Gas Turbine Start up and Evap Coolers

Hello Friends,

I've observed that during startup of gas turbine high ambient temperature would not allow turbine to speed up from 82 percent so Evap coolers are taken in service to reduce ambient temperature . What is the relationship between turbine speed and ambient temperature during start up.
syed taha ahmed,

It would be nice to know what manufacturer's turbine and size of unit you are referring to, as well as the turbine control system being used to control and protect the turbine.

Usually when this happens with GE-design heavy duty gas turbines of the B/E class (in other words, non F-class units), it's because the exhaust temperature hits maximum during start-up and acceleration--which is only marginally affected by ambient temperature during starting and acceleration. High exhaust temperatures can happen for a lot of reasons; some of the biggest ones are:

--dirty axial compressor
--dirty turbine inlet air filters
--poor axial compressor condition (excessive clearances)
--high exhaust duct back pressure (usually when gas turbine exhausts into an HRSG (Heat Recovery Steam Generator--a "boiler")
--improperly adjusted Control Constants (usually when trying to get turbine to rated speed faster than normal)
--improperly calibrated fuel control valve LVDTs (if so equipped), or improperly scaled liquid fuel flow divider feedback
--changes in fuel heating content (usually increases in calorific value)
--excessive exhaust temperature spreads during acceleration (which are really bad for the combustion hardware and hot gas path parts, but also reduce the turbine efficiency because of non-uniform and reduced hot gas flows through the turbine section)

There is a small relationship between ambient temperature-corrected turbine speed during start-up and acceleration, but it's usually used primarily as a bias to compensate for ambient temperature during firing. Cold ambients usually require a little extra fuel to help reliably establish flame; hot ambients usually don't require as much fuel to reliably establish flame, so by making small changes to the amount of fuel admitted when trying to establish flame the effects of thermal stress on the hot gas path parts (combustion liners; transition pieces; first stage turbine nozzles; and exhaust diffuser) can be reduced. (GE calls is part of their "low cycle fatigue" control scheme--trying to reduce the harmful effects of thermal stresses during starting and shutdown.)

Some "creative" people have discovered that by using the evaporative cooler to cool the axial compressor inlet air temperature during starting and acceleration they can "ride through" the exhaust temperature limit situation without having to address the real cause(s) of the problem. While this isn't necessarily a harmful thing (if there isn't a lot of carry-over (water droplets) from the evap cooler during low-speed operation, which there usually is), in the long run it's only going to work for a limited period of time if the underlying conditions aren't corrected. Usually, using the evap coolers during acceleration only minimally decreases the exhaust temperature, especially when the IGVs haven't started to open yet, so it's really just dumb luck if it works, or for how long it works. (You can watch the effects by monitoring TNH and TTXM during acceleration with the evap coolers off, and with them on--if you are working on a GE-design heavy duty gas turbine with a GE Mark* turbine control system.)

This is a brief description of what can cause the situation you seem to be describing for a GE-design heavy duty gas turbine and Mark* control system. There are several inter-dependencies and quite often this problem is the result of more than one cause. Using the evap coolers to ride through this situation, while creative, is not recommended and will probably only solve the stalled acceleration problem for a short time, especially if the root cause(s) are severe.

Hope this helps! We're glad you found, but remember: We don't know you, and we don't know what kind of turbine-generator(s) you have experience with and are working on, and we don't know what kind of turbine control system is in use at your site. So, when posting, especially in the beginning, please try to post as much information about the turbine and control system and fuels being used, etc., at your site as you can. We're not able to see and know what you can see and what you know--so help us help you by providing as much information as possible. If you are writing about trips or shutdowns, we need to know what alarms were annunciated prior to the event, during the event, and after the event, too. In chronological order. Without that information, we are really just guessing. Also, if you are writing about a trip or shutdown if you have any date you can post to the thread that would be really great (say from the Trip History Display, or a data archival/historian system).