Page 94 - Improving Machinery Reliability

66 Improving Machinery Reliability

procuring pumps that are required to comply with the standards of the American
Petroleum Institute (Le,, API 610), a capable vendor will make a diligent effort to fill
in all of the data requirements of the API specification sheet. However, the real
depth of his technical know-how will show in the way he explains exceptions taken
to API 610 or to supplementary user’s specifications. Most users are willing to waive
some specification requirements if the vendor is able to offer sound engineering rea-
sons, but only the best qualified centrifugal pump vendors-those from whom you
want to purchase-can state their reasons convincingly.

Concentrating on Problem Applications

In assessing vendor experience, the engineer responsible for vendor selection
should concentrate on pumping services that have a history of being troublesome.
The approaches proposed by the various bidders for solving typical problem applica-
tions may differ drastically and allow rapid separation of sound proposals from
potentially troublesome ones.
One of the most common problems in pump application is insufficient net positive
suction head available. Often this is not realized until vendors’ proposals have been
solicited and some vendors have failed to meet the conditions.
When NPSH is insufficient, the plant design should be reviewed to determine
whether decreasing the length or increasing the size of the suction line is feasible.
Raising suction-vessel elevations is another possibility, but economics may dictate
selection of pumps designed especially for low NPSH. When NPSH availability is
limited, the pump vendors, of course, will offer double suction pumps when possi-
ble. Also to be considered are inducer-type impellers, but it must be realized that
inducers may have a limited flow range relative to a non-inducer-type pump. Also,
inducers should never be used in erosive services. Pump performance deteriorates
rapidly as the effectiveness of the inducer is reduced by erosion, and cavitation
begins to take place.
Another solution to NPSH problems is to use a vertical deep-well pump. In order to
minimize maintenance problems with this type of pump, each proposed pump should
be checked for applicable service experience, with attention to exact model numbers,
similar pumpage characteristics, and process conditions. High-head, low-capacity ser-
vices also pose problems to engineers selecting pumps. In general, four types of
pumps are available for this application: multistage horizontal centrifugal, multistage
vertical centrifugal, reciprocating, and single-stage high-speed centrifugal. For
process units in continuous duty, experience has shown that multistage vertical and
reciprocating-type pumps require more maintenance than the other two types. If a
pump must handle a wide range of specific gravities, or if the fluid is particularly vis-
cous, a reciprocating pump may be the only answer. If there is limited NPSH avail-
able, a multistage vertical pump may be required. But for the bulk of high-head, low-
capacity applications, serious consideration should be given to high-speed pumps,
with the multi-stage horizontal centrifugal pump a good second choice.