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backburning
Posted by: (---.dyn.iinet.net.au)
Date: August 13, 2007 05:44AM

me again...... still working on the 740.
well we appear to have sorted out the bugs in the fuel system and it is firing well.
However it seems to pop back and often get a fire in the venturi tubes briefly as we approach pressure, my initial thoughts went to sealing the burner unit more carefully, which seems to improve the situation, but still not reliable,
any other thoughts??
Perhaps a pinhole in the superheater?, this could alter the pressure balance?
but then would a pinhole only show up when hot????

I'd appeciate ANY random thoughts!

locosurrey.

Re: backburning
Posted by: Mike Clark (---.leed.adsl.virgin.net)
Date: August 13, 2007 06:23PM

Dave

Possibly some small crack in the burner plate or poor sealing somewhere around the burner plate which, when the force of the gas stream is dying away, allows the flame to zip through and ignite the gas in the mixing chamber.

As I understand it the flame burns upstream into the flow of gas but when the flow of gas is fast enough (as it normally is through the small burner holes) the speed of gas flow keeps the flame pushed up on top. As the burner shuts down the flame front is able to advance through the burner plate through any holes or cracks which are larger than the burner holes and are subject to a lower gas velocity. Make sure the fuel pressure is pumped well up towards the end of the firing up process as this helps keep up the gas flow.

Another alternative is that the insulation in the grooves on the burner plate has broken away, allowing the casting to get hot enough to ignite the gas below. The reason you see this lighting back as the burner shuts down is that the considerable cooling effect of the gas coming in is withdrawn and heat built up in part of the burner plate can now escape downwards. It's quite surprising how cool the gas is in the mixing chamber when the burner is on full fire - rarely more than 100 deg C - but immediately after turning off the burner it rises to 150 or more - I once saw over 350 C after firing up to 600psi and then turning off the burner. This is well within the temperature at which fuel can ignite.

Do you get this lighting back in normal running on the road or only when you first fire up? You might be able to avoid it by making sure you move off before the boiler gets to full pressure and thereby avoid the heat soak. You can often kill such a light back by using the stack blower if you have one as this will both pull in more cooling air and increase gas flow through the burner.

Random thoughts!

Mike

Re: backburning
Posted by: (---.proxy.aol.com)
Date: August 14, 2007 03:53AM

Dave,

If the trouble only occurs after the initial lighting it may be that some of the fuel has condensed and fallen into the bottom of the burner pan when the main fuel is first admitted. This tends to sit at the bottom until the end of the initial burn and can then light back. The best thing to do at this stage is to open the stack blower slightly and have a cup of coffee! It will soon burn itself out.

Re: backburning
Posted by: Mark Drake (62.189.28.---)
Date: August 14, 2007 07:54AM

Hi Dave,

My 735 does exactly the same thing - I'm glad to hear that I'm not the only one that suffers this problem! I had assumed that it was a 'Stanley characteristic' and had not particularly taken any steps to avoid it - I just manage it on the road by shutting the fuel off for 30 seconds then relighting the burner normally.

However, I have noticed that it is much more frequent when the fuel jets are dirty, and also after 10 or 15 miles; i.e. when everything is thoroughly hot. It only happens as the automatic is shutting down. As Mike says I suspect that there is a leak / small crack in my burner plate which is allowing the flame to jump across or possibly the insulation is coming adrift letting my burner plate get hotter than it should.

A few months ago I installed a burner vaporiser pressure gauge, tee-ing off the fuel line just before the vaporiser. I have to admit that this is incredibly useful. When approaching villages / towns, I turn the burner control valve down until I have a vaporiser pressure of around 40 psi; I find that this setting keeps the burner happy whilst maintaining plenty of steam for slower driving. The steam automatic doesn't cut in either as the pressure seldom gets that high with such a low vaporiser pressure. I know that this seems like extra effort when there's already a steam automatic to do the job for you, but this combined with scrupulously clean fuel jets, does avoid backburning.

In the winter I'll drop the burner and see if there's anything sinister going on in there...

Good luck!

Mark Drake

Re: backburning
Posted by: Ian Vinton (---.range86-156.btcentralplus.com)
Date: August 14, 2007 01:38PM

I am also having problems with the burner lighting back. After a run of about 3 miles, I stop and after a short time, the nozzle catch fire.
I think there is a steam leak under the hood where the water is pumped in, I will be lifting the hood this weekend to check it out.
It may also explain why I cannot keep up the steam pressure when on the road.
I never experienced this until I fitted the new boiler which needed some tweaks and additional piping due to the holes not being positioned correctly by the boiler maker.

Re: backburning
Posted by: Mike Clark (---.hers.adsl.virgin.net)
Date: August 14, 2007 06:11PM

Going on from Peter's comment I find I only get lighting back when firing up if I hurry things too much. I generally leave the pilot on for about 15 minutes before turning on the main burner - any less that that and the fuel does condense on the cold inside of the burner.

The steam automatic is far from an "on and off" device and when the boiler pressure is within 5 psi of the working pressure the automatic lets just a small flow of fuel through (a vaporiser pressure gauge really shows up what's going on - I couldn't manage without one) and the pressure at the jets may be only 5 0r 10psi which is not enough to chase the flame back up. Also because the automatic is at the cold end of the vaporiser there is still a residue of fuel in there even when the fuel valve has been turned off and this will come out as a wisp of vapor which can easily catch fire at the jets or go bang in the burner if you have a sparker running. (Yes Ian!!). If you know that you are going to stop soon turn the burner off half a mile before so as to flush out the fuel and cool the burner plate.

Mike




Edited 2 times. Last edit at 08/15/07 06:36AM by Mike Clark.

Re: backburning
Posted by: (---.dyn.iinet.net.au)
Date: August 15, 2007 08:04AM

I am not alone!
I thank you all for the input.
Yes any air leaks certainly increase the probabability, but liberal gooing up with muffler putty appear to have fixed all of these (2 tubes!)

And also yes, we have found with VERY careful and fine tuning of the fuel valve it is manageable.
I feel also that jets being dodgey contribute, the car has been sitting for over 3 years and needs a good few flushes of fuel.

After perservering over the last few days we seem to have a good handle on the firing, we are still fixing steam leaks, but we have had 100PSI and were able to establish the correct operation of the automatic water level(thanks Jeff!)
and the pumps are working fine.
the electrics (are original) and are a mess, but then thats only minor (green steam?)
However the whole project appears to have (pardon the pun) fired the imagination of the press over here and everyone is getting excited, we'd be millionaires if we got a dollar for each ride requested!
I will send jeff some pic's for the site.
thankx again

locosurrey



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