Steam trains and other non Reversing light stuff :)

[Pete]

Over here Mudrat,

Hmm, loco boiler designs. It is a firetube boiler, the firebox has water walls around 3 sides and the boiler barrel on the fourth. Tubes / Flues run from the inner wall of the firebox , through the barrel to the smokebox. general practice used to be to run saturated and use firetubes of all the same diameter.

The trend is moving away from saturated to superheated. In minature the easiest way to do this is to add a row of superheater flues instead of the top row of tubes. The superheaters run from a header (wet header) in the smokebox, one down each flue (usually four), to near the back of the firebox to a 180 degree bend, than forward back through the same flue to combine at a second header (dry header) These are full radiant superheaters and actually not all that common in minature. Most loco's run their superheaters to just past the end of the flue. A more appropriate name would be driers

[PeeJay]

Are you building one?

[MUDRAT]

Thanks

Interesting, so basically you're using a superheated steam "drum".

I've seen this in a few ships before, but only ever on watertube boilers. They run a single very large mud drum, twin steam drums (where the water level is held) and then twin primary superheating sections to a single superheated steam drum, from where it is used in the process.

Does your train use a turbine to utilise the kinetic energy? I'm interested as to why you would choose to use superheated steam in this application. Unless droplet impaction becomes a factor (where you have a turbine moving close to the speed of sound), saturated steam has a higher energy yeild at any given flow.

How do you control the water level in the boiler, ie to prevent priming the superheaters?

[Pete]

Interesting, so basically you're using a superheated steam "drum".

I guess that is one way of looking at it, I have never had much to do with marine boilers, the closest I have come is walking through the boiler room in the HMS Belfast, and the Missouri.

Does your train use a turbine to utilise the kinetic energy? I'm interested as to why you would choose to use superheated steam in this application. Unless droplet impaction becomes a factor (where you have a turbine moving close to the speed of sound), saturated steam has a higher energy yeild at any given flow.

How do you control the water level in the boiler, ie to prevent priming the superheaters?

Classical double act locomotive cylinders. We use superheat to increase efficency. The difference in personality, fuel and water consumption between saturated and superheated loco's is quite remarkable.

I am curious how you get more energy from saturated when superheated is at the same pressure. From the loco perspective we do not run into condensers, have high thermal losses and also need to have enough energy in the exhaust for drafting the fire up the chimney.

Priming; Loco boilers are run using water gauge glasses, with the bottom of the glass slightly above the crown of the firebox. All loco's are required to have two totally separate feedwater systems. This usually includes one mechanical donkey pump, one or two live steam injectors and on smaller loco's it is common to use axle pumps with controlled bypass.

The actual control of water level is one of the things that is well practiced during the 10 hours of direct supervision. Either adjust pump bypass or speed to suit workload or intermittent use of injectors.

[Pete]

Are you building one?

Yes, I am working on a "Tich", 7 1/4" gauge 0-4-0 tank loco

I am comming up on getting my boiler design approved before I start fabrication.

I also recently bought a Loco I have been driving regularly for the past couple of years.

[MUDRAT]

As saturated steam is at saturation temperature, latent energy is yeilded from the medium during change of phase (ie from vapourous saturated steam to water droplets). Superheated steam must first be cooled to saturation temperature before any latent energy can be released. Saturated steam also has a higher density and therefore delivers more kinetic energy per m3, while mass flow will remain constant.

In all honesty I have zero experience using steam in a reciprocating process, I cannot state with any authority the differences between wet and dry steam in a piston scavenging situation, nor on the exhaust characteristics from the process. As demineralised feedwater is expensive to produce, I have always used a condense and capture system.

On the water level control front, it sounds like a simple yet effective system. The old fashion way! Watch the bloody gauge-glass! I love it.

[Pete]

In the loco context, the valve gear controls the steam admission as a percentage of piston travel. A good valve gear at maximum cutoff will admit steam for around 80 - 85 % of the stroke. The last 20% of the travel uses the steam expansivley ( I am sure there is a techie expression for that, but that is how I understand it ) In full size a loco at high speed is physically unable to evaporate enough water to maintain that sort of steam demand. See the next para;

The other side of the process, is what is term'd 'notching up' . The cutoff is reduced to allow less massflow and allow more expansive work from the steam. Good valve gear has even events right back to around 15%

My understanding of the physics is the steam has to go in hot enough so it does not start to condense in the cylinder, robbing the cylinder of the expansive energy.

In actual practice, a full size loco starts off in 'Full gear' ie. the maximum cutoff for the valve gear. Once the consist is moving the driver opens the regulator fully then controls the speed with the valve gear. They run on the minimum cutoff that will maintain the desired speed. Minimim massflow / fuel / water consumption.

The other important side of loco design is the drafting setup has to be just right to enable to fire to maintain at all cutoffs and steam demands.

