Advice on boiler size

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Dec 7, 2016
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Woodstock On Can
Hello all: I'm getting close to finishing my Stuart Triple Expansion and will want to run it on steam. I have 4 other steam engines that run nicely on air but would like to see them in steam as well . I have looked at PM Reasearch catalogue at their #2 boiler. It has 5 5/8 flues 127 sq. in heating surface and 44 cu. involume and 60 PSI safety valve. The triple has 3/4 HP cyl 1 1/4 IP cyl and 1 3/4 Lp.cyl.with a1inch stroke. Does any one have any comment on P.M. research products? I have no experience with boilers. Is a kit the way to go or is there a source of " ready to go"? Thanks for any and all advice. Colin
I was wondering if you were going to power something or only spin the engine ?
Hi Colin,
A few of the "Simplest" ideas.
  1. Buy a large Mamod or similar - for the boiler. To "idle" an expansion engine, you only need the steam at a pressure of maybe 20psi for the first cylinder, then when it gets "HOT" the other cylinders gradually add performance.
  2. Make a simple Pot boiler 3" diameter copper pipe is a good starter, and plan on working at no more than 30psi NWP. Having made the engine, you'll enjoy making the boiler and all the fittings I think? (I do).
  3. Spend the money on a proprietary boiler. It's quick, reliable and simple, but costs a bit...
  4. Make a flash boiler - but first buy the book (Experimental Flash Steam by Renson and Rayman - a good read!) and read all about it! The control and accessories seem daunting to those with no experience though.
  5. Get a second-hand pressure cooker, and suitable gas ring, then make a fitting for the steam take-off from the centre of the lid. - My Father made a 1/8"BSP fitting to go into the relief valve location - off-setting the relief valve so he had a Tee to take-off steam. Only used it a couple of times though, while he made a boiler that I am still using 40 years on!
I use a 3" diameter pot boiler and turn the gas burner close to minimum to power a twin expansion engine 1 " stroke x 3/4" bore (H.P. cylinder), It takes 5 or 10 minutes of idling before the L.P. cylinder starts to work, then it speeds-up a bit. Fun to watch. But it takes 10 minutes or so of steaming to warm the engine before it will run without an hydraulic lock from condensate. - Have you got a full set of cylinder drains?
I recommend that when you do run on steam the start-up is something like this. (You'll need to learn and tune the process as you go along).
  1. Lubricate everything on the engine. - (Harder to do this when hot and running!).
  2. Connect steam before lighting the boiler (Saves burning fingers on hot fittings!).
  3. Raise steam.
  4. Introduce steam - maybe when you get only 5~10psi from the boiler - with ALL the drain cocks open. Turn the engine by hand - use the flywheel as it is the coolest bit to touch. - DON'T wind the engine before the pressure gauge shows a few psi, as otherwise you'll suck water into the boiler past the clacks and feed-pump. - DO watch the water gauge, to be sure you are not "Priming" - I.E. sucking water into the engine. Remember, the Boiler needs the space in the top part to let steam form without droplets of water. But Too much space means the hot metal will be exposed at some risk of damaging the boiler. - Keep the water level between top and bottom of the gauge.
  5. At some point, you stop squirting water from the HP cylinder drains. Now close those drains and see if the engine will start, or keep winding ("barring" is the term used in the industry), to allow steam to warm the engine more while pressure is rising in the boiler.
  6. When the engine runs, watch the other drains and close - progressively - the IP then LP drains.
  7. Now let the steam pressure rise, watch the water level in the boiler, turn down the gas burner half way when 10% short of NWP - and keep adjusting to keep it there.
  8. You don't want the SAFETY valve to blow-off. It's noisy, messy, frightens the children and basically "why" run at the SAFE limit, when just shy of that is perfectly OK? (I run somewhere between 10psi and 30psi boiler pressure - depending on the needs of the engine. I have 40psi NWP and only get there for a steam test - or daily safety valve test).
  9. Watch the water gauge (every minute or more frequently, until is is automatic) and keep watering little and often.
  10. Watch the pressure gauge for any sudden changes.
  11. When something happens that you don't immediately understand, TURN OFF THE GAS. - That will give you time to think and work-out what is happening, and what you need to do next.
  12. Finally, ENJOY steaming. - I do.
As there are many experts out there - and I have probably missed the most important item, Please chip-in and remind me of the obvious thing I have forgotten to mention? - No prizes, just helps keep me sane.


