Minimizing vibrations in rotating equipment increases equipment service life. Some of the benefits of lower vibration levels include reduced transient loads, lower bearing temperatures, and reduced wear. In that regard, most engineers, mechanics, and technicians are familiar with the benefits of and techniques for static and dynamic balancing of rotating equipment, as well as the subsequent benefits of and techniques for proper alignment of rotating equipment when two or more items are coupled in a train. The following discussion details several less-common sources of apparent unbalance that, if they’re not recognized, can be quite vexing. Some familiarity with balancing theory is presumed but not required.
PUMPS
There are three types of unbalance in impeller-type pumps: static, dynamic, and hydrodynamic. Most RAM professionals are familiar with the first two. Static unbalance, of course, is a function of the distribution of mass of the rotating element about the center of rotation when it is not rotating. When the center of mass of the rotating element coincides with the center of rotation, the element is said to have static balance.
One way to picture static balance of a rotating element on a shaft is to imagine that the component is hanging downward from one end of the shaft. If the distribution of mass is such that the rotating element hangs straight down without leaning one way or the other, it has static balance (also known as single-plane balance).
Many years ago, when automobile tires were not as wide as today’s designs, they were statically balanced with a device called a bubble balance (which is basically a level). When a tire and wheel are mounted on this type of device, they are centered on and supported by a pointed rod. The process is much like balancing a vinyl record album on one’s pointed finger at the center hole of the album.
If the mass of the tire and wheel is not distributed symmetrically about the center of the wheel, the leveling bubble will shift away from the cross-hairs at the center in the opposite direction of the mass unbalance. The tire and wheel will lean one way or another on the bubble stand and not appear level. Weights are added along the outer part of the wheel rim to bring the bubble back towards the center. When the bubble is back in the center and the wheel and tire sit level on the bubble balance stand, the weights are permanently affixed to the wheel rim so that the tire and wheel will rotate without significant vibrational wobble.
When tires were relatively thin with respect to their overall diameter (similar to a bicycle tire and wheel) and road speeds were generally lower, static balance, i.e., single-plane balance, was sufficient for ensuring a relatively smooth ride. Today, this is no longer the case.
Static balance alone is insufficient to minimize vibrations in a rotating element that doesn’t resemble a thin disk on a symmetric shaft. When a rotating element is turning, especially at higher rpms (revolutions per minute), the distribution of mass within the element can be such that a moment develops between two or more mass “lumps” at different points along the shaft (a situation which is also known as multiple planes of unbalance). Even if the “lumps” balance each other out statically from the center of rotation, each will develop an individual centrifugal force pointing away from the axis of rotation (i.e., Fc = mw2R) when the shaft is spun. If these various “lumps” are not in the same plane perpendicular to the shaft, the resulting centrifugal forces will develop unbalanced moments in the shaft that will result in vibrations.
When pumps are manufactured or repaired, static and dynamic effects are minimized by employing a balance stand. Without going into detail, this is a machine on which the rotating element is mounted and spun to check for unbalance vibrational effects. By adding or subtracting weight from various locations on the rotating element, the rotating element is balanced both statically and dynamically so that it operates smoothly with minimal residual unbalance vibrations.
Further, a better balance is affected if the rotating element is balanced in the shop using the actual shaft, keyway elements, and any couplings that will be used in service. In this way the mass distribution effects of those items, albeit often small, are accounted for. If the actual shaft is not available, a similarly configured mandrill is substituted so that the rotating element can be mounted on the shop’s balance stand.
Balance stands and the shop work involved in minimizing static and dynamic unbalance generally take care of the inherent mass distribution irregularities within a rotating element. However, there’s a third, less-familiar unbalance effect that can sometimes come into play: Known as hydrodynamic unbalance, it only manifests when a pump operates in water or fluid.
Hydrodynamic unbalance occurs when the blades of an impeller do not equally push water or fluid through the pump. While static and dynamic unbalance typically results in vibrational forces perpendicular to the rotating shaft due to centrifugal effects produced by rotation, hydrodynamic unbalance often produces vibrational forces in the axial direction of the rotating shaft, i.e., in the same direction water or fluid is being pumped. This is in addition to the usual pulsations in the axial direction produced by the individual blades as they rotate around the shaft.
In the design and manufacture of pumps, it is expected that the impeller blades will be symmetric with respect to geometry and blade profile, and arranged symmetrically about the rotating axis. This symmetry ensures that the net hydrodynamic forces will be those that push the water or fluid evenly in the desired pathway. When pumps and impellers come directly from original equipment manufacturers (OEMs), it is rare that this is not the case. With few exceptions, OEMs do a good job in this regard.
