recently we overhauled the LP turbine after assembly of turbine the rotor was moved with hand cranking mechanism and was moving effortlessly. after that the turning gear was engaged and and machine was put on turning gear for two hours but during this seal oil system was not started and there was no hydrogen in the generator.

next day when seal oil system was started without hydrogen in the generator the barring gear did not engaged and barring/turning gear motor is tripping again and again on overload.

My question is can we engage the turning gear without starting seal oil pumps as there is no hydrogen in the generator?

Stating oil pump in service
lube oil pressure ok.

 

If you have a hydrogen cooled generator, then you should have hydrogen seal rings. Hydrogen seal rings have a very tight clearance especially at the gas side. Having said that, I recommend to put the seal oil in service prior to starting the turning gear because if not, you may cause damage to the inner diameter of seal rings.

 

You cannot turn the turbine without lubrication--this includes the seal oil system. Turning the rotor without seal oil will potentially wipe the seals, resulting in hydrogen leaks when the generator is pressurized. these seals are usually brass, and will abrade fairly quickly without lubrication. given your description, I suspect you have already wiped the seals. you need to remove and replace.

 

Recently this same question came up at my plant. Luckily we DID NOT isolate seal oil while the unit was on TG.