Monotube Flash Boiler Design

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Question for the pump experts:

Below is a graph of what I believe the Pressure out over time (or shaft rotation) of a 9 piston swash plate pump would look like. The GREEN lines show the pressure & flow rate of the 4 pistons which are actively supplying pressure and flow at any given time in the pumps rotation. Because there are 4 pistons pushing fluid out at all times, pressure at the output should never drop below the "Average" line.

Am I right or wrong ??


View attachment 156716
View attachment 156717
The graph does not represent what will happen. For instance to calculate the pressure with the base hypothesis of four active cylinders you would have each area times the force total force of the pistons to yield one pressure output. This should be a constant. The pressure will dip when one cylinder is removed and increase when its replaced by another. So the pulse frequency will be when the pistons unload and load. The more active cylinders the less the pulse will be, but still depends on the volume each cylinder produces.

Smaller pistons require a higher rpm thus a higher pulse frequency. Magnitude is a function of the volume each piston contributes. Volume is a fuction of piston diameters, rpm and horsepower delivered.

However, the purpose of the pump is to supply water at the steaming rate and enough pressure to overcome the piping and head losses. For traditional designs a feedwater valve is used and often in conjunction with a variable speed drive. To prevent cavitation a recirculation valve is used.

I guess the question is if the pump will cavitate due to the operation of the valves at the ports. If it does the pulses could be quite severe. With oil I would expect the risk to be low with water not so sure. Just because its a piston pump does not mean it will not require a minimum suction head or will not cavitate.

The design looks like it will minimize the pulses but the other factors such as valving and port size may cause other problems.
 
I suspect that pressure pulses from the input point (pump) will trvel at the speed of sound inside the liquid until the critical temperature/pressure point where steam is created. The oscillation of pressure from the sound wave will cause and oscillation of the phase change and this energy exchange may be helpful, or destructive.
There is a similar condition in gas where it is flowing - naturally through a tube and heats a "membrane" of heater input. It is called Rijke's tube resonance.
https://en.wikipedia.org/wiki/Rijke...e tube is a,an excellent example of resonance.
A very clever guy (Joan Lluch) I met on another thread developed this with the type of radiant heat burner he introduced into his boiler for a steam loco - designed for such a gas burner. https://www.mylargescale.com/threads/gas-burner-making-high-pitch-flute-like-noise.81946/
He resolved the issue by changing the air-fuel mix, such that the right conditions did not occur for the Rijke's tube resonance to occur. I think we made the flame front thicker? - I.E. When the oscillating gas can transit from hot-zone to cold-zone and back again across the heating element, it sounds - and resonates very loudly. When the cold to hot interface is too thick for the gas to travel across and back it cannot sound. I.E. the zone is thicker than the wavelength of the resonant frequency at the temperature and pressure of gas.
In a flash boiler, there is a zone where the temperature does not rise as the water is converting to steam. If this zone is very short, compared to the tube length, the Rijke's tube resonance could occur as the pressure pulsations (at the SAME frequency) stimulate the oscillation of this zone - if the zone is much shorter than the wavelength of the resonant frequency in the gaseous steam. (I think?).
The analogy being, that if the system's resonant frequency is stimulated by pump pulsation frequency, then a noise will sound - easily picked-up by the simplest sensor - the ear. As long as a resonant sound does not occur, then all is OK.
"SOUNDS SIMPLE" - - or am I being too simplistic?
When resonance occurs, (a mechanic once told me) it is the machine shouting "Help! Turn me off - before I explode!" And as long as we heed such warnings life should be good. Think of it this way - hit a bell and the sound is resonant - a single note. As a Tympanist I used to pick up a note from the band/orchestra and sing it close to the drum-head. when re-tuning to a different note. When truly in-tune the drum-head would sing back. - So loud I once had a conductor tell me to stop tuning at that quiet point in the music as he could hear it! - It was Out-of-tune to the key being played at that time. So we agreed where I should "sing quietly" to the drum, so it's "loud reply" didn't affect the music. Resonance is always LOUD as it absorbs virtually all the energy available as re-broadcasts it on a single frequency.
If not relevant or useful, just ignore this.
I can also explain why running two pumps at a SAME rotational speed - one with 2 pistons and one with a multiple of 2 times the pistons gives a higher frequency, but it still resonates at a numerically equivalent frequency. NORMAL practice in industry is to either add a damper or something to change the resonant frequency of the system, not necessarily change the driver, I.E. the pump. You plan on using variable speed to vary the flow, so if the resonant frequency of the BOILER is unchanged, you'll still hit a resonant frequency some way when the pulsations from the pump match it. A 2 piston pump may hit resonance of 16000Hz at 8000rpm, but a 9-piston pump will hit the same 16000Hz resonance at ~1778rpm. A different pump speed, but the same resonance. Change the resonance to 16Hz, and you won't run a pump slow enough to hit that resonant frequency. Similarly if the resonance of the boiler is too high for pump stimulation of resonance.
Damping, other than by addition of a tuned resonator (Helmholtz resonator), is different, as it absorbs the energy between each input pulse, so there is nothing left when the next pulse arrives. A Helmholtz resonator - tuned to prevent pump pulsations at the boiler resonant frequency from reaching the boiler, would be placed on a Tee-connection, located between the pump and boiler, such that the energy of pulsations - at resonant frequency - all goes into the resonator - and out again - such that the pulsations on the main-line do not go into the boiler. Like adding a large or suitably sized Capacitor on the output of a noisy electrical feed to eliminate the noise the is seen by equipment further along the line.
Tap the boiler (at pressure and filled with water) and listen to the note it "rings". Use your phone with software to tell you the frequency of that note - or compare to a piano - and then you'll know when resonance will occur, unless the phase change to steam is sufficient damping to eliminate of change the resonance. (The opposite of the Rijke's tube condition).
If it doesn't "ring" then the water, stress from pressure or something is either damping the noise, or raising the resonant frequency above the range of your hearing. (Speed of sound in water and metal is much higher than in air, so very high resonant frequencies). In this case, the pulsations from the pump (in the audible frequency range) are too slow to reach the boiler resonant frequency.
Also, if you start with a COPPER boiler that is annealed, it is likely to dampen the resonances in the metal, but after some use the copper work hardens, and the hard copper "rings" much better, as the internal material structure does not dampen the resonance. It then becomes prone to resonance fatigue failures. Tin is used for Organ pipes, so the pipe-metal resonance (very low, well damped) does not interfere with the air-column resonance (Audible range). But Tubular Bells are hard metal to "ring" at the metal-length resonance (Undamped, Audible range oscillation), tuned to the tube of air column length, so that resonates and amplifies the sound created by the walls of the ringing tube.
Some of the above may be useful, some inaccurate, but it is "what I know", so please correct me if I am wrong and ignore what does not affect this problem. (Is there really a resonance problem? - Or just a "measuring instrument resolution" issue?).
K2
 
