labyrinth seals

[Anatol]

Here I am again, mining the experience of all you experts

As always, thankyou!

I've been doing a bit of reading about labyrinth seals. Pretty interesting. In these days of high tech piston rings, are they useful in a small to mid size steam engine?
For piston seals or piston valve seals? If so, what dimensions are we talking about? (depth and width, etc)
thx

 

[Mechanicboy]

labyrinth seals is used in steam- and gas turbine to keep tight under high pressure. Labyrinth seals on rotating shafts provide non-contact sealing action by controlling the passage of fluid through a variety of chambers by centrifugal motion, as well as by the formation of controlled fluid vortices. At higher speeds, centrifugal motion forces the liquid towards the outside and therefore away from any passages. Similarly, if the labyrinth chambers are correctly designed, any liquid that has escaped the main chamber becomes entrapped in a labyrinth chamber, where it is forced into a vortex-like motion. This acts to prevent its escape, and also acts to repel any other fluid. Because these labyrinth seals are non-contact, they do not wear out.
Many gas turbine engines, having high rotational speeds, use labyrinth seals due to their lack of friction and long life. Because liquid-filled labyrinth seals still generate heat due to the viscosity of the seal oil, and because seal oil can contaminate the process fluids, modern high-performance gas turbines use dry gas seals which use spring-loaded rings with an inert gas in between the faces of the rings to provide the seal. This creates even lower friction and provides a liquid-free seal.

 

[Anatol]

Jens
Thanks for this helpful and informative reply.
"Because these labyrinth seals are non-contact, they do not wear out."
that seems a good reason to use them!
A lot of the questions I have about steam engine design are about relative scale. If you had, say a 10cm dia piston, what scale should the labyrinth seal be, if, say you were working with 150 psi steam. (Sorry to mix units). I'm *guessing* ~ 1x1mm, spaced ~ 5mm apart. Does that seem reasonable, or am I way out?

Do labyrinth seals completely replace piston rings? If so, why do cars have piston rings?
thx!

 

[Mechanicboy]

The labyrinth seal is for rotating shaft only. Use piston ring to keep tight.

 

[Anatol]

The labyrinth seal is for rotating shaft only. Use piston ring to keep tight.

really? interesting.
So what is the general approach to piston rings for small steam engine now?
Is there a recommended supplier, material or way of making?

 

[lohring]

Labyrinth type sealing has been extensively used in model IC engines. It takes carefully fitted piston and liners made from materials with similar thermal expansion. Piston rings are easier to get a good seal under less exacting conditions. Larger bore (over 1" or 25 mm) applications have not been successful, especially where gasoline is the fuel.

Lohring Miller

 

[Anatol]

Labyrinth type sealing has been extensively used in model IC engines. It takes carefully fitted piston and liners made from materials with similar thermal expansion. Piston rings are easier to get a good seal under less exacting conditions. Larger bore (over 1" or 25 mm) applications have not been successful, especially where gasoline is the fuel.

Lohring Miller

Now I'm confused
Jens says labyrinth seal are only used for rotating shafts.
so in the case of reciprocating, over25mm dia, piston rings are required?
thx

 

[lohring]

My opinions are based on a series of development tests on a CMB 35 cc engine. The started with a ringless piston and liner design but switched to rings. Below is a picture of a failed piston and liner from that engine. Note the labyrinth sealing grooves.

Lohring Miller

 

[Anatol]

My opinions are based on a series of development tests on a CMB 35 cc engine. The started with a ringless piston and liner design but switched to rings. Below is a picture of a failed piston and liner from that engine. Note the labyrinth sealing grooves.

Lohring Miller

Thanks Lohring.
Those grooves are tiny! So you went with rings on this project?
I'm assuming rings for steam are not identical to rings for IC, but don't quite know in what ways.

 

[Mechanicboy]

Oil groove on piston is not labyrinth sealing.. “V-Shaped” oil groove is to keep oil and improve lubricating the cylinder/piston. Can't keep tight against pressure from combustion or steam.

The labyrinth sealing is very difference from “V-Shaped” oil groove: When the shaft is rotating fast, then the fluid in the labyrinth house is rotating fast enough to keep against the pressure from steam pressure in the steam turbine. It's impossible to blow out the fluid out of labyrinth house due high power by centrifugal in the fluid.

 

[Goldflash]

labrynth piston compressors are common in gas process industry see below http://www.jsw.co.jp/en/products/reciprocating_compressor/labyrinth_piston.html

 

[lohring]

From Wikipedia:
A labyrinth seal is a type of mechanical seal that provides a tortuous path to help prevent leakage.

It's true that centrifugal forces on shafts assist this action. Again from Wikipedia:
Labyrinth seals on rotating shafts provide non-contact sealing action by controlling the passage of fluid through a variety of chambers by centrifugal motion, as well as by the formation of controlled fluid vortices.

On the CMB 35 mentioned above, the manufacturer switched to a ringed piston that worked better. They make a line of methanol/nitromethane fueled engines that use ringless pistons and are quite successful. The larger bore coupled with higher combustion temperatures makes ringless pistons less successful in larger gasoline fueled engines.

One of my dental compressor heads is based on a refrigeration compressor and has an oil sealed, ringless piston. It has an over 2" (50+ mm) bore. The other head has a ringed piston and leaks less oil after years of use

Lohring Miller

 

[Anatol]

labrynth piston compressors are common in gas process industry see below http://www.jsw.co.jp/en/products/reciprocating_compressor/labyrinth_piston.html

S0
labyrinth seals seem to be used in both reciprocating and rotating applications, but I've not yet found evidenced of effective use with steam pistons (or piston valves.)