Hi Artisi,
Your comments are most welcome, and I believe that we are probably more in agreement than most would think. Your comment on the need for true flooded-suction conditions is particularly important and prudent.
In thankful response to your comments, I think that I should provide additional clarification on my thoughts and comments.
There was a time when I would have agreed with you on the apparent absurdity of the large NPSHa margins that I have recommended to be considered, but my experiences with several major, critical pumps have lead me to think that margins of such proportions may well be wise. Indeed, I do agree with you to the extent that it is often practical and prudent to install less critical, cost sensitive pumps with less (sometimes much less) than optimum NPSHa. For an application as critical as the suject of this discussion, knowingly installing a pump with anything but very generous NPSHa margins would be beyond all possible reason.
In any case, compromised durability or performance should be a should be a considered choice and not an unpleasant surprise. After all, engineering work always involves optimizing compromises to achieve suitable results.
The experiences to which I refer involved pumps that had undergone individual witnessed performance tests at full rated speed and with numerous test data points over the entire flow range. In all cases, these pumps were installed and operated with NPSHa in the range from about 1.5 to over 3 times the demonstrated 3% NPSHr, and all suffered from substantial cavitation that resulted in excessive wear and correspondingly elevated maintenance and repair costs. The pumps all did perform adequately, and the observed operating power consumption and performance characteristics were in reasonable agreement with the test stand performance data. In all cases, the costs and practical physical considerations of the installations precluded doing anything to substantially improve the NPSHa. The use of some hard surfacing materials was about the limit of practical efforts to mitigate the effects of the cavitation. Once their rates of deterioration were recognized, the pumps could always be depended upon to run reliably between scheduled major repairs, but the service life between of these major repairs was always far less than it should have been had the NPSHa been truly adequate. In the contemporary context of the original system design and pump selection for these pumps, there was nothing remotely irresponsible about either the selection or application of the pumps, but the results were far from ideal. (One could, perhaps, observe that innocent ignorance does not always result in bliss.)
When I was in engineering school and early in my career, I never once encountered a recommendation by anyone that it would really be a good idea to provide a substantial NPSHa vs. NPSHr margin. Professors, older engineers, and pump manufacturers' representatives with whom I dealt "way back then" always insisted that NPSHa = NPSHr was entirely sufficient. Somewhat similarly, there was a common presumption that centrifugal pumps were "variable flow" devices that could be casually operated well away from BEP without any significant concern presuming that the drive motor and control valve were adequate to their tasks. Wasted energy and installation costs were matters of economic choice for determining whether multiple pumps should be used to cover especially wide flow ranges.
Experience and study over many years has revealed to me the absurdity of the presumptions mentioned in the above paragraph. Pumps are fascinating and potentially rugged machines, but the older I get, the more I recognize the importance of suitable pump selection and application within the context of the specific application. The current fad of seemingly using VFD's to compensate for poor, ill-informed, or ill-defined pump application has served to further emphasize to me the necessity of prudent pump selection and application.
I have often thought that it may be wise for the pump industry to adopt a standard notation for NPSHr in a form such as NPSHr(1%) or NPSHr(3%) to clearly indicate that the NPSHr values represent conditions where the pump's performance is already compromised by 1% or 3% due to cavitation. This would help to serve as a warning to non-engineers and non-pump-specialist engineers that no "warm, fuzzy comfort" should be attached to the NPSHr values and that NPSHa really does need to be substantially greater than NPSHr.
