Exhaust Gas Casing deformation (rotor sagging)

[GTALIBABA]

Hello everybody.

Can someone explain why is rotor sagging happening? Actually, my question is does anyone know which part of Exhaust Gas Casing deforms and cause rotor sagging effect?

Best Regards.

 

[byrdj]

not sure I understand the question.

rotor sag is just a mechanical property of the geometry and strength of material. going back to pure statics, the rotor is a beam support on each end, the sag would be the beam deflection between the supports. I use the very visible example of consider a piece of rope held on each end and the sag is easily seen. if you rotate the rope, the sag remains (at rotational speeds below critical)

 

[csk62]

Yes, I've seen the rotor sag shape referred to as "the caternary" which is the shape of a string supported on two sides, similar to how the rotor will be supported by bearings. Casings will deform from normal thermal expansion as designed.

 

[racookpe1978]

Your confusing two different problems.

Rotor sag occurs EVERY time a long heavy object (the rotor!) is supported at two ends. The middle sags down.

Material strength goes down at higher temperature = MORE sag when the rotor is hot than when it is cool. But it will ALWAYS sag.
When rotating at high speed, the rotor sag is minimized. When hot and not on the truing gear, rotor sag will occur the most. Leaving your rotor on the turning gear is ESSENTIAL in protecting the rotor and CT from damage when hot, cold, or in between.

But you indicated there might be a problem with the aft (hot or exhaust) bearing drooping or moving. Is that the problem instead?

 

[GTALIBABA]

I'm sorry i didn't specify what I mean when I say rotor sagging.
During the long turbine operation it is a standard problem with bearing drooping on exhaust side. So my question is which part of Exhaust Gas Casing deforms and cause bearing dropping/rotor sagging?

My question refers mostly on ABB/ALSTOM GT.


Best regards.

 

[ScottyUK]

racookepe,

A hot rotor usually sags up, not down. The effect is known as hogging rather than sagging and occurs due to the unequal distribution of heat within the rotor which results in differential expansion with the top of the rotor becoming longer than the bottom and leading to a thermal bend. Counter-intuitive but true.

 

[byrdj]

ScottyUK, the terminology between the various conditions is interesting.

to me, when a rotor temperorly deflects from center line due to thermal distortion, that would be a bow. while the stopped rotor defelection cause the rotor to frown, once rotating this temperory thermal distortion will result in eccentritity that will eventionally be rolled out and then only the sag condition remains and the eccentricity dimensish with the rotor in its normal smile.

just this week I have now heard two differnt slang terms to discribe temperary thermal deflections.
as stated, I call rotor distortion a bow, your "new to me" expression is hogging. On another forum the temporary distortion of the casing was called cat back where I had refered to it as the socially incorrect term of humping.

 

[ScottyUK]

I've learned a few new expressions today too: 'cat back' and 'humping', and the frown and smile.

I googled 'hogging' and it appears to have made its way as far as Canada because it is mentioned in one of the Candu training manuals. Hogging is used to descibe to both rotor and casing distortion although I've only come across it in respect of rotor distortion; in the latter instance it appears to be synonymous with 'humping'.

Just when you think you've heard it all...

 

[racookpe1978]

A hot rotor usually sags up, not down. The effect is known as hogging rather than sagging and occurs due to the unequal distribution of heat within the rotor which results in differential expansion with the top of the rotor becoming longer than the bottom and leading to a thermal bend

.

Turbine Rotors behave differently based on their environment: Gas turbines are extremely hot (the flames burn and the hot gasses are then are directed right through the turbine blades and vanes while encased inside a very tight-fitting casing, which is also hot. A steam LP turbine exhaust cooler steam through the vanes and blades down to a very cold condensor. On shutdown, the steam turbine has one side hot, one side much cooler.

Newer GT's use both rotor cooling air (inside the shaft) and vane cooling air (outside the vanes) to reduce the delta T.

All (shutdown conditions) MUST be kept on the turning gear to prevent distortion and rubbing.

if the OP has a GE/Alstom unit moving as he describes while hot, he should look at his bearing supports - it appears the bearings are moving down under long times at hot conditions under load.

 

[crshears]

"Sags up" - what a contradiction in terms!

"Humping" - I'm just not going there...

"Bow": as in bow and arrow, and clearly intended to describe any deviation from a perfectly straight line; as near as I can tell, same as eccentricity.

"Frown" and "catback" - I've never heard of either of those before.

"Hog" is for sure used here in Canada, in two different contexts; one is that of a turbine shaft at rest undergoing unequal expansion due to its upper half being hotter than its lower half. The other is that of a ship carrying cargo in only its forward holds - you know, the ones in the 'bow' [rhymes with 'how' this time] of the ship - but the ship being otherwise empty; the centre buoyancy in concert with the couples of the weight of the cargo forward and that of the machinery aft tends to lift the vessel midships, causing the vessel to take the curve of a hog's back.

"Smile" is illustrative, but an entirely unfamilar term to me; I've always used 'sag' for this - again, both in the context of the catenary of a shaft and that of a ship "sagging" due to being supported only at its ends as bow and stern are lifted by waves while the centre section, well, sags.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[chief]

Ensure the flexible bellows on your exhaust has free movement,otherwise you will have shaft and bearing problems caused by rigidity. Look under the inlet AND EXHAUST insulation blankets.Many installation sins are hidden and cause surprise problems.

Offshore Engineering&Design

 

[MachinaMan]

GT,

What model of ABB/Alstom unit is it?

We have some GT11N units that had Alstom issued bulletins about EGH cracking.
Construction on the EGHs was flimsy in certain critical areas, leading to in service cracking.
They have modified parts and a spring canister support for the EGH now to remedy the problem.
We have installed the new housings but not the spring can modification on our units.