Hi everybody,what is the relationship between inducer and NPSH? and Please give me more Mou.Best Regards

An inducer increases the NPSH Available by increasing the pressure at inlet to impeller.

The inducer has nothing to do with the NPSH availabe because that is part of the process. The main reason to include that on a Pump is to reduce the NPSH required.
Basically works as a booster Pump for the main impeller.
The principal disadvantage of the impeller is that increases in an important way the NSS.

Hi Djillali ,
There should be always a margin between NPSHa and NPSHr of the pumps, i.e: NPSHr>=NPSHa+Margine. Required Net Positive Suction Head (NPSH) of a centrifugal pump may be costly or sometimes impossible to improved by increasing the height/elevation of a vessel/suction drum or maintaining higher than normal liquid levels or increasing suction pressure. It may be impossible to improve NPSHr by decreasing the pump speed, ae well. Actually, sometimes running pumps faster than normal by for example variable frequency drive can actually help avoid cavitation
Inducers are sometimes a low cost alternative/means that reduce the NPSH required by centrifugal pumps but it rarely approved by end-user based on my experience. An Inducer is an axial flow impeller with blades that wrap in a helix around a central hub. An Inducer serves as a small booster pump for the main impeller. Although the inducer usually has a lower NPSH requirement than the main impeller, it may face with cavitation during normal
operation, as well. The inducer has the following disadvantages:
1-Inducers add mass, cantilevered away from the bearings,
2-It is possible to make problems worse by applying an inducer,
3-An inducer invariably has a higher suction specific speed (S) than the adjacent
impeller.
I hope the above answers to your question.

Hi Shahab,
I read your response with interest in particular that sometimes increasing pump speed using a VSD can "actually help avoid cavitation". Could you please explain in more detail how this happens?

I think by increasing the speed the chances of cavitation shall also increase.

Hi Anis,
This is my premise but I am willing to be convinced otherwise.
In my logic if a pump has already reached it's cavitation point then increasing the speed increases NPSHr.

Shahab
By varying the speed by any means , Affinity law will come into play, Hence your statement is totally wrong. It is otherwise.
Decrease in speed will definitely will improve suction condition hence less NPSR.

Anis & Mahipal,
It is good that we are in harmony on this front. I thought I had been missing something on rotodynamic technology over many years.

An inducer is a tool, useful in those cases where the NPSH is a critical condition. Not in all situations an inducer is the right option because it depends on performance curve of the pump.

Hi All,
Thanks to your feedback. However, please note that as general, I agree with you that lowering the speed will improve NPSHr. However, please note that the shape of the NPSHR curve past the BEP point is important. If the actual pump has a linear shaped NPSHR curve with flow, then running the pump slower, against a given limitation of the NPSHA, would indeed help a cavitation problem to happen. It depends on intersection point of NPSHA with and NPSHr curve and actual shape of NPSHR curve. Another problem is by lowering the speed the NPSHr may be improved, but the desired head may be sacrificed.

Hi;
I'm just curious if using inducer has any effect on performance curve (H Vs Q) and BEP or not?

The aim of the inducer is boost the static suction head of the pump, without severely affecting the head or flow characteristics of the pump. The power absorbed by the inducer should be minimal. One would ideally like to say the pump is not affected by the inducer, but the sad reality is that inducers have good on design performance, but poor off design performance. This effectively limits the window of operation of the pump (more so with helical inducers - axial inducers are not too bad across the curve). Also not to mention the potential for adverse effects of recirculation etc. As a result, the inducer reduces the NPSHr of the pump. This allows the pump to run systems with lower NPSHa or run at higher flows/speeds without cavitation (as opposed to the same un-induced pump).
I love inducers, they are really a nice challenge to design hydraulically. I have yet to come across a source that has a one stop guide to their design.

Regards,
Zac - BEng(Mech)(Hons)