9FA gas turbine seal oil system supply from lube oil pump. if lube oil system fail or stopped auxiliary AC seal oil pump will cut in. if it fail DC seal oil pump will start, but it's hardwired interlocked with AC auxiliary pump. that mean if DC started immediately AC will stop.

why we cannot run both pumps for short time as smooth transfer?

 

m.a,

Usually, the DC Seal Oil Pump motor is directly coupled to the AC Seal Oil Pump motor which is directly coupled to the Auxiliary Seal Oil Pump. Essentially they are all on one shaft (the pump and two motors).

To answer your direct question: It's a <b>really bad</b> idea to be spinning a DC motor rotor/armature (with the AC motor) with voltage applied to the DC motor field and then energize the DC rotor/armature. (The DC fields of all DC motors on GE-design heavy duty gas turbines are continuously energized, whether they are running or not; helps keep moisture from condensing in the motor, and aids in starting.) If the DC rotor/armature is energized when it is being spun by the AC motor lots of sparks usually fly and brushes are damaged and the brush rigging is usually damaged and the commutator can also be damaged. I've seen people defeat the interlock and <b>TRY</b> to run the DC motor when the AC is running ("just for a test") and end up purging hydrogen and replacing the DC motor (with a new one) and the coupling between the AC motor and the DC motor, having to re-machine the AC motor shaft, and having to replace several cells in the battery that blew their tops and cracked their cases.

Many people mistakenly believe the purpose of the DC seal oil pump motor is to maintain seal oil pressure in the event of loss of AC power until such time as AC power can be restored--and that's not the purpose of the DC pump motor at all. The purpose of the DC seal oil pump motor is to maintain seal oil pressure in the event of loss of AC only for a brief period <i>while hydrogen is being purged from the generator casing</i>, so that hydrogen doesn't leak out of the generator casing long the shaft where it can become flammable/explosive in air. The DC motor will sometimes be started during coastdown and run for several minutes after reaching zero speed (if there's no AC, there's no turning gear, usually) until purging starts--again, this is just to prevent hydrogen from getting out of the casing along the shaft and starting a fire or causing an explosion.

In the "old days" the hydrogen purity sensors were powered by AC, so if AC was lost there was no way to determine or monitor hydrogen purity even if the seals could be maintained--possibly indefinitely until AC could be restored. So, the philosophy was to just purge the hydrogen from the generator casing to prevent possible purity problems (not very likely, really), and to prevent possible hydrogen leakage as the DC battery ran down with the DC seal oil and DC emergency oil pumps running.

I think some of the newer hydrogen control panels use DC power for the hydrogen purity monitors, so while this may not be an issue (monitoring hydrogen purity when the DC seal oil pump is running), the indefinite period of being able to re-establish AC power to the systems is still a huge issue. If you've never seen pictures of the aftermath of a hydrogen fire that occurred when hydrogen leaked out of the shaft seals, you wouldn't forget it. It's not pretty!

Hope this helps!

 

thanks CSA

but in our configuration we have 2 pumps with separate motors.
what you think, it was interlocked by mistake?

 

I would suspect that if the two motors are not on the same shady the motor interlock was improperly supplied.