By Allan R. Budris
Vertical pumps are quite different from other pumps because they have more flexible motor and pump discharge casings than comparable horizontal pumps and a more flexible attachment of these casings to the foundation. This is in addition to having very long spaghetti-like line-shafting that connects the motor to the belowground liquid-end pump bowl assembly. Finally, there are typically less stringent balancing, shaft straightness, and motor shaft alignment tolerances. Because of this unique construction, vertical turbine pumps (VTPs) are more prone to structural and shaft vibration problems, especially pumps with low axial shaft thrust and those coupled to a variable speed drive.
Understanding these design differences, which are detailed below, and performing comprehensive vibration analyses, especially on larger pumps, can point out operational and design changes that can help avoid the resulting unique VTP vibration problems.
Deep Well vs. Short-Set Pumps
Despite these vulnerabilities, most deep set vertical turbine well pumps do not experience vibration problems as compared to many shorter set industrial applications (as shown in Fig. 1), which are more prone to these unique VTP vibration issues. One of the main reasons for this difference is the high shaft axial thrust experienced by deep set pumps. Unlike the typical horizontal pump, VTP impellers are not hydraulically balanced with back wearing rings or pump-out vanes. This results in a high axial thrust, which is typically supported by the motor thrust bearing at the top of the pump. In addition, deep set pumps require more of these unbalanced bowl stages to generate the high pressure needed to pump the water to the surface. The weight of the added impellers and line shafting further increases axial thrust, which actually has a positive benefit for these flexible pumps: It provides a strong restoring moment that can suppress lateral shaft vibration forces, improve the shaft alignment, reduce the amplitude of the vibration, and increase the critical speed.
On the other hand, short-set industrial VTPs typically generate less pressure and have much lighter shafting, which minimizes the axial thrust and resulting restoring moments. This makes them more prone to excessive shaft vibration if the shaft speed or vane pass frequency is close to one of the many line-shaft vibration modes (free shaft ends, fixed shaft ends or free-fixed ends), especially if one of these shaft vibration modes corresponds to a VTP structural natural frequency.
The VTP column shaft bearing spacing required to stay below the first (lowest) shaft critical speed (free shaft ends) is shown in Fig. 2. It is desirable for shorter set industrial VTPs to stay below this critical speed, especially if the axial thrust is low. In order to stay below this critical speed, it may be necessary to either shorten the bearing spam, increase the shaft diameter, or both. As an example, at a speed of 1,800 RPM, the maximum column bearing spacing required to stay below the first critical speed for a 1 11/16” shaft diameter is five feet.
Coupling Effect
In order to maintain the necessary shaft alignment with typical VTP line-shaft threaded couplings, a high axial shaft thrust is normally needed for shaft straightening, since the tolerances necessary for the small shaft ends (which butt together in the typical threaded shaft couplings) to maintain the desirable shaft straightness would be unreasonable. Such misalignment further adds to shaft vibration. However, if a high axial thrust is not present, such as with a short-set industrial pump, this shaft misalignment problem can still be managed by the use of clamp-type shaft couplings in place of threaded couplings to insure proper shaft alignment.
VTP Aboveground Structure
As mentioned above, the typical VTP aboveground structure tends to be quite flexible, with a heavy motor on top of a long motor pedestal and discharge head. This can be further weakened by a light base foundation with a mass less than five times the weight of the supported equipment. This makes it fairly likely that the pump will operate at a structure critical speed, especially with variable speed drives, and/or poor pump piping that can add external forces to the discharge head. This often necessitates a vibration analysis to avoid having a problem.
When performing a critical speed analysis of an aboveground structure, the following should be considered:
- Pump structure
- Pump piping
- Lack of pipe supports close to the pump when piping is hard-coupled to the pump
- Motor or drive reed frequency (VTPs)
- Operation close to a resonant frequency
- Miscellaneous damping effects
- Variable speed units: With all of the VTP reed and line shaft vibration modes, the odds of operating at a pump critical speed is quite high. Often, certain speed ranges may have to be locked out of the VFD to avoid operating close to a natural frequency.
Other Causes of High Vibration
In addition to vibration caused by the stationary aboveground pump structure, line shafting and impeller rotational components, hydraulic disturbances can also add to the vibration.
Although cavitation is not as much of a problem with VTPs (since the bottom portion of the bowl assembly is submerged and there is less likelihood of having high suction energy), it can still add to VTP pump vibration.
Suction pressure pulsations can also increase VTP vibration when high suction energy pumps are operated in their low flow suction recirculation region (see Fig. 3).
Vibration Analysis
Normally, a VTP vibration analysis of the stationary structure, the line-shafting, and the pump and motor rotors should be done simultaneously using finite element analysis. The goal is to determine at least all natural frequencies and mode shapes up to 1.25 times the number of impeller vanes times the running speed. The foundation mass and stiffness within a radial distance at least equal to the height of the top of the motor, relative to the level of attachment of the baseplate to the floor, should be evaluated. The vendor having unit responsibility may perform a lateral dynamic analysis.
Typically engineered pumps of 100 hp or greater, especially variable speed units and/or tall discharge assembly units with L/D greater than 4.0, should be analyzed.
Conclusions
As discussed, larger vertical turbine pumps, especially short-set industrial, variable speed units, and units with tall aboveground structures (and flexible foundations), should strongly be considered for a vibration analysis prior to installation. In addition, further consideration should be given to shorter (below critical speed) bearing spans and/or larger line shaft diameters, as well as rigid, clamp type column shaft couplings.
Allan R. Budris, PE, is an independent consulting engineer who specializes in training, failure analysis, troubleshooting, reliability, efficiency audits and litigation support on pumps and pumping systems. With offices in Washington, N.J., he can be contacted via e-mail at [email protected].
Reference:
Marscher, Willian D. “An End-User’s Guide to Centrifugal Pump Rotordynamics,” Proceedings of the Twenty-Third International Pump Users Symposium, 2007.