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Hi, the attached write up is for smaller steam plants but the theory is roughly the same
I know there is a difference but for a show engine it will give a rough idea
The SBA Steam Boat Association UK have some good books for engine boiler sizing for the real thing


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Thankyou Steamchick and fcheslop. Very good advice. If my computer would let me download those files it would no doubt be better. Just one question. What the devil is a Mamod? Thankyou very much. Colin
Hi Colin, sorry, common phrases in the UK. "Steam world": Mamod, Meccano, Willesco and others are manufacturing names of "Boys' Toys" boilers and steam engines. Search Wikipedie and e&@y and you'll find lots of cheap stuff - if your engine is just going to "idle" for 10 minutes or so.... you can adapt a boiler that makes steam - in very small amounts - pretty safely. (Steam is never REALLY safe - it is a medium carrying lots of energy, and the mischievous stuff is always trying to escape! - A bit like compressed air, but steam can permanently damage flesh by Scalding, it is so hot!).
But they are too small for "real work" such as in a boat.
You could do a lot worse than but a proper boiler making book (e.g. K.N.Harris - Model Boilers and Boilermaking) that explains in fairly simple words how to size a boiler and select what you want - to make or buy.
Personally, I think that if you have made the engine you have proven your manufacturing skills so you can make a boiler and all the fittings. - There is just "silver soldering" to learn - which will find many helpers when you come to do it.
The triple has 3/4 HP cyl 1 1/4 IP cyl and 1 3/4 Lp.cyl.with a1inch stroke . Steamchick: Thanks for taking the time to look into this for me. Iv'e reached the point with this engine where the valve timing has to be set and lots of other "fiddleley bits" made. Spring and other projects are coming so there is not a great rush to get a boiler, rather something to gather info on should something come available. Stay well in the Land of Fog & Gales. Colin
Thanks Colin, Just checked out the window: The fog has cleared - now we have Rain and Gales! But it is warmer. Not 5 degrees C but 8 degrees C... And still daylight ay 4pm? = MUST BE SUMMER! Hoorah! 4 weeks ago we had "Winter". We had nearly 1 week of -2 or -3 degrees C. The 5 cm snow was 5 days before the weather changed and it cleared. - Jeeze! That was a bad winter! But crocusses are blooming now. Late this year, usually we have the daffodils by now.
But Sunderland does not translate as "Sun on the land" - but "Sunder" = between and "land" = habitable land. This was the land no-one could sensibly live on until the 19th. century industrial revolution could drain the marshes from the flat-topped rock beneath into the river... and pump water from the mines for the coal. So it was a fine (cheap) location to build a new town for building ships and shipping coal... But we do get Sun occaisionally..
Never been to Woodstock, On, Can, but stayed overnight in a $20 motel in London, On, 20-odd years ago... I seem to remember there were a lot of Donut & coffee shops around those parts? Only place we found for an evening meal was a good Chinese restaurant... - then more Donuts & coffee for breakfast.. That's only a half-hour or so from you I guess?
A QUICK look at the steam you need...
Based on running at 1000rpm at 20psi - which is a complete guess - I reckon the volume of steam per crank rev. = 0.884 which needs the boiler to boil 1.22 cu,in./minute:
Now I don't know if I am interpreting the books correctly, as I have not found an explanation of the steam table I have written anywhere.... but I think that needs about 0.9kW of heat. I would allow 1.2 kW of heat as the minimum burner as there will be losses, so "BIGGER IS BETTER". You could manage with a smaller boiler - say 2 1/2" diameter x 6 in long if you are sure you won't need more than slow free-running the engine. - That is "Large Mamod" sized, with the simplest meths burner.
But I would propose a 3in diameter horizontal boiler, 5 or 6 in long, and a single up-and-back loop for steam drier/heater, with 3in by 5 in. ceramic heater beneath it, using a number 12 or maybe 16 jet - with butane fuel - will easily power the engine. Or this could be a simple vertical boiler of the same dimensions and a 3 in diameter burner, but then you would not manage a no 16 jet and may find it limited to do more than just "free-running". (These are what I have already for similar sized engines).
With most engines, the "as new" friction is higher than "run-in" so it is nice to have reserve on the boiler & burner. And particularly with Compound engines a superheater is very beneficial, as that way the steam doesn't all turn to hot water in the first cylinder. (With the remaining "expansion" cylinders
acting as a water pump!). And you need to lag connection pipes between boiler and engine - which should be one size larger than you think and as short as possible - otherwise the steam is half-used just getting to the engine before it gets to warm the cylinder!
A lot of fun. Keep in touch and I'll advise with what I know.
Incidentally, if anyone else wants to teach me more - I'd love to learn! (especially the "energy to steam" calcs). There is a lot of physics in steam plant - but a lot of "black art" as well. And loads of people with experience know better than engineers, because they have been there and done it. I am just trying to find the right way to make the numbers meet reality...
Colin, I have been checking a few more "numbers": IF you made a 2 1/2" diameter horizontal boiler, 6in long, with 9 x flue tubes 1/4" ID, then you would have enough heating surface with the fire under the boiler and the flue gases passing through the tubes to a chimney at least 3/4" ID.
If you made a vertical boiler 2 1/2" dia, you cuould probably get twice as many flue tubes, and a bit more (maybe 30% more?) steaming ability, if the burner has the power. (The smaller area of the end of the vertical boiler is a disadvantage compared to the same "metal" making a horizontal boiler. But the vertical boiler can be filled with flue tubes, whereas the horizontal boiler can only be half-full of flue tubes, thus the heat-exchange area for gases is reduced a bit on the horizontal boiler.).
Of course, if you made a 3" dia boiler with more flues then you would have "spare boiler power" for the next engine!
In the down load section Sandys 3inch vertical boiler and burner drawings
He also set out all calculations on the mem forum
K N Harris books are available for free online. The calc for firetubes is incorrect . The correct formula is on Sandys drawings as the fire tubes are subjected to external pressure
T = (PD / (2S +P)) + 0.005D And for maximum safe working pressure: - P = S[(2T - 0.01D) / (D - (T - 0.005D))] Where: - S = The maximum allowable safe stress for the material . T = Thickness.
Good luck
Steamchick and fceslop: Great information, thanks. Yes London is only 1/2 hour away and coffee shops on every corner. It's Canada Eh! Winter is going away here too. Was up to 15C here to-day but thats unusual We'll be back below 0C in the next few days. Going to put the Stuart away for a bit. Have finally recieved parts for 1965 Honda S600 Sportscar that I am restoring. Parts came from Germany! Pretty rare car here ,I think the S800 was available in greater numbers on your side of the pond.I shall file away info on the boiler and keep my eye out for anything that comes available. Stay well &warm Regards Colin
yes, the Harris book is a must.

