gas turbine purge air

M

Thread Starter

mgulshan

gas turbine frame 5 axial compressor. PLZ ANY BODY TELL ME WHAT IS PURGE AIR AND WHAT IS OPERATION OF PURGE AIR
 
Purge air is generally compressor discharge air used to purge lines of combustible vapors as well as to cool components. For example, CPD used to purge the gas fuel system when operating on liquid fuel, the purge air is supposed to ensure that there is no combustible gas/air mixture present in the piping being purged, and it also serves to cool the gas fuel tip(s) of the fuel nozzle(s) in the combustor.

You should be able to find a Piping Schematic (P&ID) for the Purge Air system, and some kind of system description in the Service/Operation Manual.

If you still need more information, you're going to need to provide more details about your question.
 
Thank you for this info.

Meanwhile, I am working on the Engineering stage of GE FRAME6B, and the Purge air line is taking on the PID to DLN Fuel gas skid. I understand the first point that it will be used to cool the fuel gas tips of the fuel gas nozzles in the combustor. BUT I will like to know if in that same line is used for purging combustible vapor?
 
As in the previous reply, purge air is used to push out any combustible gases (vapours) in the fuel nozzle, for cooling the fuel nozzles, AND to prevent backflow of hot combustion gases into the fuel nozzles and manifolds which can result in circulating hot combustion gases between combustors and severe damage.

Purge air, when first introduced into the gas fuel manifolds/nozzles, pushes out any remaining gas fuel (when transferring from liquid fuel to gas fuel). That's why the purge valves, VA13-n, are usually set to open relatively slowly so as not to suddenly push a lot of fuel into the combustors which can result in a load spike and an exhaust temperature spike.

Hope this helps!
 
The F5001 i am working on currently has had a combustion problem for many years. Investigations show that when the LPG system was decommissioned many years ago. GE disabled the GAS purge, not the Distillate purge system, e.g. VA-13 1 & 2 are disabled locally although the logic is still in the Mk5.

Now, the fuel nozzles suffer from extreme carbonization. I am suspecting that the purge system should be recommissioned to give the gas caps cooling air & to introduce the missing compressor discharge air back into the liner.
 
Mick J,

Something seems amiss here. Most of the units with LPG I have worked on have used the same gas fuel delivery system (manifold and nozzles) that are used to deliver natural gas; they just use separate Stop-Ratio and Gas Control Valve SRV & GCV) assemblies--one for LPG and one for Natural Gas. It would seem--if this is the case--that if one were switching from natural gas to LPG, or vice versa, that purging the gas fuel delivery system through VA13-1/VA13-2 would not be possible. However, purging the gas fuel delivery system when operating on liquid fuel would be advisable.

Also, VA13-1 and VA13-2 each usually have a pair of limit switches to indicate when the valves are fully open or fully closed, as well as a solenoid-operated vent valve between the two block-and-bleed valves (VA13-1 and VA13-2) AND a pressure switch to monitor the pressure between the two valves. I could envision that it would be possible to "disable" the two block-and-bleed valves, and even the vent valve by blocking the air flow to the valves, or by electrically isolating the valves and even the pressure switches--but the sequencing in the Mark V would require a lot more modification to prevent alarms when the valves weren't in the proper position(s) and/or there was or wasn't pressure in the intervalve area when there shouldn't or should be pressure.

Can you determine from the carbon deposits where the material causing the carbonization came from? If the liquid fuel nozzles were not properly being purged when running on gaseous fuel that can cause "dribbling" which can result in carbonization of the liquid fuel. But, that's usually pretty obvious from the patterns on the faces of the fuel nozzles.

I have also seen lubricating oil, hydraulic fluid, diesel, and gasoline entrained in natural gas from the pipeline cause carbonization of the gas fuel tips, when there is no "knock-out drum" ("cyclone separator") upstream of the turbine.

And, I wonder what would happen if the LPG wasn't properly heated/vapourized prior to being admitted to the gas fuel delivery system.... Actually, I think there would be high exhaust temperature alarms when condensate made it ways into the combustors, if not overtemperature trips.

And, natural gas can also have a "high" dewpoint temperature, that can result in condensation as it flows through valves, orifices and nozzles. (Every pressure drop through an orifice, a valve or a fuel nozzle results in a temperature drop--that can cause condensation.)

Usually, most units which I have worked on that run on LPG have to start or shut down on some other fuel--not all, but most. So, purging would seem to be necessary and advisable, if possible. Again, we don't have much to work on here, and we don't have access to the P&IDs or the Control Specification to understand the details of the unit and auxiliaries.

Some natural gas these days is very, very dirty--containing lots of sulfur and even dust and dirt.

