Hello Folks,
I need some information/guidance on what the standard flow testing procedure is of gas turbine's fuel nozzle. In our facility a very peculiar method of testing is used which I can neither decipher nor find any supporting reference. I am attaching the picture of test bench used at our facility, we eventually increase air pressure at out test bench and measure delta across flow line in mmwc. If the values of delta measured at different inlet pressure fall within range (God Knows where that range has come from), we say the nozzle is OK.

I will be really grateful if anyone can explain this / share supporting document.

 

Carry out you measurements as described and record the data. The world wide web has countless resources to better understand the test.

 

The most common reason for testing fuel nozzle flow-rates is to make sure that a set of nozzles (10 nozzles for a Frame 5) is within a certain flow-rate range of each other, AND for deriving a plan for how to place the nozzles on the unit so that the highest flowing nozzles are not placed directly next to the lowest flowing nozzles. All of this is done to try to limit the exhaust temperature spreads on start-up after the nozzles are installed. The tighter the range of flow-rates of a set of nozzles, and the better placement choices, the lower the exhaust temperature spreads will be on start-up.

The usual flow-rate range (many years (decades) ago) was 10%--all of the fuel nozzles in a set were to test within 10% of each other, more or less. In more recent times, many sites have decided to tighten that up, some considerably. With some very good results. Usually.

The differential pressure across an orifice can be used to calculate a flow-rate. By plotting flowrates at different inlet pressures a graph can be created over the expected flow through a nozzle, and can indicate any problems at different flows (the graph should be a smooth line; if it's not that could indicate a potential problem).

While it's simply impossible to believe for many, not every GE-design Frame 5 heavy duty gas turbine is like every other GE-design Frame 5 heavy duty gas turbine. Yes, they all suck and squeeze and burn and blow. But the fuels they burn can be quite different, so the fuel nozzles can have very different orifice sizes, which can result in very different flow-rates. When a site has a set of "numbers" it's usually for that particular unit or site based on the fuel(s) the unit burn(s).

The Operations & Maintenance Manuals provided with gas turbines provided by GE (GE--not one of their packagers) will have information about performing fuel nozzle testing. It's not always the best, but with a knowledge of what you're testing for and the "mechanics" (physics) of testing, you can derive your own understanding of the test procedures and processes and necessities. NOT all test stands are created equally or are the same--but they all serve the same basic purpose (described above).

That looks like a pretty decent test rig (stand), and well-used. Someone knew what they were doing. They even created a wrench to remove/tighten the outer fuel gas tip.... And there's a container of "never seize"--a very important component to proper fuel nozzle reassembly (and subsequent disassembly). I'd say the people doing the work know what they're doing.

Watch.

Listen.

Read.

Learn.

Think. Critically.

Trust science and proven techniques. There can be LOTS of new-fangled test set-ups, some of which are easier to perform and some of which are prone to miscalculation and errors. But they "look" better to unknowing souls. Doesn't make them so. In the end, all the principles are the same, only the equipment differs. Talk WITH the people who've been doing it in the past. Unfortunately, while many of them are good or very good testers and recorders--and troubleshooters--many just can't explain it to others. BUT, they have very good information to share. Probably better than any procedure or manual.

Communication. The World Wide Web. But, sharing information by communication is the best.

 

The most common reason for testing fuel nozzle flow-rates is to make sure that a set of nozzles (10 nozzles for a Frame 5) is within a certain flow-rate range of each other, AND for deriving a plan for how to place the nozzles on the unit so that the highest flowing nozzles are not placed directly next to the lowest flowing nozzles. All of this is done to try to limit the exhaust temperature spreads on start-up after the nozzles are installed. The tighter the range of flow-rates of a set of nozzles, and the better placement choices, the lower the exhaust temperature spreads will be on start-up.

The usual flow-rate range (many years (decades) ago) was 10%--all of the fuel nozzles in a set were to test within 10% of each other, more or less. In more recent times, many sites have decided to tighten that up, some considerably. With some very good results. Usually.

The differential pressure across an orifice can be used to calculate a flow-rate. By plotting flowrates at different inlet pressures a graph can be created over the expected flow through a nozzle, and can indicate any problems at different flows (the graph should be a smooth line; if it's not that could indicate a potential problem).

While it's simply impossible to believe for many, not every GE-design Frame 5 heavy duty gas turbine is like every other GE-design Frame 5 heavy duty gas turbine. Yes, they all suck and squeeze and burn and blow. But the fuels they burn can be quite different, so the fuel nozzles can have very different orifice sizes, which can result in very different flow-rates. When a site has a set of "numbers" it's usually for that particular unit or site based on the fuel(s) the unit burn(s).

The Operations & Maintenance Manuals provided with gas turbines provided by GE (GE--not one of their packagers) will have information about performing fuel nozzle testing. It's not always the best, but with a knowledge of what you're testing for and the "mechanics" (physics) of testing, you can derive your own understanding of the test procedures and processes and necessities. NOT all test stands are created equally or are the same--but they all serve the same basic purpose (described above).

