Steam turbine alignment 1

[norske]

Hi everyone,

In a coupled steam turbine alternator arrangement (3000RPM, 10MVAe) should the DE bearings be slightly lower than the two NDE bearings? That is should both the turbine shaft and alternator shaft be slighly angled downward towards one another at the coupling? If this is so why is it done?

Many thanks to all in advance who humor a somewhat mystified electrical person.

 

[byrdj]

Remeber, different companys have different ideals about how things should be designed. The Company I am familar with (GE) believed that solid coupled rotors should operate in a catennar (sic) curve. That is, a rotor supported on the two ends will sag in the center and the two ends go up. Yes, even at running speed the rotor will operate in this sagged condition. Now place the two rotors end to end with the 4 bearings at same level, the two rotor will sag in the center and the couplings ends go up and cause the coupling faces to be open at the top. Bolting the coupling together will put a bending stress in both rotors. By having the bearings set in this smiling curve, the seperate coupling faces will be fair and when bolted together, the bending in the coupling will be minimun. (I'm much better at discriptions with a white board and multiple colors)

I do Know other companys do not use the curve for setting there bearings, maybe they design the coupling shafts more felixable.

 

[byrdj]

additional to first post

1) shows two uncoupled rotors with the bearings in line. the coupling face is open on the top.
2) shows the coupling bolted together with bearings in line. the deflection to close the coupling will be a cyclic stress when spinning.
3) to make the coupling face fair, the out board bearings are raised. when the coupling is bolted together with the bearings in a curve, the cyclic stress at the coupling is reduced

some other related comments
The generator bearings are set to have its coupling face vertical. The turbine bearings may require addition "cold" off set to achieve proper alignment at rated. there is usually a compromise between fair coupling and magnitude of coupled deflection.

 

[norske]

Byrd,

Many thanks for your replies, I now understand this. I tried to respond to you with a note of thanks after your first reply (I agree entirely a white board makes this much easier). I could not work out how to get an email to you so I hope you get read this.

Many thanks once again

 

[byrdj]

It took be a while to figure out how to add the gif, but a drawing explains better than words.

While turbine bearing loading and alignment is not my expertise, I can't see where a 10MW unit would have enough rotor sag to require to get fair coupling alignment. Do you know how much are your bearings offset? I would only expect about 0.0002 X span, ie a 100" span between bearings might need about 0.020" offset.