But, still, one has to wonder why some "partial" disabling of the gas purge system was done, and without being able to see the P&IDs for the unit in question--and without better understanding what you mean by "combustion problems"--we can't say too much more. Please try to understand the source of the carbonization (liquid fuel; entrained liquids in one or the other gas fuel source; condensation of liquids entrained in the one or the other gas fuel source; etc.).

One more thing about "commissioning".... Sometimes (usually) the time available for commissioning is very short (because of mechanical construction issues using too much time). And, if there are problems with fuel supply systems during commissioning (incomplete; poor design issues; unavailability of fuels; intermittent availability of fuels; etc.) that cause things to be left in an incomplete state. The thought is that at some time in the future when "things get finished" or design issues get resolved, or fuels become available, that the systems and functions will be finally and fully commissioned. And, sometimes, that just never happens, and there's no monies for people to return to complete the commissioning. (Don't ask how I know this.) So, things get left unfinished or just abandoned because of conditions and situations that are not fully understood years, sometimes decades, later.

Please write back to let us know what you find and how you fare in troubleshooting and resolving the issues! If you need further assistance, we are going to need to understand exactly what kinds of combustion problems the unit has, and some idea of how the two gaseous fuels are delivered to the turbine. Some people post scans of P&IDs to free web-sharing sites, such as www.tinypic.com, and then post a URL to them here (since we can't post images or attachments to control.com).
 
Ah! I think you misunderstood, my fault, there is no gas used on this machine anymore. It is distillate only.

Therefore when the gas fuel side was decommissioned many years ago, they isolated VA13 -1 & 2 & associated equipment. (they even removed the gas mimic page from the screens) this results in no purge/cooling air cooling the fuel nozzles when running on distillate, hence the poor combustion & over heating of the nozzle 'gas' caps. its just my opinion, i was wanting confirmation of this theory or am i miles away?
 
Mick J,

One would presume that if the gas fuel system was decommissioned it was also removed--meaning the manifold surrounding the axial compressor casing was removed, and the pigtails/hoses from the piping to the fuel nozzles were also removed, and that blank flanges were placed on the gas fuel inlet flanges of the fuel nozzles.

This would mean there was no way for hot combustion gases to circulate between combustors through the gas manifold. This would also mean that purge air from VA13-1 & -2 couldn't get to the gas fuel passages of the fuel nozzles, which would also reduce the cooling of the gas fuel tip of the fuel nozzle.

In my personal opinion, the lack of purge air--which does provide some cooling--but whose secondary function is to prevent hot combustion gases from flowing backwards through the gas fuel passages of the fuel nozzles, through the pigtails/hoses and into the manifold and circulated between combustors--is not a problem as long as blind flanges were placed on the gas fuel inlet flanges of the fuel nozzles.

The primary purpose of purge air is to purge any fuel from the passages it is flowing into and through so that there is no chance for that fuel to combust. The secondary purpose is to prevent backflow through the passages, and possible circulation to other combustors which have higher or lower pressures. And the last, and really the least, purpose is to cool the passages. That's really just kind of a side benefit (remember purge air will be coming from the atomizing air compressor discharge, so it will be pretty hot, just not as hot as the temperature inside the combustion liner. (It's all relative.)

If it's carbonization you are most worried about--and that's what it seemed from you original post--then it would seem that there is some problem with "dribbling" of liquid fuel during shutdown. If the unit only runs on liquid fuel, it would seem the most likely time when liquid fuel could carbonize on the fuel nozzle tips would be during and after shutdown. It might also happen during starting, too, but I would think the highest likelihood would be during shutdown.

Improper atomization of liquid fuel during running operation could also cause carbonization of liquid fuel on the nozzle tips. It's entirely possible there is insufficient atomizing air flowing to the fuel nozzles when the unit is running on liquid fuel. There was some kind of "bypass" solenoid-operated valve on the atomizing air compressor that may have been overlooked when the gas fuel system was decommissioned. (I don't recall if it defaulted to maximum atomizing air flow, or to reduced atomizing air flow.)

Again, I read you original post as concern about heavy carbonization on the fuel nozzle tips. Whatever is carbonizing on the fuel nozzle tips is not being properly atomized or burned during running, or is dribbling into the combustors after the unit is shut down (or possibly during start-up, also). It's been said MANY times before on control.com: The liquid fuel check valves are the weakest link of the liquid fuel delivery system--and they are NOT controlled or monitored by the turbine control system. They are just expected to perform properly at all times, and the poppet-style check valves commonly used on GE-design heavy duty gas turbine liquid fuel systems are prone to failure. Some packagers of GE-design heavy duty gas turbines also used two poppet-style check valves in series (with cracking pressures rated for half the desired cracking pressure), and if one of them failed then the pressure would be half of what was necessary for proper mechanical atomization of the liquid fuel as it passed through the liquid fuel portion of the fuel nozzle. (Liquid fuel is mechanically atomized as it passes through the liquid fuel portion of the fuel nozzle, and then it is further atomized by atomizing air pressure/flow as it exits the fuel nozzle before it combusts. Both atomizations are critical to complete combustion.

