How do you seal a rotor underwater?

[HydroGuy]

Lets say I will be operating a generator underwater in a "box" sitting on the river floor. Turbine blades will be spinning a rotor attached to said generator, and obviously the turbine blades are exposed to the water and the generator is sealed in the box. I'm having trouble identifying what type of component could allow the rotor to spin freely yet keep all water out. The box will be at depths no greater than 20 ft underwater, and in fresh water only.

I imagine the solution is something similar to a submarine's propeller seal... but my background is in electrical engineering, and I'm not sure where to look. Any help is sincerely appreciated...

 

[turbo]

You can look up Thordon bearings for marine applications - they are often used in hydro-generator retrofits where previously the bearing was lignum vitae or some other similar material. The problem I see is that you will have to supply some kind of pressurized gas to the enclosure to prevent seepage. No seal alone is going to be 100% effective. Garlock makes some all-graphite packing that would do the job, too, but they would still require some sort of gas purging to keep water out of the generator.

Normally, hydro-generators are designed with the electrical components high and dry - that way the turbine shaft can be submerged so you can use the water both as a lubricating film and coolant for the bearings/seals. If you get the rigid Thordon material, it acts as both a bearing and a seal, simplifying your design considerably. Softer seal materials (such as graphite) will work OK, but that would require you to come up with a more complex design, including bearings that must be lubricated and cooled.

 

[Bob S]

I suspect that you need one or two "O" ring seals around a smooth drive shaft. These should be slightly compressed, and well lubricated. Too much compression may produce excessive friction. You may want to pressurize the box with the generator in it. You may want to have an air tube that you can use to maintain this pressure.

 

[Danger]

Referring back to an issue about sealing VOV thrusters a couple of years back, one of the most practical suggestions was to simply flood the motor with mineral oil and not bother sealing it. I wonder if that approach might work just as well for a generator.

 

[Danger]

That was supposed to be ROV thrusters. I still have a problem with Safari crashing whenever I hit 'edit', so I had to start a new post to correct it.

 

[Dannicus]

Submarine shaft seals leak a bit and the moisture is pumped overboard. You could do the same, but you'll have to deal with moisture inside of the enclosure.

 

[Bob S]

Referring back to an issue about sealing VOV thrusters a couple of years back, one of the most practical suggestions was to simply flood the motor with mineral oil and not bother sealing it. I wonder if that approach might work just as well for a generator.

The generator efficiency depends in part on the closeness of the rotor and stator magnetic components, and nowdays the tolerances on generator components are very tight, so the gap could be a few mils. Bigger generators sometimes use hydrogen cooling to lower the viscosity (relative to air) and provide convection cooling capacity. I suspect that mineral oil has very high viscosity, and will be too viscous for this application.

 

[Topher925]

The "old school" way of sealing a shaft in a box for marine applications was just use to use a greased sleeve or "grease box". This method consists of having the shaft run through a long high tolerance tube and then filling it with very high viscosity grease. I don't know if grease boxes are still used today. I rekon there's something like it that is used for inboards.

I think Turbo has the right idea though. Its best to keep everything electrical well above water level and not to worry about sealing. I believe this is the most popular method for marine turbines used for bridges and such.