Gas turbine in operation: During continous operation of gas turbine, ejector is used to maintain sub atmospheric pressure in lube oil tank

Gas turbine shutdown or gas turbine trips: Mist eliminator fan (works as substitute for ejector) comes in service to maintain sub atmospheric pressure inside lube oil tank

Upon gas turbine shutdown or trip: If in case mist eliminator fan not came in service, what will happen to gas turbine

 

Ejector, eh...?

My experience with GE-design heavy duty gas turbines began in the days before negative-pressure mist eliminators. Back then a large diameter (6 inches or so) was run to the top of the exhaust stack and the vapours from the L.O. tank/reservoir were just allowed to enter the atmosphere at that point. Of course, when the unit wasn't running, if the L.O. was hot enough to produce vapours (such as after a trip or shutdown), that would mean that in a few hours, especially if it was cold outside, the vapours would start to condense in the area around the tip of the pipe, and that was messy. AND, the vapours were also visible to the naked eye, which wasn't acceptable to the clean-air contingent.

The next evolution were electro-static precipitators, which caused the vapours to (try to) condense in the pipe BEFORE they reached the top of the stack, and drain back to the L.O. reservoir. These were also messy, inefficient, required maintenance and repair, and just weren't very good.

That's when the negative pressure systems came into being. Large vessels with filters and/or passages inside and a motor-driven fan to draw air/vapours from the tank/reservoir into the vessel, condense, and discharge an invisible plume to the atmosphere. (I say "invisible" because even if there's not visible plume there is almost always some oil vapours entrained in the discharge, and they inevitably condense on nearby surfaces and cause a mess.)

A path for oil vapours from the tank/reservoir is necessary to minimize the pressure in the tank/reservoir as well as to prevent the heat from potentially causing a fire or explosion--as oil vapours above a certain temperature can be easily ignited and burn or explode.

For this reason, it's important to respond quickly to any failure of a system to reduce the pressure and remove the vapours from a L.O. tank/reservoir. How quick is quickly? Well, how hot is the oil? How large is the space above the oil in the tank/reservoir? How hot are other pipes and equipment/components in the tank/reservoir?

GE-design heavy duty gas turbines won't suffer greatly if the mist eliminator doesn't continue to work whenever one or more of the L.O. pumps are running. BUT, the condition (non-running/functioning mist eliminator) should be quickly troubleshot and resolved, as in a few hours or so, especially if the oil is hot such as shortly after a trip or shutdown.

Finally, many generators use L.O. as the shaft sealing medium for hydrogen-cooled generators. This means that hydrogen is in contact with and will be entrained in the L.O. flow from the generator bearings/seals and make its way back to the L.O. tank/reservoir. Loop-seals and bearing drain enlargement tanks are only so effective--meaning they will never remove all of the hydrogen from the L.O./seal oil, some will make its way back to the tank reservoir. And, the design of some L.O. tanks/reservoirs allows for pockets of hydrogen to collect and build up in the tops of the tanks/reservoirs. This is another reason for negative pressure mist eliminators--part of the "mist" may be hydrogen, which, of course, can be explosive when ignited when the concentration exceeds a certain level.

You really need to consult the Operations & Maintenance Manuals provided with the gas turbine(s) at your site to see what their recommendation and requirements, if any, are about negative pressure mist eliminators and how long they can be out of service. The turbine control system might even prevent a START if the motor-driven mist eliminator system is not operating during acceleration until such time as the ejector system can "take over."

Hope this helps!

 

@Nirpendra Singh Is this a GE gas turbine? or any other manufacturer

 

A shortcut to asking/telling about GT manufacturers could be using the terms “GE” & “SOB” (Some Other Brand).



What say the group?

 

Some may confuse "SOB" with Supervisor. Not mine of course. My supervisor's a swell guy! (He reads this site too).

 

@Nirpendra Singh Is this a GE gas turbine? or any other manufacturer

This is Siemens makr

 

This is Siemens make

 

What type /model brand serie of this unit ...

Dont you have any OEM operation& maintenance manual...

Is there kind of lube oil tank breather system flame trap ( primary and secondary usually two are installed..)

The operations /coordiantions in case of a failure /loss of the ejector or blower should be described in that manual...
And it is surely related to the Flame trap operation too for the safety of the plant!

Looking to hear back from you
ControlsGuy25

 

Hello

Any update on this issue...

 

HELLO

Check Bypass line or Bypass valve /check valve if all are ok according to OEM specifications...

 

Can you hceck SG382/383 and tell us how flow looks..

 

A Tripped an would mean loss of vacuum in the LO tank. Hence the oil will not drain as quickly as it should. There is a chance that the oil may breach the mechanical seals, escape onto the shaft and potentially cause a fire or an explosion. (Especially the Air Comp Discharge/Combustion Chamber side)