Part 2. Centrifugal pumps and component reliability.
By Ed Butts, PE, CPI
We kicked off a three-part series on reliability in pumping and water systems last month with an introduction to reliability factors, pertinent definitions, and why this is important in today’s world.
Let’s continue the discussion this month by looking at centrifugal pumps and component reliability.
Water Pumping System Components
The conditions in which a pump operates can have a tremendous impact on its service lifespan and reliability. The pumping system which often consists of hydraulic, electrical, and mechanical components can extend or decrease the components’ useful life.
Reliability of a pumping system is generally determined by the following interrelated factors for each component in the system—in this case, highlighting the pump:
- Process (pump) engineering: Ascertaining the required type and duty of pump and driver, application, environmental conditions, and the conditions of service (COS) (flowrate × total dynamic head)
- Component (pump) design through prototype stage (described in Part 1)
- Component (pump) manufacturing through delivery: Material selection, casting, fabrication, machining, assembly, quality control, performance testing and certification, packaging, and shipping/delivery
- Component (pump) installation: Support means, anchoring, alignment to driver, startup, and testing
- Ancillary equipment and installation: Piping, valving, driver (motor or engine), electrical, miscellaneous
- Operation and maintenance: Planning and scheduling, inspection, and monitoring, perform needed repairs
- Operating variances: Change of the COS due to process or field revisions (flowrate and head), electrical issues, and wear.
Similar steps apply to every component used in a water system, and each component—including the pump, driver, valves, piping, and electrical switchgear—must generally include and pass through each of the steps shown above in the uniqueness that applies to the component and its service conditions.
It is important to understand the dependence of a pump’s mean time between failure (MTBF) or of its components in what is commonly called a “series system.”
The major components to consider in the overall pump MTBF rate, in order of probable failure:
The shaft seal (normally mechanical) or packing
- Pump bearings
- Coupling to motor
- Pump shaft.
However, this potential component failure list can be further expanded to include other components including the pump impellers, driver, wearing rings, and casing as well as the piping, baseplate, alignment, and a host of other operational issues that can influence pump reliability.
The individual reliability exhibited by each component when multiplied together creates the pumping system’s overall reliability, just as the individual links in a chain determine the strength or weakness of the entire chain.
In addition to the design, manufacturing, and installation reliability of each component, the way it is used in the pumping system can also significantly impact the component’s reliability, and in turn, system reliability.
Besides the reduced energy effectiveness and reliability from excessively low or high flow operation (as previously discussed in Part 1), there is another reliability issue with parallel operation using dissimilar or worn pumps with similar performance characteristics.
Pumps operate at the same head when they operate together in parallel. However, if one of the units is designed for much higher head or exhibits greater wear than the other unit, this can push one pump to a near shutoff condition while the other unit performs virtually all the work, which could cause the pump at near shutoff head to overheat and fail.
Finally, there is another issue to be considered when selecting pumps to operate in parallel, and that is the shape of the pump’s head-capacity (H-Q) curve. If the pump H-Q curve droops as the flow is reduced but then the head starts to rise towards shutoff head (at a zero flowrate), a second identical pump may not be able to pump, and could therefore also run at shutoff head, overheat, and possibly fail.
Even if the second pump were to activate, it would usually increase the system’s flowrate by only a minor degree. Pumps with head-capacity curves that droop towards shutoff head or display a head dip at a higher flowrate should generally not be operated in parallel, especially with a pump that displays a steadily rising or substantially different H-Q curve shape.
The reliability of a water system that consists of several components can be analyzed using techniques such as reliability block diagrams and fault trees, a common method of reliability determination.
Centrifugal Pump Failure Modes
As principal water system components, centrifugal water pumps are excellent examples of the need for using appropriate design, model and prototype testing, and operational techniques.
A 1996 report from the Finnish Technical Research Center titled “Expert Systems for Diagnosis and Performance of Centrifugal Pumps” indicated that out of 1690 pumps evaluated from 20 sites, the average pumping system operating efficiency was below 40% and that more than 10% of the systems surveyed operated below 10% efficiency.