That looks like a pretty decent test rig (stand), and well-used. Someone knew what they were doing. They even created a wrench to remove/tighten the outer fuel gas tip.... And there's a container of "never seize"--a very important component to proper fuel nozzle reassembly (and subsequent disassembly). I'd say the people doing the work know what they're doing.

Watch.

Listen.

Read.

Learn.

Think. Critically.

Trust science and proven techniques. There can be LOTS of new-fangled test set-ups, some of which are easier to perform and some of which are prone to miscalculation and errors. But they "look" better to unknowing souls. Doesn't make them so. In the end, all the principles are the same, only the equipment differs. Talk WITH the people who've been doing it in the past. Unfortunately, while many of them are good or very good testers and recorders--and troubleshooters--many just can't explain it to others. BUT, they have very good information to share. Probably better than any procedure or manual.

Communication. The World Wide Web. But, sharing information by communication is the best.

Thank you for your reply.

When I looked at this test bench and the testing methodology I was certain that the data of individual nozzle itself is not significant but the range of data of all nozzles collected through testing should have close tolerance to ensure even flow/combustion. And the second thing, which you have also pointed out, is that we should not place those two nozzle adjacent to each other in which data variance is high.

The thing which is bothering me now is should we test it at the same pressure which reflect the operating condition ? because we are testing it at even less than half of what the operating condition is. The second thing is can we adjust / ream the nozzle's swirl plate hole if the data indicates low flow. At our site , when the flow data is low, our technicians unintentionally increase the dia of swirl plate holes thinking that they are doing cleaning. I believe we should not adopt this practice as these holes are precision machined and disturbing the profile can have adverse effect at higher operating pressure.

Your guidance will be appreciated.

 

SOOOO many questions....

The answer to the question of what pressure to test at is: Can the test bench air supply provide the pressure/flow to do the test at the normal operating pressures? (I suspect not, but that's just a wild suspicion.) Remember, in a typical GE-design Frame 5 heavy duty gas turbine the pressure inside the combustor is somewhere around 125-140 psig, and the fuel gas pressure at the nozzle can be around 200-250 psig (depending on fuel gas supply pressure (upstream of the SRV), the SRV internals, and the fuel gas composition (BTU content mostly, but also other combustible and non-combustible gases in the fuel gas being burned). So, the delta across the fuel nozzle is about 75-125 psig, and that's "operating pressure" in my experience. (Granted, I'm no flow expert but these are the typical pressures and delta pressures (differential pressures) being experienced at or near Base Load for most machines burning some kind of natural gas.)

The answer to the other question is: Where are you buying the fuel gas swirl tips from? And are you SURE the orifice diameters being provided meet the procurement specification? If not, why not? If the swirl tip orifice dimensions of incoming fuel nozzles aren't being measured and recorded, why not?

Look, all of this is simply an attempt to reduce the exhaust temperature spreads to the lowest possible values after re-assembly. There are literally thousands of GE-design heavy duty gas turbines of all sizes that run with all sorts of exhaust temperature spreads day in and day out, and depending on the version of Mark* turbine control system being used on these machines (IF a Mark* turbine control system is being used!) there may--or many not--be a combustion monitor function in the control system that trips the turbine on a high-high exhaust temperature spread. So, it's incumbent on trained and experienced operators AND THEIR SUPERVISORS to monitor the exhaust temperature spreads and take appropriate action when it is necessary. The EXACT dimension of the gas swirl tip orifices--and the precision of the method used to achieve the "required" dimension--isn't really critical, even if unit performance is critical (including heat rate and fuel consumption). What shouldn't happen is that the orifices get to such a large diameter that the SRV can't maintain the proper P2 pressure upstream of the GCV, or the GCV operation falls outside the normal operating range (which can have knock-on effects on Droop Speed Control and operation (loading/unloading rates, when Base Load is reached, etc.).

I used to work with a guy that often said, "This ain't rocket science." When I was young and just getting started in the business I was appalled at that saying/attitude. But, over decades, I came to see it was very true. For the newer F-class and H-class and HA-class machines--that's not so. Those things are designed to be engineering marvels, and they are grossly complicated and the ARES (MBC) algorithms are very unforgiving, as are some of the hot gas path components. But, GE-design Frame 5 heaby duty gas turbines are the WORKHORSES OF THE GAS TURBINE FLEET around the world. Those machines can--and do--withstand more abuse and neglect (unintended and forced) than anyone can imagine and they just keep on making power in spite of the conditions. At some point, they can just give up and fail, but it takes a lot. And, in the interim they just keep on making power. And for many of them (GE-design Frame 5 heavy duty gas turbines with conventional combustors (which are what's shown in the photograph)) the fuel-air mixture is not modified or known or even calculated. And some inexpensive Toyota automobiles have more sophisticated fuel injection systems and engine computers than the Mark*. They are engineering marvels that have a LONG and storied heritage and legacy, and while they seem complicated they really aren't (especially the conventional combustor-equipped GE-design Frame 5 heavy duty gas turbines). The designers of the 1950's and 1960's and 1970's were geniuses of simplicity (almost too much so at times)--but they knew analog electronics and developed some very good implementations that stretched into the digital world because THEY JUST WORKED.

