Hello

I am looking for some information on GE Gas Turbine (Model MS9001E DLN) Safety System, in particular how the "Gas Turbine Exhaust Purging" function works. We have a GE safety document which suggests that this function monitors the GT speed during pre-ignition phase. If the speed threshold is not maintained, then the system resets the purge time counter and the purging phase is repeated. I understand all that. However, it also states that if the purge sequence fails because a dangerous condition is detected by the speed sensors then the purge sequence is aborted and restarted. And, in case of a trip, the safety PLC activates the trip sequence of the machine by closing the speed ratio valve and the gas control valves.

My questions are:

1) what is the purpose of this trip considering there would be no fuel gas present during pre-ignition phase?
2) what dangerous condition could be present during purging that would require initiating a trip?
3) how is speed signal used in implementing this trip, i.e. is it low speed trip or a high speed trip or a rate of change trip?

Is there more detailed explanation available of start-up sequences of Mark VI safety system, as the GE documentation seems very primitive.

 

nnc20,

This sounds like someone's (GE Belfort's!!!) swing at reducing the turbine purge time requirement during starting--to help turbines get on-line faster so they can bid into markets where renewables are falling of quickly and make LOTS of money.

In the past, there was just a fixed amount of time the unit would have to "crank" (be spun by the starting means) to use the air flow through the axial compressor to purge combustible gases from the turbine and exhaust (including any HRSG and the exhaust stack). Owners and operators are complaining MADLY about the length of time this requires--they want to get on line as quickly as is mechanically possible and 6 minutes, or 10 minutes, or purging is just too long for them. So, LOTS of different methods are being employed to reduce purge times as much as possible. Some sites are using oxygen sensors in the exhaust to try to detect any low oxygen conditions (too much stratification in exhaust flows in my personal opinion, especially when the temperature of air through the unit changes during the year and with machine condition (hot or warm or cold iron). There's even US NFPA standard for this (believe it or not--it's true, though!).

The whole thing about speed and time is that there are some very good calculations/estimates of how much air flow moves through a machine when it's on cooldown and during starting (accelerating) to purge speed and while purging, and so it sounds like GE Belfort is trying to do something with this method to ensure a "proper" purge. And because it's, one, something new(er), and, two, it would certainly be considered proprietary by GE (particularly GE Belfort), there isn't probably going to be very much documentation on it.

Really, the "best" documentation you're going to find is what is programmed in the application code of the Mark VIe/Mark VIes. Takes a little getting used to when you first start trying to read and understand the programming, but if you have any experience with control systems (and a design engineer should have some experience with this, one would think) it begins to make sense pretty fast and usually you will find some "notes" or "comments" in the code which may be of help--though take ANY note or comment with a huge grain of salt because lots of time when code is being changed a lot (as this would have been or is) engineers don't always change or modify notes/comments as they should.... And, sometimes, signal name descriptions are also pretty lame. Same as with any control system; just a little moreso with GE Belfort stuff (in my experience).

That's all I can add--unfortunately. I may be 100% wrong about what this code is trying to do, and I certainly can't make any comment about what it is (or is not) doing without being able to analyze it personally.

Hope this helps!!! Apologies if it doesn't.