Monotube Flash Boiler Design

[Steamchick]

Hi Toymaker,
I think you have also proven that the "work hardening" by progressive hydraulic pressure applications does just a little for the strength of the boiler? I.E. your 1600psi burst to ~2000psi burst? = is only about 25% increase...
Different sources have different opinions of Copper strength, (see below~) but for "Boiler design" I use ASME... - even though I am in the UK. - because I could get the information "more easily" than British standards...
https://www.copper.org/applications...It consists of 99.9 percent,, and H04 (hard).https://www.matweb.com/search/datasheet_print.aspx?matguid=9aebe83845c04c1db5126fada6f76f7eK2

 

[Toymaker]

Hi Toymaker,
I think you have also proven that the "work hardening" by progressive hydraulic pressure applications does just a little for the strength of the boiler? I.E. your 1600psi burst to ~2000psi burst? = is only about 25% increase...
Different sources have different opinions of Copper strength, (see below~) but for "Boiler design" I use ASME... - even though I am in the UK. - because I could get the information "more easily" than British standards...
https://www.copper.org/applications...It consists of 99.9 percent,, and H04 (hard).https://www.matweb.com/search/datasheet_print.aspx?matguid=9aebe83845c04c1db5126fada6f76f7eK2

A 25% increase in strength isn't bad, and worth doing IMO.

I will use smaller steps for the boiler's hydro test and more repetition steps at 1000 psi.

 

[Toymaker]

Aha! This demonstrates the advocacy of ASME that use the UTS is de-rated to a MAX allowable (design) stress value for the copper at room temperature of 6700psi, which at 400deg. F becomes 3000psi...
When you insert your Copper tube sizes into the hoop stress formula, what stress is achieved for the burst at 21000psi internal pressure?
ASME has a formula, considering various factors:
Stress in tube , s = P/2 x (D/t-0.8)
Where:
s = Stress in tube, psi
P = internal pressure in tube, psi,
D = OD of tube, inches,
t = initial thickness of tube wall, inches,
and the "0.8" is a 2 x a temperature coefficient factor for non-ferrous materials of 0.4 for Copper... (Table 1, of ASME Section II part D.).
You may have to determine t from sectioning a sample of bent tube (after manufacture, before pressure testing), about at the location of the burst. (outer surface of bend).
Hope this helps?
K2

I think your Stress in tube formula is just Barlow's formula solved for s, along with a safety factor.

I was, still am, a little surprised the rupture occurred on the side side of the tube and not the outside bend where logic would place the thinnest wall. I can only guess there was a pre-existing defect in the tube at that location or perhaps I stressed that point with the torch when I annealed that area.

I'm quite pleased the tube was able to handle 2100 psi before bursting. Even de-rating for temperature, 2100 x 0.8 = 1,680 psi which is 3.3 times my planned 500 psi NWP. I would have been happy with just 1000 psi.

 

[Steamchick]

Hi Toymaker.
While I agree with you in principle,
In practice I would be wanting a Factor of Safety of at least 6....
As I have no experience of what could happen with a failure when using your large and powerful burner with the volume of refrigerant you are planning...
Boilers are rated by "Bar-litres" of steam, so taking the WHOLE volume of the system that is filled with liquid and vapour, mutiplied by your NWP of 500psi = 34Bar, you will determine the bar-litres of your system.
As this is "unbreathable" gas, if the heat stored in the metal and continuing burner heat was to expand a FULL system to gas at atmospheric pressure, what volume would the gas fill?
I guess the volume is somewhere in the 10 pages of the thread, but I don't know where to look.
GUESSING the boiler is 10l... the rest of the system another 5l. Your 15 litre system volume rated at 34Bar gives a system "capacity" of 510bar-litres, so you potentially have over 500litres of gas at atmospheric pressure.
= a room 2.5m high, and 200sq.m area or about 14m square. That's a lot of space to fill with a suffocating gas. And if outside, you'll likely suffocate more than just you and your neighbour's dog...
So please consider improving the safety factor... and do your own risk assessment on the possibility of a full system of gas released rapidly through a burst joint or pipe wall.
This is huge compared to a model locomotive with a 20bar-litre steam boiler. Or maybe my sums are wrong?
Please do the sums to plan how you will survive a catastrophic gas leak?
I didn't ask. Will this gas extinguish the burner? - Or burn? I just considered the possible volume of breathable air that could be displaced, suffocating anyone present.
An interesting system this one!
K2

