boiler insulation / wood staves

[Lopez Mike]

I wish I knew the relative thermal conductivity of wood and glass wool. I could sleep leaning against my lagging without discomfort.

What I have not yet sorted out is how to do a tidy job of insulating my throttle pipe. Right now I have it wrapped with some glass insulating tape. Maybe a 1/4" thick. and then wrapped over that with a layer of maybe 3/16" cotton cord. Not pleasing to look upon.

[mtnman]

Here's some new space-age insulating material I just read about.

http://www.buyaerogel.com/product/pyrogel-xt/

[DetroiTug]

On my VFT with wetleg, there is only .400" thick white oak staves. It works well, wood is a great insulator. The reason I suggested 2" for Light housekeeper is; on my firedoor which was originally uninsulated, it would glow red at night and burn my face after a day of firing. I used 1" fiberfrax sandwiched in the door which made a big difference, but it still gets very warm when hard-firing. Not near as hot as before though. My buddy has a dryleg with 2" and then stainless steel over the outside of that and it stays pretty cool.

Mike, pipewrapping is pretty easy just time consuming. On my steam lines, I used regular old clothesline - partial polyester type - the cheap stuff. I had insulating tape under that and yes it looked pretty bad. Just do the piping in sections and cut off enough line to do each as you go. Start by laying the line underneath then wrapping over the top of it. The same way that wind eyes on fishing rods. The elbows are more difficult, I tried to do four wraps from the end of the elbow and then overlap and do four more neat wraps. It's tedious. Just the clothesline, it will take the heat, then when done just paint it all.

-Ron

[lighthousekeeper]

It has been suggested that I only insulate the top of the boiler were the water is, that having a dry fire box is too hot to insulate. So my question is do some vertical tube boilers only have insulation and wood only on the top of the boiler? thanks

[steamdon-jr]

absolutely, if you do not have a wet leg then you must create an airspace and/or insulation prior to wood.

[fredrosse]

With a dry furnace wrapper, very hot steel will not last for long, stainless is necessary. If the wrapper is kept cool somehow, with minimal insulation and air circulation around the wrapper, then steel can be ok.

If the wrapper is well insulated on the outside, then its temperature will rise much closer to fire temperature, and only stainless steel can take this high temp. If the insulation is between the fire and the wrapper, then a very high temp insulation is needed, and it will be prone to damage and wastage with wood/coal firing, with fuel constantly hitting the unprotected insulation.

I use dense firebrick lining the furnace, because it is strong enough to hold up with solid fuel firing, but the thermal insulation provided by dense firebrick is not very good. Outside the firebrick, a layer of hi-temp blanket, then the wrapper sheet, either stainless or ordinary steel, depending on the actual operating temp.

There are discussions on this forum about cooling the ash pan, so you don't burn up the bottom of your boat. Be sure to check this.

Wood insulation works OK for all boiler external surfaces that have steam or water on the other side of the insulated surface. If the surface faces fire or furnace, or very hot flue gas, then better insulation is required.

[cyberbadger]

For everyone's reference - I have been in contact with lighthousekeeper for many months.

My first boiler which I have never put in a boat is the same basic design and from the same boiler maker. The major difference is that lighthousekeepers is 20" external diameter, and the one I bought was 16" external diameter. I thought this may shed some light (Infrared that is) to illuminate the situation - Pictures are worth a thousand words. I'm giving you 4 in one. So by that math I just wrote a 4000 word essay.

I have provided this image to lighthousekeeper months ago.

Again - same general VFT dry leg design by the same boiler maker. Mine was a few inches less in diameter. This was operating on a cart in my driveway.

Do not take the temperatures from these FLIR Infared Camera images as accurate. The emissivity setting is not correct,min/max temperatures, etc. Just look at the pictures as relative temperature between regions of the boiler lighthousekeeper has in his boat. Note that there isn't much of a gradient between the water in the pressure vessel and the firebox/furnace, it's very sharp.

I applaud and congratulate lighthousekeeper for doing a semi successful sea trial. I salute you lighthousekeeper - you beat me first to the water and your launch is prettier then mine will be.