the bore and stroke of the high pressure cylinder only will give you the required amount of steam per cycle, as it is a triple expansion engine, knowing this figure, you calculate the amount per min according to the rpm, about 500 for such large model. this gives the amount of water which has to be evaporated par min at the pressure you need, something around 5 -6 bar (75-85 psi) to fully benefit the triple expansion of steam.
this last data give the heating surface of the boiler, and its size, which would be rather man, and coal heated.
a beautiful engine requires a boiler that doesn't make her ashamed.

of course your engine would run with a plain pressure cooker too, it is a matter of personal choice...
Hi Colin,
Plenty of us can make boilers (for material costs I guess?) if you don't want to invest in silver soldering tackle, etc. but then some of us enjoy using those skills. Making the fittings is a good exercise, especially making a safety valve - which should be to a proprietary design (for safety!) - as it needs to be precisely made, which it what you obviously enjoy doing.
yes, the Harris book is a must.

the bore and stroke of the high pressure cylinder only will give you the required amount of steam per cycle, as it is a triple expansion engine, knowing this figure, you calculate the amount per min according to the rpm, about 500 for such large model. this gives the amount of water which has to be evaporated par min at the pressure you need, something around 5 -6 bar (75-85 psi) to fully benefit the triple expansion of steam.
Fair comment about pressure. But many users - especially when building their first - want a quick, simple, if temporary expedient to problems while they work out the next step. I have made nearly a dozen boilers, because every engine needs a different one. Some have been re-builds of boiler material from potentially lethal boilers! So a pressure cooker is a possible - and reasonable safe - steam source that will idle a good model.
I have a twin compound (similar size), that takes 20psi to run as a single when warming the 2nd cylinder, but after 10 or 15 minutes the second cylinder starts to work, when the engine accelerates, and steam can be dropped to 15 psi for idling. I run it at club shows using a 3 inch horizontal Cornish boiler, and 2.5kW ceramic burner. Usually turned well down because the engine idles.., does no work.
Of course, for the compound to do real work - such as drive a boat will take a few more kW of input heat and higher pressure.
But why coal? Unless you are sitting next to the boiler, as with stationary plant or locomotive, it is a lot of clart to avoid the boat running out of fire -in the middle of the pond?
But good ideas.
Also, I think my engine is the same 3/4" bore and 1" stroke for the HP cylinder as Colin's engine. Not so big compared to my friend's 1 1/2" bore and 2" stroke single that needs 40 psi to run, as it hasn't run in yet.
a coal burner simply because it is more realistic, particularly with a "high level" engine as the triple expansion, nothing can beat a roaring bed of red glowing coal...

but obviously at shows and get together, I prefer a butane burner, far more practical, and allows to chat with visitors !