 

[Toymaker]

K2, I have to admire your consistency. You've been consistently against the use of any other working fluid other than water from the very start, and you're still against those working fluids today, despite their wide-spread use across the planet.
Does the UK have any cars or trucks running on LPG? They're quite popular here in Thailand as LPG is cheaper than gas or diesel. A single LPG tank in a car holds between 25 to 140 liters of highly flammable liquefied propane. Like R123, LPG is also unbreathable and heavier-than-air should a tank burst from a collision, so if the propane doesn't ignite and burn you to death, the gas will suffocate you. And still people drive around at freeway speeds every day of the year with far more extremely hazardous liquid/gas in their family autos than will be present in my boiler.

Yes, using any Freon in a boiler comes with potential risks, but these risks can be easily and safely managed. Part of my risk mitigation plan is use water in the boiler for all initial tests and until I'm satisfied the ECU is working properly and will quickly shut down any potential over temps or pressures.

Total volume inside the monotube boiler is 4.14 liters. I cannot imagine needing more than 15 liters total if you include the resiviour tank and condenser. Also, R123 doesn't boil until it reaches nearly 28 C (82 F), which is one of several reasons I chose this Freon; most leaks will likely be in a liquid state, which will then slowly evaporate.

The only way to get R123 to burn is under extreme pressure, and since any leak into the combustion chamber would result in instantaneous ambient pressure of the R123, the most likely outcome is the Freon extinguishing the burner's flame.

 

[Steamchick]

Thanks Toymaker, I am glad you have considered these factors.
I was exploring ideas, not trying to stop your project. Simply Freon gas is beyond my experience. The only thing approaching that was a factory where I designed equipment that held a huge volume of SF6 gas, which is highly corrosive with moisture, as in the air passages and lungs. It's as bad as Phosgene used to kill people in the First World War.... So I was taught to err on the side of caution and safety...
So you have 4.14litres of Freon (liquid, as it is under pressure?) in a container (the boiler and pipework to the turbine, condenser etc.). and the pressure is 500psi.
So I would simply rate this as a 141bar-litre "boiler".
So how much space would this gas fill if allowed to fill a closed space at atmospheric pressure? Say at 35C = not hotter than I have just experienced in Greece. (I don't know how hot you get where you are?). I am considering a "Gaseous state" as if the leak was inside the boiler chamber, where there is adequate heat from the burner and hot metal to vapourise the Freon.
I am not trying to persuade you to use water, but that is my only liquid-vapour boiler experience... that I am using just for comparison. When it comes to boilers, a boiler of 5L at 6 bar between your thighs (on a railway locomotive) is a common sight on the rail-track at my local club, so these things can be managed safely, even so close to the "family jewels".
Enjoy your project, even if I am asking stupid questions. I am only trying to learn from you.

Incidentally, I found an interesting hot oil burner...


