The following article was an effort to combine theory with actual practical statistics. Many things change, but many also stay the same. What would one see as a “big splash of change” in pumps in the past 25 years?
The biggest change is the ever-evolving trend toward instrumentation, computer control, data acquisition systems and similar automation-related technologies. However, what did not change is “how pumps feel about it.”
The number one and two forerunners of failures are mechanical seals and bearings—or perhaps some would say bearings and seals. Despite all the developments in technologies, bearings and seals keep failing the same way they did 25 years ago. There have not been any really significant, ground-shaking new bearing designs or fundamentally different seal designs that have come on the market.
Bearing and seal failures have never been an exact science—when bearing housing vibrations begin to increase from 0.05 inches per second (in/sec) to 0.1 in/sec, no guru of reliability in the world can tell you that their life will be shortened in half. If vibration went up from 0.2 in/sec to 0.8 in/sec, all specialists will tell you that the failure is “imminent.” However, I have seen cases where the pump kept running for another seven years, still at 0.8 in/sec vibrations, and was eventually changed to another design for completely unrelated reasons.
So, these are the trends. Ironically, we can repeat the old phrase adapting it to pumps: “The more pumps change, the more they stay the same!”
In the future, what we need to do as a community is to begin quantifying reliability with real field data and link that to theory—combining both. The statistical approach to reliability has been for years in a rudimentary form, and we need to address it. If we do, we will begin to learn what to tell a maintenance manager when he sees vibrations go up from 0.05 in/sec to 0.1 in/sec—to predict if the bearing, based on this analysis, will fail next Tuesday or in 10 years.
Proper lubrication is a key to long, trouble-free life of centrifugal pump bearings. In recent years, the issue of lubrication has received renewed attention from pump users in chemical plants, pulp and paper mills, refineries and other industries.
Budgetary pressures have forced many plants to reduce maintenance capital. Many knowledgeable maintenance workers have been laid off.
Not surprisingly, the ability to maintain pumping equipment properly is reduced, resulting in increased outages, lost production and rising maintenance costs.
Users have started to look to pump manufacturers to pick up the slack and help solve pump reliability problems, extend component life, and increase mean time between failure (MTBF) and mean time between scheduled maintenance (MTBSM).
Statistics show (Ref. 1) that most pump failures are related to bearings and seals. In this article, we will look at bearings, analyzing how design changes affect bearing life in a quantifiable way.
The need for improved pump reliability and increased MTBF led to a new design. When comparing the cross sections of two single-stage, end-suction ANSI pumps, both have identical wet ends (impeller and casing), but the power end and the seal chambers are different.
Improvements in the seal chamber are significant. The new design has a larger chamber to ensure better heat transfer and cooler operation of the mechanical seals. The previous design incorporated a tight stuffing box.
The new power end design features approximately three times the volume of the oil sump, an oil level sight glass to assure the proper oil level versus the constant level oiler, improved cooling via a finned cooler insert versus bottom cooling pockets, labyrinth oilframe seals versus lip seals, and stiffer footing for reduced vibrations.
A testing program has been conducted to compare the two designs under extremely adverse operating conditions, such as running endurance testing at overspeed and below minimum flow. This program resulted in quantifiable correlations between changes in pump design and their effect on life extension. Feedback from users comparing two designs was also obtained, specifically in relation to the operating temperature of the bearing frame surfaces.
Analysis of the Power End
With regard to the power end, the belief that “the bigger the better” is not uncommon in the pumping community. This idea has some merit, but manufacturers often overlook the importance of quantifying the benefits of a particular design or modification. Frequently, little information is given as to how much life extension can be obtained by, say, having a deeper sump, or how much added value and savings can be realized from the increased bearing frame heat transfer surface.
It is clear that a systematic approach to identify, measure and improve pump component design is impossible without a proper balance of theory, experimentation, user feedback and data from real world installations. Theory and experimentation should be balanced by clear communication between manufacturers and users.
Increased Frame Outside Heat Transfer Surface
Heat is transferred from the pump bearings to the oil and through the housing frame walls to the outside air. Some of the heat is also conducted through the casing to and from the pumpage, depending on the temperature of each. Typically, the difference in temperatures is small for the pumping conditions of chemical plants, and the effects are omitted for simplicity.
Our investigation has shown (Ref. 3) that the larger surface area can result in a nearly 40 F reduction in bearing operating temperature. The cooler bearings, in this case, result in approximately 13 percent longer life.