One thing that is going to happen to liquid fuel-only machines during shutdown is that since the liquid fuel can't be purged from the liquid fuel passages of the fuel nozzles after fuel is shut off to the fuel nozzles is it's going to get heated up inside the fuel nozzles (from the residual heat in the combustors) and it's going to expand, and dribble out of the tips, where it's going to carbonize. I don't think there's any way to avoid that.... Unless the liquid fuel downstream of the liquid fuel check valve can be purged out of the fuel nozzles as soon as fuel flow to the nozzles is shut off--and it's purged completely!--it would seem that there is going to be some fuel that is inevitably going to be dribbling out of the fuel nozzles and on to the fuel nozzle tips. And, even if the fuel was purged out of the fuel nozzles, it would be purged into the combustors--which would mean it would fall on to the inside of the combustion liners, where it would also evaporate--causing smoking!--and carbonization on the liners below the fuel nozzles.

Hope this helps! As long as there's no way for hot combustion gases to flow backwards through the gas passages of the fuel nozzles I don't think cooling of those passages is really required. And, again, cooling is the least required function, and it's just a relative cooling anyway--because purge air comes from the atomizing air system (if I recall correctly), and atomizing air compressor discharge air is pretty hot.
 
Few points, the purge air is from the compressor discharge, not the atomizing air (which after the pre-cooler is only around 75C). The check valves are all brand new & no leakage was found. The atomizing air bypass is only for when running on gas.

So, back to the job of the purge air, i agree its to banish combustion gasses from the gas manifold & pipework up to the GSV/SRV. however, the manual states it is part of the cooling function of the gas cap & being so, the amount of air to each gas cap (which is compressor discharge air, remember) is almost 7 bar. now, when the liner air flow is calculated, I'm sure 7 bar of additional air (to each fuel nozzle) is taken into account, this is now missing by decommissioning SA13 -1 & 2.

So the gas cap is hotter & the combustion has lost 7 bar of air. BTW, the nozzles are from brand new nozzles to badly coked in one start & only running for 12hrs.
 
Mick J,

Few points; atomizing air pre-cooler discharge temperature should be approximately 125 deg C--because there should be no possibility of moisture condensation at the inlet to the atomizing air compressor.

A liquid fuel only machine will not have a gas tip, and so would be the gas fuel tip would have NO orifices and passages in it. Usually, the swirler vanes are present to add a swirling motion to the combustion air passing through the swirler vanes to help with combustion of the liquid fuel (by assisting with atomization).

Usually, when gas fuel is removed from a turbine that also runs liquid fuel the gas fuel manifold(s) around the axial compressor that supply fuel to the fuel nozzles are also removed, and flange blinds are bolted to the fuel nozzle flanges where the pigtails/hoses from the manifolds would be connected to pass gas fuel (and purge air) to the gas fuel passages and orifices of the gas fuel nozzle tip. Failure to remove the gas fuel manifolds and pigtails/hoses provides paths for hot combustion gases to flow backwards into the gas fuel orifices of the gas fuel tips, through the pigtails/hoses and into the manifold and then they can circulate through the manifold to combustors with lower pressures. This is secondarily what purge air was supposed to prevent--the backflow of hot combustion gases from one combustor or combustors with higher pressures inside the combustor(s) to a combustor or combustors with lower pressures. It's the flow of hot combustion gases that causes the damage to the tips.

Purge air pushes the combustible vapours out of the fuel nozzles and into the combustors, where it's burned, to prevent carbonization and combustion in the fuel nozzle passages AND the pigtails/hoses and manifolds. I have seen flexible hoses that were burned through because of the combustion of fuel in the hoses that was not properly purged out of the hoses (they are the weakest portion of the manifold/nozzle fuel delivery system and can least stand the heat). The backflow of hot combustion gases into the flexible hoses (from the gas fuel orifices in the gas fuel tips of the fuel nozzles), to combustors with lower internal pressures, was responsible for the high temperatures that resulted in the hoses getting so hot the fuel which had not been properly purged and causing the fuel to ignite.

What's not clear from the description of the site, and which seems to be a large part of the problem, is whether or not the gas fuel manifold(s) were removed from the unit and flange blinds were installed on the gas fuel flanges of the fuel nozzles. Without a path for the hot combustion gases to flow backwards into the nozzle tips there will not be a problem with the nozzle tips being exposed to the heat of the combustor. The liquid fuel flame ball SHOULD NOT be on the tip of the fuel nozzled--it should be a slight distance from the metal tip, which will allow for combustion air (CPD air) to help with convection cooling of the nozzle tips. BUT, if there is a path for hot combustion gases to flow backwards into the fuel nozzles the hot combustion gases--hotter than CPD--will and have been known to cause deformation and melting of the nozzle tips.