And in the UK, the Propane tanks are steel, with an Hydraulic test pressure of 400psi.
Stay happy, and safe,
K2

 

[Toymaker]

Thanks Toymaker, I am glad you have considered these factors.
I was exploring ideas, not trying to stop your project. Simply Freon gas is beyond my experience. The only thing approaching that was a factory where I designed equipment that held a huge volume of SF6 gas, which is highly corrosive with moisture, as in the air passages and lungs. It's as bad as Phosgene used to kill people in the First World War.... So I was taught to err on the side of caution and safety...
So you have 4.14litres of Freon (liquid, as it is under pressure?) in a container (the boiler and pipework to the turbine, condenser etc.). and the pressure is 500psi.
So I would simply rate this as a 141bar-litre "boiler".
So how much space would this gas fill if allowed to fill a closed space at atmospheric pressure? Say at 35C = not hotter than I have just experienced in Greece. (I don't know how hot you get where you are?). I am considering a "Gaseous state" as if the leak was inside the boiler chamber, where there is adequate heat from the burner and hot metal to vapourise the Freon.
I am not trying to persuade you to use water, but that is my only liquid-vapour boiler experience... that I am using just for comparison. When it comes to boilers, a boiler of 5L at 6 bar between your thighs (on a railway locomotive) is a common sight on the rail-track at my local club, so these things can be managed safely, even so close to the "family jewels".
Enjoy your project, even if I am asking stupid questions. I am only trying to learn from you.

Incidentally, I found an interesting hot oil burner...


And in the UK, the Propane tanks are steel, with an Hydraulic test pressure of 400psi.
Stay happy, and safe,
K2

Your closed space volume question is purely academic. As you have pointed out, my project compared to model boilers is on the large side,...in fact, it's a car sized boiler and will never, ever be in a totally enclosed area. Thailand has car ports, not garages, so my project will always be operated outdoors. Still, I will try to answer your question of volume expansion from a liquid to vapor.

In the event of a boiler tube rupture resulting in the loss of all 4 liters within the monotube, the hot R123 gases would immediately expand dropping R123's temperature to room temp. So we're really looking at the volume difference between 4 L of liquid R123 vs 4 L of vapor R123 at room temp. (Actually, the density of R123 at 183 C and 500 psi is less than it's density at 34 C and near 1 bar pressure, resulting in less volume per kg, but I will use room temp volume/kg as a worst case condition)

One liter of R123 liquid weighs 1.44 kg at 34 C, so 4 L weighs 5.76 kg. (R123 Calculator)
At 34 C the volume/mass of R123 vapor is 0.126 cubic meters per kg, so for 5.76 kg the volume at 34 C would be 0.726 cubic meters. (from DuPont Thermodynamic Properties we've previously used)
Even if 20 L of liquid R123 were vaporized the volume at room temp would be 3.63 cubic meters, which is smaller than a typical bedroom closet.

Another reason to use Freons in small power generators is their low volumetric expansion rate compared to steam. While steam turbines need 10 or 20 rows of blades to extract all the energy from steam, (because steam expands a lot), turbines using Freons need only 3 rows max to allow the Freon vapor to return to room temp.

Finally, I like the oil burner,...very creative design that induces reverse flow of some of the combustion gases. I do wonder how long the soft steel he uses will last.

 

[Steamchick]

Thanks Toymaker.
Now I understand.... (I think). Anyway, I believe you. As you explained Freon works quite differently from water. That's what I didn't appreciate.
I agree that 3.63 cu.m of gas won't gas half the village!
And you expressed a concern some time back - and explained you had a lot of controls to prevent Freon exceeding 185deg,C (or something like that), to avoid temperature degradation of the gas into some more dangerous stuff....
Should a leak occur, and a jet of gas issued from the "boiler" onto hot metal of the burner, or into the hot exhaust gases, where would the gas be expelled - Up the chimney? Away from harm? What else could happen? I guess you have thought it through but I am curious, as without the bits I recall, I would have simply expected the gas to be burned in the exhaust...
I think your explanation of Freon explained something that puzzled me years ago.
Around 25 years ago, I had a car with air-conditioning, and the heat exchanger became damaged and leaked. I made a zinc repair on the aluminium, and had the system refilled, but a few years later it leaked again. So I went to the Scrap yard and bought a replacement from a similar car. The guy simply undid the connectors to the heat exchanger, and there was a loud "Whoosh" from the gas escaping rapidly! (he was inside a large workshop and I was outside). I sounded similar to opening the blow-down valve on an air compressor when emptying the receiver. I was surprised he wasn't gassed, but 10 minutes later, he appeared with the part, I paid him and left.
I assumed that the fill of gas was simply too little to affect him as it dispersed in the air?
Cheers,
K2

