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GE 9FA Gas Fuel Purge System
When 9FA gas turbine is on diffusion mode or premixed mode (DLN2.0+), continuous purging occur in the PM4 manifold and D5 manifold. I want to know, how much air pressure is maintained during purge?

My curiosity is, I know in base load compressor discharge pressure is around 225 PSI. But when base load, turbine is on Premixed mode. Thats why there is continuous purge in the D5 manifold. I don't understand, if by D5 manifold, continuous purge pressure is 225 PSI, how the flame in combustion chamber establish? Because, 225 PSI is so high pressure, flame can be blown off. So, How this is happening?

Again referring GE Document, "GEK 110832a", in description, it is said that, all 63PG (1A/1B/1C/3A/3B/3C) pressure switches are used for give proper block and bleed system. again referring, "0414 - Device Summary", it is said that, all switches will start acting, while cavity pressure will be higher than, 50 PSI. I am so much confused, how is it possible to maintain 50 PSI? because CPD pressure is around 225 PSI during base load.

I will be very grateful to someone , who will kindly make it clear to me.

Thanx a lot

1 out of 1 members thought this post was helpful...

niazbibiyana,

The answer to this question--and to many questions--lies in reviewing and analyzing the P&IDs for the various systems. When one is trying to understand these kinds of questions it's always best to refer to the P&IDs. In fact, every operator and technician should have their own copies of the P&IDs, and those copies should have lots of colorful markings (from highlighter pens) and lots of notes--both of which means the owner of the drawings has taken the time to review and understand the P&IDs, which means the owner likely has a very good understanding of the systems and how they operate and what the various components do and what their purpose is.

I believe you'll find that when running on gas fuel purge air comes from axial compressor discharge--so it is basically CPD. The pressure inside the combustor (where purge air "discharges") is slightly less than CPD (I think the pressure is a very low, single-digit percentage less than CPD). And, it has to be less than CPD--otherwise there could be no flow (if the pressure in the combustor was higher than CPD, the flow would be from the combustor to axial compressor discharge--and that doesn't happen!). So, with a very low differential pressure (between CPD and combustor pressure) the flow is not that great to begin with, and it's purpose is just to ensure that hot combustion gases don't flow backwards into the fuel nozzles and manifolds where can can cause very bad damage.

The pressure switches you mentioned are sensing the pressure between the double block valves that are OPEN during purging. One block valve is used to shut off the flow of axial compressor discharge pressure to the manifold/nozzles, and the other block valve is used to shut off the flow of fuel gas to the axial compressor (which should NEVER happen). And, there's s solenoid-operated bleed (vent) valve that is to be CLOSED when the double block valves are OPEN (this arrangement of two block valves and a bleed (vent) valve is commonly referred to as a "double block-and-bleed valve arrangement). And since there is flow through the double block valves from the axial compressor discharge to the combustor when they are open, there will be pressure--CPD, to be exact--between the valves. And that's perfectly normal, and acceptable--and to be expected. And, the pressure switch isn't trying to sense pressure when there SHOULD BE pressure--only when there SHOULD NOT BE pressure. Read on.

When there is gas fuel flowing in the manifolds and nozzles that are otherwise being purged, the double block valves are CLOSED and the bleed valve (solenoid-operated vent valve) is OPEN. So, there should be NO pressure between the double block valves, especially since the vent valve is open to vent any pressure between the double block valves when there is gas flowing in the manifolds. If any pressure builds up inside the cavity between the double block valves that means one (or both) of the two block valves is leaking--and the leak is so bad it is pressurizing the cavity between the double block valves even with the solenoid-operated vent valve open. And, that's NOT normal, acceptable or desirable--so the pressure switches will actuate and alarm and trip or runback the unit. Since it's not possible to tell which double block valve is leaking AND since it could be the valve which is blocking gas fuel, there's potential for a very bad situation--hence the alarm, and trip or runback.

Does this satisfy your curiosity? The pressure switches are there to sense pressure when the double block valves are closed and the bleed (vent) valves are open--when there SHOULD NOT BE pressure. They are not there to sense when there SHOULD BE pressure. And if there is pressure between the double block valves when the bleed (vent) valve is open that means the leak is relatively substantial, and since it could be leaking through the block valve that is supposed to be blocking (isolating) gas fuel, pressure in the cavity is deemed to be dangerous.

