7 replies to this topic

[sri]

I was reading an excellent write up by Mr Art on NPSH which was attached in one of his posts. I'm attaching the same file again.
I have certain doubts in the examples given.
1. In example no.3, tank bottom elevation given as +3 ft. However, pump suction nozzle is above tank bottom. Considering no credit for liquid level in tank, head due to tank bottom elevation should have been 0 ft.
2. Examples 3 & 4 have been cited as 'flooded suction'. However all the four examples looks to be of flooded suction.
May someone pl clarify.

Thanks

Attached Files

•  Arts_NPSH_Tutorial.doc   167KB   117 downloads

[sri]

still expecting someone to clear the doubt please.

[djack77494]

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.

[Qalander (Chem)]

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

[fallah]

I think in examples of 1&2 and as per relevant sketches, the "Pump suction elevation" should be 0.00 ft.

[djack77494]

I think in examples of 1&2 and as per relevant sketches, the "Pump suction elevation" should be 0.00 ft.

Using the pump suction centerline as the datum plane would be acceptable. The important point is to be consistant, using the same datum plane throughout the calculation.

[Qalander (Chem)]

I think in examples of 1&2 and as per relevant sketches, the "Pump suction elevation" should be 0.00 ft.

Using the pump suction centerline as the datum plane would be acceptable. The important point is to be consistant, using the same datum plane throughout the calculation.

Or with your permission Doug just to re-phrase;
The pump centerline should serve as basic grade level benchmark w.r.t. NPSH related issues.
Regards
Qalander

[sri]

Thanks djack, qualander & fallah for your valuable time.