 

[Toymaker]

Thanks Toymaker.
Now I understand.... (I think). Anyway, I believe you. As you explained Freon works quite differently from water. That's what I didn't appreciate.
I agree that 3.63 cu.m of gas won't gas half the village!
And you expressed a concern some time back - and explained you had a lot of controls to prevent Freon exceeding 185deg,C (or something like that), to avoid temperature degradation of the gas into some more dangerous stuff....
Should a leak occur, and a jet of gas issued from the "boiler" onto hot metal of the burner, or into the hot exhaust gases, where would the gas be expelled - Up the chimney? Away from harm? What else could happen? I guess you have thought it through but I am curious, as without the bits I recall, I would have simply expected the gas to be burned in the exhaust...

R123 decomposes into other chemicals when its over-heated, and some of those chemicals are toxic. The two most common decomp chemicals are hydrochloric & hydrofluoric acids, which inside the hot combustion gases quickly react with the other products of combustion becoming far less toxic.

I think your explanation of Freon explained something that puzzled me years ago.
Around 25 years ago, I had a car with air-conditioning, and the heat exchanger became damaged and leaked. I made a zinc repair on the aluminium, and had the system refilled, but a few years later it leaked again. So I went to the Scrap yard and bought a replacement from a similar car. The guy simply undid the connectors to the heat exchanger, and there was a loud "Whoosh" from the gas escaping rapidly! (he was inside a large workshop and I was outside). I sounded similar to opening the blow-down valve on an air compressor when emptying the receiver. I was surprised he wasn't gassed, but 10 minutes later, he appeared with the part, I paid him and left.
I assumed that the fill of gas was simply too little to affect him as it dispersed in the air?
Cheers,
K2

Most likely your air conditioner used R-134a which is non-toxic except in highly concentrated levels, and it's non-flammable.

 

[TimTaylor]

https://refrigerants.com/wp-content/uploads/2019/12/SDS-R123.pdf
see specifically sections 5, 6, 8 & 10

The flame temperature of your burner will be well above the auto ignition temperature of R123. Also R123 will decompose at temperatures above 250C, generating HCl, HF, and carbonyl halides such as phosgene.

 

[Toymaker]

https://refrigerants.com/wp-content/uploads/2019/12/SDS-R123.pdf
see specifically sections 5, 6, 8 & 10

The flame temperature of your burner will be well above the auto ignition temperature of R123. Also R123 will decompose at temperatures above 250C, generating HCl, HF, and carbonyl halides such as phosgene.

Any R123 that is burned as a result of leaks into the combustion chamber is a good outcome. The safety data sheet doesn't say that burning R123 decomposes into HCl and HF, rather it states when the temperature of R123 is raised above 250 C, (inside a sealed container) it will decompose into HCl & HF.

HCl and HF are both highly reactive acids which at the high temps inside the combustion chamber, will quickly react with other chemical products of combustion, thereby decomposing almost as quickly as they are produced.
Phosgene decomposes at 250 C, meaning the combustion gases will instantly decompose it.

 

[Toymaker]

Boiler Hydrostatic Test:

Ran the hydrostatic test on the monotube boiler this morning and I'm quite happy with the outcome.

Started by quickly going to 100 psi to check of leaks,...none found.
Next, increase to 400 psi, hold and again look for leaks,...none found.
Release pressure and go to 450psi, release pressure and go to 500 psi, the NWP. Still no leaks.
Release pressure and go 520 psi, release pressure then go to 540 psi, release pressure and go to 560 psi, etc., etc, increasing pressure by 20 psi steps all the way up to 700 psi. At 700 psi I began carefully checking for signs of ballooning at all the brazed joints, as somewhere between 700 and 800 psi I noticed the annealed areas on the sample tube began to balloon.

