Balancing Impeller Only.....Risky?

[GrimesFrank]

Purchasing a 3"x1-1/2"x11" (OP: 155 gpm @ 320ft)centrifugal pump/motor ass'y. Manufacturer wants me to accept them balancing the impeller only. I asked for the whole assembly to be balanced; shaft, rotor, bearings, etc.

A) Why is that such a difficult request? Am I being overly cautious?
B) What is the real risk of just having the impeller balanced since it is the major part of the off balance inertia risk? Probability of the whole thing shaking apart at the seams?

Pumps are not my forte so any help you can provide is appreciated.

Frank "Grimey" Grimes
You can only trust statistics 90% of the time.

 

[pmover]

no, the request is not difficult; the difficulty lays with the mfg - whether or not mfg is competent or has a quality control problem. simply do not issue purchase order to pump mfg that does not comply with specs or a request such as this. there are other pump mfgs. the entire rotating assembly should be included as part of the balancing process.

good luck!
-pmover

 

[pumpking]

While agreeing in principle with the need to balance both Impeller and shaft, with todays manufacturing technologies, it is not really necassery to test the complete assembly, expecially with the pump size stated. It will only be running at 2 pole speed, and so shaft, bearing tests etc will probably not even show.

With the current balancing practices used in the Industry, they are well proved to be be reliable and not a hinderance to reliability.

 

[JJPellin]

I would not see any problem with just the impeller being balanced. If the shaft is within API or ANSI specifications for run-out, then it must be straight and thus should be naturally in balance. I don't recall every having any rotor balanced on its own bearings. If this is a ball bearing machine, that is probably not a good idea. The bearings in a normal balance machine are precision bearing of a much finer quality that the pump bearings. Even if you wanted your impeller balanced on your shaft, I would not suggest using the pump bearings. You would probably get a better balance without the pump bearings. If you rephrased the request and asked to have the shaft balanced and the impeller balanced as two separate items, the pump company would probably do that for a small additional charge. But I would not think it worth the extra cost to balance the shaft.

 

[TenPenny]

By this size, I'm guessing you're talking about an overhung pump, like an ANSI pump. In order to balance the shaft/bearings/impeller, the manufacturer will have to put the bearings, sleeve if used, and impeller, on the shaft, balance it, then remove most of these components, then assemble the pump. In my opinion, the work involved in doing this far exceeds any benefit. The design of the pump may even require the bearings to be removed in order to assemble the pump, if they were installed for balancing.

In my 20 yrs experience, on a pump this size, balancing the impeller only is enough; ask for G2.5, and you're good to go. We have many plants here who are extremely fussy about vibration levels, and all overhung pumps are balanced impellers only. It's simply not practical to do it any other way. Shafts are so well made now, you'd be extremely unusual if you had a problem. (Assuming, of course, that you're talking of a mainstream, reputable manufacturer).

 

[GrimesFrank]

TenPenny and others,
Thank-you.
The manuf. is Hayward Gordon, I believe them to be a reputable comany with a long history. It is indeed of the overhung variety.
Impeller balancing alone, is what I will accept.
Could you take a look at this thread?
thread407-175883
Is ANSI/HI 1.6-2003 the new "standard" in pump testing?

Frank "Grimey" Grimes
You can only trust statistics 90% of the time.

 

[Tmoose]

I'd ask for documentation of runout and high spots of the arbors or shafts and impeller during balancing. IF the runout and high spots are similar at installation the impeller will still be "balanced." Any change in centering will eat away at the balance tolerance. With the popular "screw on" impellers the centering can change with every assembly. Some thoughtful work with a dial indicator will answer questions that might never be asked, and avoid expensive problems for everyone.