High Speed Balancing

Well, the 'centrifugal' force at a point on the outside of your rotor goes as f = r•ω², so at 8500 rpms it is spinning 10 times faster and the 'centrifugal' force is 100 times greater than at 850 rpms. So, when it is tested at 850 rpms it is only seeing about 1 percent of the force it will see at 8500 rpms.

Also, if the rotor is not dynamically balanced then there will be a vibrational energy that goes as (as I recall; I'm a bit rusty on this) E_vib = 1/2·Δm·(r_Δm)²·ω², so running at 6000 to 10000 rpms can generate a lot of vibration for even a small unbalanced mass. [Δm being the net unbalanced mass and r_Δm being the effective radius for this unbalanced mass.]

Lot of good info here...

http://www.testdevices.com/services/rotor_testing/growth_measurement_at_speed

A lot of plants that have rotor work performed often do a high speed balance under vacuum. This insures that they will not have bad vibrations during operation. Some plants opt for a low speed balance and that is performed at approximately 300 rpms. Low speed balance usually gives good results especially on 1800 to 3600 rpm units. on higher speed units like yours, you can do a low speed balance but you would get better results shipping it to a facility that does high speed balancing. It is a cheap insurance policy.

First, let me stress that I am not a balancing specialist, so maybe I make it too easy and more factors are to be considered but this knowledge has served me enough so far :-).

I have been involved many times in LSB as mechanical supervisor :-) and this my experience:

There are two ways to balance the rotors.

Low speed balancing (LSB) and high speed balancing (HSB).

LSB is easier to perform as it does not require to get the rotor rotated at its nominal rpm (that it is called LSB :-). LSB uses the fact that when flexible bearing support is used the rotor resonance speeds appear at lower speed (for 3000 rpm rotor let's ~ 200 rpm first and ~ 130 rpm second - I do not remeber exact rpm :-) and it is unique feature of each rotor ).

Such speed can be obtain in each workshop without big troubles. Yes, it can be some issue when we want to balance huge LP rotors - 40 or 50 tons of weight but even in such circumstances LSB can be done.

The HSB we have when the rotor is going to be balanced at its working speed.

For 50 Hz machines it is 3000 rpm.

To perform such balancing, the bunker is needed as two main problem we have to cope with: safety and friction.

Safety:

There are big centrifugal forces during HSB and staff or equipment have to be safe,

Friction:

at such speed the friction between moving rotor surfaces and air is significant and rotor gets warm. That is why the vacuum is created inside the bunker and temperature can be control.

Let's explain why the HSB is good.

At the LSB is impossible to expose the rotor to all centrifugal forces which are going to be appear at nominal rotor speed what mean the rotor will experience them being closed inside the casing.

There would be to late to act if something was wrong with rotor. Maybe "to late" is not good word - it would be expensive and time consuming to open the casing again and check the rotor and after maybe second balancing would necessary?

This is specially important if during rotor overhaul any manual work had been done on rotor or machining works had taken place.

The manual works can be: rotor reblading, rotor welding i.e. Only after HSB the quality of the works done on the rotor can be checked and if found not up to mark, can be fixed.

Additional HSB benefit is that trip device can be set. Certainly, it is possible to remove it from rotor and send for separate setting when only LSB was scheduled or even to set it during first start up of the turbine after overhaul.

I bet MIDOFNOWHERE wish he hadn't asked!

Bazzer

These people say yes to high speed balancing...

http://www.elliott-turbo.com/Files/Admin/Literature/at-speed(2).pdf

Intuitive reasoning behind their opinion really.

There is a lot more information, both academic and commercial, available by way of an internet search.

It would be good to read about some real life experiences on this topic. The potential for disaster is tantalizing.