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Author Topic: Flash steam plant control.  (Read 96269 times)

flashtwo

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Flash steam plant control.
« on: March 05, 2009, 08:58:36 pm »

Hi,
I've opened this new thread in response to the interest shown (by Proteus and Bunkerbarge et al.) in my flash steam control system, elements of which I have described in other threads.

In 2006 I had some vague idea about building a boiler which didn't fall foul of the Pressure vessel regs. and insurance. I visited the 2006 Warwick Model Exhibition and luckily found Steamboat Phil with his display of flash steam boats, old and new. With Phil's good natured encouragement, off I set to build a "controlled" flash boiler that could sedately power a Windermere launch at a few knots, but not to break any high speed records.

It took two hours to wind the 15 foot x 1/4" copper pipe into a volume about 3" dia. 9" long boiler. That was the easy part. The rest of the story revolves around learning to program PIC controllers, building the Stuart D10 from castings, building the electric pumps and servo-controlled gas valve.

There were many conflicting ideas on how to control a flash boiler - do you control the steam pressure by the pumps or by the gas valve? The PIC controllers were ideally suited for experimenting with the different methods, because, once the controller board was built, the different control methods could be downloaded without having to resort to the soldering iron or lathe.

I've got the stage where I can go from cold (2degC) to engine running at 200RPM in three minutes  using 8 grams of butane/propane, tick-over at 200RPM lightly loaded at 6.5psi, using 1.2grams/min and run at 300RPM heavily loaded at 38psi using 4.5grams/min.

The only control signal I transmit is the engine speed demand signal (the boat will have separate channels for valve gear, drains and rudder control) and the PIC based controller monitors the boiler outlet steam temperature and pressure, gas cylinder pressure and the economiser outlet temperature. The controller then calcualtes the feed water requirement and gas valve servo position.

The speed regulation is purely done on feed water and gas supply, since there is no throttle / regulator between the boiler and the engine. If I set the demand RPM to 400 then the pumps gradually increase the feed flow and the gas valve opens to maintain the steam temperature - the speed then increases and hovers around 385 to 415 RPM.

That's the potted story to start the thread.

Ian

oh, and by the way, surprisingly, you don't bother controlling the steam pressure!
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kno3

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Re: Flash steam plant control.
« Reply #1 on: March 05, 2009, 10:36:55 pm »

Pictures?
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Bunkerbarge

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Re: Flash steam plant control.
« Reply #2 on: March 06, 2009, 12:22:24 am »

Now that sounds like an interesting process Ian.  By the sound of it the unit works on similar lines to what we used to have on cargo ships as a stand by boiler, which was known as a "Stones" vapour boiler.

You basically pumped water at very high pressure around a coil which formed the furnace into which the burner fired it's flame.  You could raise steam from cold in about two minutes and the units were completely automated.  I remember the water pump needed to be a high pressure pump and they were basically a ceramic piston pump of the same type as is still used nowadays in high presure water washing units.

They were great when they were working OK but if anything went wrong they tended to fail in spectacular fashion!!

Looking forward to some pictures if possible.
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Circlip

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Re: Flash steam plant control.
« Reply #3 on: March 06, 2009, 01:50:36 pm »

The early "How to" types of model boiler relied on the pump delivering water through a clack with a by-pass to limit the amount and the clack being sealed by the back pressure of "Flashed" steam due to it hitting really "Warm" tubes.  O0

  The "Safety" of this operation as opposed to the more normal Kettle types is that in the event of a failure in the Flash system there is no water flying about and the pressure is released instantly.

   Regards   another Ian.
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flashtwo

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Re: Flash steam plant control.
« Reply #4 on: March 06, 2009, 09:11:11 pm »

Hi,

Here are a number of photos. they show the boiler with the casing removed and the controller with its developement display unit. The display unit is not installed on the boat and the steam plant can run without it.

I'll shoot some better pics outside in the daylight, since the soot covered boiler tubes seem to absorb all the light!

The complexity of the traditional mechanical controls resides within the software, which is difficult to show as you can quite imagine. A simple line count of the source code is in the region of 20,000 lines, which includes commentary lines and some spacing.

Regarding the term "coil boiler" - the first time I heard the term was at the Ally Pally in January, when a visitor mentioned that he used to work for a comapany that made them.

