In previous Pumping Prescriptions columns, we have looked at vertical pumps from various perspectives, including alignment, proper centering of the rotor shafting ("The Downside of Perfection in Vertical Pumps Repairs," May 2009) and the extent of an overhaul repair job ("Repair, Overhaul and Upgrade of Vertical Turbine Pumps," January 2008).
This month, we will look at why these aspects are so important to vertical turbine pump designs specifically, and similarities and differences between vertical turbine pumps and other pump types.
While the entire scope of centrifugal pumps is extensive, it is helpful, for simplification purposes, to consider the following general classifications:
a. End suction
b. Between bearings split case
c. Horizontal multistage
d. Vertical turbines
Both (a) and (b) are similarly different from both (c) and (d). Even more significantly, (a), (b) and (c) are similar in the transmission of their axial load, which is taken by the pump bearing and not a motor bearing. While some vertical turbine pumps feature an independent pump bearing pot, they are relatively rare. Whether the motor is hollow or solid shaft, the motor bearing takes the load, not the pump.
Vertical Turbine Alignment
Without a thrust bearing of its own and since it is positioned vertically, the pump's apparent symmetry might suggest that even a radial bearing is unnecessary. The pump bushings, placed along the shafting, are not true bearings, but rotor guides with relatively ample (as compared to a true hydrodynamic bearing) clearance between the rotor journal and the shaft. A true 4 in hydrodynamic bearing (oil lubricated, for example) typically has 0.004 in running clearance (one thousand per inch of shaft), while a guide ("bumper") bushing may have three to four times that amount-obviously with no ability to sustain a hydrodynamic loading.
This is why proper alignment, choice of the materials for the shafting and its bushings, proper restoration of the fits and concentricity of the stationary parts during overhauls are important. Bushings, unable to withstand any substantial side loads, will quickly wear, or fail, if the shaft comes in close proximity due to incorrect eccentricity of the line piping fits. True hydrodynamic bearings need to be well-aligned, and vertical turbine pumps equipped with bumper bushings should likewise be aligned. Without proper alignment, many problems arise.
When one tries to grasp the uniqueness of vertical pump design, remember the manner in which the axial and radial loads are absorbed, as it may help in understanding the subject.
The Absence of Upper Flush-A Case Study
Another important aspect to address regarding vertical pump reliability is the absence of upper flush. The upper packing, if left without flush, may result in unexpected failure. If the eccentricity between the upper packing gland is not established properly, contact may result. If it happens and flushing water is not present, there will be no packing lubrication. The remaining water within the clearance between the sleeve and the mating bushing will quickly vaporize, pushing the packing out, breaking the gland and the bolts, and likely causing more damage.
Remember the components' concentricity is critical to successful pump operation. Concentricity is often as critical, if not even more critical, than dimensional tolerance. While proper clearances between components-for example, shafts (or their sleeves) to journals-are important, it is the components' concentricity (or lack of) that makes a difference between proper pump operation and failure.
While seemingly one of the smallest or least noticeable parts, the packing box is often a key element to a pump's successful operation. In this particular situation, the packing box (item 10 in Figure 2) did not appear to have been trued up (machined) or cleaned during the last repair, judging by its rusty appearance and rough steps and edges.
The packing box was likely sitting off-center.
The pump also arrived from the repair shop with the packings already installed. The correct procedure would be to first center the shaft within the packing box (the instruction manual should clearly illustrate this), then align the motor shaft to the pump shaft with the packing finalized. This procedure ensures proper concentricity of the shaft to the packing box, and of the box to the pump main casing to which the box is attached.
Apparently, a similarly incorrect installation procedure might have happened with other pumps at the facility. A sister unit had a motor sitting "off-lined" in an apparent attempt to align the shafts given the "tilted" (not centered prior to packing) pump rotor.
The situation was likely caused by similarly pre-packed pumps that arrived from repair shop(s) and were aligned to the motor in an eccentric state, as noted above. This causes the clearance between the shaft sleeve and bushing to be uneven, with one side likely coming too close and possibly contacting, especially if the shaft (may have) excessive runout.