Page 189 - Improving Machinery Reliability

160 Improving Machinery Reliability

Table 3-14
Replacement Wording Proposed for API-610 7th Edition, Paragraph 2.9.1.5
Specification Amendment for Heavy-Duty Centrifugal Pumps
(This S.A. is appropriate for attachment to inquiry or purchase document.)
2.9 Bearings and bearing housings
2.9.1 Bearings for horizontal pumps
2.9. I .5 (Exception)
a. If bail-type thrust bearings are used, the vendor shall verify the adequacy of duplex, sin-
gle-row, 40" (0.7 radian) angular-contact type (7000 series) bearings, installed back-to-
back (DB). This verification shall include selection of proper preload values to prevent
skidding of the unloaded bearing over the entire operating range of the pump.
b. If a., above, points to inadequacies or potential problems, the vendor shall offer suitably
preloaded sets of ball-type thrust bearings with dissimilar contact angles (e.g., 40"/15").
c. Sets of properly preloaded, back-to-back mounted 15" or 29" angular contact bearings are
acceptable for pumps with double-flow impellers located between two bearing housings.
d. In each case, the need for and extent of preload shall be determined by the vendor to suit
the application and meet the bearing life requirements of 2.9. I. I.

DN value of the bearing, with D being the bearing bore (in mm), and N representing
the shaft speed (in rpm). For DN values below 80,000, we agreed to a preference for
sealed, lifetime lubricated bearings. The range from 80,000 to 108,000 we labeled
the gray area, and from 108,000 to 300,000 we defined a clear advantage for
regreasable bearings. (It has been a long-standing practice for bearing manufacturers
to recommend oil lubrication for bearings with DN values greater than 300,000.)
With DN values of 80,000 obtained, for example, by a 40 mm bore diameter radial
ball bearing operating at 2,000 rpm, Figure 3-48 recommends regreasing intervals of
approximately 9,000 hours-about one year. If such a bearing is provided with seals
and is therefore lifetime lubricated, we anticipate it to become unserviceable after
roughly two and a half times the stipulated regreasing period. These experience fig-
ures have been validated by some users and are reasonably correct. For a radial ball
bearing with a DN of 200,000, say 100 mm and 2,000 rpm, the recommended
regreasing interval shown in Figure 3-48 would be slightly less than 3,000 hours, or
four months of continuous operation. Using the two and a half times rule, we might
expect frequent failures after about ten months of operation-clearly not acceptable
for most process plants. Thus, pay attention to DN values when deciding how and
when grease lubrication is applied or reapplied.

Marginal Lubrication: A Factor in Pump Failures
As indicated earlier, a pump manufacturer will occasionally supply pumps with
inherent design vulnerabilities. In fact, we must assume that the designs of at least
some pump manufacturers who give bearing temperature concerns as the reason for
disliking lightly preloaded or high-contact angle bearings are really suffering from
marginal lubrication. In one instance, we found a manufacturer supplying pumps
with excessive bearing inner race interference fit. Had he chosen to furnish pre-
loaded bearings as well, early bearing distress would have been virtually certain.