That makes sense to me, especially the difference between the axial and double acting pump.

The reason both triplex and axial pumps have 3 pistons is the pressure wave overlap. If those double acting pumps of yours are holding up to the heat, what about driving 3 off of jack shafts from a common motor. Then you'd have 6 pulses that you could overlap?

I tried to find a chart showing actual piston velocities in a fixed swash plate system, but all I could find was papers on variable flow swash plate units, where the piston travel is not fixed.

Seeing the actual actuation behavior of the pistons on a graph would have been nice.


In terms of resonance between the pump output and coils, I personally think it would be obvious. The license plate on my bike has a couple points where it resonates with the little bike engine and it causes a very audible racket. your system should act in kind, with resonance... well... resonating.


It seems to me that if you added a couple of accelerometers to your unit, in places they won't get cooked, an arduino should give you the pressure spike induced system vibrations as a simple to graph read out. Any sudden spikes in the vibration would let you know if you hit resonance.

To be clear it wouldn't likely show the pump graph but the overall system "noise" as I assume that the tubes generate lots of vibration as they vaporize the water.

The units are cheap so it wouldn't break the bank, if you are concerned about resonance.


Does that make sense?

Yes, adding accelerometers to sense & measure vibration is a good idea, especially during the development phase.

And you're right about the boiler making a fair bit of vibrations as the water first begins to boil, but it quiets down once the boiler is putting out a steady flow of steam.

Oh boy I can't wait for this move to be over. Then you guys can dog pile my projects 😉

Move ?? What move ??

Since, as I recall correct me if I am wrong, you're boiler is intended to operate above the critical point, you might have more cushioning then you think as you will have the water phase changes to absorb the pulses?