But when pumps that have been in service are sent out for repair due to erosion or cavitation damage of their impeller blades, rigorous attention to and re-establishment of the original blade designs isn’t a given. While worn blades often have damage at the leading and trailing edges, they don’t all wear exactly the same way. If, for example, the wear on a leading edge appears relatively small in comparison to the blade, repair personnel may simply grind off the thinned or spongy area, i.e., the cavitation damage, without faithfully restoring the original profile of the leading edge. This approach sometimes shortens the width of a blade, which, affects the net amount of push it produces.
Or, perhaps, an attempt is made to re-establish the damaged blade material that has been ground away by brazing or welding new material onto the leading edge that has a different roundness or flow roughness. This can make a difference in the flow and drag characteristics of the blade, or where on the blade cavitation will occur and how much cavitation will occur. Another example is if the leading edge of a blade is trimmed back such that the leading edge, where the blade first bites into the water or fluid, is now recessed as compared to the other blades.
After repairs are made to the blades and shroud of an impeller (if it has a shroud) at various workstations in a shop, the impeller will typically be sent over to the balance-stand workstation. Dynamic balancing is often the last step in the re-manufacture or repair of a pump impeller. Unfortunately, it doesn’t necessarily detect or correct any blade symmetry irregularities like those noted above. To be clear, an impeller does not need to have faithful blade symmetry to be dynamically balanced. Further, the person doing dynamic balancing of the impeller may not notice whether the impeller blades are all flow-symmetric with one another.
However, there is a balance-related indicator that may point to problems in the flow symmetry of the impeller when the repair or refurbishment work has affected hydrodynamic-flow balance. Be wary of when the amount of weight added to the rotating element to affect balance during dynamic balancing is significantly larger than normal, or when the amount of weight subtracted by grinding or drilling is similarly larger than normal, or perhaps both effects are present, Normal in this case is as compared to the weight added or subtracted when the impeller was initially balanced after the original manufacture, when the blades were more symmetric with each other.
Weight for balance is often added or subtracted on the interior side of the impeller hub, or on the outer shroud (if it has a shroud). It is a gross error to add or subtract weight to affect dynamic balance on the blades themselves, as this will also affect flow.
Besides inspecting the amount of balancing weight, it’s good practice to inspect a refurbished or repaired impeller prior to installation. Among other things, measure the blades and examine their profiles vis-à-vis the OEM drawings. If the OEM dimensions and blade profiles have been re-established, hydrodynamic unbalance will be minimized. Running fingertips across the leading edges of the impeller blades to feel their smoothness may not be scientific, but it is a quick way to check whether there are significant differences in the flow coefficients of friction among the blades. While these inspections are performed after the fact, examination of rotating shaft bushings located near an impeller for unusual wear can indicate whether the impeller has had significant hydrodynamic unbalance.
Anecdotally, hydrodynamic unbalance as described above has most often been observed by the author in large-diameter, single-stage impeller pumps that have been repaired several times.
AIR MOVERS
Air movers, i.e., large fans, can have types of unbalance similar to those seen in impeller-type pumps. The fundamental difference between impeller pumps and large fans, of course, is that the fluid being moved is generally air or gas instead of water or liquid. Thus, the term aerodynamic unbalance is applied when the amount of air pushed out by the individual blades is significantly unequal, resulting in a noticeable vibration.
As with pumps, static and dynamic unbalance with respect to the mass distribution in the rotation element is usually minimized by mounting the rotating element in a balance stand and affecting multi-plane balancing. Because the air mover would otherwise blow air around the shop when balancing work is done, which is usually undesirable, the rotating component may be shrouded in a thin membrane, like cellophane, that does not nominally affect mass related balance. This blocks air flow so that static and dynamic balancing can be affected. Vibrational effects due to air flow, otherwise, give false indications as to where mass should be added or removed to affect static and dynamic balance. Some shops may even use a chamber to draw a partial vacuum to do the same thing.
Like pumps, aerodynamic balance occurs when all the fan blades push air equally in the desired direction. Unbalance can occur in service when repair work or refurbishment has affected the contour or geometry of a blade, or perhaps even a group of blades. As noted previously regarding pumps, it is possible for an air mover rotor to have dynamic balance, but not have aerodynamic balance.
Additionally, some fan blades move air laden with ash, welding fumes, or other particles. Over time, those blades can become coated with such particles. If the buildup of material is uneven, unbalance can develop. In such cases, the unbalance is both due to the unsymmetrical distribution of dirt on the blades, and due to the change in air flow around blades that no longer have the OEM profile.
