sri,
Addressing your points 1 & 2,
1) I think it should be understood that the liquid level in the tank is at least at or above the outlet nozzle feeding the pump. Thus the liquid level in the tank, relative to grade, would be +3 ft, same as the pump suction centerline. (With an internal diptube and sump, this is not necessarily true, but it is pretty common.)
2) I concur with your observation that all 4 examples are really "flooded suction" situations. In fact, if you don't have a flooded suction, then you are "pumping" air or vapors, typically undesirable. Perhaps my usage of the word is not conventional, but I use the term "flooded suction" to emphasize the absence of a vortex or other undesirable phenomena that would introduce a gas or vapor into the entry of the pump suction line.
Comment to Art: I believe your reference elevations for examples 1 & 2 are not consistant. I concur that taking the liquid to be at the bottom of the vessel is conservative and good approach. This elevation, like the pump's centerline, should be relative to grade, which differs from the sketch.
Additional comment to Art: Excellent NPSH presentation.
Dear sri/ djack77494 Hello/ Good Evening,
Although your perception in way for 'flooded suction' seems justified, but there is a
Minimum NPSHa value 20~25 ft considered as benchmark Flooded suction criteria.
Art kindly correct me if this is not the case.
In all fainess Last line of Doug's post is whole heartedly seconded by me.Regards
Qalander