spark plug and flame detectors

A

Thread Starter

Ayed

could you please explain the followings
How the spark plug generates spark?
How the flame detector detects the flame?
 
S
Air is an insulator. If the voltage gets high enough, it will punch a hole in the insulator and current will flow. You connect a source of very high voltage like an ignition coil or a spark transformer to the terminals of a spark plug and it will spark.

Your question about flame detection is more involved. There are many flame detection technologies. Flame rectification, UV detection, IR detection and so on, and instrumentation made for each.
 
I know I'm going to be made to regret this, but here goes another overly long explanation anyway. I don't believe Ayed would have asked this question if he didn't want to know all about the spark plugs used in GE-design heavy duty gas turbines--not just how the spark is generated. While electrically similar to the spark plugs in any internal combustion engine, they are unique in many other ways that is probably what Ayed is really asking about in his overly simplified question. So, rather than have an extended banter and more overly simplified questions I'm choosing to answer the questions he didn't ask, and to provide some additional information that may eventually be asked as a result.

(By copy to Ayed, if you find any of the information provided in response to *any* of the questions you pose here on control.com helpful or lacking please provide some feedback--positive or negative. If you can take a few minutes, or less in this case, to pose a question or two, you can take a couple of minutes, or less, to say thanks or you need more information or you meant something other than what you asked. Thanks!)

In a GE-design heavy duty gas turbine it's necessary to establish flame in the combustors during starting. At the appropriate time during a start sequence, either 110 VAC or 220 VAC is applied through a relay to an ignition transformer which increases the voltage to many thousands of volts (to be honest, I don't know if it's AC volts or DC volts, and it doesn't really matter--read on).

This high voltage is then applied through highly insulated (electrical and heat insulation) leads to the spark plugs, or ignitors, plural because there are usually two ignitors for redundancy purposes.

These ignitors have a center electrode that the high voltage passes through into the combustion zone inside the combustor. This high voltage electrode is insulated from earth (ground) by means of a ceramic insulator in the ignitor.

At the tip of the electrode there is a small gap between one or two small "tangs" that are earthed (grounded). The high voltage jumps the gap creating a spark. The spark then ignites the fuel. A spark can be created by AC (alternating current) or DC (direct current), and one is probably better than the other, but they will both generate sparks as they jump the gap to earth.

A spark is actually the ionization of the air by the voltage that is crossing an air gap from one conductor to another (in this case, the high voltage conductor in the center of the ignitor to the earth tang, or tab).

In a GE-design heavy duty gas turbine, the spark plugs are left energized for the duration of the firing period (anywhere from 20 to 60 seconds depending on the type of turbine, age of the machine, type of fuel and type of combustor) even if flame has been detected. This is to ensure that if the flame dies out during the firing or warm-up period that the spark remains to try to re-establish flame in that combustor.

Now, there are only two combustors with spark plugs, but there are many combustors (anywhere from four to 18 depending on the size and type of the turbine). In the combustors where flame is present the pressure is higher and there are tubes called cross-fire tubes that connect each of the combustors.

The hot gases in the combustors with flame pass through the cross-fire tubes (because pressure wants to go from an area of higher pressure to an area of lower pressure because of natural physics) and so ignites the fuel in the adjacent combustors. When flame is present in two adjacent combustors, the pressure in then is relatively equal and there is no longer any flow through the cross-fire tubes.

When flame is established in every combustor, then there is no flow through any cross-fire tube; they have served their purpose and are essentially "shut off" by the equal pressures in each combustor. (As a side note, when units with conventional combustors reach operating speed there is so much air rushing past the ends of the cross-fire tubes that it effectively seals the ends of the cross-fire tubes even if flame goes out in a combustor or there is a fuel restriction in one or more combustors that results in uneven combustor pressures.)

The center body of ignitors used in most GE-design heavy duty gas turbines with conventional combustors, the part with the ceramically-insulated electrode, is designed to move axially. As the pressure in the combustors increases with speed and fuel flow the center body of the ignitor is pushed out. The causes the tip to be pulled back from inside of the combustor so that it doesn't get burned by the hot combustion gases.