I have read a few tales of inconsiderate drivers having the concept explained by their fireman with the much loved 'contact' counselling becaue the poor fireman had to stoke all the extra, wasted coal.

In minature I have only come across one loco that could be driven effectively on the valve gear and I take every opportunity I get

Most minatures suffer from either poor valve gear design, poor steam circuit with the associated losses, poor drafting setup or just drivers who simply do not care.

Char costs around $400 / Ton and good black coal is about the same. I choose to save the 30% fuel burn notching up and superheating gives ..

[MUDRAT]

Yes, I can see how condensation inside the piston would be a bad thing, and certainly the flow characteristics of superheated steam are much more condusive to "drafting" - I guess a locomotive doesn't have an Induced Draft fan!

While I still maintain saturated steam would have a higher energy yeild per kg per second, I can see that using superheated steam would be more condusive to the process. Amazing! I love steam.

Another question - why use a firetube boiler? Is it because the full-scale locomotives used these designs? You could turn the model train world on its head with a Diesel or kerosine powered twin-drum watertube furnace!

Obviously there must be some kind of low-level trip on the little boiler, how does this work?

And when are you coming back to Wagga - we must meet up. As you can tell, I'm nuts about boilers. Are there any good steam/boiler/turbine forums you have discovered?

[MUDRAT]

Here's my "locomotive" - 60MW bark boiler. I can't get a photo of the bigger one next to it. It's about 30 meters tall from the hearth to the penthouse. The big boiler (Chemical Recovery Furnace) is 50 meters tall.

[Pete]

Fire tubes, hmmm. Probably a lot of reasons including habit, efficency, driver comfort, restrictive boiler code, reserve boiler energy.

The classical water wall loco boiler is the style used exclusivley on silver soldered or brazed copper boilers (under 25 litres by the code and under 6" by practicality) Over that size the boilers are usually welded steel and most people use a design known as Briggs for the firebox. Difficult to describe without pictures but basically instead of a continuous water cavity around the firebox they use vertical tubes on a manifold around the sides and usually nothing but a steel plate on the backhead. Usual practice is to pack the gaps with fireclay.

Personally I really dislike this idea, the sides and rear of the firebox can glow a dull red at night with the lack of continuous water cooling, this is great in Tassie in the middle of winter, it keeps your toes and other more important bits nice and toasty. Anywhere warmer than that, and the boiler becomes very unplesant to sit behind with the radiant heat. My biggest beef is that the water tubes are very difficult to inspect adequatley on the bi annual boiler inspection. I am a boiler inspector under training with my club ATM so I am learning more and more about this as I go along.

Water level trip; there is provision in the code to fit fusible plugs to the firebox crown sheet however it is not mandatory mainly because it is difficult to make reliable plugs in minature and the boiler must be well maintained / clean of scale for the plugs to be reliable.

I will not be using them in my boiler, I will rely on myself to maintain the water level. With the threat of having to make a new boiler if I stuff up. Being silver soldered, the expected failure mode is the crown stays soften then pull through collapsing the crown. Having said this, it is very unusual to leave a boiler unattended for more than 5 mins and even then the owner is always in hearing distance for safety valves.

Personally I am very interested in kero firing, I like the idea of a clean burning fuel and I also have access to ample waste aviation turbine fuel I need to find some info on reliable burner systems that will scale down..

The Wagga trip I try to make is in November however with the new addition to the garage (the Brumby ) and the EJ driveline mod comming the minister for war and finance has decreed there will be no interstate trips till the ute is finished... and I still have not found a good Liberty driveline yet.

Forums, I am a subscriber to COALS - conversations of Australian live steamers. It has been quiet lately, but there is a lot of experience lurking there. Including the chairman and secretary of the AMBSC - Australian minature boiler safety comittee. The group who maintain the code our bilers are made to

I also keep half an eye on yahoo groups ; Steam_tech, TheTeslaturbineList, SAR_Livesteam, live-steam, live_steam. Like all old forums they can be quite stagnant with general chit chat but there i usually some very bright people lurking aswell.

Running at 100 PSI the firetube boiler heems to be the most efficent, but I am always open to design ideas. My little 'Tich' is running on air, so I always have the option of making it a test bed for different ideas. Main potential sticking point is whatever the design I need to be able to make it fit in the code. Luckily the inspector I am training under is very flexiable and the Boiler code has the catch all clause of ; It if is not in the code but not specifically prohibited then I have the option to goto AS 1200 and 1228

[MUDRAT]

Well I love watertube boilers cos I guess that's just what I'm used to - I've always found them stable and quite efficient - for my purposes that is.

I also think that a firetube boiler would be better for small applications like the loco, where a compact simple design would be superior to a complicated multi-drum arrangement. Have you got any pics of a miniature loco firetube boiler under construction or inspection?