Ian.
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flashtwo

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Re: Flash steam plant control.
« Reply #5 on: March 06, 2009, 09:12:27 pm »

Here's the controller photos.

Ian
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flashtwo

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Re: Flash steam plant control.
« Reply #6 on: March 08, 2009, 09:24:04 am »

Hi,

I've come to grips with a photoshop package and I have produced some better pictures.

Displayed are:
a)  The gas valve with its servo and the gas cylinder pressure transmitter,
b)  The boiler, economiser, oil separator and D10 engine.
c)  The inside of the boiler casing showing the insulation and expanded metal mesh.

Ian.
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Bunkerbarge

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Re: Flash steam plant control.
« Reply #7 on: March 08, 2009, 05:41:40 pm »

That's a pretty impressive piece of engineering there.  Very interesting to see you developing such a project and, as you say, translating all the old mechanical processes, which were notoriously unreliable in real life, into software.
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logoman

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Re: Flash steam plant control.
« Reply #8 on: March 08, 2009, 09:07:06 pm »

lovely to see it again, please keep us updated with the progress.
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mogogear

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Re: Flash steam plant control.
« Reply #9 on: March 12, 2009, 12:48:57 am »

As my inexperience with flash steam systems will be obvious...In your system .. is this a closed system that recaptures the "spent " exhaust steam and it's condensate replenishes the "coil" boiler? --Well- I guess it can't since it would contain traces of lubrication oil.

I was thinking of a system similar to refrigeration --except heating to expand-- then power through the engine- then condensate back into liquid form... and start the cycle over again...a weak mind on my part!!

Great engineering job!!
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BarryM

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Re: Flash steam plant control.
« Reply #10 on: March 12, 2009, 10:00:07 am »

Mogogear,

You're describing a closed feed system as is common in full-size steam plant  but is seldom practical in model format.

Regards

Barry M
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SteamboatPhil

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Re: Flash steam plant control.
« Reply #11 on: March 12, 2009, 10:16:01 am »

Ian's plant is a total loss system, as BaryM has said a closed system in mode form is next to impossible to do and make efficient.
l
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Bunkerbarge

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Re: Flash steam plant control.
« Reply #12 on: March 12, 2009, 10:23:16 am »

That's about it Barry.  Modern steam plant be it main propulsion or auxilliary systems always recover and recondense the steam.  Water consumption would be unbelieveable if you didn't and you couldn't make enough feed water to keep up with it!!

To translate this to a model boat though you would have to be considering a condenser unit and all it's associated auxilliaries and controls, not to mention seperating the lube oil system from the steam system.  As the model is never more than about ten minutes from the bank it is a lot easier to simply use the feed water and allow it to exhaust.  Having a feed tank on board can extend the duration to well over an hour, which is beyond most lubricator and seperator tank capacities so most modellers can comfortably work around this and put up with a total loss system.
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mogogear

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Re: Flash steam plant control.
« Reply #13 on: March 13, 2009, 05:32:41 am »

Ok.....So Ian... could you pin-point the inlet for water to your tubed boiler? Is it that small solenoid directly in front of boiler ?   I am very well schooled now that you will need a constant flow of water from some where to supply your Coiled volcano ........
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flashtwo

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Re: Flash steam plant control.
« Reply #14 on: March 13, 2009, 09:35:19 am »

Hi,

The feed supply will come directly from the lake, through a settler tank to allow any grit to settle out, into a common suction manifold from whence the two electric pumps draw their supply. The pumps have their own non-return valves and deliver to a common boiler supply manifold. One of the pumps is for standby duty.

The route through the boiler is up through the economiser in the stack (the vertical coil where it can pickup to 74degC of temperature), down to a thermocouple pocket,where the economiser outlet temperature is measured, and then into the steam generating tube. The steam exits via a manifold, which houses a safety valve (rarely ever used since the software shuts the gas valve well before high temperatures or pressures are reached), boiler outlet pressure and temperature tapping points and finally the engine steam supply port.

I'm not concerned about mineral build-up in the boiler, since it is continuous "blow-down" with fresh water pumped through the entire single tubed system.

