First re the brass corrosion or more properly de-zincification, and the Midwest boiler.
De-zincification is a process which happens from the inside out, so external inspection tells you nothing, - the first indication is when it fails! The mode of failure can be interestingly varied also. I think Susans scenario of "Ka-boom", may be a bit dramatic, but is always a possibility, I feel that a leak, and let down of pressure is more likely, however that, spraying steam and boiling water all over, can be pretty nasty too.
By the way the de-zincification process does not need iron or any other "foreign" metal to be present.
My own experience with de-zincification was in an old copper 7.25 inch gauge loco boiler, with a brass steam turret approximately 1 inch cube, with an integral 1/2 inch threaded stem into the boiler bush, and internal threaded take offs in three of the sides. I tested the complete boiler hydraulically to 200 psi with no problems apparrent. There were no external signs whatsoever of any possible problems wih the steam turret or any other parts.
I then attempted to unscrew the steam turret, and the whole block simply crumbled into coarse spongy lumps. I wonder how much longer the thing would have operated under steam before the turret let go under steam pressure of 100 psi or so.
The commercial makers of toy brass boilers - Mamod, Willesco, etc., use a particular brass alloy which is not really prone to the de-zincification process, so they are more safe than someone using any old brass would be. I hope that the Midwest people use such an alloy too.
That Midwest boiler worries me in that the ones I've seen have no safety valve - they rely on the plastic steam pipe popping off the stub it fits onto, if the pressure gets too high. That sounds possible, but sure as eggs someone will wire or clamp it on, or replace the plastic with brass or copper soldered on. I know of at least one accident in the USA where a spectator child was burnt when the steam pipe popped of the engine end in a model steam boat. The boiler was over filled, and the free end of the pipe was whipping around spraying boiling water and steam all over.
The issue of the thickness of the flue pipe wall I think is probably a red herring - the conductivity across a thick wall will be reduced (VERY slightly!), but as heat transfer is much more limited at the metal/water and flue gas/metal interface, the effect of metal conductivity will be negligible.
The real problem iwth the Midwest and very many other small vertical boilers is one of totally inadequate heating surface, combined with ineffective draft and gas flow arrangements. A Sterno fuel system wouldn't help either. The proposed three wick alcohol (Methylated Spirit UK) burner, would help considerably. I'd be inclined to put several more flues in the thing and use any other way of improving both heating surface and draft. If this works and you are considering this stage of modifications, then I start to wonder why persist with the original boiler? It might be easier to build a right one from scratch in the first place rather than bodge up the original.
Jim Gregg.
At 07:13 AM 9/4/03 -0400, you wrote:
Susan,
I'm not too worried about brass corrosion in this instance. The chimney tube/main flue of the Midwest boiler is brass to start with, so adding some brass fire tubes should make no difference in the ultimate life of this boiler. I can easily inspect the brass water tubes every time I refuel the boiler because the tubes are right there in the fire space, exposed every time I lift the boiler to put in some fuel. Also, I am scrupulous about drying the boiler completely after the last run of the day. I empty the boiler while everything is still hot and leave the filler cap off, only replacing it after filling the boiler when next I run the engine. There is no iron at all in my boiler so there should be little or no electrolytic corrosion taking place that would leach the zinc out of the brass.
Should the water tubes fail, boiler pressure would quickly dissipate. The water tubes are in the fire space so the gusher of water from such a failure should put out the fire right away. The open ends of the water tubes are completely in the waterspace so little steam would be released through the water tubes if they fail. Also, there is no throttle or restriction (other than the engine itself) in the steam takeoff line that would allow high steam pressures to build in this boiler. Therefore, any excess steam would more likely exit through the steam line, not through a failed water tube.
I thank you for your concern. I share your interest in keeping this hobby as safe as can be. This hobby is supposed to be fun, not an opportunity for carelessness to cause personal injury or property damage.
Casey Sterbenz
From: Susan Parker <[EMAIL PROTECTED]> Reply-To: [EMAIL PROTECTED] To: [EMAIL PROTECTED] Subject: Re: BPE boiler mods? Date: Tue, 02 Sep 2003 19:25:17 +0100
Hi Casey,
Hope you are keeping well.
Date: Mon, 01 Sep 2003 20:58:12 -0400 From: "Casey Sterbenz" <[EMAIL PROTECTED]> Subject: Re: BPE boiler mods?
I added three water tubes to my Midwest boiler. I cleaned out some brass ball point pen refill tubes, annealed them, bent them into a "U" shape, then soldered them into the bottom tube plate of the boiler.
Oops, if this means what I think it means this is not safe. The zinc in the brass can leach out over time leaving a spongy matrix which will fail.
I.e. Ka..BOOM..!!!
I would very much advise you to remove these and replace with copper.
Brass is fine for burners and smokeboxes, BUT not for use in a boiler itself or any fittings (e.g. bushes, stays, etc.).
[snip]
Best wishes, Susan.
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