The Steamboating Forum

swedtug wrote:Yes I suspect that they will be glowing dull red sometimes, so it is probably important to exaggerate the pipe wall thickness .

And be holding full boiler pressure, with no way to isolate them....?

Certainly would not want to be getting the spec of that wrong would you!

Daniel

I do not have a superheater but have considered it, yes theycan glow red, you mention no isolation....I would have a bypass to isolate the superheater with a safety valve in the superheater so if you closed it off by accident it would not pressurize....no bypass = no getting home if the superheater fails.

Yes I have, of course, an isolation valve, and I have the steam trottle before the superheater, and a bypass pipe to put there if something goes wrong. the bypass pipe has to be bolted in with flanges and is no fun to do with a hot tube, but probably more fun than to paddle a 30 ft tug

Johan

Ok, so various options there. I have a Wikipedia based knowledge of locomotive superheaters, which appear in later life to have had the valve on the outlet 'front end valve' for increased response from the regulator which woild I think be very desirable given while we do a lot of going along we also do a fair bit of manoeuvering and starting/stopping at locks, junctions etc.

All slighlty hyperthetical as this point as there is plenty of work that ort to be done well before this is even considered, but where is the fun in that!

I found the below photos from installations on steam cars, I guess it would look a bit the same, a bend tube, or maybe two in parallel, hung below the bottom tube plate. I expect some calcs could be done and everything!

http://www.stanleysteamers.com/photoalb ... heater.JPG
http://www.stanleymotorcarriage.com/Par ... heater.jpg

All very interesting anyway!


Daniel

Here you can see the superheater tubes under the boiler, if you look carefully you can see the small superheater tubes to the atomising burner in the Middle.

"I personally don't think a second safety valve is required on small boat boilers, an alternative is to mount the (only) safety valve on the superheater outlet header, but with a modest size of superheater the drum valve should suffice."

This is certainly not allowed in USA practice under the ASME Code rules, the main safety valve must be directly connected to the boiler drum, with the shortest connecting pipe practical. A second relief valve at the superheater outlet may or may not be required, depending on circumstances.

With respect to overheating the superheater tubing, one method is to have the superheater tubing in a flue gas zone which is at a moderate temperature, so that, even with no steam flow, the tubes will not overheat. This takes much more heat exchange surface area compared to a superheater placed in a hot flue gas zone, however the hot zone superheater usually can overheat badly if steam flow is interrupted. On power plants where the heat source is not shutdown when process/turbine steam flow is stopped, a bypass system is provided, dumping steam as waste just to keep the superheater with flowing steam, protecting the superheater tubes from overheating. Either plan could be workable on a small plant, but with operating complication on a hot zone superheater. No special controls would be needed with the superheater located where the flue gas temperature is fairly low at 600F-800F (315C - 425C), and the proper superheater tubing material is selected.

Red heat is at about 1100F (600C), and almost all stainless steels are not permitted (per ASME Boiler Code) for use in boilers above this temperature. Carbon steels are generally not permitted above 840F (450C), and copper alloys, 406F (208C) maximum.

AFAIK, stainless/nickel alloy tubes are permitted in super heater service since they are not subject to chloride corrosion w/o boiler water contact.

Controlling super heater temperatures is tricky on a small steamboat boiler due to varying steam demand and firing changes. One possible mechanism is to use a bimetallic coil or quartz rod to control routing part of the steam leaving the super heater back through a tube through the steam drum prior to mixing with the un-rerouted portion. This would regulate the temperature of the steam mixture, and help prevent lubrication problems in the engine.

In any case, w/ a super heater fire-up has to be done carefully, even on water tube boilers, since there's no steam flow cooling the super heater until the boiler is making steam. One mechanism that is used in full size practice is to bypass the flue gases around the section containing the superheater; this is harder to arrange w/ our small boilers. Also, radiant heat can cause superheaters to fail quickly if there's no steam passing through them.

In general, a mild super heat helps ensure relatively dry steam conditions at the engine throttle. High pressure, high temperature plants are interesting from an engineering standpoint, but not really something I want to be next to while relaxing. Terry Williams' steam outboard would see 800 PSI , 800F sometimes....

- Bart

Certainly while an increase in power, from improved efficiency would be excellent, if it was an expense of a huge amount of hassle its worse than where we are now. It would have to be a reliable/conservative as it could practically be.

Obviously the boiler exists and is not about to be changed, 25 years old its in mint condition and returned only a few years ago when it was in fact found the tubes where still basically fine. There are I think 175 x 1" vertical fire tubes and obviously no 'flue' tubes for a superheater.

How hot do you need to dry the steam, it there any millage in a coil in the smoke dome?


Daniel

How hot do you need to dry the steam, it there any millage in a coil in the smoke dome?

There certainly can be a small benefit w/ such a coil, and there's obviously no danger of overheating unless the
boiler is grossly over-fired. Even w/ a large underworked boiler, at steady state the flue temperature will
be enough above the steam temperature to support some heat transfer. Of course, w/ a large fire tube
boiler, carrying the water level a little lower yields the same result .

- Bart