Dear Experts

First of all, I am sorry for my english, I am not fluently. Right now, we operate some of GE Frame 9 Power plants, we have some difficulities in doing preventive maintenance, until right now, (except when we do periodic maintenance), we just did breakdown maintenance when spark plug in trouble.

How can we do some good preventive maintenance about it?
Thank u

 

Are you talking about the spark plugs used for coventional combustors (the retractable spark plugs) or the 'high energy ignitor' spark plugs used with DLN combustors?

Have you tried contacting the manufacturer to see if they have any literature or recommendations? They usually have some information, unless, like most of the high energy ignitors they don't really have replaceable or user-serviceable components, and even sell kits.

But, you didn't tell us what kind of failure you experienced; was it because the spark plug didn't retract after firing? Was it because the ceramic insulator was cracked or missing? Was it because a portion of the electrode was missing? Was it a problem with high-voltage connector?

They're not very complicated devices, and the maintenance concerns should be relatively easy to determine even without any written documentation or procedure. For the retractable spark plugs, the mechanism should operate smoothly without binding. The electrodes should be in good condition. The ceramic insulator should be in good condition. The plug connector should be in good condition. If you're not checking these things during inspection outages, then you're asking for trouble.

The high energy ignitors I've worked with don't have any user replaceable parts, so the only thing to check is the "gap" on the end. When replacing them, the insertion depth into the combustion liner is *critical* so pay attention to it very closely. The dimension is stated very clearly on the combustor drawings, and can usually only be measured using a piece of brazing rod. If inserted too deeply, it will burn up prematurely, If not inserted far enough into the liner, there will be a failure to ignite the primary during a shutdown, usually resulting in a turbine trip.

 

Thank you very much for the reply

Yes, we use conventional combustor or the retractable spark plugs. During periodic inspection (in CI, HGPI, etc) we always inspect
the quality of spark plug and its cable. But, we have the problems with the cable insulator, when the ignition got in trouble, the cable looks like got some overheat and we have a experience, that the insulator cable (not the ceramic one but it made by some good plastic/PVC)already melted.

The questions are:

1. How can the insulator cable get some overheat, sometimes melted ? If i am not mistaken, the cable is designed for hot area.

2. How to do some predictive maintenance for the conventional spark plug,what kind of aspecta that we have to consider, to avoid spark plug's trouble.

Thank u, and please.. help me

 

Thank you very much for the reply

Dear Mr. CSA,

We use conventional spark plugs (the rectractable spark plugs). During inspection outages, we always check the qualitiy condition of the spark plug and its cable.

However, our power plant is a peaker power plant, not continuous running power plant. So, the mechanism of spark plug will be up and down during start up and shut down the unit. We have some experiences, that when the spark plug got in trouble, usually the PVC insulator was broke down, and some experience showed to us that the spark plug cable itself was broke down.

We need your advice, about how to make some good preventative maintenance or predictive maintenance to make our spark plug operates well. What kind of aspects that we have to consider it ?

Thank u very much

 

High temperature cable does *NOT* mean it can be left laying on a hot combustion can or compressor discharge casing or combustion wrapper and be expected to survive.

The cable should be properly supported, allowing for the flex required for the retractable spark plug operation, but never allowed to be in contact with hot combustion cans or compressor discharge casing or combustion wrapper.

A megohmeter could be used to check the cable if you suspect problems (note you will have to disconnect both ends of the cable to meggar it). There is no "maintenance" to be done with the cable once the teflon has been damaged by excessive heat; it must be replaced.

But, the root cause of the problem is likely that because someone considered the cable to be high temperature rated, that it could be left laying in contact with the combustion can or the compressor discharge casing or the combustion wrapper, and that's just *NOT* the case. It must be properly supported to allow sufficient flex but *NOT* allow it to come in contact with any hot turbine components. Air flow in the compartment will be sufficient to prevent damage to the cable if properly supported, but can't protect it when it's laying on a hot can or casing or wrapper.