Double block-and-bleed valve arrangements are used when its critical to have positive isolation (shut-off) between two systems--in this case, the axial compressor discharge and the gas fuel manifold(s). If gas fuel (which is at a much higher pressure than CPD) should flow into the axial compressor discharge the results could be explosive, catastrophic and even deadly. So, the pressure switch is used to monitor for pressure when there should be NO pressure--especially when the bleed (vent) valve is open.

Hope this helps!

Dear CSA sir,

I am very very satisfied to your answer. I actually wanted to know this. As you suggest, for following the P&ID, I always follow this.

Thanks a lot. But sir, you said that, "The pressure inside the combustor (where purge air "discharges") is slightly less than CPD (I think the pressure is a very low, single-digit percentage less than CPD). " Do you know, the approximate amount of pressure for GE 9FA ,DLN2.0+ turbine on base load?

Again, As sir, you are a C & I expert, do you know? why in fuel purge system, on D5 purge line the last valve "VA13-2" has some control function to control the opening percentage of that VA13-2 valve? but in PM4 purge line, the last valve "VA13-6" does not have this type of control. Do you know the exact reason?

Thanx again.. Have a nice day

1 out of 1 members thought this post was helpful...

niazbibiyana,

You're welcome. And thank you for the feedback.

I'm not very familiar with DLN 2.0 or any of its variants; it was a version which wasn't produced for very long before the change to 2.6, and (fortunately) I never had to work on any 2.0-equipped units. I had a course on DLN 2.0 in the late 1990's, and I understand the basics--but that's about all. I've never seen a P&ID or any information about DLN 2.0+.

When purge air is being re-introduced after gas fuel has been flowing in the manifold/nozzles, the concept is to open the purge valves (the double block valves) slowly. There is fuel trapped in the manifold and nozzles and if the purge valves open quickly they will blow the fuel in very quickly--which will cause a load spike and an exhaust temperature spike. By slowing the opening rate of the purge valves the fuel is not purged out so fast and this minimizes any load- or exhaust temperature spike.

But, that's about as much as I know about purging--and "D5" is not something I recognize or have seen on a P&ID. If you could scan and post a copy of the gas fuel P&ID to a (free) Web-hosting site, such as tinypic.com, and then post the URL/link to the posting that would be very much appreciated. I might be able to posit some thoughts after reviewing the P&ID--but that's just a guess on my part.

Dear CSA sir,

This the 9FA DL2 Gas Fuel purge P&ID.
http://i58.tinypic.com/seneyu.jpg

Thanx for your response. But, after seeing this diagram, if possible, please kindly, more clear me, about the function of D5 purge line last valve.

Thanx again

1 out of 1 members thought this post was helpful...

niazbibiyana,

Thanks very much! The resolution of the drawing is not very good; when I zoom in the drawing the print is too grainy to be able to read it (it's very small print for such a large drawing).

Also, there is only one page of the set; it seems there are three pages, total, in the Gas Fuel P&ID. I also wonder if there is a Fuel Purge P&ID which might be useful also.

Dear CSA Sir,

Should I send you full P&ID of DLN 2.0+ gas fuel purge system? I am uploading P&ID of "Gas Fuel Purge System" and "Gas fuel System" on a free file sharing site for you.

http://www.mediafire.com/view/7o0kaewbn8qfn3x/Fuel_Purge.pdf

http://www.mediafire.com/view/76cyze32fcks90n/Gas_Fuel.pdf

Thanx a lot again. Sir, I here uploaded for you. But , kindly inform me in D5 purge line the last valve "VA13-2" has some control function to control the opening percentage of that VA13-2 valve? but in PM4 purge line, the last valve "VA13-6" does not have this type of control. Do you know, why? if you know, please give me some idea.

niazbibiyana,

> But, kindly inform me in D5 purge line the last valve "VA13-2"
> has some control function to control the opening percentage
> of that VA13-2 valve? but in PM4 purge line, the last valve
> "VA13-6" does not have this type of control. Do you know,
> why? if you know, please give me some idea.