Back to increasing pressure in 20 psi steps all the way to 1000 psi, with no signs of ballooning !! This a major improvement compared to results seen in both sample tubes. This is just one test, but it does suggest that small steps of increasing pressures of 20 psi resulted in work hardening the copper tube significantly. I'm now confident the yield strength of my boiler tubes is at least 1000 psi.

If I can make the time needed for one more hydro test on a sample tube, I'm curious to try the 20 psi step procedure on sample tube which would then be tested to burst pressure.

 

[Steamchick]

Well done Toymaker.
I have learned a lot about the progressive work hardening from your explanation. When doing repairs to "no good" boilers, I usually end-up heating only a portion to annealing point for silver soldering. Especially if just sealing a leaky boiler tube. I think I have experienced a few odd failures after that from "differential" cooling that sets-up stresses in the copper, and has cracked otherwise sound joints. But I have also been concerned because some parts of old boilers may be well work hardened and when just locally heating a zone, which anneals the local zone, upon cooling or Hydraulically testing, there may be some differential stresses that could cause some material over-stressing at the transition zone between hard and annealed metal. This may account for some "odd" leaks that have appeared on previously sound joints?
So in future I shall be following a small-step by small-step build-up of hydraulic pressure.
Thanks!
K2

 

[Toymaker]

Boiler Hydrostatic Test, addendum:

Not sure if this is important or if it had any affect on the work hardening process, but beginning at 700 psi (the observed yield limit from the sample testing) I began taking additional time between steps; while the pressure was left at it's high value, I stood up, walked 3 short steps to the boiler from the hand pump, and carefully examined the 5 brazed joints for any signs of ballooning. I then walked back to the pump, released the pressure and began re-pressurizing. The additional time taken after reaching the yield point, to examine the joints for ballooning resulted in the boiler tube having just a few more seconds of static time at high pressure. Did this make a difference?? I may never know with certainty.

 

[Toymaker]

Well done Toymaker.
I have learned a lot about the progressive work hardening from your explanation. When doing repairs to "no good" boilers, I usually end-up heating only a portion to annealing point for silver soldering. Especially if just sealing a leaky boiler tube. I think I have experienced a few odd failures after that from "differential" cooling that sets-up stresses in the copper, and has cracked otherwise sound joints. But I have also been concerned because some parts of old boilers may be well work hardened and when just locally heating a zone, which anneals the local zone, upon cooling or Hydraulically testing, there may be some differential stresses that could cause some material over-stressing at the transition zone between hard and annealed metal. This may account for some "odd" leaks that have appeared on previously sound joints?
So in future I shall be following a small-step by small-step build-up of hydraulic pressure.
Thanks!
K2

I would have never tried increasing pressure by many small steps had you not mentioned in one of your posts that this is the recommended procedure in the UK (or at your club?), so a big Thank You for that very helpful info.

 

[Steamchick]

Hi Toymaker, Don't thank me, I have learned buckets more from you! It is why we all share knowledge on this website. You are really a good teacher to me. As an "ex-Industrial engineer" - and having Designed, Supervised Manufacturing, installed, sold, managed Quality and Regulation matters, managed Contracts, etc. etc. in a long and varied career, the "Boiler and Model making" I do is just something to keep my brain from stagnating in retirement. And the Design and Regulating matters are most challenging. (For my wee brain).
But the mental exercise of sharing "what I think I know" and learning "from all who know other stuff" is what keeps me entertained. So I am glad to add a little knowledge to the great pool of knowledge from all on this site, as much of what anyone writes is read by many, and there are so many with varied expertise that we all must be careful not to give wrong information (sadly, I sometimes do... but others are quick to correct my errors). I am impressed that much of what you have "deduced through rational thinking" is what has been deduced by others and documented as the right thing to do.
So I am equally grateful for our exchange of views and mutual teaching.
K2

