Monotube Flash Boiler Design

[Toymaker]

Exactly what I see - - - - when I click on either the 'see full' or 'download' button I am directed to the either 'log in or sign up' page.

OK, well, since that page allowed me to download the pdf, I don't think I'm violating any copyright laws by attaching it to this post.

 

[ajoeiam]

OK, well, since that page allowed me to download the pdf, I don't think I'm violating any copyright laws by attaching it to this post.

Thanks.

 

[Steamchick]

Hi Toymaker. Wth your interesting background I understand better why you are making this turbine, but with your experience had you considered making a gas turbine? I am part way through another biography of Whittle (the latest, I think the 3rd I have read).. and the trials and tribulations he experienced making his jet engines. You are having an easier time than he did! He also was surrounded by "doubters" who simply did not really appreciate how good his design work was.... guess you understand that situation.
K2

 

[Toymaker]

Hi Toymaker. Wth your interesting background I understand better why you are making this turbine, but with your experience had you considered making a gas turbine? I am part way through another biography of Whittle (the latest, I think the 3rd I have read).. and the trials and tribulations he experienced making his jet engines. You are having an easier time than he did! He also was surrounded by "doubters" who simply did not really appreciate how good his design work was.... guess you understand that situation.
K2

Yes, I have considered designing and building a gas turbine, and my knowledge of what would be required to complete such an endeavor prevents me from pursuing that idea.

However, for a previous hobby, I have designed and built a few accessories for existing turbines, such as the UDF shown below. The white blades in the photos below are what is known as a Propfan or a UDF (Un-Ducted Fan); they produce far more thrust than a traditional propeller and don't have the drag of a typical jet engine's ducting. The engine is from a T62 apu (auxiliary power unit) and is rated at a conservative 80 SHP (Shaft Horse Power);...in practice, these little engines would easily put out 100+ SHP. Blades were an epoxy composite structure made from layers of fiberglass and Kevlar covering the 7075 Aluminum root-spar.

 

[HMEL]

I'm enjoying following this discussion, and like K2 am learning lots.

But a small but I hope relevant point- as far as I know a copper boiler doesn't normally fail catastrophically, but relatively gracefully (I do emphasize relatively gracefully- we're talking milliseconds instead of microseconds) with the copper tearing rather than fracturing. This allows the pressure to collapse (again) relatively slowly.
Unlike steels which tend to fracture and drop the pressure almost instantaneously, usually with catastrophic results. IE, the entire body of fluid is suddenly at close to atmospheric pressure and can no longer be a liquid, resulting in a big bang!

The severity of failure and its rate depends on the operating pressure and temperature not the material. The material properties determine at what state failures occur. Steel boilers operate at much higher pressures and carry considerable more working fluid in them. I have seen copper tubing explode violently. Structural failures are instantaneous once the yield points are crossed. Not only is the working fluid released violently but valves, anything associated with it and shrapnel are also coming your way. Many heat exchangers use copper alloys and a great deal of care goes into their design and selection especially the fired ones.

 

[Toymaker]

The severity of failure and its rate depends on the operating pressure and temperature not the material. The material properties determine at what state failures occur. Steel boilers operate at much higher pressures and carry considerable more working fluid in them. I have seen copper tubing explode violently. Structural failures are instantaneous once the yield points are crossed. Not only is the working fluid released violently but valves, anything associated with it and shrapnel are also coming your way. Many heat exchangers use copper alloys and a great deal of care goes into their design and selection especially the fired ones.

Do you have any details concerning the copper tube explosion you witnessed; tube diameter, pressure, temperature? Pictures of the aftermath?

I believe it's also important to mention that boiler type & design have a large impact on how destructive a rupture failure becomes. When a drum or tank style boiler ruptures, all the steam is released instantaneously, whereas when a monotube ruptures, steam located many feet or meters distance from the rupture take a few milliseconds to travel the length of the tube to escape. As a result, many steam car clubs prohibit drum or tank style boilers from operating during events where people are nearby, such as public shows; only monotube boilers are allowed to operate as they are deemed safe to operate in a public setting.

 

[HMEL]

Do you have any details concerning the copper tube explosion you witnessed; tube diameter, pressure, temperature? Pictures of the aftermath?

