Hello,

this is my first introduction to this forum. The reason for my first appearance is a series of event. We recently shutdown our Frame-5 gas turbine with MARK-V control system for routine maintenance. An observation came up during maintenance that some oil was found in atomizing air circuit. Currently gas turbine is only operated on Gas fuel. When gas turbine was operated on Auto mode gas turbine speed went to 14HS and after two minutes it actuated over speed trip at 5600 rpm. One combustors were open oil contents were quite obvious. Atomizing compressor seal was found damaged which caused this oil flow. We plugged the atomizing air circuit and started gas turbine every thing went fine with a successful start up. Now I have two questions

1) Why did acceleration or speed control FSR did not closed the GCV valve if gas turbine was being accelerated by oil? No alarm annunciated in this activity it seems as MARK-V control did not have any answer for this sort of problem. Please explain in detail.
How FSRMIN worked in this acceleration and speed control scenario and if speed is increased by an external fuel what safety is there in control to eradicate this problem, DCS graph showed a continuous increase in Fuel gas flow through GCV.

2) SRV hunting was very obvious from 2 to 9 bar P2 during this activity, Is this over-speed trip due to parallel fuel supply of oil or it can also be caused by SRV hunting and oil is just a diversion.

Thanks
Peg.

 

Hello,

I've been hoping someone else would respond to this one....

First, it's *NOT* recommended to operate a dual fuel unit without Atomizing Air for extended periods of time. The Atomizing Air flow actually cools the atomizing air passages and prevents hot combustion gases from flowing back into the Atomizing Air manifold (there are no check valves in the Atomizing Air system). Combustion gas back-flows through the AA manifold can cause overheating of the manifold, pigtails, and the AA passages of the nozzles themselves.

Second, the role of FSRMIN when at rated speed (or above) is to prevent loss of flame on load rejection. It is quite often not set properly (both too high, and too low). If it's not set properly, then if the Speedtronic tries to reduce fuel flow the GCV is "limited" by the FSRMIN.

Further, as speed increases the P2 pressure reference increases, and so the actual P2 pressure will increase, and even if the GCV is being held constant the flow through the GCV will increase as P2 pressure increases.

I would say you have experienced multiple issues at one time. I would say that lube oil entrained in the AA system--unless it was quite a lot--wouldn't necessarily add to the speed. More likely, it would only increase the torque slightly but would produce exhaust smoke which otherwise wouldn't be present.

If you also had excessive SRV hunting, depending on the frequency of the hunting combined with a high FSRMIN value that could cause an overspeed condition.

Now, before you ask how to properly set FSRMIN, well, that's not so easy. To set it properly you need to load the unit (producing power) and then open the generator breaker under load. This is called a "load rejection" test. The desired result is that the turbine speed won't increase more than 3-4% before decreasing back to 100.3%. If turbine speed increases much more than 107-108% or more, then FSRMIN is set too high.

If the turbine loses flame when performing a load rejection test then FSRMIN is set too low.

And, if either undesired result is experienced it's necessary to change FSRMIN appropriately and re-test to make sure the change isn't excessive. And, most sites (and utilities) don't like load rejection tests. They're hard on the turbine and generator breaker. They can result in trips (loss of flame or overspeed). They're not fun, and nobody likes repeating them.

Hope this helps. Again, I wouldn't expect the energy added to the machine from burning lube oil wouldn't add much to the speed. Certainly if the lube oil leak was excessive it could cause a certain rise in speed, but not necessarily enough on its own to cause the overspeed. The more likely cause of the overspeed and overspeed trip was a high FSRMIN combined with a hunting SRV.

And, again: It's not recommended to operate without AA flow--even on gas fuel. (Of course you won't be able to operate efficiently, if at all, on liquid fuel without AA flow.) You might cause "collateral" damage, unintended, but damage nonetheless.

A lot of times problems like these are the result of more than one condition and that seems to be the case in this instance.

As for preventing this problem, it's pretty difficult to do so. Especially if FSRMIN isn't set properly to begin with.

Hope this helps.

 

Like you, I was hoping someone else would answer this..... and you did.

Good answer, well done again and a Merry Christmas

 

Thanks, Bob Johnston.

And Happy Holidays to you, and to everyone!