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Cladding Problem - Need Advice

Posted: Tue Jun 18, 2019 11:01 pm
by Centurion

I have a Benson Mtn. boiler on a Elliott Bay hull. During the winter, all the straps holding the cladding on the boiler let loose (corroded pop rivets on hardware) letting the cladding fall loose around the boiler.

I've repaired the strap fittings but have no idea how to collect all the loose cladding boards to get them into position to reinstall the straps. It seems like I can't get the slats tight enough together to get the straps back in place. Any cladding experts out there?


Re: Cladding Problem - Need Advice

Posted: Wed Jun 19, 2019 1:00 pm
by DetroiTug
Your friendly ratchet strap. :)


Re: Cladding Problem - Need Advice

Posted: Wed Jun 19, 2019 1:55 pm
by PeteThePen1

Can I add a rider to Ron's sensible suggestion - a couple of willing helpers. My own experience of slats is that they seem to fall all over the place as if somebody has turned up the Gravity reading just as soon as you put a few in place.

May I also hijack the thread on a slightly different but insulation related question? Does anybody know, or know how to work out, the temperature of exhaust steam given a particular temperature and pressure at the engine? My boiler is meant to deliver steam at 150 psi (10bar) which is about 185 degrees C. The engine is a twin 2.125 X 2.25 per cylinder. I am seeking to insulate the exhaust piping for safety reasons and see that some inexpensive commercial insulation is rated up to 150 degrees C.



Re: Cladding Problem - Need Advice

Posted: Thu Jun 20, 2019 6:43 am
by fredrosse
If you have exhaust at atmospheric pressure, then the exhaust will be wet steam at almost exactly 100C (212F). Virtually impossible to be any higher temperature unless the exhaust is somehow pressurized. No need for insulation rated for any higher temperature. If you condense with vacuum exhaust, then exhaust temperature will be lower, but occasionally you will have atmospheric exhaust pressure anyway.

That exhaust temperature can be insulated with many types of rubber foam, or fiberglass materials, ordinary cotton cloth, but not synthetic fabrics like polyester cloth. An easy solution is to wrap the hot pipe with cloth or fiberglass material about 12mm thick (1/2 inch), then a final covering of white cotton cloth saturated with Plaster of Paris. This will provide a durable hard surface, similar to the old time casts we got when we had a broken bone decades ago (now they use something similar, but more modern). The Plaster of Paris hardens in about one minute, so plan ahead and rehearse the wrap before actually applying the wet cloth/plaster coating. I use this on main steam lines, with fiberglass insulation, and it has been holding up well for almost ten years now.

Another suitable material is several layers of Aluminium foil, crinkled and wrapped around the pipe in several layers to 12mm (1/2 inch) nominal thickness with lots of air gaps between the several foil layers. Then proceed with the Cotton/Plaster final coating if you don't like the looks of the Aluminum foil. That arrangement is good for steam temperatures well above 400C (750F).

Re: Cladding Problem - Need Advice

Posted: Thu Jun 20, 2019 1:36 pm
by PeteThePen1
Thanks Fred

That is exactly the advice I need. All I have to do is get on and implement it. I will report back when the overhaul is complete.



Re: Cladding Problem - Need Advice

Posted: Sun Jun 23, 2019 9:49 pm
by barts
The exhaust steam temperature is going to be determined by the number of expansions of the steam. If your engine was running at 100% cut-off, the exhaust steam would be at a temperature determined by the constant-heat expansion (same as a throttling valve) of the input steam. 150 psi saturated steam has a heat content of 1,196.0 btu/lb and is at 366 F. Keeping heat content constant and setting pressure = 15 psia, we see we now have steam at 306.9 F - super heated, of course, as we'd expect. This is why our engines squeak when we throttle down after running hard - the steam is very dry. Of course, you won't run your engine at 100% cut-off, and your insulation is certainly good for the worst possible case of 152C.

The tricky part of figuring out steam cycles is determining what sort of expansion we have. Typically, we model the expansion of the steam with piston motion as adiabatic (constant entropy, since it's supposedly reversible). Expansion through a valve (throttle valve or exhaust valve) is modeled as constant heat, since no energy flows out or in of the steam. These are approximations, of course - there are losses everywhere.

- Bart

Useful steam table calculator: ... /propSteam

Re: Cladding Problem - Need Advice

Posted: Wed Jun 26, 2019 5:24 am
by fredrosse
While an adiabatic expansion model often serves as an approximate representation of steam engine cylinder pressure vs. stroke, the actual conditions are very far from this representation. It is a well established fact that the largest source of steam engine efficiency loss is initial cylinder condensation during admission of steam into the cylinder.

The admission temperature is high, the exhaust temperature is relatively low, and the metal surfaces inside the cylinder will equalize somewhere in between these temperatures. The heat transfer coefficient for steam condensing on these surfaces is very very high, so a relatively large amount of condensation is formed on steam admission.

When the exhaust valve opens, the steam inside the cylinder suddenly drops to exhaust pressure, and any remaining live steam suddenly expands to exhaust pressure, with subsequent drop in steam temperature. Simultaneous with this process, the wetness collected on the hot cylinder surfaces (initial cylinder condensation) is rapidly evaporated, and this is at the corresponding saturation pressure, in the case of atmospheric exhaust this will be 212F (100C).

The result is almost always exhaust that is wet (a saturated steam/water mixture) and at the saturation temperature corresponding to exhaust pressure.

As Bart correctly points out, a pure steady (and insulated) throttling process, where no external work is done, results in outlet steam temperature that is well above the corresponding saturation temperature as a function of exhaust pressure, however this is not the case with the intermittent cylinder process occurring in a steam engine, even with 100% cutoff. For a steam engine to approximate the pure throttling process, the valves would have to constantly leak so much steam that the flow would just blow through the cylinder space like an open valve.