I believe it's also important to mention that boiler type & design have a large impact on how destructive a rupture failure becomes. When a drum or tank style boiler ruptures, all the steam is released instantaneously, whereas when a monotube ruptures, steam located many feet or meters distance from the rupture take a few milliseconds to travel the length of the tube to escape. As a result, many steam car clubs prohibit drum or tank style boilers from operating during events where people are nearby, such as public shows; only monotube boilers are allowed to operate as they are deemed safe to operate in a public setting.

No dont have any pictures and the situation was an over pressure occurred due to an operational problem. Copper tubing can take surprisingly high pressures at low temperatures. And it would have normally been ok. But if you look at the limitations of brass valves you will get an idea of the temperature threshold that copper and copper alloys can not exceed. But the point is the copper failed catastrophically and violently.

I learned not to take pictures unless I had instructions to investigate an accident. Lawyers really like that stuff and they even like to review your notes if they know you have them.

But the point is every system has a weak point and monotube boilers are no different. What most likely happens with monotubes is poor water quality and high solids build-up. Keeping the boilers small and with low working volumes helps but they can still burn the hell out of you. There are two general types of failures those on the steam side and those on the fire side. I have seen both. You can not just wave your hand and say because I have this system I am safe. Your operational parameters must be within good engineering limits. We have a long history of boiler safety in this country because we have a long history of boiler disasters in this country. I have some old photographs of engines and boilers that leveled buildings and anything around them.

However, I consider model boilers and older antique systems a special animal as they usually require someone who really knows the system. But I have also seem some designs I would not stand anywhere close to them.

 

[Steamchick]

HMEL,
I too have dismantled a copper boiler and re-used the material (in a different design - silver soldered) because I could see where the original maker had continually developed leaks in a riveted external flange joint, and with cross-tubes in a firetube (Cornish Boiler style) because the whole thing had been overstressed through operating at his idea of the pressure need to run the boat, not a NWP developed from calculations based on the material sizes he had used.
I have also repaired 2 boilers and de-rated a half-dozen or more, that were not fit for their "in service NWPs" in models. All I have really done is change the NWP from "what they used" - with a factor of safety in some cases of just over 2... - to a Factor of Safety of at least 8 - by calculation.
In another case a boiler "supposed to be for a NWP of over 100psi" wasn't "safe" by calculation but needed de-rating to less than 55 psi to get a FOS of 8. It had been "made by a friend" who increased sizes from the design of his boiler (from 5" diameter to 6" diameter) without increasing ANY material thicknesses. I had to disappoint the new owner as he needed over 80 psi for his model... Another was de-rated from 45psi NWP to 7 psi NWP... so none of the owner's models would run with that boiler! It needed stays at 1/2" spacing, not the 1 1/2" actually fitted.
Yet another was made by someone ( a college lecturer) because he found some "interesting material" and decided to make a complicated water-tube design with it. That was de-rated from his proposed NWP of 45~60psi down to 12psi... to get the FOS= 8 by calculation of the "new material". 60+ water tubes silver soldered into 3 drums for a boiler that will not run his models...
A guy at the local club - very recently - had his boiler distort when a hydraulic test at 1.7 x his NWP was applied. It was supposedly to a "standard" design, but was made to twice the linear sizes of the drawing, using some materials that were different thicknesses. £200 of materials and months of work has made some pretty scrap.
So many boiler makers have wasted efforts - or put themselves at high risk due to failures (some had actually leaked!) because they did not understand the strength of their materials. All this shows what amateurs can do.
But you have an excellent understanding of material strength, (better than I) so for a powerful machine as you are designing, containing a gas that can be very dangerous if it leaks or overheats, all I am suggesting is that you increase your factor of safety to something that Regulators recommend.... For your safety.
Perhaps I simply feel the point made in the article you attached in post #8: I.E.
"Although the supercritical Rankine cycle can obtain a better thermal match than the organic Rankine cycle, the supercritical Rankine cycle normally needs high pressure, which may lead to difficulties in operation and a safety concern." - Is your 500psi (34Barg) the "supercritical" design? - I guess not?
Guessing that you have at least 75cu,in of liquid (about 1.25 litres? - Maybe my sums are wrong?) in the tubes in the firebox, at 500psi, have you calculated how much space the vapour would occupy if a catastrophic release occurred? But this is perhaps irrelevant? - "It must not leak or fail" is the design objective.

