Vertical Multistage Pump Rotor Balancing (Without First Stage Impeller)

[Tilas]

Hi,

We have currently purchased one small multistage(16 stages) 50 Hz pump for water service and was asking the Vendor to have a complete rotor balancing for it. However the Vendor replied that they would only balance the shaft, stage impellers with keys (not including the half coupling and suction-first stage impellers) at operating speed (Bal Grade ISO 2.5). They said that the couplings and first stage impellers will be balanced separately at Bal Grade ISO 2.5).

Will it be difficult for OEM's to balance the rotor with the coupling hub and first stage impellers for Vertical Pump Rotors?

Thanks in advance,
Kristian

 

[Artisi]

What is small?

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.)

 

[Tilas]

Hi Artisi,

It it's driver is only 5.5 kW. Cap 5 m3/hr rated head at 93.4 meters. 1450 rpm.

Thanks

 

[Artisi]

probably standard off-the-shelf pump unit so sounds to me like complete overkill.
More likely hydraulic imbalance will be greater than any problems with the rotor balance.

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.)

 

[Tilas]

Hi Artisi,

Thanks for the reply. I will consider accepting their proposal in this case.

But is it possible to have also just for my understanding ? I thought it is not possible due to constrains in balancing their balancing machine or something(for the coupling hub and the first stage impeller.

 

[Artisi]

Coupling could be balanced separately but cannot understand why only the first stage impeller is balanced and its role in the overall scheme balance.

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.)

 

[Tilas]

Hi Artisi,

The complete note of the Vendor goes like:

"Balancing at balancing speed of the machine during testing. Without coupling and suction impeller. Suction impeller and coupling will be balanced separately. Rotor means pump shaft and stage impellers with keys."

The suction impeller has a different diameter (150 mm) with and the rest of the stage impellers have the same (146 mm).

The stage impellers will not be balanced separately but only together with the shaft and keys. Only the suction impeller and coupling will be balanced individually.

Do you think this is ok? I mean without balancing the stage impellers (only being covered in a balancing with the shaft and the keys).

Thanks

 

[Artisi]

Can you identify the pump brand or at least give a clue. Cannot understand why a unit of this size at 1450rpm needs such testing. (actually I have a fair idea why but it would be impolite to say so )
If it were 200kw at 2950 rpm it would be understandable.
However what you are
asking / proposing shouldn't be a 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.)

 

[Tilas]

Hi Artisi,

Hahaha. Thanks for being gentle on me

It is actually a comment from my Client(Plant Owner), and I being the EPC contractor wanted to accommodate his request as far as possible. Thus I wanted to know if it is possible to begin with and if there really is some merit on doing this.

The pumps are KSB pumps WKB 40/16 Model.

Your replies was so helpful. Thanks

 

[JJPellin]

For a sixteen stage pump on a very thin shaft, it would not be a good idea to spin the entire rotor for assembly balance. If it was in our shop, we would balance each impeller individually to 4W/n (only because we balance everything to this standard) and not attempt any sort of assembly balance. In an outside shop, I would likely ask for each impeller to be balanced individually to ISO 2.5 and build it. It is more important that the shaft is straight and the bowl bushings and wear rings are concentric.

Johnny Pellin

 

[EdStainless]

Stuff this small we never balanced. The entire unit can be deflected enough to cause bigger problems than impeller balance.
Mechanical clearances and hydraulic uniformity are the bigger issues.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[rotw]

Possibly the suction impeller is shrink fitted to the shaft while the other impellers are key assembled. Just a guess.
Anyway what Artisi suggested seems to me logical. Means for this power range / duty client request could very well be over the top...

 

[1gibson]

Does the pump have a tail bearing (bearing below the 1st impeller?) If not, that's likely where the shaft is supported on the balance machine, so that's why you can't have the 1st impeller on.

Regardless, note that API 610 only requires rotor balance on BB pumps, and it should not be applied to vertical pumps. It's not realistic due to the shaft length in most cases. Nobody should be doing that as a standard, or without additional cost, and it's not worth paying for.

Why was rotor balance specified?

 

[CouplingGuru]

Coupling balance ISO G2.5 at 1450 rpms? that is just throwing money away. It could be done, but it is a waste. Unless you get up around 3000 rpm range balancing across a coupling isn't needed. And if manufactured and selected appropriately balancing isn't even required up to 3600 rpm but you need to have the right coupling properly applied.

When it comes to couplings we are always here to help.

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