Steam Leakage through Labyrinth - Steam Turbine 1

[6748]

Questions may be very basic. Since the supplier is not willing to give the correct data, I am raising it in this forum.

01. What is the duty/purpose of Labyrinth seals is a steam turbine?
02. What should be the percentage leakage allowed through this labyrinth?
03. Is the leakage coming out after delivering some motive power to the turbine? OR is it coming out without contributing to the motive power of the turbine?
04. We have labyrinth seals at the High pressure and low pressure sides.
04. For a 1.5 MW turbine, the steam flow is 15000Kgs/hr of steam with the following thermal conditions.
Inlet 45 Kg/cm2g at 420 Deg C.
Backpressure 3.5 Kg/cm2g
What is the normally allowable leakage thru Labyrinth?

6748

 

[byrdj]

That is a good question. O can't answer all, but I'll start

labyrinth seals are the latestest in the evolution of shaft seals for turbines. that do not contact the shaft and normally work good. prior shaft seals were contacting carbon and water impellers.

the purpose of labyrinth seals is to prevent air from entering turbine during startup on the HP end and the LP end all the time if under vacumm.

there are noramly a minium of 3 rings of seals and 2 supplied pressures.

working from the outside in, this side has air, on the other side of ring 1 there should be a vacumn source pulling to a seperate gland exhuaster. normally only a couple inches water at each seal, maybe 10 to 15 inches water at gland exhauster.

the other side of ring 2 will be a low pressure (3 to 5 psig) source of sealing steam. the steam that goes towards ring 1 will mix with the air coming accross ring ring and go to gland exhuaster to seperated water from air.

The other side of ring 3 will be inside the turbine. at startup this will be at vacumn. thus the steam supplied between ring 1 and 2 will enter the turbine and mix with throttle steam.

Once the turbine is running, the steam inside turbine at HP will be greater than supplied sealing steam and throttled steam will go accross ring 3 and will provide this source. If the LP secction of the turbine is at vacumn, it will always need a source of sealing steam, thus it will use the HP leak by.

I said a minumn of 3 rings, if the HP throttle steam at ring 3 is too great, a 4th ring is added. the steam that makes it between rings 3 and 4 is routed to a bleed steam at a low stage, so the steam flow accros 3 towards 2 does not over supply the sealing pressure.

I don't know if 3.5 kg/cmg is pressure or vacumn and don't have time this am to look.

So if you have a gland exhuaster, there should be no puffing of steam out of the shaft.

hope this helps, the rest of your questions will require a little research for me maybe another will be able to respond

 

[byrdj]

continue..

when doing a heat balance, Sealing steam is a loss. Since it is either supplied before the SVs or it is bleed at the HP wheel.

I don't have ASME PT6 avaialble, but I think it provides generilized corrections for sealing stem lossed. The one heat balance I have at the house shows a 0.25% for the HP seal feeding the LP seals.

I don't know if this was part of your question, but there are interstages seals. If that is the concern, they to are losses

So is your concern steam blowing out of the turbine or are you trying to account for a poor heat balance?

 

[6748]

Thanks Mr byrdj,

01. This is NOT a condensing turbine.
02. This works with a backpressure of 3.5 Kg/cm2 gauge pressure.The LP steam comes out of the turbine at 250 Deg C.
03. The turbine insides, steam lines etc., will never come under vacuum.
04. My problem is very very excessive bleed off from the outlet pipe that is meant for discharging the labyrinth vent steam to atmosphere.
05. There is no gland extractor.
06. What I gathered from yr response is -- The HP labyrinth bleed off is diverted to the LP labyrinth. But what we have is TWO discharge lines. One coming from the HP labyrinth side and one coming from the LP labyrinth side. The HP bleed off line is venting out superheated ( colorless ) steam with a loud noise.
07. We have made a rough estimate of the steam loss. It is around 1500 Kgs per hr of steam for a input steam of 11000 Kgs/hr of High Pressure steam. This wastage of steam makes the power generation to cost more.

Now I have to get back to the Turbine supplier and get more information to understand the problem correctly.

6748

 

[byrdj]

13% is a lot of wasted steam, but I guess It depends upon the plant and turbine design.

I'm assuming now,

It sounds like your design will always be steaming out of the two discharge pipes. At the HP end, there should be more rings inside the turbine, thus a longer path down the shaft for the escaping steam. That would reduce the flowrate of wasted steam.

Now, it is very easy to damage the teeth of the ring and open up the clearance, thus excessive waste. this could occur from impoprper alignment of the rotor to the casing during build or by a high vibration event. Does this seems likely.

It occurs very often with other turbines. So much so, that Our company makes Labyrinth packing rings that have one special long tooth that will push the ring back in the event of rotor contact and save the sharp teeth from being pushed or worn during minor transiets "Guardian design"

Also, does the turbine have interstage bleeds (extractions) that the waste from the HP end steam conditions be matched with and added back into the process. at the least, the HP waste could be put back in with the LP exhuast.

 

[rmw]

It is a poor design. The HP bleed off should be being fed back into a lower pressure point or a lower pressure seal. The LP seal you can't do much about because you have no lower pressure point to take the bleed steam to.

If you piped your HP steam bleed off to your LP seals you could get more work out of it before it vented.

There may be safety and overspeed ramifications of doing that. I would need to see your heat balance before casting a final judgement.

The labrynth is just a tortuous path designed to make it hard for the steam to get by and therefore get out, but it is not a perfect seal. Are your seals set to proper clearances? Are they rubbed out? Both will affect leakage.

rmw

 

[6748]

01.We have stopped the turbine.
02.The turbine manufacturer is the one who has installed and commisssioned the turbine along with the alternator.
03. We have called the manufacturer to the Works to check all the sources that can vent steam and reduce the wastage.
04. As per the suggestions made by "byrdj" and "rmw", we will ask the supplier to look into the feasibility of re-routing the HP bleed off to an useful discharge.

6748

 

[byrdj]

it is hard to believe the design would have that much seal blow by. the manufacturer should know the exected. i would still believe the packing rings have been damaged and radial clearances opened