Page 174 - Improving Machinery Reliability

146 Improving Machinery Reliability

400
.
5 350
.9 Aluminum Composition I
5 300
c
5 Babbitt Composition I
8 250
A
c
g 200
II)
.-
E 150
E
100
1
Babbitt Cornposition 111
50
' I ' ' - BabbittThickness, mm
0.05 0.10 0.15 0.20 0.25 0.30
Figure 3-40. Maximum allowable pressures for babbitted bearings.

Reliability Review for Centrifugal Pumps
Significant design differences can exist among centrifugal pumps in spite of their
compliance with such accepted industry standards as API 610 or ANSI B73.1 and
B73.2. A number of reliability improvement or verification steps can be recommend-
ed, and three typical examples of design weaknesses are illustrated later.
Potential design weaknesses can be discovered in the course of examining dimen-
sionally accurate cross-sectional drawings. There are two compelling reasons to con-
duct this drawing review during the bid-evaluation phase of a project: first, some
pump manufacturers may not be able to respond to user requests for accurate draw-
ings after the order is placed; and second, the design weakness could be significant
enough to require extensive redesign. In the latter case, the purchaser may be better
off selecting a different pump model.
Looking at pump components, we note that rolling-element bearings are not
always selected for satisfactory operation over a wide range of operating conditions.
This is where user experience and better selection guidelines can be of value.
Improvements can come from better engineering and better dissemination or presen-
tation of pertinent data. Bearing-related improvements are described later.

Service Life is Influenced by NPSH and Suction-Specific Speed Criteria
It is beyond the scope of this text to examine pump hydraulic performance in
detail. Hence, a brief overview will suffice.
Many of the major contractors and experienced machinery engineers in the petro-
chemical industry are aware that centrifugal pumps with high suction-specific speeds