"Blast tube" material for oil boiler

[sako243]

We've an old commercial boiler which is basically a series of cast water jackets bolted together and a big flame through the middle. Simple, effective and easy to maintain and despite going out of production in the 80s most parts are readily available.

Inside the chambers there is what I refer to as the "blast tube" which (our understanding) is there to effectively be a sacrificial protection tube for the water jackets. It's a simple 220odx300x2mm tube with some little feet welded on it.

As you can see the bottom has blown out a few years later (I had this one rolled up by friend). Our theory is that with mild steel condensation and water content is an issue and "flash" (literally) rusting the bottom where the condensation settles.

I don't mind making these up every few years as they don't really cost me anything but I'm wondering making it in stainless would mean it lasts longer if it is water being an issue? If so what grade?

Guess it is time to practice stainless TIG...

 

[addjunkie]

Stainless doesnt like temperature cycling, 304 and 316 both loose strength and corrosion resistance when heat cycled. Both are mildly prone to micro cracking too. That said will it last longer than mild probably…304L would be better than 304.

If it were me, Id get a few of mild steel made up at the same time and just change them out as a service part.

 

[winchman]

That looks like the flame hasn't been set up correctly causing the tube to melt

 

[sako243]

I did wonder about the temperature cycling with stainless but forgot to ask. Thanks for clarifying.

The mild is simple and I can't complain about it since I got the last one rolled for free same day. He even delivered it and welded up the seam for me.

 

[sako243]

That looks like the flame hasn't been set up correctly causing the tube to melt

Prior to installing this one the jets etc., were all renewed by someone I'd trust to do a good job so I'm pretty happy they were set up correctly.

The boiler does live down in the cellar which does get damp and certainly high humidity.

 

[addjunkie]

Prior to installing this one the jets etc., were all renewed by someone I'd trust to do a good job so I'm pretty happy they were set up correctly.

The boiler does live down in the cellar which does get damp and certainly high humidity.

Humidity is always present in combustion in simplistic terms for every litre of juice you burn, will produce roughly a litre of moisture in the exhaust.

 

[daleyd]

The company I used to work for made thermal oxidisers and industrial dryers - all the stuff around the burners was 253MA grade stainless steel which is good for high temp applications, but may not be easy to find.

I have a load of 253 MA welding rods if you do get hold of some!!

 

[hotponyshoes]

There are several grades of stainless (and various other materials) that are suitable for high temps & heat cycles.
None of them are particularly easy to work with if you wanted to roll it from sheet.
Some have particular weld and post weld requirements and not many are found in the skip.
You would be lucky to find a tube the correct size and ordering one to spec would be eeexxxpensive.

Having said that, if it's only acting as a heat shield you might be fine using one of the exotic materials at well under the existing 2mm thickness.

I'd be doing as suggested, get a few cheap ones made up at once and replace when required.

 

[Mikes]

That looks to have been caused by flame impingement. Possibly the wrong nozzle has been fitted. If you look at the nozzle, it will have two bits of information on it which are critical. 1st is the throughput and second is the angle. Perhaps the wrong angle nozzle has been fitted (too wide). Do you have the previous nozzle that was taken out to compare the new to? Wouldn't have been the first time that a service has resulted in the wrong nozzle being fitted!

 

[winchman]

Impingement thats the word I was looking for. Looking at the amount of debris in the boiler I would have the set up tested more often as the heat transfer won't be good.
I have seen one burner with an asda bag stuck to the fan inlet that was interesting

 

[Turbo]

That looks to have been caused by flame impingement. Possibly the wrong nozzle has been fitted. If you look at the nozzle, it will have two bits of information on it which are critical. 1st is the throughput and second is the angle. Perhaps the wrong angle nozzle has been fitted (too wide). Do you have the previous nozzle that was taken out to compare the new to? Wouldn't have been the first time that a service has resulted in the wrong nozzle being fitted!

Have to agree, that looks like someone used the wrong nozzle or the burner head is not far enough into the boiler.

It only takes one boiler service man to fit the wrong nozzle, maybe he didn't have the correct one in stock.
Then all future service men will normally fit that same incorrect nozzle. Very few will bother looking up the correct nozzle and go by what's already fitted.

Is there any chance that the tube you got made is too long?