When the unit shuts down and the pressure inside the combustor decreases there is a large spring that pushes the center body back into the combustor, making it ready for the next start sequence.

Now, as for how the flame detectors work, you can use any World Wide Web search engine to research flame detectors, or UV (ultra-violet) flame detectors, or Geiger-Mueller flame detectors. Basically, the majority of GE-design heavy duty gas turbines in operation around the world use the Geiger-Mueller type of flame detectors, which when exposed to the type of UV radiation most commonly produced by the combustion of fossil fuels produce a frequency output that is proportional to the intensity of the UV radiation (the "strength" of the flame). There's no rocket science or unusual characteristics of these flame detectors that need additional explanation above what can be found in many other places on the World Wide Web, complete with schematics and diagrams and probably even videos somewhere!
 
I forgot to include some possible important pieces of information about the ignitors (spark plugs).

Most GE-design heavy duty gas turbines with conventional combustors have a single ignition transformer with two outputs (some a two ignition transformers each with a single output).

There is no way to tell if one or the other or both ignitor(s) is(are) working properly without removing it(them), applying voltage to the transformer(s) and observing the spark and intensity.

Sometimes, if the ambient area around the turbine is relatively quiet, one can hear the arcing of the spark plugs inside the combustor when energized when the unit is not being cranked. In other words, with the unit not running or being cranked <b>after having been properly purged</b> and the turbine compartment ventilation fan(s) not running and without a lot of other ambient noise, it's possible to crawl inside the turbine compartment (with the fire protection system disabled!) to hear the spark plugs arcing when they are energized temporarily. But, this should only be done with the proper precautions and it does not prove the intensity of the arcing as it can't be observed.

The proper method for testing involves normal electrical safety measures (ensuring the ignitors can't be energized while they are being removed and re-installed; personal protective equipment and safe personnel distances from energized components; etc.). Once removed, the ignitors must be earthed (grounded) by laying them in contact with an unpainted metal surface of the turbine or walkway outside the turbine compartment. With the high voltage ignitor lead attached, they must then be energized in the normal fashion by applying "low" voltage AC to the ignitor transformer(s) while ensuring that no one is touching the ignitors and is a safe distance away. A spark should be visible. Sometimes, it's a constant "buzzing" type of spark; sometimes it's a popping type of spark. But, the spark of both ignitors should be relatively equal and of "good" intensity, meaning that it should be bright and visible even in sunlight. Generally, it should not be intermittent, popping/clicking less than once per second (for the typical style of ignitor used with conventional combustors).

Ignitors do require maintenance. The ceramic insulators can crack over time and with excessive heat. The retraction springs can weaken and even break. The ares where metal contacts metal when the center body slides can become worn and even chafed, preventing smooth action. The earth (ground) tangs and/or the high voltage electrodes can deteriorate over time, increasing the gap to such an extent that the voltage cannot jump the gap and so there will be no spark.

Lastly, it's pretty common for mechanics to mistreat any device or instrument they are removing or replacing as if it were a bolt or a flange which can (and should!) be dropped, kicked, thrown, and generally abused. Additionally, because the center body is spring-loaded many mechanics derive extreme pleasure by grabbing the end of the center body, pulling it out to the stop, and releasing it and watching the internal spring pull it back down. Not once or even thrice, but many times in succession. Before telling their mates about this new "toy" and how much fun it is to play with and watching them do the same several times, as well. Great fun, that!

This generally damages the ceramic insulator, even after only one or two pulls and releases. But, hey, that's not mechanics' work to fix them; it's the instrumentation and controls tech's work. Mechanics just get paid to keep removing and installing them as often as necessary.
 
CSA, I will take the time to thank you for the overly long explanation for which many of us should be thankful for, and have grown accustomed to getting from you.

I had a general understanding of the spark plugs and ignitors, as well for the flame detectors. But with any subject I never really get an in depth idea until a failure makes me study it more deeply. Your explanation of the operation of the crossfire tubes was a simple subject that I knew just worked, but had never considered the dynamics of how it does its job. Thanks for that.
 
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