Steam turbine low pressure blades 2

[thickhead]

The turbine I operate at work was found to have up to 1mm of metal reduction on each of the final three LP blade rows. This meant there was not much of the thinned section left. Why are these final stages fitted with tip-thinned blades, is it something to do with enthalpy or velocity or condensation at this low pressure?
Any help will be appreciated

 

[rmw]

If your system has any Mechanical Engineers in it get them to explain to you about steam expansion into the moisture region. The fact of your having asked the question that you did tells me that you are unaware of this which is common to all turbines exhausting into a deep vacuum. I am kind of old school and it is easiest for me to demonstrate by using what is called a "Mollier Diagram."

The blade design has to do with hydrodynamics aerodynamics and vector forces and all blading in your turbine has some type of airfoil design. The airfoil profile changes as the specific volume of the steam changes through the steam path.

Last row blades (buckets in some turbine types) are usually furnished with some kind of wear resistant provisions in the last stages; replaceable wear strips, hard surfacing, or a combination of both.

Yours is a problem that all turbines exhausting into a deep vacuum exhibit and last stage blade wear is a constant fight for turbine operators. It is not a matter of if, but when.

rmw

 

[TheBlacksmith]

Exactly. I am one of those old timers and moisture came to mind reading the question. I also threaten the newbies with a Mollier diagram - most of them can't understand it, but I just draw the stean cycle on it.

 

[thickhead]

TheBlacksmith - If you wanted a little chat with rmw who offered me assistance, why didn't you email him or something??

rmw, thanks for your help but I think I worded my first question incorrectly:
The top half casing was removed after the rotor could not be turned by hand the day after a normal shutdown (below 100c). The turning gear threw its hand in after an hours normal cooldown cycle.

To keep it simple - the Manufactures engineers (German), and our Company engineers (French), have concluded that the loss of LP blade material was because of an on-line rub. The cause of the rub was case distortion. This was caused by exhaust line stresses. This because of incorrect spring hangers, and wrongly installed bellows at the turbine and at the condenser.

So while we were happily and innocently generating power with no indication of abnormality, we effectively machined 1mm of material off the tips which normally have a 3mm clearance.

Not being a steam engineer, I asked the Engineers on site why these stages of LP blades are manufactured in this way but the language difficulties just got in the way. I have Googled 'Steam Turbine LP blade tip thinning' in dozens of different ways and haven't found an answer.

I am familiar with Mollier diagrams and steam tables, but I am not familiar with this blade design. And I'm begining to think a few knowledgeable sentences from somebody who is ...... may not be forthcoming

 

[byrdj]

I still do not see the question

is there a band around the outer diameter of the bucket (blade) tips?

the statements I read are

the end of the buckets have been worn 1mm due to rub

there should be 3mm radial clearance

So what is the question related to bucket design

 

[thickhead]

I will try and upload an image

 

[byrdj]

looking at the photo

I see what looks like thin ragged metal at the tips on the discharge side of the bucket. I am not familair with a free standing bucket with a "mini shroud" on the discharge side.

 

[thickhead]

Thanks for looking byrdj, it would appear that not many people have. My initial thought was maybe they machine it like that so that if there is a tip rub, the full surface area of the blade won't have to come into contact with the nozzle carrier. But surely thats a bit simplistic?

 

[thickhead]

I meant the full surface area of the blade tip

 

[thickhead]

I’ve found something else, but the penny still won’t drop, perhaps a light will come on in someone elses nut:

The clearance and rim losses of the HP blades are reduced to a minimum by using shrouds, whereas the LP rows have conically twisted blades, the blade ends of which are tip-thinned on one side … regarding sides with a low clearance loss factor, the blade tips are provided with a one-sided thinning.

 

[byrdj]

Are the stationary tip "seals" spring return or solid.

If spring return, then the thin tip could be sacrificial, even if the seals were solid.

from just looks, would these stages be concidered more "reaction" verses "impulse". If more reaction, then there is a need to have some type of blade tip sealing and possible the design was that thin tip ahead of rotation

 

[rmw]

Is it my old tired eyes or do I see the remains of tennons on the 24 row? If so, was the shroud removed for this picture or did it "go away"? If not and in either case, I extend my previous remarks to include solid particle erosion. The type of thinning I see here I wouldn't attribute to moisture. That is more of a 'feathering' type erosion. If there was a shroud there and it "went away" that could explain the severe damage to the last row.

rmw

 

[thickhead]

HP, MP, LP are all reaction.
HP and MP blade shrouds have caulked labyrinths to seal against the nozzle boxes, but the LP has nothing in that assembly.

The surface area of the ‘lip’ on the LP blades which is described as ‘tip-thinning’ can’t be much more than about one fifth of the whole blade tip surface area. I just can’t get my head around it. Wanting to know rather than needing to know is becoming a bit of a pain so all comments are very welcome.

I’ve emailed the manufacturer but they won’t talk to me – the conflict of ‘is it their fault or is it the constructors who are to blame’ has closed many mouths!

 

[thickhead]

rmw, byrdj
I have dug out a photo of the rotor assembly before it was damged, I hope it uploads ok

 

[chief]

Reaction turbines use either end-tightened blading ,( where the area between the fixed and moving blades is sealed by shroud tips, which are adjustable by means of a worm gear arrangement), or tip tightened blading, where the clearance beween casing and turbine blade tip. is in the region of 70 thou ins for the LP BLADES.
It would appear from the photgraphs that tip tightened blades were used in this installation. The blades are secured from excess vibration by binding wire usually silver soldered to each blade. The tips are ground to a knive edge to limit damage in the case of rubbing against the casing, which appears to be the case in this instant.

Offshore Engineering&Design

 

[thickhead]

Chief said: The tips are ground to a knive edge to limit damage in the case of rubbing against the casing, which appears to be the case in this instant.

That is it then - what seemed overly simplistic is the answer. Thanks very much Chief