Big bearings, Big loads, and Maintenance

[Shannow]

So as to not drag Bryanccfshr's thread completely off the rails.

http://www.bobistheoilguy.com/forums/ubb...242#Post1212245

At work, I've got some big bearings, with some big loads on them. Up to 15 longitudinally along the train, ranging from 6" daiameter up to 20".

Generator bearings are 20" diameter, 19"long, and hold around 30 tonnes of mass, spinning at 3,000RPM.

There are three oil systems. (well 4 if you count the hydrogen seal oil, sealing the hydrogen inside the generator)

Main Oil - provides running oil to the bearings, and some hydraulic oil to other systems.
Bearing Oil - provides running oil to the bearings only.
Jacking Oil - provides hydrostatic lift to the bearings when the shaft is not turning fast enough to manufacture a wedge greater than the height of the asperities.

Shaft spins at 3000RPM, on ISO 32 oil that leaves the bearings at around 85C.

When off line, the shaft rotates at 4RPM until cool. Jacking oil provides the lift to the heaviest loaded bearings, at about a gallon a minute to each bearing. This is only on the heaviest loaded bearings (1 through 10 on the 15 bearing machine, and 5 though 10 on the 11 bearing machine, which has tilting pad journals for the first four, that can lift the shaft during barring).

On run-up, the machine is barred at 4RPM on jacking oil (lift), and Main Oil (lube/cooling etc.), at 800-1000RPM, the jacking oil is stopped. On rundown, it comes back in at 400RPM (I've never seen damage to a bearing when the shaft has rotated to a stop from speed without jacking oil).

Would never try shaft turning (barring), or attempt run-up without it.

During maintenance, the jacking oil is used to lift the shaft to rotate it for alignments etc...takes 10-15 minutes for the oil to escape from between the shaft and journal to get a reliable dial gauge reading on it.

Some maintenance crews use STP on the bearings pour it over the shaft, wrap a sling from the overhead crane (think strap type oil filter wrench), and lift/turn at the same time. The STP appears to squeeze so slowly that a single application will last days.

 

[Bryanccfshr]

Very interesting. Next time I have the oppurtunity to work around our Turbine crews I will take note.

 

[tom slick]

how does the jacking oil work? is it a seperate system?

 

[Shannow]

tom slick, yep, it's essentially a hydraulic circuit, running at 2600 psi.

The oil is individually piped to the bottom of the bearing, where there's a coin sized recess to receive it. Pressure lifts the shaft before the shaft is rotated.

Often there are flow control valves to each bearing to ensure that a damaged bearing or misalignment doesn't starve the rest of the bearings.

The developed pressure at the bearings (i.e. what it really takes to lift the shafts is 5MPa (725 psi).

After maintenance, with the machine on barring (4RPM), you can tell how successful the alignment was, by watching the pressure on the jacking oil system.

Poor concentricity between coupled shaft demonstrates as a fluctuating 1 per rev cycle of the pressure, as the weight on the bearings changes across the coupling.

Poor bearing heights results in a split across the coupling (may have 4MPa on one side of the coupling and 6 on the other....I've seen 10MPa and 0, still with no vibration.

On the wedge (i.e. at speed, jacking off), the pressure under the shaft is 1MPa.

 

[tom slick]

So does it has a special set of bearing surfaces for jacking? or it is the regular oil system with a jacking circuit?

Really interesting stuff, very different from aircraft turbines.

 

[Shannow]

Nope, they are all plain bearings (well the jacked ones), with the industry standard 1 thou per inch of clearance.

Most are "lemon bore", where the bearings are bored to finish size with a shim between the half joint, giving a side clearance twice the vertical, but the bearing halves being semicircular, and larger than the shaft.

The jacking oil is oil that's pumped in at BDC, and forced itself between the bearing face and the shaft.

Regular bearing oil is supplied either at the half joint, or at TDC depending on design.

 

[tom slick]

The engines I used to do major work on had 5 bearings on the main assemblies, 3 roller and 2 ball. common oiling system with pressure and scavenge pumps. We could spin them all we wanted but we are only talking a thousand pounds or so of rotating assembly.