Although exceptions apply for smaller pumps and pumps of low specific speed, most industrial or municipal pumps above a best efficiency point (BEP) associated with 100 GPM should generally be able to achieve between 70% to 85% efficiency. This underscores the low operating efficiency present in many applications and the importance of improving efficiency whenever feasible.
Pumps are generally designed for specific flow ranges and speeds in alignment with their specific speed rating. When a pump is properly sized and operating optimally at its BEP or within the best efficiency window (BEW) and at optimum speed, liquid flow is constant and the radial forces acting on the impeller are reasonably balanced, and therefore, the lowest.
The intensity of the radial forces that are generated around the periphery of an impeller is a function of the total operating head and the width and diameter of the impeller. Although they operate relatively balanced at or near the BEP or within the BEW, force imbalance can increase quickly as operation moves to either left or right of the BEP on the pump curve.
This is because the impeller’s discharge angle was originally designed to match the volute or diffuser’s optimum inlet angle at the associated BEP flowrate. This allows the pump to experience the highest efficiency along with the lowest and most balanced radial force and lowest resulting vibration.
If the pump operates too far away from its BEP at an increased or reduced flowrate, an imbalance of volute or bowl pressure and resulting forces will occur inside the pump around the impeller.
Operating at a lower or higher speed than the pump was designed for can also impact the pump’s performance and increase vibration and cavitation potential along with the impeller’s discharge (radial) force, particularly if the pump is operating within a critical speed range. This force imbalance causes greater shaft deflection, suction and discharge recirculation, excessive loads on bearings and packing or mechanical seals, often resulting in excessive vibration and greater heat.
Vertical pumps possess their own unique set of challenges in addition to the wear potential often observed in the bowl assembly from sand, misalignment, or dry running conditions. The lineshaft that exists between the driver and bowl assembly is normally held in tension by the weight of the impeller stack against the upper adjustment means, but excessive bearing spacing, worn bearings, too small of lineshaft for the load, and inadequate lubrication can result in “shaft whip” between the intermediate column bearings, leading to rapid unit failure.
Each of these anomalies will significantly reduce the life of the pump and increase overhaul cycles and the likelihood of premature failure.
Centrifugal Pump Reliability Factors
The individual reliability and life of a centrifugal pump typically reaches a peak value at the BEP or BEW but decreases sharply as the actual operating point deviates from this region in either direction. Thus, the operating point plays a decisive and critical role in sustained pump reliability.
If the unit cannot consistently operate at the BEP or BEW, maintaining performance within the preferred operating range is the second-best option. The preferred operating range is a flowrate typically between 75% to 120% of the pump’s BEP flowrate. Extended operating time at either reduced load or an overload will also likely increase the component failure rate.
Cavitation is another potential operating anomaly that can rapidly reduce pump reliability. This can occur at flows higher or lower than at the BEP or BEW flowrate and can quickly degrade pump performance and destroy impellers and volutes/bowls if permitted to continuously operate at this condition uncorrected.
As evidenced in Figure 1, the reliability curve for a typical centrifugal pump is a bell-shaped curve that coincides with the BEP and a MTBF of –1. It also has limits in both directions generally associated with reduced bearing and seal life as well as low flow cavitation and increased suction and discharge recirculation and operating temperature. This is because the flow reduces and approaches shutoff head to conditions of reduced bearing and seal life and increased vibration and cavitation potential as the flow increases and extends to runout flow.
Many engineers and pump system designers often tend to specify oversized well pumps with excessive horsepower drivers on the assumption that it is better to have too much power for the application or additional unplanned well lift (added head) than not enough. Thus, if the flowrate of the system is excessive, the presumption is that the pump can simply be throttled back on the discharge side to reduce the flow and move it to the left on the pump curve and meet the desired operating condition.
This approach, although common in practice, creates an inefficient design and ends up as an overly expensive and unreliable pumping system. It not only increases the head and relative energy costs for the pumping system, but significantly reduces the operating life and reliability of the equipment. And that will most likely increase the frequency and occurrence of repair, often leading to ultimate unit failure.