I believe you are referring to the I/P (current to pressure) positioner on VA13-2 that isn't present on VA13-6? The only guess I can hazard is that possibly the system designers though that at some time it would be necessary to be able to control the purge valve position using a 4-20 mA (current) signal. Sh. 1 of the Purge System drawing has two notes, 5 and 8, that are very important--and I presume, as you suspected earlier, this is how it's hoped that the diffusion flame will not be "blown out"--and how it's hoped that the fuel being blown into the combustor during the start of purging will not cause a large load swing.

Device 65GP-E1P is likely connected to a 4-20 mA output of the Speedtronic turbine control panel which is used to control the intermediate position of VA13-2 by sending a current signal to the I/P positioner (which the I/P converts to a pressure reference that is proportional to stroke, which is then compared to the actual stroke and adjusts the air pressure to keep the actual stroke (position) equal to the reference from the Speedtronic). By controlling the opening (stroke; position) of VA13-2 the flow-rate through the valve can be controlled and the amount of purge air flowing into the combustor through the five diffusion flame nozzles in each combustor can be controlled (limited).

So, 65EP-G1P should be listed on the Device Summary, and should also likely be connected to a milliamp output of the Speedtronic turbine control panel. You can review the sequencing/application code in the Speedtronic to determine how the output signal is derived by working backwards from the milliamp output.

Hope this helps!

By niazbibiyana on 13 October, 2015 - 11:39 am
1 out of 1 members thought this post was helpful...

Dear CSA sir,

As per your question, from device summary about VA13-2 position limit switches


33PG-3 : FUEL GAS SYSTEM PURGE VALVE LIMIT SWITCH
NORM=(2) NO-CES; CLOSED WITH VALVE CLOSED;

33PG-4 : FUEL GAS SYSTEM PURGE VALVE LIMIT SWITCH
NORM=(2) NO-CES; CLOSED WITH VALVE 17% OPEN;

33PG-15 : FUEL GAS SYSTEM PURGE VALVE LIMIT SWITCH
NORM=(2) NO-CES; CLOSED WITH VALVE 55% OPEN;

33PG-16 : FUEL GAS SYSTEM PURGE VALVE LIMIT SWITCH
NORM=(2) NO-CES; CLOSED WITH VALVE CLOSED;SPARE


Our GT expert told me few days ago, when D5 nozzle start purging, at first VA13-2 is opened 55%. then it starts reducing up to 17% and then continue purging from this position. But he could not explain me the exact reason to control the VA13-2 position. But I think at first it starts purging from 17% opening of VA13-2. Then slowly starts opening up to 55% ( Of course when GT is in Premixed Mode). Am I right?

In our Cimplicity screen, the VA13-2 valve position is not showing in percentage. So, I could not see the exact position from Cimplicity.

It is our bad luck to enter in toolboxST to check any logic. Because our management dont gave us the permission to touch toolbox ST.

Thanx a lot for your response. Have a nice day.

NIAZ

1 out of 1 members thought this post was helpful...

niazbibiyana,

Thanks for the information on the limit switches. And, without being able to see the application code running in the Mark VIe of the turbine at your site it's impossible for us to say exactly how the unit at your site is controlled and protected.

I will say that if the Mark VIe only has the limit switch feedback from the VA13-2 that the control scheme must be very clunky and complicated (something the engineers at GE Belfort, France, are very inclined to do--overly complicate something.)

As for what is or isn't on the HMI display--that's an oversight on the provider's part, not to at least show which limit switch is "active" during operation to indicate valve position (presuming there's no other valve position feedback to the Mark VIe).

>It is our bad luck to enter in toolboxST to check any logic.
>Because our management don't gave us the permission to touch
>toolbox ST.

This is one of my personal pet peeves--and reflects a total lack of understanding of how ToolboxST works. Not allowing ambitious and self-motivated individuals to have access to ToolboxST to be able to learn and monitor operation and eventually become better operators and technicians is just simply short-sighted and misguided. ToolboxST has password-protected levels that prevent inadvertent modification of parameters or forcing of signals--and if you don't people to modify things (unintentionally or intentionally without permission) then don't give people the password(s). But, don't block their access to be able to learn and understand--and even become a better troubleshooter.

Unfortunately, this is all too common. Sorry to hear about this; perhaps you could suggest that the knowledgeable people with access to ToolboxST be made available periodically to answer written questions and help others to be better operators and technicians (and supervisors, even)?