When those types of fan blades are repaired, new material is sometimes added by welding or brazing to replace eroded material, which, in turn, can change blade “stickiness.” Thus, with respect to the particles suspended in the air being moved, this process may lead to a blade surface that allows faster or slower buildup of particles (depending upon the particulars of the replacement material and its surface-finish qualities). In either case, mass distribution irregularities develop while the air mover is in service.
As with pumps, comparison of the repaired areas of an air mover with the OEM specifications for dimensions and surface finish will help identify any of these issues prior to installation.
ELECTRIC MOTORS
Electric motors that have been tested in the shop and shown to have minimal dynamic vibration levels, can sometimes exhibit unbalance vibrations in service, notably just after alignment work has been completed. In such cases, alignment measurements between the electric motor driver and the machine to which It has been coupled may even have been very good . However, the vibration measurements suggest that there is still a mass unbalance in the driver that wasn’t there before. This can be a real puzzler.
Electric motors have two axes of rotation: The first is related to the mass distribution of the shaft and rotor. Like pumps and air movers, the amount of residual unbalance in the motor’s rotating element is minimal when the center of the moment of inertia of the element coincides with the center of inertia of the shaft. If they do not coincide, or nearly coincide and the shaft is tightly constrained in place by bearings, the eccentricity between the two moments of inertia will act like a mass unbalance. Fortunately, most electric motor manufacturers and re-wind shops keep this eccentricity to a reasonable minimum.
The second axis of rotation mentioned above is related to the air gap and magnetic center of the motor. The rotor portion of an electric motor operates within a magnetic field. The magnetic field also has a center where it likes to rotate, and the rotor tries to center itself there when operating. If the mass moment of inertia center, that is, the mass-related center of rotation, and the center of the shaft do not coincide reasonably close to the magnetic center, the rotor may try to shift between the magnetic center and the other centers.
The OEM and electric-motor shops are generally cognizant of adjusting the air gap and dynamic balance so that all those centers are reasonably coincident. However, sometimes when an electric-motor drive is being aligned with another equipment item, instead of adjusting the motor’s frame to bring it into alignment, the bearings within the driver are adjusted. If the rotor bearings are moved enough, this may change the air gap and shift the rotor away from its magnetic center. This may cause the rotor to oscillate in some fashion, 1x, or 2x, or even intermittently, between the two axis of rotation positions. When the air gap is inadvertently changed in this fashion and the problem isn’t recognized, alignment of the electric-motor driver to another piece of equipment can seem like an endless job.
NEW ANCILLARY CONNECTIONS OR DISCONNECTIONS
Obviously, vibrational forces produced in rotating equipment, such as in a pump and motor driver combination, are transmitted to the supporting frame, skid, pipework, and conduit. In general ancillary connections to the equipment should be designed with sufficient damping, rigidity, or flexibility to not amplify those vibrational forces. Generally they are so designed.
During the service life of equipment, though, its mounting skid, connected pipework, or power conduit might be changed. In the process, vibrational-resonance frequencies may inadvertently be changed, as might crucial rigidity or damping characteristics. Consequently, vibrations that were previously measured in a piece of equipment may no longer be solely a function of just the pump, the motor driver, or their alignment. Sometimes such changes result in an amplification effect. Often this effect is non-linear.
If field-balancing work is being done on the pump, driver, or their respective alignment, minimizing the vibrations of the individual pump, driver, or their alignments to each other may not do much to reduce overall vibration levels. In fact, erratic vibration measurements may occur that seem to defy cause and effect expectations. Removing and re-balancing the pump or re-balancing the driver, or re-working their respective alignment to each other may not affect the outcome as needed.
Unfortunately, changes such as those noted above will sometimes allow vibrational forces from other equipment to be transmitted into the pump and driver combination, which heretofore had been isolated from them. Moreover, work to minimize vibrations on the pump, the driver, or their alignment will not minimize vibrational forces transmitted into the pump-driver system from other sources.
In essence, what may have been just a three-part system with respect to vibrations, i.e., the pump, the driver, and the alignment between the two, has become a multi-part vibrational system. It could now include an amplification factor; vibrational forces transmitted through a new connection from a different piece of equipment; or both of those things at the same time. Accordingly, unless the amplification effect or the transmission of new vibrational forces is addressed, hours of field work attempting to minimize vibrations in the pump and driver will be wasted.TRR
ABOUT THE AUTHOR
Randall Noon is a registered professional engineer and author of several books and articles about failure analysis. He has conducted root-cause investigations for four decades, in both nuclear and non-nuclear power facilities. Contact him at [email protected].
Tags: reliability availability, maintenance, RAM, root-cause analysis, fluid leaks, leak investigation, leak monitoring