However, unless you get a different boiler lighthousekeeper- even with the best material insulation, you can't run away from the fact that it's a dry leg VFT - take the advice of the folks here they know it better then I do but you can't just wrap the firebox/furnace with some better insulation outside.

-CB

[fredrosse]

cyberbadger, great photos, and revealing. Certianly worth many words. I wrote a note to a person selling boilers on e-bay, I was interested in buying his boiler, until I found out it had a serious violation of boiler construction rules. The note is repeated below:

"I know free advice often is ignored, but I offer it anyway. I am a professional engineer, working in the steam power industry for several decades, and often use the ASME Boiler and Pressure Vessel Code. The boiler rules require that the shell extension beyond the tubesheet should not exceed 1/8 inch if exposed to furnace gasses above 850F, and not otherwise water cooled.

It appears to me that your design has the shell extending well into the furnace space where it can be overheated by the fire. One simple remedy is to have the boiler shell length the same as the firetube lengths, and attach a separate furnace tube, butted up against the boiler shell tube, but not directly welded to it. The attachment of these two parts can be made with tabs on the outside diameter of the shell & furnace, perhaps four flat bars, 1/8 x 1 inch, about 4 inches long, spaced at 90 degree intervals, with a weld at each end of the flat bar (one inch long weld beads). One weld on the boiler shell, one weld on the furnace tube, each weld about 2 inches from the furnace/shell butt joint. That would allow the furnace tube to go to higher temperature, and freely expand without adding stress to the boiler pressure vessel. A similar bolted arrangement could also be easily made."

The reason for this ASME rule is because the high temperature furnace section runs much hotter than the tubesheet (as clearly illustrated in the photos), and hence forces powerful thermal expansion on the tubesheet weld. Every time the fire is running, a much higher stress is imposed on this critical weld, tyring to tear the boiler shell away from the tubesheet. This stress condition also has complete reversals when the boiler is shutdown, another serious issue can result here, fatugue stress, which may result in failure after a certian number of repeating cycles, depending on the properties of the metal.

If this is the case with the illustrated boiler, or any similar ones, then the modification suggested above should be implemented. The suggested modification (cutting the shell to separate the furnace section from the boiler section) would take about an hour with a common 4-1/2 inch cutoff tool, after proper measurements and work setup had been completed. One other method would be to insulate the lower furnace tube section from the fire, and thus preventing this tube section from getting significantly hotter than the boiler steam/water section. This fix however is far less reliable, and reduces furnace usable volume significantly.

[DetroiTug]

"However, unless you get a different boiler lighthousekeeper- even with the best material insulation, you can't run away from the fact that it's a dry leg VFT"

Nothing inherently wrong with a properly built dryleg VFT and it depends on the exact design of each which is better - wet or dry. Some things to consider: Wetleg- smaller firebox with smaller fire and it all comes down to how much fire and then there are fewer tubes for the diameter by comparison.

Dryleg- Much larger fire, more BTU's, larger crown sheet facilitating more tubes. This is rather like the Compound vs simple debate, but it's really decided on a case by case basis.

At some point in the range of boiler sizes, the firebox diameter of a wetleg becomes so small that very little fire can be built rendering the whole plant very inefficient.

I have a large wetleg with a 19" diameter firebox, I seriously question what that water leg is worth and how much it is costing. Considering combustion begins halfway up the firebox - cold air being drawn in the lower portion. I ran some numbers on it a while back and I think I'd be getting much more heat transfer from another ring of tubes.

One can look to the steam car boilers, none of the contemporary boilers of that period other than a few prototypes early on, had a waterleg of any sort. These were some of the best engineers of the day and they apparently opted for more tubes and insulating the burner housing.

Re: Insulating the outside of the firebox with Fiberfrax only, a lot of boilers set up that way. Fiberfrax is good to around 2500°F

Those are great pics showing the heat signature.

-Ron

[Mike Rometer]

(quote) Considering combustion begins halfway up the firebox - cold air being drawn in the lower portion. (quote)

Well how about a semi wet-leg then?