exit angle of an impeller

[mec5216]

Hello

Is there any way I could measure the exit angle of an impeller??

Thank you!

 

[Artisi]

yes and but why?

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.)

 

[mec5216]

If so, how can I do that?

 

[LittleInch]

Ask a vendor for a sectional drawing or ask them the same question.

Or take one apart and measure it. Curious as to why you want to know though...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DubMac]

I used to use protractors when I measured angles.

 

[mec5216]

By trimming an impeller, its exit angle is changed a bit. I was wondering if this changing in the exit angle could be so great that the new head due to the impeller trimming would not follow the affinity laws anylonger.

 

[mec5216]

DubMac, how do you measure the exit angle using a protractor ?

 

[Artisi]

"By trimming an impeller, its exit angle is changed a bit. I was wondering if this changing in the exit angle could be so great that the new head due to the impeller trimming would not follow the affinity laws any longer."

Yes and no - applying the affinity laws with any accuracy is only applicable for approx.10% - maybe 20% reduction in diameter depending on impeller design,beyond this change the affinity laws become a little inaccurate making it difficult to predict H /Q /P with any certainty, however some pump companies will have curves for different diameters based on test results, these can be used with reliability.

However, in the real world AND if talking about standard medium sized process pumps most "pump people" would apply the affinity laws without to much concern and if not sure would probably not make the full calculated trim first time round - especially for a critical application - but trim and test pump performance in situ, if over performing it's easy to trim a little more. Now if the pump should under perform then there are a few "trick" to get back some flow- but this in another discussion.

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]

So, you trimmed but forgot to underfile (or didn't go a great job with the underfile) and now you are trying to figure out why you are low in head?

I'll check back after about a dozen more posts to see if any information about the problem you are trying to solve somehow appears.

 

[LittleInch]

How much did you "trim" it by??

Was it still within the max and min impellor size given by the pump vendor??

Normally this is the two thick lines on the pump curve. Yes the angle will change, but within the max / min impellor bands, flow curves will normally be close to an iterative curve which mirrors the min / max diam curve, e.g. min is 160mm, max 200mm, you're at 180mm, you should be half way between the min / max curve so at any flow point, the head will be half way beteeen the min and max curve. Of course all curves have an accuracy limit on them any way, but normally suppliers will give you a little more than your duty point.

Of course below the min diameter everything changes....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DubMac]

Per Artisi, the margin of uncertainty in affinity laws introduced by differing vane angles is much smaller than the margin of error in the affinity laws themselves. There are other forces at work, such as varying radial forces due to trim, which may have the same or greater effect than vane angle change.

As he says, if you are truly in an exacting situation, you will do no better than utilizing empirical data. Cut the impeller just outside of what the affinity laws tell you to; measure accurately, and start the iterative process down to dead nuts accuracy.

Is this just a theoretical exercise??? Not that there'e anything wrong with that.... if not could you share why you desire such accuracy?

 

[ApplicationENG]

Alng the same line as Artisi,

Who is the pump OEM? Ask them for the pump curve to the impeller diameter you just cut it to. They will need the material, speed, model, diameter and pump size. if you have all this, it should take them 5 min. to get back.

 

[PumpSmart]

Yes,
Made a front view of tour impeller, measure angle of vane line and tangencial line in eacn vane.
average angle could work very well.
As per desing, you could calculate Ns and go to pump handbook to see how much angle will be.

You may underfile or overfiled vanes an change performance curves.