It does appear to me that the safety of your system is similar to the safety of refrigeration plant in millions of homes, except they are cooler and use steel tubing, not copper. Also similar in "stored energy" terms to a regular model boiler, at maybe 50 "bar litres" (a similar order of magnitude to a 7 1/2in gauge locomotive?). Although the power (I seem to recall 80kW?) of your system is more like a Domestic car at approaching full power!
So are we too sensitive to Safety? I think "not", but that is my opinion.. Just for the domestic refrigerator and the family car a hell of a lot of Design and Testing work goes on to make these things as safe as they are in service. And notwithstanding, some manufacturers have made mistakes and the products have hurt or killed their owners. Proving that "To err is Human".
K2

 

[Gary C]

Soft correction purely for clarification.

"the brake and clutch fluids are above their flash-point, so instantaneously ignite. No spark, flame, etc, just a spray of fluid onto pure temperature of hot metal... And the spray ignites."

In the above analogy, the correct term is "ignition temperature" or auto ignition temperature or spontaneous combustion.

Flash point is: "the lowest temperature at which a liquid gives off sufficient vapours to cause an ignitable vapour/air mixture, which can then be ignited from an external source of ignition".

Kind regards

 

[Steamchick]

Soft correction purely for clarification.

"the brake and clutch fluids are above their flash-point, so instantaneously ignite. No spark, flame, etc, just a spray of fluid onto pure temperature of hot metal... And the spray ignites."

In the above analogy, the correct term is "ignition temperature" or auto ignition temperature or spontaneous combustion.

Flash point is: "the lowest temperature at which a liquid gives off sufficient vapours to cause an ignitable vapour/air mixture, which can then be ignited from an external source of ignition".

Kind regards

Thanks Gary. Glad someone is correcting my errors.. It was 35 years ago-ish... and the crash test Engineer was explaining the reason behind various "safety features" - such as the location of various components in "safe" zones. E.g. why some cars have a remote hydraulic fluid reservoir, instead of mounted directly on top of the Master cylinder. Probably irrelevant for Toymaker's design....
Except a "Cautionary note" if the boiler tubing should ever leak or fail. The R123 fluid would instantly vaporise and see burner metal and flame and gases above the chemical breakdown temperature where it produces HF and HCL gases. I don't know if it would be exothermic or endothermic.
K2

 

[Richard Hed]

Thanks Gary. Glad someone is correcting my errors.. It was 35 years ago-ish... and the crash test Engineer was explaining the reason behind various "safety features" - such as the location of various components in "safe" zones. E.g. why some cars have a remote hydraulic fluid reservoir, instead of mounted directly on top of the Master cylinder. Probably irrelevant for Toymaker's design....
Except a "Cautionary note" if the boiler tubing should ever leak or fail. The R123 fluid would instantly vaporise and see burner metal and flame and gases above the chemical breakdown temperature where it produces HF and HCL gases. I don't know if it would be exothermic or endothermic.
K2

I have a question I have been pondering for a long time that hyou may be able to help with:

If hyou have a tube (all of this is just for argument sake and all made up from nothing) that, say has 100,000 psi tensile strength, has 1/10" walls and 1" diameter and is infinitely long, one might consider tha5t the total pressure of a 1 psi pressure inside would be infinite against6 the walls. Obviously this is misleading. So if we reduce this tube to say, a mile long with the same psi, the total pressure against the walls would be substantial, however, it is not this total pressure against the walls that is important, it is, rather, a length related to the 1" diameter that will make the important dimension that we need to know about. I have always thot that given a 1" diameter, that the important length would be 1" in length for making the calculations for the pressures the materrial will bear.

Can you guide me to the correct thimpfking?

