Steam turbine low pressure blades
Steam turbine low pressure blades
(OP)
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
Any help will be appreciated





RE: Steam turbine low pressure blades
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
RE: Steam turbine low pressure blades
RE: Steam turbine low pressure blades
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
RE: Steam turbine low pressure blades
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
RE: Steam turbine low pressure blades
RE: Steam turbine low pressure blades
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.
RE: Steam turbine low pressure blades
RE: Steam turbine low pressure blades
RE: Steam turbine low pressure blades
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.
RE: Steam turbine low pressure blades
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
RE: Steam turbine low pressure blades
rmw
RE: Steam turbine low pressure blades
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!
RE: Steam turbine low pressure blades
I have dug out a photo of the rotor assembly before it was damged, I hope it uploads ok
RE: Steam turbine low pressure 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
RE: Steam turbine low pressure blades
That is it then - what seemed overly simplistic is the answer. Thanks very much Chief