 

Dear Mr. CSA

Thank you very much for your solutions.

 

We operate four DLN 2.6 7FA's, each having two retractable ignitors. After a few years of experience and a number of ignitor failures, our operators are now supposed to pull the ignitors to verify that they will retract before startup. After startup, they are supposed to verify that they have, in fact, retracted fully. If full retraction hasn't occurred, they're supposed to wiggle and pull on it until it does come out.

Of course, some operators don't bother with these checks, and it only takes a few times of getting stuck in the partially retracted position to deform the tip to where it gets so big that it can't retract at all any more, even if you wiggle and pull on it.

What I've seen is that these "mushroom headed" ignitors invariably have a groove cut into their OD which indicates that the combustor liners were either not lined up properly to begin with, or the liners have shifted during operation. The liner presses against the ignitor causing it to stay in the extended position, vibration cuts the groove, and then the tip deforms from the intense heat.

When this happens we change them out, and I personally try to center the ignitor within the hole in the liner as best as I can. I also encourage the mechanics to align the liners to the ignitors during combustor assembly by installing an ignitor as a guide.

For maintenance and repair, we also built a little tri-pod pressure test stand that allows us to pressure test the ignitors to verify that the cylinder and piston seals are holding reasonably well. Sometimes the piston needs lapping to get it to seal properly in the retracted position. This reduces hot-gas leakage that tends to cook the cable connector.

I've also found cylinder seals (the metal O-rings on the top and bottom of the cylinder) get crushed or not even installed at all, causing leakage. They're cheap and easy to preplace.

 

Dear Pals,

We are operating GE frame 7FA DLN 2.6 gas turbines. we are experiencing spark plug failure after doing offline water wash. Our engineering team recommed to remove the spark plug while doing offline water wash.

GE offline wash procedure there is no recommendation to remove the spark plug.

Is is useful to remove the spark plug during offline water wash to prevent spark plug failure?

Please help

Best regards

Parag
Kingdom of Saudi Arabia

 

Pal,

You say you experience "failures" but you don't describe exactly what the failure mode is!!!

Have you tried removing the ignitors during an off-line water wash to see if that helps the problem?

Are you getting the infamous "Ignitor Excitor Channel Failed" process alarm?

How is the high energy ignitor (these are different from the "spark plugs" used in conventional combustors and are differentiated by referring to them as ignitors) failing? Is it just that the ignitor tips get wet and if it's removed and dried off and reinstalled that it starts working again?

Or is that the ignitor tip is damaged?

Are you certain that the ignitor isn't being fired during the water washing (this has happened inadvertently on occasion!)?

Is the ignitor exciter failing after a water wash?

Two things to note: The ignitors are usually installed in the "top" of the machine and in the upper section of the combustors and as such, they should not be submerged or immersed in detergent/water.

Seconds, there are generally two ignitors, and it's not normal for both to fail at the same time.

After an off-line water wash using detergent, there is supposed to be a rinse, and then the unit is supposed to be started after that to dry it out, even if it's just FIRE mode for about 20 minutes or so. Is there some long time lag after the rinse before the unit is being fired to dry it out?

What kind of detergent are you using?

What is the ratio of detergent to water? If you're using the manufacturer's recommended ratio, you're probably using too much detergent (remember: they want you to use the maximum amount every time you wash so that you buy more!). I've even sites that use more than the manufacturer's recommended ratio, their (flawed) thinking being that more is better! And they complained and complained and complained about how long the rinses took!

How long is the rinse taking?

So, there's definitely something you aren't telling us or that you haven't discovered yet.

Try to provide as much detail as you can when posting a problem here if you want the quickest and most concise response. Try to anticipate the questions you might be asked. Tell us about any troubleshooting you might have done and the results of that troubleshooting.

Help us help you, and you'll be much happier in the Kingdom.