 

[Steamchick]

H Richard after a quick read, I smiled because I think I understand your error. You are mixing forces and pressure. So you are considering length, which is not needed when comparing pressure and stress in the material. An infinite force derived from 1 psi x an infinite length, divided by an infinite cross section of material does not compute... in my head anyway. The pressure is 1 psi whatever the length. The force increases with length. The critical stress is Called hoop stress. Effectively the force resisted in a ring of the tube wall, divided by the cross-secton of that ring material . Your 1 inch of tube is an example of a ring. Toymaker taught me in an earlier post that hoop stress is calculated by Barlow's formula. I just knew it as the hoop stress calculation. Wikipedia explains that much better than I can, my explanation confuses me, so I suggest you read that first , then we can answer any other questions.
Is that OK?
K2

 

[Richard Hed]

H Richard after a quick read, I smiled because I think I understand your error. You are mixing forces and pressure. So you are considering length, which is not needed when comparing pressure and stress in the material. An infinite force derived from 1 psi x an infinite length, divided by an infinite cross section of material does not compute... in my head anyway. The pressure is 1 psi whatever the length. The force increases with length. The critical stress is Called hoop stress. Effectively the force resisted in a ring of the tube wall, divided by the cross-secton of that ring material . Your 1 inch of tube is an example of a ring. Toymaker taught me in an earlier post that hoop stress is calculated by Barlow's formula. I just knew it as the hoop stress calculation. Wikipedia explains that much better than I can, my explanation confuses me, so I suggest you read that first , then we can answer any other questions.
Is that OK?
K2

Ah, well that is quite simple enough, except for sigma, the allowable stress. If I am right, allowable stress is related to tensile strength.

 

[Toymaker]

Ah, well that is quite simple enough, except for sigma, the allowable stress. If I am right, allowable stress is related to tensile strength.

Yes, allowable stress can refer to tensile strength (or Ultimate Tensile Strength), but it can also refer to Yield strength, which is actually more important then UTS. When the Yield strength of a metal is exceeded, the metal is permanently distorted, like when you pull on a spring and stretch it so far that it never goes back to it's original shape. When the Yield strength of a boiler is exceeded, but not so much that it explodes, the boiler walls have been stretched, making them permanently a little thinner. Each time the boiler's Yield strength is exceeded, the walls get a little thinner, until one day the walls are so thin they break.

 

[Toymaker]

H Richard after a quick read, I smiled because I think I understand your error. You are mixing forces and pressure. So you are considering length, which is not needed when comparing pressure and stress in the material. An infinite force derived from 1 psi x an infinite length, divided by an infinite cross section of material does not compute... in my head anyway. The pressure is 1 psi whatever the length. The force increases with length. The critical stress is Called hoop stress. Effectively the force resisted in a ring of the tube wall, divided by the cross-secton of that ring material . Your 1 inch of tube is an example of a ring. Toymaker taught me in an earlier post that hoop stress is calculated by Barlow's formula. I just knew it as the hoop stress calculation. Wikipedia explains that much better than I can, my explanation confuses me, so I suggest you read that first , then we can answer any other questions.
Is that OK?
K2

Along with the magnitude of a force (ie 20 lbs) there is also a direction. When you step onto your bathroom scales in the morning to weigh yourself, the direction of the force your body is applying to the scales is down, towards the center of the planet. But the direction of the force a pressure exerts is in all directions at once. Think of blowing up a balloon. The balloon expands in all directions, because the force is being applied uniformly in all directions.

 

[Steamchick]

Hi Richard,
I have a few moments to expand on your questions.Barlow's Formula relates the internal pressure that a pipe can withstand to its dimensions and the strength of its materials. The formula is P= (2*T*S/D), where: P = pressure. S = allowable stress.

Now the allowable stress is an odd one.
0.1% Tensile stress is a good number to use for copper - I think, maybe others know better - but in a job in the 1970s all the design work with aluminium (anneal 99%pure electrical grade and various structural alloys and tempers) we used 0.2% proof stress.
https://blog.thepipingmart.com/metals/what-is-annealed-copper-properties-uses-and-composition/
Temperature affects Copper drastically, for boilers especially.
https://blog.thepipingmart.com/meta...res increase, the tensile,—a decrease of 85%.The strength of copper is determined by its tensile strength, which is measured in pounds per square inch (psi). As temperatures increase, the tensile strength of copper decreases significantly. At room temperature (70°F), copper has an average tensile strength of 24,000 psi. When heated up to 500°F, however, that number drops to just 3,500 psi—a decrease of 85%. This means that heating up copper causes it to become extremely weak and brittle.

