life
Greater radiating surfaces dissipate more heat
Oil stays in the sump longer, allowing more cooling
There is more time for contaminants to settle and be collected on the magnetic tip of a drain plug.
Labyrinth oil seal advantages
Labyrinth seals provide unlimited life and full-time protection against contamination
Noncontacting design does not produce any frictional heat that could be transmitted to the lubricating oil resulting in cooler oil temperature and extended bearing life.