a ring at pump inlet to improve the NPSH? 2

[SJEC]

We have ordered a large liquid centrifigul pump (over 500HP) .
After running the test, the NPSH can not meet the requirment.

So the vendor offer two options to us.
1. add a inducer which I can understand but is not allowed by the client's spec.
2. install a fixed ring at inlet of pump. It will be just in front of the impelle but fixed on the cast.
The ring looks like a gear, it has multipul teeth toward the center of pump. The vendor explain it will break vortex etc.

The question is if any body heard about it? What is mechaism for this ring? Is there any article to explain how it works?

Thanks

 

[Artisi]

By what margin does the pump miss the NSPHr and how / where was the test undertaken onsite or in certified test facility.

Fitting a vortex breaker or flow straighhtener (nothing new here) is appropriate only if pre-rotation is causing a problem, it doesn't improve NPSHr. If the approach flow to the impeller eye is already straight and steady then this won't overcome your problem.

Unless the vendor has established that pre-rotation is a problem further investigation is needed to establish the real problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1gibson]

3. Rework the impeller.
4. If they tried that, scrap it and get a new impeller
5. If that doesn't work, have them tell you exactly how/why they sold you a pump with NPSHR they can't achieve.
6. Then explore their alternate solutions, and request (if they haven't already offered) extended warranty coverage.

If time is an issue (always is) then start with 5, ask them to get started on new casting for 4 while they work on 3.

 

[DubMac]

The use of inducers and "straightening vanes" to improve NPSH characteristics has long been an area of contention in the pump world. Although many people are wary of inducers (must admit me included), many fine companies use them; go look at Lawrence, Sundyne, Goulds for some marketing info.

The "science" behind NPSH improvement devices is very, very complex and not worth you trying to delve into at this time. Ask the salesman to explain the hydraulic principles behind the "device" he is proposing and I gaurantee you will get a blank look. Most likely he or she will tell you "well we've done it before many times and it just works".

The preferred solution would be to tell them to provide the pump they sold you (meets NPSHR). Most likely however you are under the constraints of time and may need to work with them to overcome the problem. Ask them for specific installations where they have used this before and have them confirm for you that it works.

If you don't have time for a new pump and must work with them, you absolutely need to make it very clear that they are responsible for any substandard performance incurred in the pump. That would obviously include cavitation damage but also vibration, seal issues, not meeting flow/head/efficiency.

Get a 2 year warranty and a spare impeller with rings if you accept.

 

[JJPellin]

It would make more sense to me to change the profile of the leading edge of the vanes to improve the NPSH performance. There was a good paper on this subject at the International Pump User's Symposium this year. Changing from a round profile to an elliptical profile resulted in a substantial improvement. Changing from an elliptical to a parabolic profile produced even more benefit.

Of course, if a profile change is used to meet the spec, it needs to be well documented so that some unfortunate person in the future does not accidentally un-do the upgrade during an impeller replacement.

This is the paper I am referring to:

INFLUENCE OF IMPELLER LEADING EDGE PROFILES ON CAVITATION AND SUCTION PERFORMANCE

Ravi Balasubramanian
Simon Bradshaw
Eugene Sabini


Johnny Pellin

 

[zerosequence]

What is the anticipated installed situation, in particular how is the NPSHa developed? How does it compare to the test situation? How far off from passing was the NPSHr test? Did they just throttle a valve on the suction? Are they using boost pumps. Was the test done via a tank and vacuum suppresion?

 

[SJEC]

Thanks for all of above advice.

the pump have been tested, yes, the NPSHr became lower than requirement after installation of the ring ( I even do not know how to call it).
However, the power efficiency became lower than expacted number.

Right now, we do not have any other choice.
Since the project will not allow you to spend time in another impeller.
we have to accept it.

In the meanwhile, we are considering purchas another impeller as a spare.

 

[Artisi]

1. what pump type are you discussing?
2. does it meet flow and head requirements?
3. you haven't answered re test location - is it a test loop or a site test?
4. do you have any pump curves - ie, bid curve and test curve.

If you want good advice you need to supply good information, there are some pretty smart people in this forum but we aren't mind readers.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[SJEC]

Artisi

1 pump type OH2
2 YES, both meet requirement now.(after adding the ring)
3 on test loop. the pump is still in manufactory's shop.
4 The curve is availabe. but I am sorrym, can not post the test curve here.

Thanks

 

[PumpSmart]

Any photo of the ring?
A pump mfg that can not meet NPSH test, why kind of manufacturer it is?.
It seems that it is the first time they test this pump.
Please indicate pump mfg, model, size, NPSHr you want, NPSHr they tell. What is the NPSHa?
Some mfg has a low NPSH impeller design that fixed in regular pump.

regards