Monotube Flash Boiler Design

[HMEL]

We are too much used to consider heat the only output of a chemical reaction.
But - for one instance - in batteries (rechargeable type) the output of chemical reactions is electric energy which at it's turn can generate mechanical output; while heat generation during charging-discharging is a waste. And here you have also a cycle; yes, very different from thermodynamic one ... it is not my point to discuss differences.

The battery is an energy storage device not a heat engine. It does not work form a high temperature heat reservoir to a low temperature reservoir. The construct of a cycle with regards to heat engines is very specific. The word cycle has a different meaning.

 

[Richard Hed]

Perhaps when ASME was first founded it was meant to help boiler makers, whom had great fabrication skills but were light on metallurgy, math and physics, make safe boilers that didn't go boom. And to ensure those old boilers never went boom, even when the welds aren't quite as good as they should be, and quality of the steel wasn't as good as it was sold to be, they made the safety factors ridiculously high. Fast forward to today; If NASA followed ASME rules for pressure vessels for their LOX and fuel tanks, none of their rockets would ever lift off the pad,...they would be far too heavy. This should be proof enough that safe, reliable pressure vessels can be made with much smaller safety factors than those listed by ASME.

I believe somewhere on ASME's time-line they became more interested in profit than in good engineering. Perhaps after a few states passed laws mandating ASME codes be followed or you couldn't get certified and sell your boilers, ASME realized they could charge as much as they wanted for copies of their standards. Following codes and standards with built-in safety factors of 8 will certainly ensure a boiler wont explode, but IMHO, just blindly following codes and standards isn't good engineering; the engineers at NASA, Space X, etc prove this with ever launch.

Often organizations like this are meant to keep small concerns out of the business, only large concerns could afford it. Here's something too: The FDA was orginally created to stop meat processors from poisoning troops after the Spanish/American war in the early 1900's--it's function now is not that at all, it's too often a tool for the corporations to stop competition. There are many examples like this. i would thimpfk, however, that there would be some way to get those rules for free such as borrowing them from a university library./

 

[HMEL]

Often organizations like this are meant to keep small concerns out of the business, only large concerns could afford it. Here's something too: The FDA was orginally created to stop meat processors from poisoning troops after the Spanish/American war in the early 1900's--it's function now is not that at all, it's too often a tool for the corporations to stop competition. There are many examples like this. i would thimpfk, however, that there would be some way to get those rules for free such as borrowing them from a university library./

I have taken the 16 hour ASME exam for a certified inspector and delt with many of the rules I have the following comments.
1. The ASME are American standards complied and written by engineers who have a broad and highly technical background. The committees that are formed by the engineers do have a diverse opinion of how things should be done.
2. It is against the law to specify particular products for purchase. However products that have a specified danger should meet certain standards. ASME does not force anyone to adopt them.
3. States adopt these standards because in general they are better then they can write. Not all States use them in a pure form and add or delete to those standards.
4. Safety factors are a design function. I know for a fact that NASA uses ASME standards for pressure vessels but they use special materials that are light weight such as carbon fiber and other light weight expensive alloys. These factors have been developed due to experience as well math formulations. When knowledge is not available over design is prudent
5. The purchase of these rules and regulations is very expensive but they can be found online for specific topics.

And finally if you choose to build something that does not meet those standards you are doing so at your own risk. And the people who are most insistent that you follow them is the insurance company who is going to cover you financial loss when things go south (maybe if your ducks are in a row!)

 

[Toymaker]

I have taken the 16 hour ASME exam for a certified inspector and delt with many of the rules I have the following comments.
1. The ASME are American standards complied and written by engineers who have a broad and highly technical background. The committees that are formed by the engineers do have a diverse opinion of how things should be done.
2. It is against the law to specify particular products for purchase. However products that have a specified danger should meet certain standards. ASME does not force anyone to adopt them.
3. States adopt these standards because in general they are better then they can write. Not all States use them in a pure form and add or delete to those standards.
4. Safety factors are a design function. I know for a fact that NASA uses ASME standards for pressure vessels but they use special materials that are light weight such as carbon fiber and other light weight expensive alloys. These factors have been developed due to experience as well math formulations. When knowledge is not available over design is prudent
5. The purchase of these rules and regulations is very expensive but they can be found online for specific topics.

And finally if you choose to build something that does not meet those standards you are doing so at your own risk. And the people who are most insistent that you follow them is the insurance company who is going to cover you financial loss when things go south (maybe if your ducks are in a row!)

HMEL, I have no doubts that ASME engineers are well educated, well meaning professionals,...but even the best engineers can made to look bad by poor management. I think I've made it very clear that my biggest beef with ASME is their blatant price gouging, and that's managements doing.

NASA writes and follows their own specs, which often reference other specs, such as ASME, but NASA gives themselves exclusions for flight hardware. So, yes, NASA does use ASME standards, but only as part of their own NASA written standards. Here are two examples of NASA standards: GuideforCertifying PressureVesselsandSystems
NASA REQUIREMENTS FOR GROUND BASED NON-CODE METALLIC PRESSURE VESSELS

Please share your online sources for free ASME codes,... when I looked, I found nothing.

HMEL, as I stated back in post #157, I cannot afford to even look at ASME's codes, as they're simply out of my price range. So the boiler I've built may, or may not, meet ASME codes,....TimTaylor seems to believe I've messed with quite a laundry list of ASME codes,....but again, I have no way to check; this isn't a choice I've made to not meet ASME standards, rather it's a choice ASME made to make their data unaffordable to most.

I have done my best to follow the laws of physics and mechanical engineering, and never ever exceed the yield strength of copper tubing (even at boiler temperature), which means I'm a long way from even getting close to exceeding the ultimate tensile strength. That's my safety standard.
As a final safety measure, I'm currently gathering all the pieces and parts needed to perform a hydrostatic test on my monotube boiler. If it passes my tests, I'll proceed to boiling a little water.

 

[Steamchick]

Hi Toymaker, I managed to download some of ASME but only the older versions. But it is then possible to check the list of updates to see what has changes for the clauses you actually need.
My latest download was 2015 version of volume 1.
I am away from my PC for a week so cannot send it or a link. Not for a week anyhow.
K2

 

[Toymaker]

Hi Toymaker, I managed to download some of ASME but only the older versions. But it is then possible to check the list of updates to see what has changes for the clauses you actually need.
My latest download was 2015 version of volume 1.
I am away from my PC for a week so cannot send it or a link. Not for a week anyhow.
K2

Thanks for the offer to help, just be careful as I believe even the older documents are still covered by copyright, and any organization that charges $550 and up for a book or CD may not hesitate to prosecute violators.

 

[Steamchick]

Hi Toymaker, as you are not in a country that is bound by ASME., you are probably doing nothing wrong with your calculations. Life (human) is a pretty simple thing to create, (not discussed on this site) but also easy to destroy. A Factor of Safety of 2.5 on Yield strength and FOS of 7 on UTS is not so bad. So it makes your job un-CERTIFIABLE in the USA and some other countries, but is not necessarily dangerous. But between us, as "experts" trying to advise on your build, we have to be careful what we say on this public forum. The forum would stop us commenting if we blatantly advised something dangerous or illegal.
I am a member of a "hobby" model engineering club. The boiler tester has authority to test Steam boilers for models - and up to full size - providing they comply with a certified design or a set of suitable calculations. But not air receivers, tube boilers (like yours) and multi-drum designs.
Yet the required calculations are only for around half of the stress calculations I recently submitted, and he advised that in future I do not declare the calculations for the parameters not covered by the stated regs.Because the regulators have not yet agreed what limits should be adopted for those parameters! Or maybe it would stop people enjoying their hobby because so many would have uncertifiable boilers?
I am in the UK where ASME is not required... but their rules are much easier to comprehend than UK regs, and very nearly the same, when it comes to model boiler design.
All in all, the only further advice I can suggest is that you also check your calculations for any thinning of tube walls that could have occurred geometrically due to the bending and shaping of tubes.
In my calculations I simply take a worst case scenario that the inner radiusof the bend does not compress, but the outer radius of the bend stretches to the increased arc length, therefore thinning the material proportionately.
Looking at your design, I have not seen any side penetrations in the tube for the coil. That is good. Otherwise ASME would require a Stress Concentration Factor of 3.3 to be applied when calculating hoop stresses.
Cheers.
K2

 

[Steamchick]

Thanks for advice on copyright laws..... I'll respect that and delete my downloads.... You can never know who is reading all we type in the interweb... C something I. A. .?

 

[Toymaker]

Thanks for advice on copyright laws..... I'll respect that and delete my downloads.... You can never know who is reading all we type in the interweb... C something I. A. .?

I'm not a lawyer, so the following is my opinion: at least in the movie industry, copyright owners don't usually go after those people that have illegally downloaded a few movies to watch in their own homes, instead, they go after the distributors that are placing those movies on web sites making them freely available to the public. So I doubt ASME will come after you for simply downloading one of their docs and having it on your computer. They would be more likely to sue you, and those you got the document from, for widely distributing it to others. Also ASME may have granted permission for their older docs to be distributed by whomever you got your copy from,... we don't know.

If there are any actual lawyers reading this thread, their advice would certainly be welcome

 

[HMEL]

HMEL, I have no doubts that ASME engineers are well educated, well meaning professionals,...but even the best engineers can made to look bad by poor management. I think I've made it very clear that my biggest beef with ASME is their blatant price gouging, and that's managements doing.

NASA writes and follows their own specs, which often reference other specs, such as ASME, but NASA gives themselves exclusions for flight hardware. So, yes, NASA does use ASME standards, but only as part of their own NASA written standards. Here are two examples of NASA standards: GuideforCertifying PressureVesselsandSystems
NASA REQUIREMENTS FOR GROUND BASED NON-CODE METALLIC PRESSURE VESSELS

Please share your online sources for free ASME codes,... when I looked, I found nothing.

HMEL, as I stated back in post #157, I cannot afford to even look at ASME's codes, as they're simply out of my price range. So the boiler I've built may, or may not, meet ASME codes,....TimTaylor seems to believe I've messed with quite a laundry list of ASME codes,....but again, I have no way to check; this isn't a choice I've made to not meet ASME standards, rather it's a choice ASME made to make their data unaffordable to most.

I have done my best to follow the laws of physics and mechanical engineering, and never ever exceed the yield strength of copper tubing (even at boiler temperature), which means I'm a long way from even getting close to exceeding the ultimate tensile strength. That's my safety standard.
As a final safety measure, I'm currently gathering all the pieces and parts needed to perform a hydrostatic test on my monotube boiler. If it passes my tests, I'll proceed to boiling a little water.

https://www.freestandardsdownload.com › free-asme-standards-download

Here is a source but you will have to search what you want. Now there is a way that is acceptable to test your system with the ASME procedure However it is a potential destructive test. You hydro the unit under question to an acceptable pressure above the nominal design pressure. I dont remember the specific multiplier but lets say you use 1.5 above that value. Remember the working pressure is generally 75% of the Nominal design pressure. I have used it once to qualify a part and there is also the loop hole of using all boiler certified piping materials. Its tricky but I did this with a starch cooker that had to cross borders without an ASME plate on it. The design however was simple.

ASME sells sections of the code and there range in price from 50 to 500 bucks.

If it was me I would hydro the system and be done with it. I actually believe the pressure parts will be easier to solve than using a rankine cycle with refrigerant.

You are building a prototype and at this stage how you build it is the pressing question. The safety factor you choose is your decision but hydro is probably best bet without doing the math.
HMEL

 

[Toymaker]

https://www.freestandardsdownload.com › free-asme-standards-download

Here is a source but you will have to search what you want. Now there is a way that is acceptable to test your system with the ASME procedure However it is a potential destructive test. You hydro the unit under question to an acceptable pressure above the nominal design pressure. I dont remember the specific multiplier but lets say you use 1.5 above that value. Remember the working pressure is generally 75% of the Nominal design pressure. I have used it once to qualify a part and there is also the loop hole of using all boiler certified piping materials. Its tricky but I did this with a starch cooker that had to cross borders without an ASME plate on it. The design however was simple.

ASME sells sections of the code and there range in price from 50 to 500 bucks.

If it was me I would hydro the system and be done with it. I actually believe the pressure parts will be easier to solve than using a rankine cycle with refrigerant.

You are building a prototype and at this stage how you build it is the pressing question. The safety factor you choose is your decision but hydro is probably best bet without doing the math.
HMEL

Thanks for the help HMEL, very much appreciated.

Living in Thailand as I do, all the tubing I can buy is made in China, and I don't trust their quality control that wall thickness will always be as they claim, so I will most definitely hydro the boiler. I have a hand operated hydraulic pump capable of 10,000 psi. My plan is to first hydro a short section of left-over copper tube up to, but just below, the yield limit, then measure the OD for any changes. Then I will destructively hydro that test section of left-over copper tube to determine it's actual burst pressure limit; the math says that should be around 3,800 psi. I will be on the other side of a concrete wall while I'm hand pumping the hydraulic unit. I will then test the actual boiler to something below the yield limit.

 

[Toymaker]

Initial test results:

I hydrostatically tested a short length of left-over tube used to make the boiler, and I was more than a little surprised at the preliminary results. The test sample had the smallest radius bend used in the boiler, which means the thinnest wall thickness on the outer-most wall (measured at 0.028"); this should be the weakest section of tube in the boiler. Below shows the curved test sample attached to the the hydraulic hand pump. The diameter at the middle and both ends were measured, and marked on the tube before testing, these were measured again after testing at 3 different pressures. Photo below shows the test set-up.


Pressure was slowly increased to 500 psi, the planned max working pressure. Tube was examined for leaks, none were found. After a few minutes of stable 500 psi, pressure was released and the three OD markings were measured for any permanent expansion,...there were none.


Pressure was again increased to max pressure for the guage, 1,000 psi,...still no leaks. Pressure released and 3 OD points measured,...again, no change. At this point, I'm pleasantly surprised. Barlow's formula for a 0.028" wall says the max yield pressure should be 986, and I'm not seeing any signs of permanent changes to the OD.



The plan was to destructively test this sample to determine burst pressure, that required a different guage with a much higher max limit; the guage below exceeds the pumps rated limit of 10,000 psi.

Once again, pressure is slowly increased, first to 1,000 psi then tried to go to 2,000 but only made it to about 1,400 maybe 1,600 psi, not sure. The brazed, star-crimped seal used the close the end of the tube sprung a pin-hole leak ending my testing for today. If you look at the full test set-up photo, the crimp that later failed is clearly visible. I must buy a proper end cap and braze it in place before I resume testing this little sample.

Again, I'm pleasantly surprised as the three measured points are still showing no change, not even 0.001".

At this point, I've proven the copper tube used in my boiler has a room temperature yield strength in excess of 1,000 psi.

 

[Scott_M]

Hi Toymaker
I have been thinking about your large volume upcoming hydrotest. And not wanting to pollute your boiler with hyd. oil or loose it all together.
If your hydraulic pump came with with a hyd. cylinder you could use its ram to "compress" a vessel with the test water in it. Possibly make a new end cap for the hyd. cylinder with some threads on the outside to screw into another cylinder that will have the water and connections to your boiler. I think it would be very doable, getting it sealed up would be the hard part.

Food for thought

Have been following along with great interest !

Scott
Ps I know you can't compress water

 

[Toymaker]

Hi Toymaker
I have been thinking about your large volume upcoming hydrotest. And not wanting to pollute your boiler with hyd. oil or loose it all together.
If your hydraulic pump came with with a hyd. cylinder you could use its ram to "compress" a vessel with the test water in it. Possibly make a new end cap for the hyd. cylinder with some threads on the outside to screw into another cylinder that will have the water and connections to your boiler. I think it would be very doable, getting it sealed up would be the hard part.

Food for thought

Have been following along with great interest !

Scott
Ps I know you can't compress water

Thanks for your suggestion on how to separate the water and oil using an auxiliary ram; and I agree with you that it should be doable. A double acting ram, with water on one side and oil on the other should be perfect.

However, I came up with a much easier solution which I successfully used during the test. I kept the pump filled with oil and filled the short test tube with water. Since both oil and water are incompressible fluids, very little oil actually flows into the tube during testing. Even with the leak in the tube, and my continued pumping to re-pressurize the tube to find the leak, when I disassembled the test setup and drained the tube, less than 1cc of oil came out of the tube, the rest was water.

For the future hydro test of the boiler, even if a leak occurs there should be a movement of only 1 or 2 cc of oil into the boiler tubes which shouldn't be too difficult to clean out.

Today is Sunday here in Thailand and all the HVAC stores that sell copper tube supplies, like the end cap I need, are closed, so there will be a full day's delay before I can resume testing.

 

[Toymaker]

I asked a question in this New Post that applies to all copper boilers, including this one. I'm looking for feedback.

 

[Steamchick]

Hi Toymaker, I am pleased to read your hydraulic testing post. When I get home, I'll see what the ASME calculation would be for an hydraulic test of a 500 psi NWP. Copper vessel. I guess something like 1500psi? Without permanent distortion. As your copper assembly is annealed in parts from making joints, the recommendations for de-annealing while hydraulic testing are to start around 50% of the Normal Working Pressure, then release the pressure, and go to 60%NWP, and release, etc. in the "10% " increments up to the full test pressure. Releasing the pressure each time so the annealing is work hardened progressively by the pressure-release cycles. When at final test pressure, hold it (locked supply valve if there is one?) And monitor for any hydraulic pressure drop after 20 minutes. The test-part isolation valve may be necessary if the hydraulic pump has any leak-back which could cause the pressure to not be maintained without pumping.
Hope this procedure helps?
Ken

 

[Toymaker]

Hi Toymaker, I am pleased to read your hydraulic testing post. When I get home, I'll see what the ASME calculation would be for an hydraulic test of a 500 psi NWP. Copper vessel. I guess something like 1500psi? Without permanent distortion. As your copper assembly is annealed in parts from making joints, the recommendations for de-annealing while hydraulic testing are to start around 50% of the Normal Working Pressure, then release the pressure, and go to 60%NWP, and release, etc. in the "10% " increments up to the full test pressure. Releasing the pressure each time so the annealing is work hardened progressively by the pressure-release cycles. When at final test pressure, hold it (locked supply valve if there is one?) And monitor for any hydraulic pressure drop after 20 minutes. The test-part isolation valve may be necessary if the hydraulic pump has any leak-back which could cause the pressure to not be maintained without pumping.
Hope this procedure helps?
Ken

Yes, your incremental pressure increase procedure is very helpful and confirms my suspicions which I voiced in a different thread I just posted a few hours ago. I suspected hydro testing would result in work hardening the copper but I had no confirmation for that theory,...now I do, so many thanks.

 

[Toymaker]

Destructive Test, Initial Results

First, I realized a mistake I made during the first test was that my 3 measured OD locations were all in non-annealed sections. For this second test I intentionally dented the sample tube in the annealed area to provide a quick visual way to show the yield limit had been reached. This worked and revealed a yield limit of between 700 and 800 psi for the annealed areas. Because I wanted to find the yield limit of the annealed sections, I did not attempt to work harden the copper by increasing the pressure in steps with zero pressure in between those steps.

Below shows before and after pressure test. Notice the dents on the left side of the tube are completely expanded on the "after" pressure photo on the right.



Below is a clear view of the small, 0.54" long rupture which occurred at 1600 psi in the annealed section.



I will a run a 3rd test using a new test sample with an annealed section and follow a step-release-step pressure increase to determine if this copper tube can be work hardened to increase the ultimate tensile strength.

 

[Toymaker]

Destructive Test, Final Results

On the first pressure increase I went to 800 psi straight away, as I had again placed a substantial dent in the annealed section of the tube and this time I wanted to see if I could "repair" the dent with pressure. I have a similar dent in my boiler tubing and I want to see if I can safely "re-inflate" it; the answer is yes, but not without permanently weakening that section of tube.

After the dent was re-inflated to my satisfaction I released the pressure and began stepping through increasing pressures starting at 500 psi, then 600, then 700, up to 1000 psi, which is the limit of my smaller pressure guage. I then repeated going up to 1000 then release, then 1000 again and release, ten times.
Change guages and go to 1100, release. Next 1200 then release, all the way to 2000 psi.

At 2000 psi I noticed the annealed sections of the tube were beginning to balloon; the other sections of the tube showed no signs of OD change. I suspected I was quickly reaching max pressure.

Next pressure stop would have been 2100 psi but just as the needle hit 2100 psi, the tube burst. Notice the ballooning at both ends of the tube.

 

[Steamchick]

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