Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
8 NPSH The expressions NPSHR and NPSHA are accepted abbreviations for net positive suction head required and net positive suction head available. Probably more has been written on NPSH than any other subject involving pumps. With so much literature available, one might assume that NPSH and its relationship to cavitation is well understood. Nothing could be further from the truth. To this day, NPSH is still misunderstood, misused, and misapplied, resulting in either costly over-design of new systems or unreliable operation of existing pump installations. Avoiding these problems requires accurate prediction of NPSHR, supply of sufficient NPSHA, and attention to suction piping approach. NPSHR can be considered the suction pressure required by the pump for safe, reliable operation, Establishing NPSHA Establishing NPSHA, which is the head available characteristic of the system that provides flow of liquid to the pump, is the responsibility of the system designer. As NPSHR increases with pump capacity, normal practice is to establish NPSHA at the operating condition, then add a reasonable margin to accommodate any anticipated increase in pumping capacity. All too often, future operating problems begin here. It is not unusual for the ultimate user to add some anticipated increase in capacity, then for insurance the contractor designing the system adds even more. When the pump designer finally gets the data sheet, he designs the impeller inlet and suction nozzle geometry for operating capacity, plus user margin, plus contractor margin. If these margins are not carefully con- 85
86 Centrifugal Pumps: Design and Application trolled, the result can be an over-sized pump that operates well to the left of BEP (Figure 8-1). Such over-designed pumps are vulnerable to surging, recircuiation, cavitation, noise, and vibration. This is particularly true with high-suction specific speed pumps above 11,000 where the inlet geometry has already been extended for minimum NPSH. For minimum energy consumption and trouble-free operation, pumps should ideally be operated between 80% and 100% BEP. As this is not always possible or practical, pumps will often operate at lower flows. It is therefore important that the minimum flow for continuous trouble free operation be carefully considered by the pump designer. Minimum flow is influenced by physical pump size, margin between NPSHA and NPSHR, impeller inlet geometry, suction nozzle geometry, mode of operation, and last but not least, the liquid being pumped. With so many variables it is not unusual to find recommended minimum flows ranging from 10% to 60% BEP. Figure 8-1. Margins of safety result in oversized pump.
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86 <strong>Centrifugal</strong> <strong>Pumps</strong>: <strong>Design</strong> <strong>and</strong> <strong>Application</strong><br />
troll<strong>ed</strong>, the result can be an over-siz<strong>ed</strong> pump that operates well to the left<br />
of BEP (Figure 8-1). Such over-design<strong>ed</strong> pumps are vulnerable to surging,<br />
recircuiation, cavitation, noise, <strong>and</strong> vibration. This is particularly<br />
true with high-suction specific spe<strong>ed</strong> pumps above 11,000 where the inlet<br />
geometry has already been extend<strong>ed</strong> for minimum NPSH.<br />
For minimum energy consumption <strong>and</strong> trouble-free operation, pumps<br />
should ideally be operat<strong>ed</strong> between 80% <strong>and</strong> 100% BEP. As this is not<br />
always possible or practical, pumps will often operate at lower flows. It<br />
is therefore important that the minimum flow for continuous trouble free<br />
operation be carefully consider<strong>ed</strong> by the pump designer. Minimum flow<br />
is influenc<strong>ed</strong> by physical pump size, margin between NPSHA <strong>and</strong><br />
NPSHR, impeller inlet geometry, suction nozzle geometry, mode of operation,<br />
<strong>and</strong> last but not least, the liquid being pump<strong>ed</strong>. With so many<br />
variables it is not unusual to find recommend<strong>ed</strong> minimum flows ranging<br />
from 10% to 60% BEP.<br />
Figure 8-1. Margins of safety result in oversiz<strong>ed</strong> pump.
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