I'm very saddened to hear about this situation; very, very sad, indeed. It comes from a completely misguided, short-sighted and ill-informed perspective--and in many cases causes morale and motivation to be a problem for operators and technicians (and supervisors).

1 out of 1 members thought this post was helpful...

Ok, Dear CSA sir. Thank you very much

niazbibiyana,

Thanks for the feedback!

(And, please stop calling me 'Sir.' I work for a living just like most everyone else on this forum.)

1 out of 1 members thought this post was helpful...

OK, Dear CSA......:)

niazbibiyana,

People will start talking (as people will do) if you keep using 'Dear' also.

My colleagues call me CSA. (That's one of the nicer things they call me.)

Ok. CSA. :D

Can I get your Linkedin/Twitter/Facebook Profile ID?

1 out of 1 members thought this post was helpful...

niazbibiyana,

The pressure drop across the combustion liner is somewhere in the range of approximately 2-4%, so if CPD was 225 psig the pressure inside the combustion liner would be between 216 - and 220.5 psig.

Dear CSA sir,

I am so much grateful to you for your continuous response. Believe me, I am new on GE 9FA Gas turbine. only 6 months experienced. Previously, I did a job on Wartsila engine based plant. By the way, Can you please tell me, How you calculate this?

"somewhere in the range of approximately 2-4%"

Thanx a lot

1 out of 1 members thought this post was helpful...

niazbibiyana,

I've been able to download the drawings, and when I get some "free" time I will have a good look over them.

GE measures pressures inside the combustors in their Combustion Engineering Laboratory, and that's how the general difference is known. It's not calculated, it's just a number derived from empirical data gathered over years and years of testing and monitoring in the lab, and also at some selected sites in the field.

It's not something that's measured or monitored on running turbines; it's just a known- and understood given (because gases can only flow from an area of higher pressure to an area of lower pressure--another one of those proved physical "laws").

Thank you very much CSA sir. Please, when you have a free time, please have a look, and kindly make me understand my desired things.

Have a nice day.

1 out of 1 members thought this post was helpful...

niazbibiyana,

Would you please post Sh. 1 of the Gas Fuel system P&ID? Many times there are Notes which can be very helpful.

Thank you in advance.

Dear CSA sir,

I am now out of office for 8-9 days leave. If you don't mind , after reach Office I will send you again.

Many many thanks for your fast reply.... Have a nice day

niazbibiyana,

It would also be helpful to know what each of the four limit switches on VA13-2 do (in other words, what position they are to indicate)? Perhaps the sequencing/application code in the Speedtronic turbine control panel adjusts the milliamp output to 65GP-E1P to make the valve move to one of the four positions indicated by the four limit switches. (GE Belfort would definitely do something like this....!)

Dear CSA,

I am not a control engineer... would like to understand the functionality of purge valves in DLN 1 frames Three GCV arrangement.

I want to know the purge valve open close position during different combustion modes.

I mean just tell me position of VA-13/VA-14 during primary, Lean Lean, Secondary and premix mode....

My understanding is that these are continuously purging during premix mode....

Regards
Novice

Novice,

Is the unit dual-fuel (gas/distillate)?

Regards,
CSA

Novice,

I don't have access to any P&IDs as I write this. The topic of GE-design heavy-duty gas turbine fuel purge systems has been covered many times before on control.com (there is a 'Search' feature at the far right of the Menu bar at the top of every desktop control.com webpage).

Gas fuel purge systems have primary and "follow-on" functions. The primary function is to make sure there is no fuel in the fuel supply lines to the fuel nozzle(s) so that hot combustion gases cannot ignite the fuel and cause damage or an explosion. (Gas fuel liquids, as well as other combustible liquids which might be entrained in or condense in supply piping need to be purged as well as any combustible gases.) Second, the flow of purge air also serves to provide some cooling to the fuel supply lines and fuel nozzles so that hot combustion gases don't damage them. Third, the flow of purge air prevents hot combustion gases from flowing backwards into the fuel nozzle and supply piping. But, the primary purpose is to make sure there is no combustible gases or liquids in the fuel nozzles and the fuel supply lines to the fuel nozzles. (While you didn't ask about liquid fuel purge, the purpose and follow-on functions are similar--except one of the most important things about purging liquid fuel out of the supply piping and nozzles is to prevent the liquid fuel from "carbonizing" ("cooking" and hardening) in the liquid fuel supply piping and liquid fuel nozzles.)