 

[Steamchick]

Toymaker, you asked "The additional time taken after reaching the yield point, to examine the joints for ballooning resulted in the boiler tube having just a few more seconds of static time at high pressure. Did this make a difference??"
I can't explain the physics of what the material is doing, but I do recall that the Coppersmith at one place I worked (He shaped copper pipes 3 inches diameter that were at a NWP of 28bar...) told me that when he bent tubes he not only had to keep annealing as he made the bends, but he had to let everything "rest overnight" to "ease the stresses" - and then correct any distortion that occurred while "resting"... These pipes had flanges at both ends and were bolted to an air receiver and large (10in dia piston) Air actuator valve block at the other end. They were custom made and fitted so the flanges were bolted to adjacent components with gaskets and there was virtually no stress when all bolted together in the equipment.
So perhaps the couple of minutes "resting between pressure cycles" was a good thing? It probably needs a Metallurgist to explain this one to us?
K2

 

[Toymaker]

Failure Analysis

I went back to re-examine the second test sample to see what I could learn.

I sliced the tube in half at the failure point which allowed to take good metal thickness measurements at the rupture edges. Wall thickness had thinned to 0.020" around all the edges. The measured OD was made from top to bottom as viewed in the photo below; this method avoids including the bubble-like bulge at the actual rupture point. Recall the burst occurred at 1200 psi. Using Barlow's formula and solving for S (tensile strength), we get:

OD x P/ 2 T = S = 21,600 psi ultimate tensile strength of the annealed area which had been partially work hardened.

21,600 psi falls between accepted yield strength and ultimate tensile strength of hardened copper, so generally seems acceptable. Also shows that some work hardening had taken place.

 

[NapierDeltic]

Hi Toymaker, I see a tooling mark along the tube which I suppose comes from bending process.
Could be stress from bending tool and wall thinning (in that area seem to be very high contact forces and material push outwards) the reason for failure happening in that neutral fiber (or close to) of tube?
But other than that, bulging looks quite uniform.

 

[Steamchick]

Curious... in my experience (not a lot really!) of bending tubes, the tube hardly compresses the inside radius at all, but stretches all the material from the inside radius of the bending die to the outside. So for calculation purposes for Barlow's formula/Hoop stress calculations, I always take a worst case" that the outer diameter wall has been stretched from inner diameter material of known thickness, so the original tube wall thickness is reduced by the ratio of ID/OD. (Circumference ratio). The thinner value is then used in my designs. As the thinnest wall is on the outside diameter of the bend I expected a split failure to occur there, but now I think there are other factors affecting the tube, to generate the split on the side...
I guess, that Toymaker's bending dies (a wheel and die? - or set of dies and wheels) has some imperfection in the way they meet at the mid-line. Perhaps a slight mis-alignment? Or not quite the correct dies for the tube OD? - as the mid-line is quite pronounced.
Perhaps this is causing some trapping of material so it does not slide correctly in the dies/rolls and is causing some local thinning that should not occur? The apparent line/fold along the mid-line where rolls/dies meet will form a discontinuity in the wall of the tube, and a shape and work-hardened stiffness in the tube forming a stress concentration that has made the stresses focus where we did not expect the tube to fail.
As the tube is bent, the inside zone will be slightly compressed, and slightly work hardened in the process, and the outer half or more will be stretched, and work hardened proportionally to the amount of stretching. I suspect the split has occurred near to where there is a natural neutral axis between the compression and tension zones and consequential work hardening during bending. So stresses have been focused by the "fold-line" stress concentration into an area of lower work hardening from the bending process.
But that is purely my hypothetical guess!
Conclusion, improve the bender so the fold-line does not appear where dies and rolls meet. Or is it too late to make a new coil?
K2