I agree with your sentiments and I still do not really know why 9.6.1 was withdrawn.My problem is that I am investigating the case of a 3x350MW coal fired sub-critical plant where the EPC contract says the BFP NPSH margin should be 1m or 10% whichever is greater and a bank's engineer who says it should be 80%.At the moment I am proceeding on the basis that the EPC contract requirements apply to both continuous and transient conditions such as following a turbine trip and and the bank's engineer's requirement applies only to continuous operation.I2I,I am not exactly clear how "too much NPSH margin" can cause adverse effects except as mentioned by Artisi.
Without knowing the specifics of the plant, 10% is probably too low and 80% is most likely overkill on the margin.  Most that I've been involved with operated in the 30%-50% range.  You need to look at NPSH not only at design conditions but also at runout.  Also confirm whether the margin is stated in terms of the 3% NPSH. Excessive margins lead to larger pumps with larger suction passages.  The larger suction passages allow for a lower NPSHR, but result in higher suction specific speeds.  Since the suction is oversized, suction recirculation (and probably discharge too) will commence at much higher flowrates limiting the range of operation.  With boiler feed pumps/deaerators you also have to consider transient operation during load swings/abnormal trips/etc. 
I received ANSI/HI 9.6.1 today and have to admit that for

2011-08-14

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