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K2, do you have a reference for this data? I've never heard of a metal having compressive strength that is 20% of the tensile strength. Metals (especially ductile metals like copper) have nearly equal tensile and compressive strengths, in my experience. I'd like to learn more about this phenomenon.

Cylinders collapsing when they "shouldn't", on the other hand, I have lots of experience with. That happens because they don't fail in yield; they fail under elastic buckling. And elastic buckling is very sensitive to geometric imperfections.

Anyway, if you can point me in the direction of that strength data for copper, I'd be grateful.


Hi Carl,
I'll try and show some stuff...
Copper reduces strength with temperature increase... e.g. at 200C compared to 20C. :
But this data table quotes Compressive strength and Tensile strength ranges...
In this table, the compressive strength of copper is defined as a minimum of 45MPa. And a max of 330MPa. These figures are for everything from fully annealed to hardest condition at room temperature. (20C).
The Tensile strength is quoted as 210MPa to 380MPa - under the same conditions:
Therefore I deduced that the "worst case" for Annealed copper is the compressive strength of 45MPa in the same condition as tensile strength of 210MPa. : I.E. Compressive strength of 21.4% of the Tensile strength.
This correlated with the University graph I had previously studied where the Compressive strength versus Tensile strength at 200C followed a similar value.

ASME Code Table 1B of Section 2, part D, requires the "limiting stress" in tension at 400deg.F. to be 3000psi.
Considering the "Compressive strength" (worst case at 21.4% of Tensile strength) I therefore use the limiting factor of stress for tubes in compression to be 898psi.
Of course, you will note that I have not considered any variation of tube wall thickness, deformation, ovality or other factors that are major contributors to failure of tubes in compression (Uniform external pressure). But I do consider the ASME Stress Concentration Factor "standard value" of 3.3 whenever there are any penetrations of the tube - such as a firing hole, hole for cross-tubes in a flue or fire-tube, etc. as every discontinuity of stress (such as a hole = weakening, or reinforcement = strengthening) becomes a focal point where compression failure is likely to be initiated.
I agree that "once initiated" the tube fails in Buckling. It happened to me on one Hydraulic test, where I was going to 4 x NWP, and the tube collapsed at about 3.8 x NWP. My pre-test calculations had used the "tensile strength limit" for hoop stress, so I should have been OK for more than 8 x the NWP, but I suspect a deformity of the tube initiated the failure. However, that is when I found the "Min Compressive strength" to be so much lower than the tensile strength... and believe this to be significant. (Is it? - I really am not sure if I am right or wrong?).
But a "better educated Engineer" than I can explain if I am right or wrong? (I have a Physics degree, which is NOT the same as an Engineering degree!). I am glad of your expertise and input. (I sometimes confuse "Apples and Bananas" on subjects where I am not an expert!).
Hello guys, back again. How much water should be in my boiler, I know it’s a bit of a silly question but I meant volume wise in relation to the total capacity and where to site the water level gauge.

best regards sutty
Hello guys, back again. How much water should be in my boiler, I know it’s a bit of a silly question but I meant volume wise in relation to the total capacity and where to site the water level gauge.

best regards sutty

Water level in boiler is in generally 2/3.
Hi Sutty. I suggest max water level (below the top of the gauge glass) at least 2 cm below the steam outlet bush. To minimise Priming (Unwanted water carried over with the steam!). The bottom of the gauge MUST BE ABOVE the highest bit of silver soldered joint... I guess the top plate of the firebox. Maybe 1 cm or more....
In normal use you must be able to see the meniscus between steam and water. So the glass must have marks for top and bottom levels, and be longer than the water level difference.
Any boiler that is allowed to drop the water level to the silver soldering at the top of the firebox is likely to fail, by heating the silver solder so it can't withhold the steam pressure. Then blows away !

Cheers Ken, and does the glass tube diameter have to be a certain size ?

regards sutty
Yes, glass tube has to be capably of safely withstanding the NWP. I use 5mm, or 3/16 in OD, but the tiny bore can cause mis-readings due to surface tension, meniscus effects..... If fitted from the side, above water level, to side, below water level with direct connections in bushes, then just make sure the connection holes are at least as big as the glass bore. But I have some boilers, where the top it ting is from a bush in the top of the boiler, or bottom fitting is a longer tube so the gauge is more remote for the boiler. These always need a parallel tube alongside the gauge to stabilise readings.
I'll take photos tomorrow.
Hi Sutty,
Some photos of boiler water gauges:
The first is a 5" vertical boiler I "inherited" to fix-up for the local club to use - as a regular "test anything" boiler. (NWP 30psi)

The gauge (second photo) is affixed to 2 bushes in the side of the boiler shell.
This boiler is similar to yours with a firebox surrounded by a water jacket, and the bottom water-gauge bush is above the top-plate of the firebox.

The 3rd photo is of a horizontal loco boiler, 3" dia, the gauge being wholly above the top of the firebox (above the top of the firing hole door).