 

[Shannow]

Our machines are located about 30 feet off the ground (condensers beneath them), with the oil tank on the basement.

Gravity gets it back to the tank...

Was surprised at how two guys swinging off the blading could rotate a 40 tonne shaft in "V" stands if the V was lined with teflon sheet, and STP poured over the top of the bearings.

Main rotating assembly is about 200 Tonnes, spinning on 11 bearings. The bearing heights and angles form a "catenery", which has the coupled shafts "sag" into the middle, reducing the need for a beefy thrust assembly.

 

[Shannow]

Originally Posted By: tom slick
The engines I used to do major work on had 5 bearings on the main assemblies, 3 roller and 2 ball. common oiling system with pressure and scavenge pumps. We could spin them all we wanted but we are only talking a thousand pounds or so of rotating assembly.


So what's the arrangement ?

Concentric shafts, or axial ?

Balls as thrust, rollers to take expansion ?

The precession forces as planes maneuver would be interesting to model.

 

[tom slick]

All of the bearings/shafts are axial arrangment, no gear reduction etc. It's a turbofan (GE F-110 series) so it had two rotating assemblies that rotated the same direction. The two ball bearings take thrust from the compressor and the fan. The three roller bearings supported the fan and two turbines.
one interesting arrangement is the high pressure turbine was supported by the low pressure turnbine shaft. The the low pressure turbine was supported at the rear and held the weight of both. Both spin, the high pressure turbine around 14k RPM and the low pressure turbine spun about 5k rpm IIRC.

Side note, with the forces involved at high speeds the compressor blades stretch and twist constantly. The high pressure turbine blades were air cooled by forcing air out of holes in the blades.

 

[Shannow]

Took me an hour to find this.

The 9 compressor stages, and only 3 turbine reinforces my thermo lecturer's statement that gas turbines rely on compressor efficiency to even function.

Any information on how they govern the high speed and low speed shaft ?

 

[tom slick]

the compressor (high speed) is governed by fuel flow and turbine temperature, nothing mechanical. It has a magnetic sensor to give RPM feedback and a couple temperature sensors that are optical and thermocouple. The Fan/Low Pressure Turbine are just along for the ride.
The first stage of turbine (high pressure turbine) powers the compressor. The second 2 stages of turbine power the fan. IIRC there is a 600*F temp drop between the first stage turbine and the second stage turbine. some because of cooling and also the turbine using that energy.

Older engines had more stages of turbine. The Allison T-56 engines I worked on had 4 stages and its compressor was smaller (physically)

 

[tom slick]

Here is a better illustration

 

[Kestas]

Does hydrogen present a problem for the bearing material? Under load and elevated temperatures, hydrogen can destroy a bearing in short order. We have a lot of experience forensically examining such bearings.

 

[Shannow]

Kestas, there are no roller bearings, just plain white metal.

The hydrogen seals are a white metal face, with central oil feed, in direct contact with the hydrogen, and we've not had any issues with them.

Your circulating currents bit in the other thread was interesting.

In the steam turbines, the steam passing across the blades builds a static charge. If the shaft is unearthed, then this charge will find it's way across the oil film, and spark erode the bearings.

Saw one case where the earthing brush was inadvertently painted, and spark erosion destroyed the Governor drive over the course of a week.

But there can be only one Earth, or the circulating currents can become huge.

 

[Kestas]

I thought bearing damage from static charge was a rare occurrence. I guess it's more prevalent than I imagined. I've personally only seen one case of electrical damage from static electricity. The application was a belt driven a/c pulley. It was in no way electrically connected to the motor. I was investigating other issues with the bearing when I cam across electrical damage.

Another rare occurence for bearing electrical damage is from lightning strikes. With our windmill business ramping up, I'll look forward to documenting such a case.

 

[Shannow]

http://www.gepower.com/prod_serv/products/oc/en/orbit/downloads/orbit_v27n207_electrostatic.pdf

Has some detail on it.