We have a GE frame 9E gas turbine, with Accessory gearbox of Flender AA30CZ. We were facing high vibrations in accessory gear box. during the combustion inspection of GT we inspected that there are pitting marks on half bearings of the Accessory gear box main shaft.

It appears to because of some leakage current. can anybody have seen this type of problem?

What can be the reason.

Regards
BSJhala

 

This is a GREAT question for the gearbox manufacturer! They would likely be in the best position to have seen this kind of problem before, on this or other applications.

Electrons are funny things. Undesirable flows can be induced in some very unlikely situations. I've seen one problem caused by high air flows through a vapor extractor when there was high humidity in the air above the L.O. tank. These kinds of situations are usually combination of things, none of which on their own would result in a problem, but when combined under the proper set of circumstances will result in some seemingly unexplainable problem.

But, without understanding the particulars of your turbine and its configuration and layout it's really difficult to comment.

Again, this is the kind of question that a lot of people seem reluctant to pose to the manufacturer or supplier, but, if posed in a "Help us figure this out, please" manner instead of "You're equipment is crap" manner will usually result in some a very helpful and informative response. Manufacturers really are interested in having good products with good reputations and in today's environment of cost-reduction and competition sometimes some corners are cut.

We've all heard of component suppliers providing parts that, on the surface, meet the specification and are alleged to have been tested to meet the specification, but in fact weren't. Sourcing parts does not relieve the purchaser of performing quality control and analysis on the parts they receive from suppliers, though many seem to think that writing QC responsibilities into the specifications does!

I'm not saying that's what's happened here, but it wouldn't be outside the realm of possibility in today's business environment. The trade journals are filled with stories of Customers and suppliers/manufacturers working together to solve problems and both benefiting in ways that were unexpected.

 

Electrical pitting can be electrostatic or electromagnetic in origin. Both sources result in pitting damage, they differ in origin and destructive capabilities. Electrostatic shaft current (direct current) is the milder of the two. Damage progresses slowly, and it generally occurs at the location with the lowest resistance to ground. It can also be attributed to charged lubricant, charged drive belts or impinging particles.

Gear and Bearing Electrical pitting damage can be caused by intermittent arcing between the stationary bearing and rotating machine components. Because of the small oil film thicknesses relative to design clearances, the arcing commonly occurs through the bearings. Although the rotating and other stationary members can also be affected, the most severe pitting occurs in the soft babbitt of bearings. The pits may appear frosted or they may be blackened due to oil deposits. As pitting progresses, the individual pits lose their characteristic appearance as they begin to overlap. Once the bearing surface becomes incapable of supporting an oil film, the bearing will wipe.

George D. Lankford
Director, International Project Management
Philadelphia Gear Corporation Onsite Technical Services

www.philagear.com

 

Excellent post. Just a couple of questions, please.

What would be remedy for electrostatic pitting? Most of these gear boxes do not have drive belts connected to them; the Main Hydraulic Pump and the High-pressure Liquid Fuel Pump and the Atomizing Air Compressor are all directly connected to the output shafts of the Acc. Gear. The starting means is also directly connected (through the torque converter) and the turbine shaft (through the Accessory Coupling). I've heard of electrostatic charges on steam turbines but not on gas turbines; though we don't know if these units have steam injection or water injection of any purpose.

Second, how would the lube oil particles become charged? What would be the remedy?

Third, what would be the remedy for electromagnetic pitting? What would be the source? Would it be poor grounding? I've seen some applications where the Atomizing Air Booster Pump, driven by an AC motor, is mounted on the top of the Acc. Gear Box. So, if the Gear Box is not properly grounded would that be a likely cause?

Thanks very much for your input and answers!

 

I would have a look at the earthing on the main shaft. Do you have or should you have an earthing brush of the main shaft between the generator and the gas turbine?

Most probably the shaft is building up static electricity and there is no means of discharging it (i.e. there is no effective earthing path). Thus the built up static is trying to find alternative paths, normally either the bearings or the gear trains of the accessory gearbox, depending which has the lower effective resistance.

