Page 71 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

56 Practical Machinery Managementfor Process Plants

Figure 1-23. Shaft fit vs. mounted preload on centrifugal pump bearlngs.

Improving Seal Life
Several Scandinavian pulp and paper plants routinely achieve in excess of
42 months mean-time-between-failure (MTBF). In 1993, several North
American paper mills reported a dismal four months MTBF for their hun-
dreds of mechanical seals. While the failure ratios between North America
and Scandinavia are less severely biased in such industries as hydrocarbon
processing, there are still untold improvement options available to engineers
and technicians who refuse to see their jobs as "'business as usual."
A single X in. diameter hole drilled as shown in Figure 1-24, will safe-
guard and dramatically extend the life of many mechanica1 seals by venting
air trapped during shop work or gases that might accumulate in stand-by
pumps. Of course, operator diligence and conscientious venting of centrifu-
gal pump seal housings could accomplish the same thing.
Another highly beneficial seal life extension program starts by identifying
pumpage that contacts seal faces under pressureltemperature conditions at
least 15°F away from the initial boiling point (IBP). Many of these applica-
tions, and especially those containing massive amounts of solids, are candi-
dates for dead-ended stationary seals and steep-tapered housing bores.