The Steamboating Forum

Hello all,

Like I said in my last post, I have myself an engine and a boiler. The engine I'm fairly happy with. I will need to add a reverse eccentric and Stephenson link in place of the slip eccentric but that's pretty much it there. The boiler, on the other hand, needs some real thought put into it. Here are the pictures again for reference:

http://www.flickr.com/photos/22974775@N ... 846168351/

It's a huge heavy thing, almost certainly more than I need. The designer of the hull that I'm building specified 6-8 hp. The engine supplier said I could have an 8 for the same price as a 6hp so I went bigger. The boiler is a Yarrow three-drum design with 70 sq ft heating surface. It will just barely fit in my boat. The boiler has some unusual design features to it. The casing is a cludge: there's no bottom to it, the grate is huge (38" x 29"), the stack is small (5"), the insulation is poor, etc. It's huge on the outside with a cavernous firebox. The footprint is about 40" by 42". But lets concentrate on the boiler itself for now.

Let me run through the boiler as-built. The mud drums are 4" diameter and the steam drum is 10". There are 39 tube per side of 1" each in two staggered rows. The feed water enters first into two "feed water heaters", one behind the main tubes on each side (you can see the entry in the pictures of the boiler front). They are included as heating surface. They consist of two header pipes parallel to the main tubes with 9 1" tubes running between them. The feedwater enters at the bottom of the front header and exits at the top of the rear header. At the rear of the boiler, the exit from the feed water heaters is piped downward to enter the boiler via the mud drums (refer to the picture of the boiler rear). You will also notice a third pipe at the rear of the boiler, which goes from the upper area of the steam drum to the superheater. The superheater is 3.3 sq ft consisting of three 1 1/4" tubes and is visible in the pictures of the inside of the boiler. The superheated steam exits at the front of the boiler. Simple enough.

Here are my concerns. The superheater is directly in the fire, and as it is plain steel I don't imagine it will survive much dry firing. In addition the outlet from steam drum is very near the water level, which might be acceptable in a stationary use but on a boat there will be a lot of carry-over. The solution here is to take the wet steam off the top of the steam drum instead of the rear. The only advantage to the current set up is that the superheater could be flooded for initial firing to keep it from overheating. Any opinions on the life expectancy of this kind of superheater?

Secondly, the way the feedwater heaters are routed downward to the mud drums doesn't feel right to me. At times steam might be generated in the feed water heaters that would then be trapped and impede water circulation. My solution would be to reroute the feedwater heaters to the back of the steam drum, probably using the same bung that is currently the steam outlet. Any thoughts on this?

If anyone is still following along, basically I'm trying to rejigger what I've got to make the best of it. It has even occured to me that I might cut a foot out of the length of the boiler and reweld the end. That would reduce the heating surface to about 50 sq ft, though it wouldn't reduce the width any. At that point however it might be simpler to just order a smaller boiler...

Cannot seem to get to your pictures, is this just me/my computer, or is the link bad??

Bad link, I think

Oops, it was a bad cut and paste job. The link is fixed.

"The superheater is directly in the fire, and as it is plain steel I don't imagine it will survive much dry firing."

In power plant practice, the boilers must be fired with very low flame temperature until steam is raised, and some steam flow is established in the superheater tubes. This is a difficulty here, and you need to start with a small fire, and plenty of excess air (doors open?) until steam flow can be established. Stopping the engine with a brisk fire going would damage the superheater unless steam flow continues.

" In addition the outlet from steam drum is very near the water level, which might be acceptable in a stationary use but on a boat there will be a lot of carry-over. The solution here is to take the wet steam off the top of the steam drum instead of the rear. The only advantage to the current set up is that the superheater could be flooded for initial firing to keep it from overheating."

Carryover is a real problem for some boilers. For the Margaret S I fitted a separator tank, steam enters this pressure vessel, and is spun around, water dropping to the bottom, where a common commercial steam trap takes it away. The top of this vessel has the connection to the engine, so it will never get a slug of water. 3 inch diameter, 12 inches high, 3/4 pipe connections.

"Any opinions on the life expectancy of this kind of superheater?" The steel superheater tubes should not exceed 750F-800F metal temperature, how many times will it be above this temperature, depends on operations. Metal temperature will be considerably higher than steam outlet temperature with steam flowing. If you can keep it below 750F, it should last as long as the boiler. Get one of those infared lazer thermometer guns ($28.00 USD at Harbor Freight), you can point it at the superheater tube bank and get the metal temperature.

fredrosse wrote: Carryover is a real problem for some boilers. For the Margaret S I fitted a separator tank, steam enters this pressure vessel, and is spun around, water dropping to the bottom, where a common commercial steam trap takes it away. The top of this vessel has the connection to the engine, so it will never get a slug of water. 3 inch diameter, 12 inches high, 3/4 pipe connections.

The boiler and engine were supplied with a water separator but I hope to avoid using it. For one it's about three time bigger than it needs to be, something like 6 in diameter by 18 inches tall, very heavy. Also my thinking on that is that it just presents more surface area for the steam to cool on its way to the engine. I'm hoping that with the wet steam outlet on top of the steam drum the superheater should put out sufficiently dry steam. It is around 3.5 sq ft surface area, but I'm not sure if that's big or small in relation to the boiler.

fredrosse wrote: The steel superheater tubes should not exceed 750F-800F metal temperature, how many times will it be above this temperature, depends on operations. Metal temperature will be considerably higher than steam outlet temperature with steam flowing. If you can keep it below 750F, it should last as long as the boiler. Get one of those infared lazer thermometer guns ($28.00 USD at Harbor Freight), you can point it at the superheater tube bank and get the metal temperature.

That's probably what I'll do, just try it out and see how it works. When I repipe it I might even go ahead and include a bypass line. If it fails I can just secure it and rebuild it in stainless.

What are your thoughts about the feedwater heater setup?

Without the separator tank you run a potential risk for the engine, your choice.

The feedwater heater sounds OK, however normal large boiler practice is to introduce hot feedwater into the steam space of the upper drum, this immediately heats the feedwater up to saturation conditions and drives out any disolved gasses. I think the way it is setup on your boiler is OK though, maybe a little more corrosion, but many steamboats do not introduce heater feedwater to the upper drum.

A few thoughts:

I've seen feed water dumped in in almost every possible part of the boiler and can't see that it made much difference. I've even seen it fed in at the top where it was deliberately sprayed all over the steam space, the notion being that horrible feed water would have the calcium separated out or left softer or some such crack pot idea.

I haven't had any problem with steel superheaters failing from running dry on small boilers. Possibly you might get high enough temps with extreme forced draft but stop and consider that these tubes are welded in place at thousands of degrees and if the steel can stand up to that, I doubt that our hobby boilers are going to faze them. In some good designs it arranged so that they can be replaced without tearing the boiler apart.

I do think that moving the steam outlet up as high as possible is a good idea. Some times you can arrange a perforated pipe within the steam space to act as a separator. On railway locomotives subject to rather violent motions the use of a steam dome is universal.

Sound like you got a decent unit and it won't take much to get good use out of it.

Mike

Thanks for the good advise guys. I think I will make the changes I mentioned but I'm sure they aren't necessary. The big decision to make is whether to go with the big one I have or order a smaller one for the boat and keep the big one for a shop boiler. The latter idea is tempting but right now I don't really have a place for the big one to live.

Harry