Page 249 - Centrifugal Pumps Design and Application

222 Centrifugal Pumps: Design and Application

Reduced Annular Seal Clearance. Of particular interest is the growth
of the impeller eye seal diameter. As pump speed increases, the decrease
in annular seal clearance reduces leakage loss as a percentage of input
power and improves pump efficiency. Mechanical friction losses and im-
peller disk friction losses also are reduced, but the reduced leakage loss is
dominant. These effects are most noticeable in pumps of low specific
speed. Factory testing of a 1,010 specific speed pump at 2,950 rpm and
7,000 rpm demonstrated an increase of six points of efficiency (from
49% to 55%) at the higher speed. Low speed pumps are less affected.
The difference in efficiency between 1,800 rpm and 3,600 rpm operation
for most pumps is negligible.

Comparison of Diffuser Casings with Volute Casings
Both diffuser-casing construction with inline impellers and stacked in-
ner case assembly, and volute-casing construction with opposed impel-
lers and horizontally split inner case assembly can be applied with suc-
cess to critical centrifugal pump applications. There are, however,
significant differences. A comparison of these differences follows,
Diffuser Casings
Lower Cost. The diffuser-casing construction with inline impellers re-
sults in lower manufacturing cost. Additionally, if damage occurs, a sin-
gle casing segment can be replaced instead of a complete volute case. All
series impellers and series casings can be of the same design and any
number of stages can be stacked to produce the required head. Opposed-
impeller construction requires right-hand and left-hand impellers and
right-hand and left-hand volutes. Major pattern changes or separate pat-
terns are required to produce inner casings with the desired number of
stages needed to produce the full range of head requirements.
Precision Diff users. The critical portions of the diffuser passages (those
in which high fluid velocity is converted to pressure) can be investment-
cast or machined, thus manufactured with less roughness and more preci-
sion than the typically sand-cast inner volute cases.
More Compact. The inline impeller configuration is more compact. All
else being equal, the result is a shorter rotor and a shorter pump, and
because the inline impeller design has no crossover, the inner casing and
barrel diameters also become smaller than those of the opposed impeller
construction.
Simple to Destage. If the pump differential head requirement is re-
duced, it is very easy to remove one or more stages from a pump with
inline impellers. Pumps with opposed impellers can be destaged, but the
process is more complex.