4th Photo is of a 3" dia "tank" boiler, with fire beneath the tank, so the water gauge is "where it is" - I guess without much thought of where the water should be? This only allows the boiler to be filled just over half way, so there is a large steam space. (Ullage). But the bottom of the water gauge is not far below the mid-line of the boiler, so almost all the base exposed to the fire beneath will be kept wet if the water level is on the gauge.

5th is another 4" dia vertical boiler I was asked to "fix-up" for the club, but this is only good for 15psi, so is limited in application. This is during a steam test. You can see the water gauge - situated so the boiler is filled at maximum about 2/3rds full, leaving plenty of steam space, and the bottom of the gauge is above the fire-box top-plate.

6th is a 2" dia vertical boiler (very low performance as it cannot take much of a burner in the 2" dia circle!). The water gauge is situated for about 2/3rds fill. You can see the white background with blue diagonal lines. When looking through the glass at these lines you can clearly see the water level, as when full of water the lines change direction of slope. But the lined background must be at a suitable distance from the glass.

Finally a lovely looking large boiler (Not mine!) showing location of the water gauge: Again this looks to control water level around 2/3rds full. The bottom connection is above the firebox top-plate. The top connection is in the ullage, level with the gauge connection bush. This appears to be a fairly standard practice, allowing plenty of space for the water "splash" to dissipate before the steam take-off bush on the top boiler plate.

Finally, a boiler that shows that the world doesn't have to conform to anything straight and true! It even uses a round "port-hole" glass for a water gauge!
Wonky steam engine.JPG

Hope these are useful?
Hi Sutty, I forgot to mention that all bar the last boiler use 3/16" gauge glass.
Although NOT in Regulations, I believe that the Hydraulic Test should be with gauge glasses fitted and proven to take the pressure. But I think it is at the discretion of the Tester?
Cheers Ken , changed the drawing to suit already, I'll post a pic in a bit for your views.

Regards sutty
Ken, hope we're not being a pain, here's a modded drawing. The bushes are all phosphor bronze and everything will be silver soldered, we're upgrading the firebox to 10g, there are 8 cross tubes 3/8" 16g the top and bottom boiler caps will be 10g , the flue will still be 1'' x 16g


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No pain here! (Just toothache keeping me awake!). Good stuff! Confirming the calcs based on 10g (2.588mm according to the web...) firebox tube: OK for 26psi. But that is the NWP limit! - So I suggest the NWP declared at 25psi, Safety set to fully blow by 26.5psi - if you can be so accurate...
You should also have a BLOW_DOWN VALVE, situated a low as is sensibly practical. Maybe in the foundation ring, or low down in the firebox water jacket = Another bush to be added! This is for emptying the boiler of water after use, and is usually a simple screw-down valve. BUT, do not use a domestic radiator bleed valve as anything like that will blow "HOT STEAM" directly onto your fingers when you open the valve. It needs to be a valve with a pipe so the water (which becomes STEAM when the pressure is released to atmosphere) can be ejected AWAY FROM PEOPLE & PETS.
This is the one bush/valve that is missed on most of the small boilers I have (Because I didn't know!) and "made by others" that I have repaired/re-commissioned.
ALSO... you will need to plan for a STEAMDRIER/SUPERHEATER in the steam you collect from the top of the boiler. Explanation: Steam at 25psi that is from a surface of water will be "saturated" - I.E. the slightest expansion will cause a slight pressure drop, and to expand the steam it need energy, which cools it slightly, and makes water droplets. This is the "white-stuff" you see from a kettle when boiling. But as the steam passes down the pipework to the engine, through the valve chamber and passages into the cylinder, there is a further expansion cooling and pressure drop, so by the time you get to the cylinder you have lost a significant amount of POWER. Also, the "Wetness" of the water vapour in the steam becomes a quantity of water in the cylinder(s) of the engine, added to which the steam will turn to water when it does work on the piston. So "wet" steam from a boiler can provide a lot of water in an engine, which doesn't have much room at the end of the exhaust stroke to be able to cope with water when the exhaust valve is closing. Too much water will cause an Hydraulic lock - which can stop an engine DEAD, and the rapid deceleration of all the bits will probably break something, or at least damage bearings, etc. So to reduce the "water" in the engine, the "drier" or "superheater" is pipework carrying steam from the boiler, through HOT flue gases to raise the TEMPERATURE of the steam, so it can carry more energy towards the engine. Then the energy that is lost "in transit" doesn't become "water vapour" and cause problems in engines, and more energy is available at the engine to do work. A single loop of small pipe going from the boiler outlet, down the flue to where the flue gases are hotter, then back up and out to the boiler, will dry the steam adequately. On some boilers, I have used a coil of tubing in the flue, but it restricts the flue considerably and therefore reduces the size of burner I can use...
If you use stainless steel pipe or something similar, you can feed the pipe down the flue and through the firebox. (Don't use copper as it will erode quickly and eventually leak steam!). Tubes fed through very hot gases from the fire are superheaters - and can add huge amounts of power from the fire to the steam that reaches the engine. BUT the material must be able to withstand the very high temperature for the fire.
Hope this helps?