Would it be "better" if the testers weren't filing and drilling the gas swirl tip orifices to obtain the necessary dimensions? Sure. (It would probably reduce the number of new gas swirl tips that had to be purchased.) But, in the end, those people have developed methods to keep those units running and producing power and revenue. Damn the torpedoes.

 

I assumed (bad thing to do) that the technicians making "adjustments" to gas swirl tip orifices to achieve desired flow-rates were doing so to NEW tips.... If they were removing material which had collected in the orifices of gas swirl tips removed from the unit after some period of operation, then as suggested it's a very good idea to investigate what the material is (and where it might be coming from) and how to prevent it from getting into the fuel nozzles. Coalescing filters, while expensive, can save money if the fuel nozzles get blocked ("choked") frequently and have to be removed and cleaned.

A lot of natural gas suppliers seem to be doing very little to remove contaminants from the fuel they are selling.... Some have flat out stopped doing so, and refuse to meet the specifications they originally agreed to when contracting to supply fuel gas.

 

I assumed (bad thing to do) that the technicians making "adjustments" to gas swirl tip orifices to achieve desired flow-rates were doing so to NEW tips.... If they were removing material which had collected in the orifices of gas swirl tips removed from the unit after some period of operation, then as suggested it's a very good idea to investigate what the material is (and where it might be coming from) and how to prevent it from getting into the fuel nozzles. Coalescing filters, while expensive, can save money if the fuel nozzles get blocked ("choked") frequently and have to be removed and cleaned.

A lot of natural gas suppliers seem to be doing very little to remove contaminants from the fuel they are selling.... Some have flat out stopped doing so and refuse to meet the specifications they originally agreed to when contracting to supply fuel gas.

Yes, you are right, the cleaning activity that they are performing are on removed nozzles from the unit.

The problem is that the deposited material is found on some nozzles only. Had it been a contaminant, it would have been found on each nozzle. Actually, the nozzle which are being used at our frame 5 are dual fuel, where we have plugged the atomizing air and liquid fuel ports. I believe that the absence of purge air supply during gas fuel firing is causing back flow of flame and the debris/contaminant found in some nozzle is eroded material from nozzle body. Lab analysis also reveal iron content in the sludge.

 

That's not necessarily true (that a contaminat would be found on ALL ten nozzles). If the contaminat is "heavy" it may only be found on the lower fuel nozzles. On many older GE-design heavy duty gas turbines the gas fuel enters the manifold around the axial compressor casing from the bottom or sometimes from a position just to one side or the other of the very bottom of the gas fuel manifold. Heavier contaminats may not be uniformly pushed around the manifold to all ten nozzles. Many times water or hydrocarbon-based liquids in the gas fuel supply will collect in the lower piping runs and will only get pushed up into the gas fuel manifold as total fuel flow-rate increases. And, again, the contaminants may not be uniformly pushed around the manifold and into all fuel nozzles. May times heavier contaminants can "flow" (be pushed) into the closest fuel nozzle(s) to the gas fuel manifold supply point.

In my experience plugging atomizing air and even liquid fuel "lines" in dual fuel nozzles is not really recommended, especially if it's not done correctly. It's VERY IMPORTANT NOT to set up and kind of reverse flow possibility where hot combustion gases can flow from one nozzle to another, and I've even seen this happen through liquid fuel lines back-flowing through the liquid fuel flow divider! Best to put a blind flange on the atomizing air flange of each fuel nozzle, and put a tubing cap/plug on the line where the liquid fuel enters the fuel nozzle.

You can also purchase atomizing air tips which are blanked off (the OEM made those for single-fuel (natural gas only) units, and they are available from a variety of vendors.

But, it's not 100% true that liquid contaminants will uniformly infiltrate/enter all 10 fuel nozzles; the real world just doesn't always work that way and doesn't most often. Sometimes the contaminants condense as they pass through y-strainers, gas valves (SRV and GCV) and even through piping and even through gas swirl tip orifices (where there's a pressure drop there will also be a temperature drop). (That's why "knock-out drums" or "cyclone separators" are really good at removing entrained liquids and even debris--there are usually several changes of direction as the gas fuel flows through the chamber, and usually a small pressure drop, too.) It would be very interesting to know where the nozzles with the highest concentration of build-up were removed from the machine (lower combustors or upper combustors).

And, while we're discussing entrained contaminants in gas fuel supplies it is a VERY GOOD idea to periodically test the natural gas and to review the dew point of the gas. Per the OEM specifications, gas fuel supply temperature should be at least 50 deg F above the dew point temperature to prevent condensation of liquids as the gas passes through the system (y-strainers, valves, orifices, etc.).

But, again, many natural gas suppliers are doing very little--if anything--to filter out naturally-occurring sediments (silica, mostly, I believe) from the gas they are pumping into pipelines. This is causing lots of issues with control valves AND fuel nozzles where units which have been running for 10- or 20 years had zero problems in the past and are suddenly experiencing these kinds of issues which affect reliability and availability.