Flame rectification fooled

  • Thread starter Mikej
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  • #1
A practical problem. A floor mounted boiler frp failed by cycling on and off rapidly with the main gas relay opening closing when there was no flame. This may be happening only when the igniter system is still hot.
The main earthing is through the pilot gas tube, using an aluminum pipe with a brass olive into steel. There is considerable heat curve between the pipe and the steel electrode. Could a micro amp dc current be generating by the Seebeck effect to confuse the detection system?
 

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  • #2
.Scott
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Could a micro amp DC current be generated? Absolutely.
Could it confuse the detection system? I am thinking that you are talking about the flame detection system - but perhaps not. In any case, any product detection system should be well isolated from the affects of ground currents. So my guess would be "No".
Of course, it would take a closer examination of the system to know with any certainty. But I really doubt it.
 
  • #3
Thanks Scott, I think unlikely, but replacing the controller doesn't fix it. It is stopping lockouts which I thought impossible. The issue is that the electrode would have a DC bias relative to ground. I don't know, the setup is mikeymouse, happens on several waterheaters and no engineer has yet found the reason. It is intermittent, maybe temperature related but passes every test on the manufacturers test lab..any other ideas?
 
  • #4
Tom.G
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...any product detection system should be well isolated from the affects of ground currents...
...the setup is mikeymouse, happens on several waterheaters...
The operative words are bolded above. Follow-up your hunches. Any way of even temporarily connecting a wire across the aluminum-steel joint? Try disconnecting the sensor on a problematic device. Could be an intermittent open or ground in wiring to the sensors. Also check grounding of controllers. It could even be an issue with the power source. Or someone may even have decided to use the gas supply line as an electrical ground somewhere in the building.:nb)
 
  • #5
jim hardy
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hmmm

Flame rectification? New to me, just looked it up.

http://www.achrnews.com/articles/98085-troubleshooting-a-flame-rectification-system
has a couple good troubleshooting ideas...

https://yorkcentraltechtalk.wordpress.com/2011/05/23/the-phenomenon-of-flame-rectification/ explains how it works (which i really needed)
Internally, in the control module, once flame is established, current begins flowing in the flame sensing circuit. The current energizes a relay in the control. A minimum amount of current is needed to energize this relay (microamps). When this relay pulls in, one set of contacts open to de-energize the ignition system and another set of contacts closes to keep the main gas valve energized. It is important to know the minimum microamps that are required to “lock in” the control module and keep the unit running.

If the proper amount of microamps is not present, the control will not allow the main valve to remain open and shuts the unit down. Depending on the control, it may or may not go into “retries” to prove that flame was established.

cycling on and off rapidly with the main gas relay opening closing when there was no flame.

Sounds to me like it's looking for a flame that isn't there.
Does the ignitor try to start one?
 
  • #6
Thank you for your ideas. The two Earth returns are by the pilot and main gas pipes.
The theory that the hot tri-metal junction is causing confusion in the rectification system seems to be correct. This morning:
Test at cold..eveything OK. Run the burner 20 mins...test failure.
Predict that the main gas pipe earthing is faulty. Cool and remove the burner and the holding screw is loose. Tighten and replace. This time at hot: no failure. The only difference is that the failure happens when the earthing is defaulted through the pilot's hot tri-metal junction.
I was doubtful that this would be the case, but now 90% likely.
The down side is that the securing screw will soon be loose again due to poor design.
There are thousands of these units with this inherent weakness- the resulting cycling can override the lockout...boom!
 
  • #7
jim hardy
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There are thousands of these units with this inherent weakness- the resulting cycling can override the lockout...boom!

Sounds like good troubleshooting.

Is their sales department aware of this ?

When you're sure you're right
- here's what i would do -
Take a picture,
go to internet and find the parent company
click on "Investor Relations" then "Executive Team", find Chairman of the Board

Send to him a letter in your own handwriting describing that problem
tell him how you fixed yours with a work-around
and that you await their corporate response particularly a retrofit kit to provide an improved and reliable earthing means..

A video of the gas valve cycling wouldn't show anything because gas is invisible.
But if you tie a big feather or orange streamer to the gas nozzle so it flutters, unambiguously demonstrating the danger, then your presentation has 'Punch' .

old jim
 
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