We are operating GE7FA DLN 2.6 Gas turbines,

The scenario is like this, M/C was operating on Base load, suddenly there was a alarm of "PM3 GCV out of position alarm & M/C got tripped.

In the field it was found that there was heavy oil leak from the Fuel gas compartment,, Operator didn't have any idea from where the oil was leaking, because it was spreading all over like shower. Later on it was found that PM3 GCV hydraulic oil supply line completed got detached from its location (stainless steel ferulle got sheared off).

We Switched off the hydraulic & lube oil pump after GT reached to zero speed (we must not stop the lube oil system when m/c at speed).

We checked the trend, Hydraulic oil pressure immediately dropped from 115 bar to 35 bar when M/c got trip.

My argument is whether it is advisable in such a situation to stop hydraulic even though m/c is on rotation?

because of the heavy shaft lift oil supply needed to lift the shaft to form hydro dynamic film ( in GE 7FA machine the same hydraulic supply goes to lifting shaft through solenoid v/v 20QB & pump compensator)

We resumed the lube oil supply to the bearings after 1.5 hours of M/c trip. Is it advisable to stop the lube oil supply when M/C tripped from base load? because bearings are very hot it requires some medium of cooling to save the babit metal of the bearing.

If we stop the hydraulic supply we would have save the oil spillage, for your information we topped up 31 drums of oil to restore the lube oil tank to normal level.

please give us the advise what to do in such a situation to control the lube oil spillage

Regards,
Parag

 

Isolation valves can be installed on the hydraulic oil supply lines for the GCVs/SRV and IGV. This isolation point also allows for actuator maintenance, including servo replacement, while the unit is hot and on turning gear.

Greg Ponto
Lead Combustion Turbine Specialist
Elwood Energy
Office ph. 815-423-2232
Cell ph. 815-671-2383
[email protected]

 

This is an excellent illustration of two very important aspects of maintenance & operation.

The first is the need for regular rounds by operators looking for, among other things, oil leaks that can be spotted before they become a trip and potentially damaging to the unit.

The second is the importance of <b>knowing and understanding</b> the piping schematics (P&IDs) for every system on the turbine.

I'm not saying that every situation like this one can be avoided by diligent rounds, but certainly the majority of them could be!

The turbine probably has a Jacking, or Bearing Lift Oil, Pump. If so, then it's likely not necessary that the Aux. Hyd. Pump is required for cool down or during coast down. BUT, the Piping Schematics will have the answer to that question, in addition to looking at the sequencing.

I'm not familiar with any F-class units that require hydraulic oil pressure for coast down or cool down. But, consult the drawings for your unit(s) to be sure.

This exact condition has been occurring with increasing frequency. When reassembling tubing fittings after any maintenance outage they should be carefully inspected for signs of wear or cracking. If there is any strain on the fittings (and this appears to be the biggest cause: fittings with excessive or abnormal strain from poor construction and/or reassembly practices) they should be replaced with new tubing abd fittings, possibly even flexible hoses.

An ounce of prevention is worth a pound of cure, which is just another way of saying that being proactive is better than being reactive.

Read and study the Piping Schematics for your unit. The control system is <b>NOT</b> just the Speedtronic. Unfortunately ther are way too many people who think they don't need to know the Piping Schematics for their machine (and way too many GE field people and their managers who think the same way!!!).

The most successful sites (and GE field <b>engineers</b>) are the ones who study and basically commit those drawings to memory. They don't change much from machine to machine or when the control system is upgraded.

 

Dear CSA,

Same hydraulic oil pumps is used to jack the bearing while coasting down as well as on startup.

There is a solenoid v/v & pump compensator that will increase the hydraulic pressure in bearing lifting circuit to 250 Bar.

There is no separate jacking oil pump being used in GE 7FA machine.

Normal hydraulic pressure during machine on load is 115 Bar, when coasting down at around 95% speed, solenoid v/v 20QB will energize & hydraulic oil pump pressure will increase (through pressure compensator) to 250 bar to jack the shaft.

My question is in such a scenario ( heavy oil leak ) if we stop the hydraulic during coasting down, will it harm the bearings? as shaft weight is high & jacking oil is required to form the hydro dynamic film.

Thanks,
Parag

 

In my opinion, in an emergency situation if you have to shut off the hydraulic pumps because of a massive leak, then you have to shut off the hydraulic pump. I would imagine it's not desirable for the bearings, but consider the condition where the plant loses AC and the unit comes down on the Emergency L.O. Pump. Is there an Emergency Hyd. Pump for the bearing lift oil?

So, as long as it doesn't happen (coming down without Hyd. Pump pressure for lift oil pressure) on a regular basis then it would seem that if it only happens on an infrequent and irregular emergency basis it would be okay.

I'm not a fan of the F-class machines, nor do I have a lot of experience with them. I've seen mostly early and older F-class units, and some that were packaged by the GE licensees (John Brown, Alsthom, etc.) and a lot of those had odd or unusual skids and auxiliaries (different from those provided by GE with their packages).

Once again, GE is continually told by their Customers that GE units are "too expensive" so GE is doing everything to cut costs to reduce cost and improve profit margin.

 

The best possible modification may be to convert Auxiliary Hydraulic Pump to operate by DC supply. This will help to save the GT shaft from bowing down in case of AC supply failure and costing down.

When GT is costing down, Jacking Oil Pump cuts in at around 10% frequency. The costing down time varies from machine to machine. So, ideally there is no problem in stopping the Hydraulic Pump once the oil leak is observed. But, you have to take the required corrective action very quickly and you have to start the Hydraulic Pump again before its requirement(it must be started before GT speed falls down below 10%). Though, it may turn down to very risky affair if Hydraulic Pump is not started in required time(bowing down of GT shaft and also may be bearing damage).

It is very true that GE compromised at important equipments to cut the cost. Siemens V 94.2 machines have seperate Lube Oil & Hydraulic Oil tanks and systems. Lube Oil pumps & jacking Oil Pump(Lift Oil) are connected with Lube Oil Tank and supply oil to turbine bearings, jacking oil pump and turning gear. Hydraulic Oil pumps are used to actuate Gas & Naphtha/HSD Control/Stop valves.

 

Some may believe that having two separate oil tanks and having to store two separate types of oil (lubricating oil and hydraulic oil) is some kind of improvement. There are others who believe that utilizing a common oil medium for both lubrication (including jacking, or bearing lift, oil) and hydraulic pressure systems is more than a cost-savings, it just makes sense.

If there is any compromise, it's that the AC motor-driven hydraulic pump output may be used for supplying the bearing lift (jacking) oil system. But, one has to believe that the designers considered <b>coast</b>-down without bearing lift (jacking) oil when they designed the system.

Don't confuse using a common medium for lube- and hydraulic systems with making any kind of compromise to system safety and reliability. It's just not a valid criticism. In this case, it's nothing more than a difference in design philosophy.

 

Hi, Good question to ask,
The answer to the extent of damage that can occur.

Only GE can tell. But we can fairly predict that extensive damage will not occur, as this is also the case whenever black out occurs and your Emergency Diesel Generator fails to start, since HOP is not a DC drive,
Obviously this is a highly probable event and hence may be considered by GE.