The overall mechanical design of most API-style
vertical-column pumps (Fig. 1) comes close to
meeting the expectations of modern pump users. Such pumps are
often equipped with self oil-lubricated bearing thrust
assemblies in contrast to older designs that depended on
motor-thrust bearings only. Some older vertical pumps deserve
to be closely reviewed. Based on failure history and
criticality of service, older vertical-column pumps are
candidates for upgrading at the next routine repair or maintenance opportunity. The
adequacy for large vertical pumps should include these 10
Fig. 1. Typical
pump inlet bowl and
- The dn value of the bearings (shaft rpm
multiplied by the mean bearing diameter, in mm). It should
not exceed the experience-based limit of 500,000.
- Disclosure of mechanical seal pv values
(pv = pressure x velocity). Also seal component
materials and seal balance ratios are needed. Working with a
respected manufacturer, reliability-focused users should
verify the pv values against prior experience. In
case of unusually high pv values, the locations and
names of contact persons may be needed.
The line bearings or column bushings used for shaft
stabilization (Fig. 2) should be made from
high-performance polymer materials. Many high-quality
bushings contain carbon-graphite fibers. The typical
diametral clearance should be [(0.001) (shaft diameter,
in.) + 0.002 in.] For a nominal shaft diameter of 1.6875
in., the bushing bore should be 1.6912 in. +/0.0005
in. Three or four axial grooves should be provided in the
bushing bore to counteract fretting risk during occasional,
but potentially severe, rubbing contact.
Fig. 2. Column
sandwiched between two column
flanges. A two-piece split
tapered bushing secures two
keyed shaft ends in place; a
single-piece tapered sleeve fits
over the two-piece split tapered
- A nominal diametral clearance of 0.010 in. is recommended
for the bore of labyrinth bushings not serving as
- If vibration probes are used, the probes should monitor
both high-frequency acceleration and low-frequency velocity.
Gradually developing bearing defects will show up in the
acceleration spectrum before there are velocity excursions.
During shop testing, the pump manufacturer should verify the
absence of resonant vibration. This is especially important
in variable-speed vertical pumps; resonant vibration must be
absent at all anticipated operating speeds.
- Hand-fitting of keys and bottom-radiusing of keyways
should be considered, and roll-pins should not be used for
key fixation. Improved shop practices will increase the shaft
factors of safety.
- Monitor proper assembly procedures. Bearing manufacturers
have long insisted on either supporting the bearing inner
ring while pushing on a shaft or, alternatively, while
pushing the bearing inner ring on the shaft.
- O-ring selection varies with the fluid being pumped.
Teflon wrap over nitrile rubber or Viton cores should be
considered for olefin services. The final selection should be
approved by an O-ring or mechanical-seal manufacturer.
- In cryogenic temperature environments and where dual
seals are used, low pour-point synthetic lubricants will be
advantageous as a barrier fluid.
- There is universal agreement among bearing manufacturers
that an oil spray introduced into the bearing cage (ball
separator) is the most desirable lubricant application
method. An oil spray greatly reduces the risk of overheating,
which is a primary concern in pump geometries where several
rolling element bearings are assembled as a stack of two or
more bearings. HP
Heinz P. Bloch resides in
Westminster, Colorado. His professional career began
in 1962 and included long-term assignments as Exxon
Chemicals Regional Machinery Specialist for the
US. He has authored over 520 publications, among them
18 comprehensive books on practical machinery
management, failure analysis, failure avoidance,
compressors, steam turbines, pumps, oil-mist
lubrication and practical lubrication for industry.
Mr. Bloch holds BS and MS degrees in mechanical
engineering. He is an ASME Life Fellow and maintains
registration as a Professional Engineer in New Jersey