The purge systems on a DLN-I unit (well, most of them, except maybe for those designed and built and implemented by GE Belfort, France) use axial compressor discharge air flowing continuously through the supply piping and nozzles of the fuel system which is NOT in use to push the combustible gases or liquids out of the fuel system and nozzles and to also ensure that hot combustion gases do not flow back into the nozzles and supply piping. And, because axial compressor discharge air temperature is less than the internal combustor temperature in the area of the fuel nozzles the flow of purge air also provides some cooling effect to the nozzles, in particular.

The DLN-I combustion system (on most GE-design heavy duty gas turbines equipped with DLN-I combustion systems) have three, sometimes only two, fuel supply systems. The three are Primary, Secondary and Transfer. Some units do not have a Gas Fuel Transfer Valve and are therefore referred to as "transfer-less" DLN-I combustion systems. (It sounds as if your unit may have only an SRV, GCV and Gas Fuel Splitter Valve--and no Gas Fuel Transfer valve, making it a transfer-less DLN-I gas fuel system.)

When a unit with DLN-I combustors is starting it is usually only flowing fuel through the Primary fuel supply lines and nozzles; there is no fuel flowing through the Secondary or Transfer fuel supply lines or nozzles. In this case, purge air would be flowing through the Secondary fuel supply piping and nozzles (and in some cases the Transfer fuel supply piping and nozzles). When the unit transitions to Lean-Lean combustion mode as it is loaded after start-up the purge air into the Secondary supply piping and nozzles is stopped (because there will be fuel flowing soon in the Secondary supply piping and nozzles). So, when there is fuel flowing through the Primary- and Secondary fuel supply piping and -nozzles there will be NO purge air flowing to the combustors.

When the unit transitions to Premix combustion mode, if there is purge air for the Transfer supply piping and nozzles, that purge air will be stopped as the unit transitions from Lean-Lean to Premix combustion mode (as there will temporarily be fuel gas flowing in the Transfer supply piping and nozzles). Very shortly after the transition to Premix combustion mode is complete if there is Transfer purge air, it will be re-admitted to the Transfer supply piping and nozzles.

As the unit is unloaded and transfers from Premix to Lean-Lean there will be no need for purge air for the Secondary or Primary fuel supply piping and nozzles (because fuel is flowing through both of them). As the unit is unloaded further and transitions to Primary combustion mode purge air to the Secondary fuel supply piping and nozzles will be readmitted.

If the unit is dual fuel (gas/liquid), there will usually be a purge air system for the Primary gas fuel supply piping and nozzles, which will ONLY be used when the unit is operating on liquid fuel (fully on liquid fuel). If the unit is operating

VA13-1 and -2 are for the Primary gas fuel supply piping and nozzles (and would only be used when the unit is operating on liquid fuel), and VA13-3 and -4 are for the Secondary gas fuel supply piping and nozzles and nozzles. (It's possible that the packager re-numbered the Secondary gas fuel purge valves as VA14-1 & -2; who knows these days.)

So, to recap: When the unit is running on gas fuel, there should be NO purge air flowing in the Primary gas fuel supply piping and nozzles. If the unit is running on gas fuel AND the unit is Primary combustion mode, there should be purge air flowing in the Secondary gas fuel supply piping and nozzles. If the unit is a dual-fuel unit and it is running on liquid fuel (fully on liquid fuel), there should be purge air flowing in both the Primary- and Secondary gas fuel supply piping systems and nozzles.

Hope this helps!

Dear Novice,

It is helpful when you start a new thread rather than continue a thread from several years ago that may or may not be the same type of unit that you are working with.

I assume this is a DLN1 combustion system independent valve configuration running on a GE Frame 7 or GE Frame 9 unit.

You indicated you want to know the desired position of valves VA13 and VA14 during the different combustion modes. I believe you missed some numbers as there should be more numbers and all should start with VA13-X. You need to clarify if you are talking about VA13-1, VA13-2, VA13-3 or VA13-4 and we can then best answer.