Shroudless VS Shrouded Impellers

[cryopumps]

In the industry that I work in they have always used shrouded impellers and I am now wondering why. Looking at the alternative and not using a shroud I see a a possibility of easy maching (CNC with great surface finishes), no efficiency loss from wear ring clearance, and less material so lower weight. Using the shroud we have to cast the impellers which is not accurate and we can not thoroughly check them and the surfaces are horrible. Does anyone know why we should not being using shroudless impellers.
The industry is multistage pumps for the LNG gas industry. (methane, propane, butane)

 

[Scipio]

There are two reasons I'm aware of to use shrouded over semi-open impellers. One is that shrouded impellers are less sensitive to axial positioning. A semi-open impeller's efficiency is dependant on proper positioning with respect to the front side of the suction casing, so end float adjustments and thermal effects have to be taken into consideration. Second, I believe you'll find those efficiency losses you refer to due to wear ring clearance are better than the efficiency losses you're likely to see from discharge-to-suction slippage in a semi-open design. Normally I only see semi-open impellers on small ANSI and slurry type pumps.

 

[Artisi]

Scipio's comments on the benefit of closed impeller versus open impeller and leakage may be relevant,so long as you are working in a process industry with clean product, that is, a product without ANY entrained eroding materials.
Your initial comments about open impellers and higher surface finish is also relevant, a reason why the majority of high quality small pumps are fitted with open impellers as they are easier to manufacture to give a much superior surface finish to the wetted areas and higher efficiencies.
The majoring influencing factor in the process industry in the choice of open or closed impellers is application, some industries demand open impellers as closed impellers could never be considered because of the nature of product. If your application is to pump hard and abrasive solids or abrasive slurries you will then use closed impeller pumps in rubber or hard metals to suit the application, although again, the smaller end of this will more than likely be open impeller because of manufacturing constraints.
For your industry small open impeller pumps should be ok, as you can set the right clearances between impeller and casing and or wear plate (if fitted)and this clearance should remain constant as the product is clean.

International College
Naresuan University
Phitsanulok
Thailand

 

[vanstoja]

Semi-shrouded impellers (backplate but no front shroud)have been widely used for centrifugal compressors but rarely for centrifugal pumps. If flow channel surface finish is a critical concern for efficiency or flow noise control then open impellers should be considered since fine surface finishes can be obtained by 5-axis milling machines under numerical control. If the impeller shaft is free-floating (eg. with fluid film thrust bearings)endplay control is problematic with open impellers because tip clearance losses may be too high and tip clearance vortex flows transverse to main channel throughflows can create severe rotating stall conditions at off-design flowrates. Rolling element thrust bearings with minimal shaft endplay may provide adequate tip clearance control for open impellers. One alternative if endplay clearance is a concern , is to machine the open impeller to get good surface finishes and then install a separate front shroud plate by welding/brazing.

 

[TZellers]

That's a pretty detailed post vanstoja, but I have to disagree on the "semi-shrouded impeller" comment. An impeller with a rear shroud and no front shroud is typically called a semi-open impeller (I know, half-empty or half full). They are very common in centrifugal pumps, particularly in the ANSI market (almost exclusively for chemical process pumps) and sub-ANSI (fractional HP units).

Semi-open impellers rely on the clearance between the front edge of the vanes and the casing for maintaining efficiency, whereas enclosed impellers rely on the wear ring clearances. Since the clearances open up over time, semi-open impellers have the advantage of having around 5 times the surface area to wear (area of all the vane edges compared to the a enclosed impeller's ring area). Also, semi-open impeller pumps usually have an external adjustment to reset the clearance to factory specs to reclaim the lost efficiency over time, albeit with no concern over the reduced b2 width.

Fully open impellers are usually used in non-clogging applications, where solids passage is key, and efficiency is not priority one. If you rely on the clearances between the both vane edges and the casing (front and rear), how can you adjust over a period of time? Go forward and open up the rear clearance or vice versa?

 

[Artisi]

TZellers.
You have summed it up very well and your comment on vastly increased surface area available for wear is spot-on.It is certainly easier and cheaper to adjust an impeller against the casing or wear-plate than to dismantle and replace wear-rings which in some cases also requires a machining operation.
I spent more years than I like to remember in industries using predominantly open / semi open impellers and know quiet well the failures of impeller / casing wear-rings of applications with other than CLEAN product.


International College
Naresuan University
Phitsanulok
Thailand

 

[TZellers]

Thanks Artisi. I've only been on the "inside", i.e. the sales, marketing and engineering side. So, I know the logic, just haven't had the live experience it sounds like you have.

 

[Artisi]

just to extend TZellers comments a little and expand on my own regarding pump efficiencies.
A well designed open - semi-open impeller can give good efficiency, for instance: the paper stock pumps of Allis Chalmers, Worthington, Scan Pump, Byron Jackson etc. all of which are either open or semi-open impeller design and have efficiencies of 80 - 85% + depending on pump size. These efficiencies can usually be maintained for the most part of the life of the impeller and wear plates.
The same goes for the quality built chemical process pumps with open and semi-open impellers manufactured by a large number of companies -high overall efficiency across a wide range of pump sizes and duties.
Sure - these process pumps don't get into the 90% range of some of the large water pumps- but then water pumps can't pump paper stock, sewage solids, abrasive contaminates etc.

To sum up - selecting the best pump for the application is always a trade off, you have to decide which is the right pump for the application at the best efficiency balanced against a sound engineering decision.

If the application demands an open or semi-open impeller then efficiency becomes secondary consideration whereas for a clean product pump the efficiency should be the primary consideration - all other things being equal.


International College
Naresuan University
Phitsanulok
Thailand