Cooling is a by-product of purging. Purging of combustible vapours is the primary purposes. Preventing back-flow of hot combustion gases (hotter than CPD) into the fuel nozzle orifices, pigtails/hoses and fuel manifold(s) is the secondary purpose. And, cooling is the tertiary purpose--and really, it's just a benefit of the flow of purge air through the nozzles--not a purpose. There is "combustion air" entering the head end of the combustor (CPD) that flows around and over the fuel nozzle tips that provides cooling (because CPD is cooler that the hot combustion gases which are radiating heat--against the flow of combustion air (CPD)).

Without being able to see pictures of the nozzles immediately after removal from the combustor endcovers, and after removal of each of the tips (gas fuel; atomizing air) and the liquid fuel portion of the fuel nozzles it's impossible to say what caused the carbonization. I have seen improperly assembled and improperly installed liquid fuel "cartridges" that leaked liquid fuel into the atomizing air passage and very quickly caused carbonization--a LOT of carbonization--on the tips of the fuel nozzle (the liquid fuel tip, the atomizing air tip, and the gas fuel tip). The patterns formed on the tips very clearly showed the downward flow of the fuel and carbon formation. The improperly assembly of the liquid fuel cartridges, in addition to allowing liquid fuel to leak out of the cartridge and into the AA passage, resulted in improper atomization of the liquid fuel (pressure atomization caused by the internal swirler vanes of the liquid fuel cartridge). This also caused problems with the flame being too close to the fuel nozzle tips, and because of the improper atomization and incomplete combustion added to the carbonization. (Very shortly after the machine had been re-started with the reassembled fuel nozzle cartridges high exhaust temperature spreads appeared; it was started and run twice in 24 hours, and only ran for a one- or two-hour period before the spreads became so high the unit was in danger of tripping, and the unit was manually tripped shortly after synchronization on the second start because the spreads were so high.) Because there is no purging of liquid fuel after shutdown while operating on liquid fuel, and because so much fuel had leaked out of the liquid fuel cartridges during the shutdown/cooldown period and had carbonized in the liquid fuel cartridges it caused a couple of the liquid fuel tips to be nearly completed blocked, resulting in the very high exhaust temperature spread at FSNL.

Disabling gas fuel has to include removal of the gas fuel manifold(s), associated pigtails/hoses, and blanking (blinding) of the gas fuel fuel nozzle flange--to prevent hot combustion gases from flowing backwards through the gas fuel nozzle orifices, pigtails/hoses and into lower pressure combustor(s). ESPECIALLY if the purge air is disabled BUT the gas fuel manifolds and pigtails/hoses were not removed, there is going to be a path for hot combustion gases to flow backwards and through the manifold--and THAT is going to mean the fuel nozzle tips and passages are going to be exposed to excessive heat. There are NO check valves in the gas fuel system to prevent backflow into the manifold, and pressure differentials of even a couple of psig (tenths of a barg) WILL cause hot combustion gases to flow where they ain't supposed to flow.

Again, there is combustion air (CPD) that will be flowing around the outside of the fuel nozzles at all times--regardless of which fuel is being burned, so there is some cooling of the nozzle tips even if it's not from the inside. And, the flame ball should not be ON the fuel nozzle tip, it should be a short distance from the tip.

If the gas fuel manifold(s) and pigtails/hoses are still installed on the unit, and VA13-1 &-2 have been disabled to prevent purge air from flowing through the manifolds and pigtails/hoses and gas fuel nozzle passages and orifices there is a high likelihood that hot combustion gases have flowed backwards through the gas fuel system from combustor(s) with high pressure to combustor(s) with low pressure--and this will cause the tips to experience higher-than-normal temperatures resulting in deformation. (I saw the tips of atomizing air cones liberated in a couple of hours; fortunately they were vapourized in the flame ball and high diffusion flame temperatures and didn't cause any damage to the first stage turbine nozzles and buckets.) I worked on one of the first GE-design dual fuel (distillate-gas) heavy duty gas turbines to be retrofitted with purge air for the gas fuel system; it took several months to work out all of the problems and sequencing--and in the process a lot of fuel nozzle tips and components were damaged. My millwright/pipefitter crew became VERY good at removing and re-installing fuel nozzles very quickly--like a race car pit crew! (In the beginning we had lots of leaks and problems on reassembly.)

Without a manifold and without pigtails/hoses connecting the fuel nozzles to the manifold there is not going to be a path for hot gases to flow--and so purging, and cooling, isn't going to be required. Take away the path for hot gases to flow and the need for purging/cooling goes away.
 
Top