Just done a little research though - a Briggs is a firetube boiler? Isn't it like the ones they used on those old tractors and stuff? Certainly agree that your nads'd get fried!! LOL

Oh, and one more question - what do you use for fittings? ie gauge glasses, safety valves, FW stop valves etc etc? Are these available "off the shelf"?

[Pete]

http://www.users.bigpond.com/peter_arms ... _front.jpg

http://www.users.bigpond.com/peter_arms ... r_rear.jpg

This is a Tich sized boiler laying on it's side.

I don't have any pics of Briggs boilers as we use wetback loco boilers exclusivley.
Most boiler fittings are made from scratch, in brass if on a budget, or strongly preferrable in Bronze / Stainless. Majority of commercial fittings are simply too massive and look horribly out of scale. Having said that, a narrow gauge logo looks about right with commercial fittings

There are a couple of specialist suppliers who supply either model engineers sized fittings, or their commercial fittings go down to a size where they are just small enough. One of our club members is rebuilding a loco that uses Kunkle safety valves and check valves. Also uses the smallest commercial injectors available. 1 GPM - they are truly massive. I believe they ran them from bottom of the glass, about 8 seconds to 2/3 glass and about 20 PSI pressure drop. The boiler is in the 40 Lt bracket, our code runs up to 50L in steel.

[fredsub]

wow is that an example of silver soldered drum?
this topic is great...keep it up.

[MUDRAT]

Hey that's awesome. Those 4 large tubes above the small firetubes, are they for the superheaters?

This is the website I found regarding the Briggs boiler.

[Pete]

Yes, they are superheater flues.

That link is an excellent set f briggs pictures, I have not come across that site before, cheers.

Fred ; the boiler is all silver soldered copper with bronze bushes where there are threaded penetrations.The barrel and the outer wrapper around the firebox are rolled from a single sheet and then butt strap joined along the seam.

[Pete]

Mudrat, a couple of pics of another boiler I have designed a replacement for. I am just waiting for the owner to start cutting metal.

I believe it is around the 30L capacity.

The third pic is looking up one of the vertical water tubes around the firebox. This is the other issue I have with briggs boilers, it is imposible to inspect them properly without a bore scope. Thanks work for the loan of the borescope BTW

http://users.bigpond.com/peter_armstrong/img_0516.jpg
http://users.bigpond.com/peter_armstrong/img_0517.jpg
http://users.bigpond.com/peter_armstron ... csmall.jpg

[MUDRAT]

OMG that last picture's enough to give any boiler operator a heart attack! Are those wall tubes actually generating section or economisers? Either way it's a recipe for catestrophic failure. It's one thing to bust a tube while you're sitting in your air conditioned control room, it's another to blow a tube when the boiler's sitting between your legs!

I've actually had an experience with a superheater pluggage and failure. The original boiler design called for our PO4 dosage to the steam side of the drum (why no one ever could figure out) which led to Phosphate carryover, causing the blockage. Although only one superheater tube ruptured, about 30% were plugged solid. It was in the primary section so that was an 80 bar failure.

[Andrew]

hmm
superheated steam? are you sure thats a great idea on a home project? best to stick with saturated I would reckon....

[Pete]

OMG that last picture's enough to give any boiler operator a heart attack! Are those wall tubes actually generating section or economisers?

Economiser, that's big boiler speak for superheat / reheat isn't it ?

The vertical tubes are a work around to kind of make water walls. At the time it was built, the steel code had no provisions for prototypical loco boilers (water walls all around the firebox, like the copper boiler I posted up) The guy who built that boiler came up with that idea as a work around, better then classical briggs and it still fitted in the code. I hate the way politics gets in the way of good engineering. Sadly as you can see, it is impossible to clean the water space and very difficult to even inspect the tubes. BTW, this boiler has been condemned which is why I have designed it's replacement.

The diagonal's are Arch tubes, same concept as thermic syphons just simplified. In modern boilers they have inspection openings at each end and are inspected at the hydro.

Andrew, in minature Loco's the boilers are running at 100 PSI, I think that gives a saturated steam temperature around the 140 degree mark ? Mudrat, jump in here anytime . I would be suprised if the superheated temp gets over 250C. Superheating lets the driver get a lot more expansive work out of the steam before it starts to condense, in real terms the difference in personality of the loco's is quite noticeable.

There is only a small quantity of 'superheated' steam at any given instant, only what is in the superheater plumbing and a boiler failure is going to release saturated steam only.

[Pete]

I've actually had an experience with a superheater pluggage and failure. The original boiler design called for our PO4 dosage to the steam side of the drum (why no one ever could figure out) which led to Phosphate carryover, causing the blockage.

I am curious what your feedwater treatment system involves. Our club is in the process of phasing out an ineffective treatment process and I am interested to find an industrial chemist to have a chat about a change of treatment. Currently using a tannin and alkalai process, but I think most of the problems are caused by the operator, not the treatment...