In your case it seems that the accessory gearbox bearings are the discharge path of this static electricity. I would also have a look at the GT bearings and the teeth of the accessory gearbox, especially those meshed with the main shaft, to see whether there is pitting there also.

I believe that the gearbox bearings are the victim of this situation. The cause is as indicated above. Have a chat with the designer/assembler of the package and get his opinion.

 

YES, IT MAY BE INSUFFICIENT OF EARTH GROUND THAT WHERE YOU FOUND LEAKAGE THAT COULD BE THE COURSE.

 

We also face a problem with acc gear. Frame6B gas turbine. Acc gear supplied by BHS Cincinnati Type A 619. Mai hydraulic coupling failure occurs. We operate 2 Frame6B gas turbine.In both the machines together 4 times the failure occurred in last 3 years. The machine is running since 4 years. All recommended mntce practices followed.

 

I've seen this happen twice in as many weeks to the same Frame 6B turbine many years ago just after a maintenance outage; the Nylint coupling between the Main Hyd. Pump and the Acc. Gear driving shaft would break. The first time it was verified that the Hydraulic pressure was correct after the coupling was replaced and the unit re-started.

The second time it was discovered that the pressure had been set using the relieve valve instead of the pressure compensator on the pump. In other words, someone had cranked the pump's pressure compensator setting up and then reduced the relief valve setting to achieve the desired pressure. This causes the hydraulic pump to work very hard because there is excess flow through the pump and out the relief valve.

The proper way to set the relief valve is to "crank up" (increase) the Main Hyd. Pump pressure compensator setting until the Hydraulic system pressure is equal to or higher than the relief valve's setpoint (from the Device Summary) and adjust the relief valve to relieve at the pressure in the Device Summary. Then reduce the pump pressure compensator setting until the hydraulic system pressure is equal to the normal system pressure.

The Hydraulic system is pretty much a static system when the turbine is running at a stable load. There is really only flow through the pump and servos when a position or flow change is necessary and as soon as the position feedback or the flow feedback equals the reference then the Hydraulic flow stops again.

By having a constant flow through the pump because the relief valve is relieving at a low pressure setpoint causes the power drawn by the pump through the coupling to be higher than normal, and even higher when position or flow changes require even higher Hydraulic flows.

Please write back to let us know how you find the settings of the Main Hyd. Pump pressure compensators and the Main Hyd. Pump pressure relief valves, and how you leave them after you check them.

I could also think of one other situation which might aggravate the above condition (improperly set Main Hyd. Pump relief valves) or even possibly cause a failure on its own: a non-working Hydraulic Accumulator (or a lack of a Hyd. Accumulator). The Hyd. Accumulator should help with sudden flow increases or decreases; that's its purpose. If it's not properly charge, or not properly valved in, or, as I've been seeing lately none is used on some smaller GE-design heavy duty gas turbines, then this could also seemingly cause a similar problem of excessive power being drawn by the Main Hyd. Pump.

[By the way, an Auxiliary Hydraulic Pump should be adjusted in exactly the same way: the relieve valve should not be used to set the system pressure, rather the Aux. Hyd. Pump pressure compensator should be adjusted to control the Hyd. Pressure when the Aux. Hyd. Pump is running when the unit is shut down (and the Main Hyd. Pump is not running). I've also been hearing that some smaller GE-design heavy duty gas turbines are being supplied without Aux. Hyd. Pumps (seemingly in a cost-reduction effort).]

I could also imagine this happening when an improperly rated coupling was used.

 

We also faced similar incident in same machine same gear box and had to replace few sets of Gear wheel sets, Gear box Bearings, Acc Couplings shafts (oil type). Pitting on bearings, Gear wheels and damaged Acc coupling shaft teeth observed. Later found the cause due to internal short circuit in the Generator Rotor. Also some issue in the no 5 bearing pedestal insulation. We installed grounding brushes near no 4 bearing and To Acc coupling shaft with leakage current measurement provisions.

Still we have Generator Rotor issue and can see some pitting marks in the Acc Gear bearing (drive end) and same thrust bearing during periodical inspections.

Hope this info will help you guys.(I’m not an electrical guy and I don’t know much about Electrical Discharges due to generator rotor short circuit)