I can only get to the critical point with R123, or some other Organic working fluid. The critical point of water is 374 C
and 3,200 psi,...my copper tube boiler would most certainly rupture long before I reached those numbers. The critical point for R123 is 184 C at 533 psi, which is well within the capabilities of my boiler.

Edit: I was staring right at the forest but missed it because a bunch of trees were in the way

check out fig 6

https://tud.qucosa.de/api/qucosa:71101/attachment/ATT-0/

It shows the behavior of a single piston on a fixed plate.

Actually most of the paper is useful because while it is about variable plates, it is comparing them to fixed plates.

Hope this helps.

That's a interesting paper you linked,... the primary focus is on how pump pulses cause a sort of feedback problem, where the pump pulses induce small changes in the angle of the swash plate in a variable plate system, which then causes more pump pulses. This all occurs because the swash plate angle controls use the hydraulic output from the pump.
 
Just read this: "you're right about the boiler making a fair bit of vibrations as the water first begins to boil, but it quiets down once the boiler is putting out a steady flow of steam." - Excellent! What you are demonstrating is the slow vibration of initial boiling, maybe hitting a low resonant frequency, but when the boiler develops pressure and more steam the boiling is with smaller bubbles, and a higher frequency of "input" above the low resonant frequency of the boiler so it doesn't shake rattle and roll!. You can use that to determine a resonant frequency from the low note of those vibrations (use a phone with some software if it is a better calibrated ear? - it may be sub-sonic for the ear?) and compare that to frequency of pulsations from the pump. A pump that does not reach the resonant frequency is good.
K2
 
I very much appreciate everyone's inputs, suggestions, analysis, and pointing out what you believe are my errors; you've all helped me reach the conclusion that the best technical solution is to use a different pump, one with more than two pistons.

I'm opening a new thread under "A Work In Progress" where we can all discuss (and ridicule if you must) my DIY feed pump. Hot Link: 9 Piston Feed Pump
 
Yes, adding accelerometers to sense & measure vibration is a good idea, especially during the development phase.

And you're right about the boiler making a fair bit of vibrations as the water first begins to boil, but it quiets down once the boiler is putting out a steady flow of steam.



Move ?? What move ??



I can only get to the critical point with R123, or some other Organic working fluid. The critical point of water is 374 C
and 3,200 psi,...my copper tube boiler would most certainly rupture long before I reached those numbers. The critical point for R123 is 184 C at 533 psi, which is well within the capabilities of my boiler.



That's a interesting paper you linked,... the primary focus is on how pump pulses cause a sort of feedback problem, where the pump pulses induce small changes in the angle of the swash plate in a variable plate system, which then causes more pump pulses. This all occurs because the swash plate angle controls use the hydraulic output from the pump.
Yes, adding accelerometers to sense & measure vibration is a good idea, especially during the development phase.

And you're right about the boiler making a fair bit of vibrations as the water first begins to boil, but it quiets down once the boiler is putting out a steady flow of steam.



Move ?? What move ??



I can only get to the critical point with R123, or some other Organic working fluid. The critical point of water is 374 C
and 3,200 psi,...my copper tube boiler would most certainly rupture long before I reached those numbers. The critical point for R123 is 184 C at 533 psi, which is well within the capabilities of my boiler.



That's a interesting paper you linked,... the primary focus is on how pump pulses cause a sort of feedback problem, where the pump pulses induce small changes in the angle of the swash plate in a variable plate system, which then causes more pump pulses. This all occurs because the swash plate angle controls use the hydraulic output from the pump.

I'm in the middle of a move and thus between houses with no functioning work shop, so I can't start anything. Only dream lol.

Sorry I got the criticality mixed up.
 
I'm in the middle of a move and thus between houses with no functioning work shop, so I can't start anything. Only dream lol.

Sorry I got the criticality mixed up.
I can relate,....when I moved from San Francisco to Thailand I waited for 3 long months for all my household goods, including all my hand tools and benchtop CNC mill, to arrive, then several more weeks for my crates to make it through Thai customs.
 
I can relate,....when I moved from San Francisco to Thailand I waited for 3 long months for all my household goods, including all my hand tools and benchtop CNC mill, to arrive, then several more weeks for my crates to make it through Thai customs.
Well happily it's not that rough, but the new place had a lot of hidden work. I was supposed to be happily nesting in a pile of swarf months ago!
 
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