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Upgrade the design of vertical, multistage centrifugal pumps in low-temperature services

12.01.2012  |  Bloch, H. P.,  Hydrocarbon Processing Staff, 

Keywords: [pumps] [bearings] [shaft] [bushings] [lubricating oil] [maintenance]

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 factors:

  Fig. 1. Typical vertical-
  pump inlet bowl and
  mixed-flow impeller.

  1. 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.
  2. 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.
  3. 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 bearing
      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

  4. A nominal diametral clearance of 0.010 in. is recommended for the bore of labyrinth bushings not serving as bearings.
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. In cryogenic temperature environments and where dual seals are used, low pour-point synthetic lubricants will be advantageous as a barrier fluid.
  10. 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

The author

Heinz P. Bloch resides in Westminster, Colorado. His professional career began in 1962 and included long-term assignments as Exxon Chemical’s 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 and Texas.

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Doug Knutson

It would be interesting to see Mr Bloch's comments on how these tips apply to long vertical sulphur pumps. We typical get about 13 months of use before rebuild, it operating continually. However starting and stopping seems to be the trigger to even short-life if not properly warmed and checked. Much of our problems relate to aux steam jacket temp not hot enough during winter months. Any Lessons learns or tips would be welcomed.

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