Ken, enormous help, thank you. I've added a couple more bushes to the design, one for a blow down valve and one for a water feed . I've also designed the super heater tube to run down the flue and back again from 6mm St St tube with stainless elbows through the flue.
The burners are nearly finished but I don't think we are going to use them, it was a nice little machining exercise for Edd, I read in K N Harriss's book about
a vapourising burner so we've drew one up and I'll get the material ordered.
brazing tuition next.

Best regards sutty


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I have some "home Made" vaporising burners of various sort, but really, Gas (camping gas cylinders) are suitable and much safer. Did I send you some information on Ceramic Radiant burners?
Cheers Ken, yes and they look good but I want this exercise to be two fold, 1/ to get him interested in engines (all kinds) and 2/ to get him used to the making things bit.
I tried to get him some Balsa wood model plane kits, you know the ones I mean, the ones you build on the plans then tissue and dope to skin them. When I went to both of our local model shops they just laughed, so we've now gone this route, he's slowly getting interested we knocked the burners up this morning, a bit of drilling on the miller with a dividing head and a bit of lathe work, I don't think he even noticed the geometry involved.

Thanks for all your help, I'd probably be demolishing my shed without it. Best regards sutty
Ask about anything and I'll try and answer. I made a couple of balsa and tissue planes..., when that age, and that followed making 2 boats in Balsa. Both panes flew, but I was more into making than flying/crashing/rebuilding. My Grandfather helped me make bits to "Improve" my bought plastic track cars when I was still in single figures, so I knew what a thread was before I started metalwork in school at 11.
And I still remember some of it! That's the age to start!
Another thought. A pressure burner is a nice project. Just be VERY careful when lighting it. First, you have a small tank of fuel, and it is at pressure! Even making the pump is a challenge, never mind making a gas tight pressure tank. The tank should be hydraulically tested at 6 times the pressure that the pump can develop. Then when lighting, you have to heat the burner, with wet fuel and uncontrollable flames, before turning on the fuel supply. If you have included a controllable valve. Otherwise, you have a hot burner: Is it hot enough? Too hot? Then you have to develop pressure and pump fuel through to the jet before it will ignite. If not hot enough, you'll get intermittent bursts of wet fuel blasted through the jet causing 3 foot long yellow balls of flame! WW2 flame throwers were based on this design! Get the burner too hot, and the fuel will pop and bang and self extinguish, filling the atmosphere around you with toxic fumes from broken down fuel molecules. Choking and bad headaches can result. And the clouds of gas from unburnt degraded fuel are explosive if you have another flame nearby.... (Yes, I have been there and not cooked myself. I regularly use both kerosene and petrol blow-lamps, but butane or propane gas is MUCH safer to use in a burner...).
A challenge to you, so please take care looking after your nearest and dearest Edd.
Ken, the type we were looking at doesn’t involve pressure or pumps, it’s shown in K Harri’s book page 99 plate 5.7. We can make it all from stock tube, there would be quite a bit of silver soldering on it, he’ll either get good or bored but all kinds of welding needs lots of practice.
keep the ideas coming though

regards sutty.


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Hi Sutty.
Yes, I have just looked at them. When you initially mentioned "vaporising burner" I thought you meant the one he calls a "Roaring burner". SO the difference in language between "Harris" and "Chicken" caused the mis-understanding. My cautionary tirade was all about "roaring" burners, in Harris parlance. (I call them Vaporising burners, because they have a vaporiser tube before the GAS jet - as per:
Of course, all burners vaporise the fuel, but the type you are making I should call a "wick" or "Wet" burner, as the flame is directly from the vapour above a "wicking zone". (This need not be cotton, asbestos, glass wool/fibres, etc, but can be simply a very fine stainless steel mesh! The fuel is drawn over a large surface area by surface tension, and vaporises because there is a lot of heat from a flame in close proximity.) As per:

I am sure you are following his advice re: fins on an inlet tube?

Here are some of mine...
Meths burner (Old, bought, good for cups of tea, no control beneath a 3" vertical boiler!)

Meths burner for 3" Horizontal boiler: A sheet of Meths flame rises from the wick, up inside the surface of the firebox, and without anywhere else to go passes along the length of the boiler shell to exhaust at the far end. (Good as far as it worked, but only used once, as re-fuelling was DANGEROUS, until it had cooled and I could withdraw it from the boiler! firebox. The flame was pretty good, but subject to wick size. Where it charred, the flame was reduced, so one firing was enough before I converted to GAS.

Next, a prototype (foil) and finished (wicked with glass fibre loft insulation!) for a MAMOD small 2" horizontal boiler with boiler side vents, not a directed flue. (As good as anything else I have used: But gas is more controllable).

Silver soldered tin-plate:

Finally, the Ceramic gas burner to replace the Mamod meths burners: