Monotube Flash Boiler Design

[peterl95124]

I'm not sure what point you're trying to make, as final reports of the Apollo-13 incident determined that a routine stir of an oxygen tank ignited damaged wire insulation inside the tank, causing tank pressure to rapidly increase resulting in an explosion.

IF, you're drawing attention to the possibility that my feed pump is quite capable of over-pressuring the boiler tubes, thereby rupturing them, I've addressed that possibility by installing pressure and temperature sensors on the feedwater input to the boiler and the steam output as shown in post #331 above. The ECU monitors these pressures and temperatures and prevents feed pump induced over-pressure conditions by reducing feed pump RPM whenever pressures reach the max limit. As an additional safety precaution, a purely mechanical pressure release poppet valve is installed on the steam output side of the boiler.

Perhaps I'm mis-remembering, but I thought it was the result of a pressure gauge that only went up to the designed operating range of the device, so if you were over pressure the gauge just pegged at that maximum operating pressure, the astronaut read the dial and presumed everything was OK because it didn't show *above* the max pressure. Perhaps it was a different flight.

 

[Toymaker]

Perhaps I'm mis-remembering, but I thought it was the result of a pressure gauge that only went up to the designed operating range of the device, so if you were over pressure the gauge just pegged at that maximum operating pressure, the astronaut read the dial and presumed everything was OK because it didn't show *above* the max pressure. Perhaps it was a different flight.

Regardless of your memory being right or wrong, I do understand your concern regarding pressures beyond the limits of the gauge,...it's sort of the reason I needed to change guages.

The scale on the original guage on the left reads MPa and the first line above "0" is 2.5 MPa (362 psi), the second line shows 5 MPa (725 psi) which is very close to the max 800 psi limit I wish to hold, and the 10 MPa (1450 psi) marking may just burst the copper tube. A very small needle movement on the left guage represents a very large pressure change.

By using the quage on the right, having a full scale reading of 700 psi, I will be able to much better set the PWM output driving the DC motor to provide the the exact feed pump pressure I want. The greater granularity of the larger guage will also allow me to verify the readings from both pressure sensors match the readings on the dial guage.

 

[Steamchick]

Hi Toymaker, I am pretty sure you understand what is going on and what is needed. I like what you are doing. I just wonder a bit if there is a bit of naivety in your thinking - or maybe it is my lack of understanding? - The displacement pump provides a theoretical maximum pressure based on the strength of the pump and torque of the motor. This is achieved against an orifice that restricts the flow. - Less flow, less pressure achieved. So the motor speed control should manage that.... except, you'll be working against boiler pressure: - So instead of the pump needing to produce 500psi against the flow/orifice when there is ATMOSPHERIC pressure at the orifice, you want that flow against a Back pressure of 500psi and the orifice/flow requirement.
Simply "Boiler" 500psi + "Flow" 500psi =1000psi at the pump and gauge?
Am I being stupid? It looks wrong to me, so please can you (or anyone?) explain? - There must be an Hydraulic "pumping" Engineer out there who can tell us what is right?
Thanks,
K2

 

[Toymaker]

Hi Toymaker, I am pretty sure you understand what is going on and what is needed. I like what you are doing. I just wonder a bit if there is a bit of naivety in your thinking - or maybe it is my lack of understanding? - The displacement pump provides a theoretical maximum pressure based on the strength of the pump and torque of the motor. This is achieved against an orifice that restricts the flow. - Less flow, less pressure achieved. So the motor speed control should manage that.... except, you'll be working against boiler pressure: - So instead of the pump needing to produce 500psi against the flow/orifice when there is ATMOSPHERIC pressure at the orifice, you want that flow against a Back pressure of 500psi and the orifice/flow requirement.
Simply "Boiler" 500psi + "Flow" 500psi =1000psi at the pump and gauge?
Am I being stupid? It looks wrong to me, so please can you (or anyone?) explain? - There must be an Hydraulic "pumping" Engineer out there who can tell us what is right?
Thanks,
K2

I'm not a "Pumping Engineer" but I'm fairly certain I remember most of the hydraulics lessons from my various physics classes, so I'll take a stab at explaining how I believe a monotube boiler systems work.

Imagine a single pipe, 10 meters long, with a feed pump at each end and a pressure guage in the middle of the pipe and at each feed pump. Turn on both feed pumps such that their respective pressure guages each reads 500 psi. I'm 99% certain the pressure guage in the middle of the pipe will read 500 psi, and not 1000 psi as your above equation would suggest.

At their most basic level, monotube boilers are dynamic hydraulic systems; a feed pump provides pressurized fluid flow into one end of a pipe, and steam valves at the outlet end provide the resistance. The water turning into steam before it exits through the resistance (typically a valve) doesn't change the overall hydraulic system pressure.
Pumps of equal pressure connected in parallel will increase the flow rate but not the pressure. Water being vaporized into steam inside a monotube boiler is acting as a pump, supplying pressure and additional flow due to the increased volume of steam vs water, but these two "pumps" are connected in parallel, one at each of the monotube pipe, so their combined pressure remains the same,...no increase.

Finally, I believe it was HMEL whom noted that feed pumps typically need to supply a few hundred psi above the steam output pressure in order to overcome flow resistance of the tubes.

 

[Steamchick]

Hi Toymaker. I do not have any useful hydraulic expertise... Hence my confusion?
I think there is a difference here between the static condition and the dynamic condition where fluid is flowing. I guess the feed pump (water) is a smaller bore tube than the flash tube? So you are trying to pump x amount of water at P.in pressure. If P.in = pressure in the flash boiler tube, then there must be zero flow? If HMEL as you need a few hundred psi more at the pump to get the flow required, then surely he is suggesting the pump must need to make P.steam plus P.flow? And you have sized the pump and motor to achieve P.flow at the required flow rate, but It seems to me you need to consider the pressure profile though the system, at max (mass) flow...?
I think the extra few hundred psi is to overcome the "boiler" back pressure... ?.??
It isn't as simple a a non-dynamic simple water column hydraulic system. Or not in my confused head?
K2

 

[Toymaker]

Hi Toymaker. I do not have any useful hydraulic expertise... Hence my confusion?
I think there is a difference here between the static condition and the dynamic condition where fluid is flowing. I guess the feed pump (water) is a smaller bore tube than the flash tube?

I have read on the SteamAutomobile web site that some home builders use 3 different tube sizes in their boilers, starting small and ending with the largest diameter, while others, myself included, use a single diameter for the entire boiler. Which design is better I couldn't tell you.

So you are trying to pump x amount of water at P.in pressure. If P.in = pressure in the flash boiler tube, then there must be zero flow?

IMHO, there should never be zero flow through a flash boiler as that risks melting the tubes.

If HMEL as you need a few hundred psi more at the pump to get the flow required, then surely he is suggesting the pump must need to make P.steam plus P.flow? And you have sized the pump and motor to achieve P.flow at the required flow rate, but It seems to me you need to consider the pressure profile though the system, at max (mass) flow...?

I'm comfortable knowing feed pump pressure and steam output pressure.

I think the extra few hundred psi is to overcome the "boiler" back pressure... ?.??

It's not back pressure the feed pump needs to overcome, its simple flow resistance. There are several online calculators that will determine flow rates for given tube diameters and pressures. I used this one Flow - Pressure - Tube to determine pressure and diameter of various flexible connecting tubes between feed pump and boiler.

It isn't as simple a a non-dynamic simple water column hydraulic system. Or not in my confused head?
K2

Yes, static systems are a bit simpler, but the added complexity of a dynamic system comes mostly from flow resistance through the tubes. Once flow resistance is added into the analysis, there's little to no difference.

 

[Steamchick]

OK Toymaker, perhaps you can clarify this one? - "these two "pumps" are connected in parallel, one at each of the monotube pipe, so their combined pressure remains the same," - I had them in series? Atmospheric through water pump to a higher pressure, > boiler pressure, then the "boiler" is a second "pump" raising pressure a further amount?

In my confusion (I definitely don't really understand this now):
The water at atmospheric goes into the pump and exits at 500psi because the pump is trying to push the water up to "boiler tube" pressure to get it to flow. When the pump exceeds 500psi, the water starts to flow, and as the pressure rises further it flows a lot more...? (HMEL's extra few hundred psi?). All the while the steam exiting the "boiler" part at - 500psi?
- But I thought that the change of state from water to steam required the input of a lot of energy (Latent Heat) and then some more energy to raise the steam to the temperature of 500psi?

1. Is there not an associated pressure rise during that heating phase?
2. Or does each "drop of water at 500psi" simply become a "bubble of steam at 500psi" so all the heat is effectively Latent heat?" - and the only change is the volume?

I have never tried to figure out what is really happening inside a flash tube boiler - other than water and heat in = steam out. But maybe the last paragraph (2) is what is happening?
Sorry to be a bother, but my head isn't straight yet.
K2

 

[Toymaker]

OK Toymaker, perhaps you can clarify this one? - "these two "pumps" are connected in parallel, one at each of the monotube pipe, so their combined pressure remains the same,"

I'm trying to say that the pressure produced from steam production inside the boiler tubes, is not very different than the pressure produced by a pump. It doesn't matter what mechanism produced the pressure, whether it was steam or a pump.

Atmospheric through water pump to a higher pressure, > boiler pressure, then the "boiler" is a second "pump" raising pressure a further amount?

Pressure produced by a pump is directional, pushing water through the tubes in only one direction. Pressure produced by expanding steam produces pressure in all directions, resulting in pressure pushing back against the pressure from the pump. So, no further pressure increase from the expanding steam.

In my confusion (I definitely don't really understand this now):
The water at atmospheric goes into the pump and exits at 500psi because the pump is trying to push the water up to "boiler tube" pressure to get it to flow. When the pump exceeds 500psi, the water starts to flow, and as the pressure rises further it flows a lot more...?

Yes.

(HMEL's extra few hundred psi?).

Remember, we're analyzing a dynamic system, which means there's flow resistance in the tube; 500 psi fluid pressure exiting the pump will be lower by the time it reaches the steam production area inside the boiler tubes.
The pump needs to output 600 or 700 psi because flow resistance will lower that pressure all along the tube length.

All the while the steam exiting the "boiler" part at - 500psi?

Yes !!

- But I thought that the change of state from water to steam required the input of a lot of energy (Latent Heat) and then some more energy to raise the steam to the temperature of 500psi?

1. Is there not an associated pressure rise during that heating phase?
2. Or does each "drop of water at 500psi" simply become a "bubble of steam at 500psi" so all the heat is effectively Latent heat?" - and the only change is the volume?

I have never tried to figure out what is really happening inside a flash tube boiler - other than water and heat in = steam out. But maybe the last paragraph (2) is what is happening?
Sorry to be a bother, but my head isn't straight yet.
K2

K2, for the purposes of pressure flow, by including latent heat and how individual droplets of water change state, you're way over-thinking hydraulic pressure and flow. The details of how steam produces pressure aren't important for this discussion, the only important fact is that steam produces pressure in all directions.

 

[Steamchick]

Thanks Toymaker. I should study this subject more so I can do some sums... that always helps me understand things better. From what I have read so far on flash boilers, they are a "Black art" - but hugely powerful. Babcocks boilers in Power stations are almost flash boilers, as they have continuous feed at boiler pressure plus a bit, vast arrays of "tubes in flames" - then somewhere (top-drum) where the steam can escape and be taken to superheaters: More tubes in flames/hot gases!
Thanks for the help.
K2

 

[Steamchick]

Just one thought... A flash boiler I saw at a show, in a high speed boat, had the water feed pump directly driven by the engine. So as revs increased, the water flow increased, the pump was matched to the steam demand (it just all became self regulating), and as the engine also drove the fuel pump, everything balanced. For starting, there was a separate propane burner to pre-heat the coils, and the engine was rope started to pump main burner fuel and water to the combustion chamber and coils simultaneously. I guess this works well for boats, but maybe not for cars?
K2

 

[Toymaker]

Just one thought... A flash boiler I saw at a show, in a high speed boat, had the water feed pump directly driven by the engine. So as revs increased, the water flow increased, the pump was matched to the steam demand (it just all became self regulating), and as the engine also drove the fuel pump, everything balanced. For starting, there was a separate propane burner to pre-heat the coils, and the engine was rope started to pump main burner fuel and water to the combustion chamber and coils simultaneously. I guess this works well for boats, but maybe not for cars?
K2

Most automotive feed pumps are driven directly by the primary steam engine, and use valves to direct the correct flow and pressure into the boiler. The SES feed pump used computer controlled solenoids to open and close 3 individual valves, one for each plunger, to better control the exact flow and pressure needed. The constantly changing power demands on a car's engine makes the direct drive links used in power boats, pretty much impossible.

 

[Toymaker]

Measured Pressure & Software Verification:

Finally reached a point where I was able to check pressure reading on the feed pump's dial gauge against the pressure readings from the boiler's input and output pressure sensors; everything worked as planned.

Using the feed pump to pressurize the boiler, I was able to observe readings displayed by both the pump's dial guage and the digital display from the ECU. Digital display values jumped around more than I would like, but stayed within about +/- 15 psi (30 psi swing). It's possible the sensors are reading the pulses from the 3-piston pump, and its also possible I'm reading noise on the sensor voltage, which is only 0 to 5 volts. I'll place a small capacitor on the sensor output line to smooth out any noise and re-test. Pressure readings were tested up to 900 psi.

Note that only Boiler PSI In: 503 and Blr PSI Out 513 (the light Blue line) were working for this test; ignore everything else on the digital display.

 

[Steamchick]

I see what you mean about the "digital fluctuation in pressure".
Noting the single reading you quote of " Boiler PSI In: 503 and Blr PSI Out 513" - presumably the 2 sensors and interpretation software are independent? So just fluctuating randomly? rather than there being a real pressure rise though the system? I assume this test was just water - without heat from the burner?
K2

 

[Toymaker]

Yes, this test used only the feed pump to produce the pressure, no heat was used or needed; So, even though I was able to vary the pressure from 0 to 1000+ psi, the test I did should be considered static, with no pressure rise through the system.

Yes, Input and Output pressure sensors are read, interpreted, and displayed separately; they are completely independent of each other.

The pressure sensors are rated 0 to 1000 psi and the max voltage input to the Arduino computer is 5 vdc, meaning 5 vdc represents 1000 psi. Very small differences in voltage readings result in large psi swings,...a small amount of noise on the signal line could be causing the fluctuations. I'll try a couple different capacitor values to smooth the signal.

The largest pressure swing I observed on the digital display was 30 psi, which is only 6% of the max output reading of 500 psi. That's actually pretty good,...but I'll still try for improvement

 

[minh-thanh]

Measured Pressure & Software Verification:

Finally reached a point where I was able to check pressure reading on the feed pump's dial gauge against the pressure readings from the boiler's input and output pressure sensors; everything worked as planned.

Using the feed pump to pressurize the boiler, I was able to observe readings displayed by both the pump's dial guage and the digital display from the ECU. Digital display values jumped around more than I would like, but stayed within about +/- 15 psi (30 psi swing). It's possible the sensors are reading the pulses from the 3-piston pump, and its also possible I'm reading noise on the sensor voltage, which is only 0 to 5 volts. I'll place a small capacitor on the sensor output line to smooth out any noise and re-test. Pressure readings were tested up to 900 psi.

Note that only Boiler PSI In: 503 and Blr PSI Out 513 (the light Blue line) were working for this test; ignore everything else on the digital display.

View attachment 153512

View attachment 153513

View attachment 153514

I like the way you use the software ..to control the pump, heat...especially with the parameters...shown on the screen, although I don't know much but it's quite interesting to see

 

[Simon0362]

...

The pressure sensors are rated 0 to 1000 psi and the max voltage input to the Arduino computer is 5 vdc, meaning 5 vdc represents 1000 psi. Very small differences in voltage readings result in large psi swings,...a small amount of noise on the signal line could be causing the fluctuations. I'll try a couple different capacitor values to smooth the signal.

The largest pressure swing I observed on the digital display was 30 psi, which is only 6% of the max output reading of 500 psi. That's actually pretty good,...but I'll still try for improvement

When I have done similar Arduino based sensor readings in the past, I have done say, 10 readings in quick succession and averaged the results - or discarded the outlyers to calm the effects of electrical or other noise. It all depends on whether you have enough processor cycles to do this, but I think that won't be too much of an issue here.
Fascinating thread BTW, really enjoyed reading through your work and analysis - as well as your cross technology solutions.
Simon

 

[Toymaker]

Boiler Alterations:

After the above pressure testing of the sensors I discovered a good bit of water inside the boiler housing, and after pulling the coils from the housing and applying a few hundred PSI using the feed pump, I found the problem.



The brass fitting which threads into the aluminum radiation disk and is sealed with red (high temperature) silicone sealant, sprung a leak, (notice red arrow in above photo). I had such a difficult time sealing these two connections during hydrostatic testing that I decided to replace the aluminum disk with the all-copper solution shown below.

I'm sure the seven 180 degree turns of the 5/8" tubing will cause a pressure drop, but I'm hoping the pressure loss will be more than offset by the heat absorption of this maze-like assembly. All the pieces are brazed together and have been pressure tested to 1000 psi using the feed pump to supply pressure. I used the same step-wise pressure increase method which I used on the entire boiler, which is to slowly increase pressure in small increments of 50 psi as a way to work-harden the copper tube. I suspect using the feed pump to apply hydrostatic pressure to the tubing is a better method than my previous use of a hand operated hydraulic pump, as the feed pump applies a stream of continuous pressure pulses which likely have the same work-hardening affect as does a continuous series of rapid hammer blows on metal.

So, the big question: will the braze hold up to the direct heat of the blue flames from the burner? Based on the lack of any noticeable heat damage to the aluminum disk, I believe it will. Unfortunately, I'm weeks away from being able to run any boiler tests as I'm currently in the process of replacing the electric motor on the air blower (leaf blower) with a much larger, stronger motor.

 

[Steamchick]

Just a word of caution.... something that may shorten the effective life of the copper tube array.
Right angle bends in tubing under internal pressure are always "trying to straighten the corner". Thus there is a huge stress concentration on the inside corner.
To alleviate such stress, I suggest you affix securely some ties at the ends, which will support the tube array and take the stress away from the brazed corner joint.
Otherwise, I suspect there will be fatigue induced stress cracking of the braze on the inside corners. Such leaks are not what you want. So added strength from some ties should give a much better durability of the design. = longer life! = happiness?
Additionally, the ties will be heated, so their surface area will add a small amount to the total for absorbing heat by the fluid.
I suggest silver soldering copper ties should be the right process.
K2

 

[Toymaker]

Just a word of caution.... something that may shorten the effective life of the copper tube array.
Right angle bends in tubing under internal pressure are always "trying to straighten the corner". Thus there is a huge stress concentration on the inside corner.
To alleviate such stress, I suggest you affix securely some ties at the ends, which will support the tube array and take the stress away from the brazed corner joint.
Otherwise, I suspect there will be fatigue induced stress cracking of the braze on the inside corners. Such leaks are not what you want. So added strength from some ties should give a much better durability of the design. = longer life! = happiness?
Additionally, the ties will be heated, so their surface area will add a small amount to the total for absorbing heat by the fluid.
I suggest silver soldering copper ties should be the right process.
K2

Lets see if I understand you correctly,... you're suggesting I braze (silver solder) some sort of metal structure onto each end of the tube array, something like the red lines in the below pic ? The red lines could be flat copper strips, perhaps 0.8" wide by 1/8" thick, where each "U" tube end is brazed onto the copper sheet using a generous amount of braze for added strength?

 

[Steamchick]

Yes, That is perfect!
Or even 5 or 10 mm in from the ends, on the side of the tube array should be adequate. I appreciate you would not want to affect the integrity of the corner joints in adding the ties.
In engineering "ideals" this is "horrible design" with the fabricated corner joints - as you appreciate and have already commented - but as with so many things, it is an expedient solution that can be made to work properly. Hence, I am trying to help with ideas to make the job stronger and safer without decrying what you are trying to do.
Just be VERY careful when you steam this system, as you are dealing with a lot of power in a confined space. This is not a bomb, but an escaping steam jet is a likely failure mode. If it goes up the exhaust flue it can be considered safer, but a steam jet is worse than a flame. It is invisible. A similar (multi kW) steam jet nearly cost my friend his arm, when he was doing an emergency repair and "something failed" - not exploded. The steam at 3 bar was INVISIBLE for some distance before it turned "white and steamy", but cooked his skin and muscles almost instantly in the few seconds before he fell away from it. The muscles do not grow back. A year later he had an arm, but not a very good one. It is missing a lot of muscle. Lucky the bone was not cooked too, which would have needed an amputation. He changed his job. It changed his life. So please take care.
K2