In addition, undersized piping system components, such as process piping and adding unnecessary inline control or throttling valves, also leads to higher head with resulting excessive energy usage.
When excessive energy is added to the pumping system, control elements absorb it throughout the system, generally by adding heat, noise, and vibration to the control components, reducing their operating life, and adding to system maintenance costs.
To comprehend the damage that can occur in a rotodynamic pump, it is necessary to understand the basic operational principles of rotor centralization to the stationary components. Known as the Lomakin Effect (Figure 2), it is a force created at the wear rings and throttle bushings within a centrifugal pump.
The force is the result of unequal pressure distribution around the circumference of the impeller during periods of rotor eccentricity or shaft deflection. However, the offsetting supporting force only occurs when the pump is operating at or near its BEP rated condition of head and flow.
The Lomakin Effect can sometimes be confusing because it encompasses two separate phenomena that occur at the wear rings or bushings: damping and stiffness. Damping does not directly prevent shaft deflection but minimizes rotor response to excitation forces, much in the same way shock absorbers help to smooth the ride in a car. Thus, reduced clearances increase the damping effect and results in a more stable rotor action.
Perhaps most importantly, the stiffness and damping on most pumps are located at the impeller where the pump otherwise has no direct bearing support. Typically, this means if a given wear ring clearance is reduced by 50%, the Lomakin stiffness will double.
This strategic location gives the Lomakin Effect a great deal of influence in minimizing shaft deflection. Combine the increased damping and stiffness impacts, and a pump with reduced wear ring clearance runs with lower vibration, less shaft deflection, and a longer life than a pump with standard clearances.
In the case of a vertical turbine pump, if the entire pump/motor baseplate is not perfectly aligned, plumb, and level, the rotating shaft components may contact the stationary components. Points 1 and 2, illustrated in Figure 3, identify critical areas that must comply with Hydraulic Institute (HI) standards.
In addition, the suction and discharge flange loading at points 3 and 4 on a vertical turbine canned booster pump or at the discharge flange on a vertical turbine well pump must also comply with HI standards. In many cases, it may be possible to more than double the mean time between repair/mean time between failure (MTBR/MTBF) of a standard vertical pump by upgrading repairs to a so-called “precision remanufacture.”
As a result, pump vibration will be reduced and can be verified upon restart as proof of the benefits from the upgrade; usually, the repair and upgrade costs are minor portions of the pump’s total lifecycle cost.
The highest number and frequency of multistage pump failures attributed to downtime due to the need to repair or failure of the unit are in order of occurrence: mechanical seals or packing, bearings, a malfunctioning or worn single impeller, multiple (stacked) impeller failures (generally caused from misalignment or abrasives), bowl and line shafts, and couplings.
Seals and bearings are essentially considered to be nonrepairable pump components and are generally replaced upon failure, where other pump components are often rebuilt or resurfaced.
Repair or replacement of pump to driver support brackets (i.e., discharge heads) or casing/bowls/volutes is a less common repair, particularly when wear rings are used.
Certain operating conditions or anomalies, such as high levels of abrasives (sand), cavitation, vibration, and insufficient NPSH, can impact the failure frequency and rate of a pump, particularly a well pump, and must be examined separately.
Ultrasound inspection is one of the most reliable methods of determining the physical state of water pumping equipment. Typical applications for ultrasound inspection include all parts of pumps, bearings, motors, gearboxes, and valves.
Auditing and verification of hydraulic applications and service conditions, and utilizing condition-based monitoring and lubrication of bearings and rotating equipment, are excellent ways to extend pumping system reliability. Typical failure modes for centrifugal pumps under varying service conditions are shown in Table 1 in Part 1.
This concludes the second installment of this three-part series on water and pumping system reliability. Next month, we will wrap things up with a discussion on electric motor, engine, and water system reliability and the associated failure modes for each.
Until then, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.