So ASME limit silver soldered copper boilers to 100psi Normal Working Pressure, and their Regulations for boilers derive a max stress of 3142psi for copper used in a boiler at 100psi NWP. (Kozo Hiraoka explanation in an article in Live Steam and outdoor railroading). Whereas the permissible strength is 6700psi at below 100degrees F.
Now I have a concern that the Tensile strength of Copper is around 230MPa (lowest published value I have seen from the Copper industry), but minimum quoted in COMPRESSION is ONLY 45 MPa... So when the tube is subjected to internal pressure, Barlow's formula gives one answer, but if used in COMPRESSION with External pressure (e.g. flue tube and fire tubes) then the lower strength of 45MPa gives a much lower permissible NWP for the same tube. So with Copper particularly, you must be aware of how it materially fails in Tension or Compression and due to temperature at the pressure you want to apply.
Toymaker is the expert, not I, so I am sure if I have made errors he - or someone else - can teach us all the right answer. I have just fudged my way to some understanding so I can design model copper boilers...
Cheers,
K2

 

[Richard Hed]

Hi Richard,
I have a few moments to expand on your questions.Barlow's Formula relates the internal pressure that a pipe can withstand to its dimensions and the strength of its materials. The formula is P= (2*T*S/D), where: P = pressure. S = allowable stress.

Now the allowable stress is an odd one.
0.1% Tensile stress is a good number to use for copper - I think, maybe others know better - but in a job in the 1970s all the design work with aluminium (anneal 99%pure electrical grade and various structural alloys and tempers) we used 0.2% proof stress.
https://blog.thepipingmart.com/metals/what-is-annealed-copper-properties-uses-and-composition/
Temperature affects Copper drastically, for boilers especially.
https://blog.thepipingmart.com/meta...res increase, the tensile,—a decrease of 85%.The strength of copper is determined by its tensile strength, which is measured in pounds per square inch (psi). As temperatures increase, the tensile strength of copper decreases significantly. At room temperature (70°F), copper has an average tensile strength of 24,000 psi. When heated up to 500°F, however, that number drops to just 3,500 psi—a decrease of 85%. This means that heating up copper causes it to become extremely weak and brittle.

So ASME limit silver soldered copper boilers to 100psi Normal Working Pressure, and their Regulations for boilers derive a max stress of 3142psi for copper used in a boiler at 100psi NWP. (Kozo Hiraoka explanation in an article in Live Steam and outdoor railroading). Whereas the permissible strength is 6700psi at below 100degrees F.
Now I have a concern that the Tensile strength of Copper is around 230MPa (lowest published value I have seen from the Copper industry), but minimum quoted in COMPRESSION is ONLY 45 MPa... So when the tube is subjected to internal pressure, Barlow's formula gives one answer, but if used in COMPRESSION with External pressure (e.g. flue tube and fire tubes) then the lower strength of 45MPa gives a much lower permissible NWP for the same tube. So with Copper particularly, you must be aware of how it materially fails in Tension or Compression and due to temperature at the pressure you want to apply.
Toymaker is the expert, not I, so I am sure if I have made errors he - or someone else - can teach us all the right answer. I have just fudged my way to some understanding so I can design model copper boilers...
Cheers,
K2

Well, that's really something: you have PTSD!

 

[Toymaker]

<snip>
Toymaker is the expert, <snip>
Cheers,
K2

I appreciate your vote of confidence, but I'm just an old electronics engineer, muddling his way through mechanical engineering and trying his best to use what little he learned about metallurgy in his previous jobs. UTS and Yield Strength are also very important factors when designing turbine blades, but instead of internal pressure being the important factor, it's simple tensile stress from the centrifugal force of the spinning blades.

 

[Steamchick]

Sorry Richard, I am not in touch with the acronym... PTSD. Does it mean "passed the "sad" diploma"?
Or "stupid"?
Or perhaps Post Temperature-Stress Development"?
K2

 

[Toymaker]

Sorry Richard, I am not in touch with the acronym... PTSD. Does it mean "passed the "sad" diploma"?
Or "stupid"?
Or perhaps Post Temperature-Stress Development"?
K2

Google: "acronym PTSD"