Page 177 - Improving Machinery Reliability

148 Improving Machinery Reliability

(Nsss)* can often be expected to give satisfactory operation only over a narrow oper-
ating For these pumps, vendor's quoted net positive suction head values
(NPSHR) away from the best efficiency point (BEP) are rightly questioned. Also,
reasonable explanations have lately been offered in the technical literature linking
high specific-speed impellers with recirculation phenomena which, in turn, are influ-
enced by pump geometry. Intolerable recirculation appears to exist in high head
pumps using large impeller inlet-eye areas in efforts to achieve low NPSHR. See Fig-
ure 3-41 for details.
In a 1977 technical paper,33 Irving Taylor sought to quantify probable NPSHR for
zero cavitation erosion of pumps in various services.
Providing a series of approximate curves for pumps with different suction-specific
speeds (Nsss), he alerted users to the fact that NPSH requirements for long-term sat-
isfactory operation of pumps away from the best efficiency point (BEP) may be sub-

Figure 3-41. Running a single-suction impeller at normal flow (top) avoids the internal
recirculation that occurs at low flow conditions (bottom).

- N x Q0.5 where N = pump speed, rpm
*NSSS -
(NPSH,).75 ' Q = pump flow, gpm, at BEP