The small silver "solenoid" in front of the boiler is, in fact, the boiler outlet pressure transmitter (Tx) - the long copper tube is to entrain cold condensate to stop the transmitter over-heating. The Tx is ranged 0 to 10Bar and sends an electrical signal to the controller, where it is used for high pressure shut-down and calculating the steam saturation temperature (Tsat ) for the given steam pressure. The Tsat is the actual set point to which the gas valve controls the steam temperature.

I've just finished making the 4.5 inch prop and just about to fit the valve gear and drain valve servos.

Ian.

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dreadnought72

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Re: Flash steam plant control.
« Reply #15 on: March 14, 2009, 11:44:05 am »

I'm extremely impressed - and not least by the use of microprocessors to do the hard work. I'm planning to use a couple of these in my (electric) build, and wouldn't mind hearing more about the programming you've done with them...to my mind, they offer the opportunity to allow for onboard functions and control that'd be difficult to achieve otherwise, and I hope non-programming modellers wouldn't be put off learning another skill, or by the idea of writing programs. (Far easier than programming the VCR, I think!)

Andy
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flashtwo

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Re: Flash steam plant control.
« Reply #16 on: March 14, 2009, 12:49:17 pm »

Hi Andy,

Thanks for your interest in the software side of the project, since that is the major difference between it and the conventional model boat steam plant.

As I said above, the biggest advantage of using the micro-controllers is in being able to build a common controller board and then program it for whatever application you need without major wiring rebuilds.

My starting point was the Velleman programming board from Maplins (part N61CG), which connects to the COMM port of your PC - make sure you have a COMMs port since the programmer is not a USB version. If you only have a USB connection there are other USB type programmers available.

Once you have the programming board, you then write the software code on microsoft Notebook, compile it using  MPASM, Microchip's Universal Assembler, which you can download for free from the internet and then physically download the program onto the chip using Velleman's PICprogrammer (I think thats from the disc that comes with the board).

The board has all the essentials to start programming - little push button switches and LEDs - and also some starter programs for demonstration, which you can modify and test yourself. All my programs have descended from the demo programs. One note of caution- always manage your software by ensuring you use a version sequential numbering system (V1, V2 etc.), note the major changes between versions and the date (this can all be written at the start of the code) and KEEP backups on DISCs. Never change a working version, always make a new version and change that.

You can see the dichotomy here - what starts out as a model boat forum topic rapidly descends into "computerspeak", but, for some modellers, the technology offers solutions to some awkward control problems, plus the opportunity of some innovative "on-board" action.
As an example, for an RNLI fundrasing event last year, I programmed the PIC controller on my RAF Crash Tender to send slow morse code on its search light with the message "Grace Darling"  and offered a prize for decoding it (damm -  I'll have to change the message now!).

The Administrator may have a view on this venturing off into another realm of technology far removed from what is essentially the manual craft of modelling boats.

Thanks again for your interest and I'm quite happy to answer any questions.

Ian.


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BarryM

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Re: Flash steam plant control.
« Reply #17 on: March 14, 2009, 01:11:30 pm »

"The Administrator may have a view on this venturing off into another realm of technology far removed from what is essentially the manual craft of modelling boats."

I hope not; this is all about model control and that is as pertinent as planking a hull.

Keep going,

Barry M
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steamboatmodel

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Re: Flash steam plant control.
« Reply #18 on: March 14, 2009, 03:21:42 pm »

Ian stated "
My starting point was the Velleman programming board from Maplins (part N61CG), which connects to the COMM port of your PC - make sure you have a COMMs port since the programmer is not a USB version. If you only have a USB connection there are other USB type programmers available."
I have one of Microchips PICkit 2 units which uses USB
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en023805
so far I have only played around with the demo programs on it and haven't developed anything else.
Regards,
Gerald
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nick_75au

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Re: Flash steam plant control.
« Reply #19 on: March 15, 2009, 09:38:40 am »

Hi Ian,
Very impressive work, I am interested in your feed pumps, any more detail on these please.
regards
Nick
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flashtwo

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Re: Flash steam plant control.
« Reply #20 on: March 15, 2009, 04:00:46 pm »

Hi Nick,

Here is a close up of the "duty" pump, the "standby" pump being a mirror image and located on the other side of the engine.

The first photo shows the main pump body with a 50p coin in front (no Aussie coins I'm afraid!) with the 1/4inch (6.35mm) ram going to the right. The crank is connect via a 100:1 gear box to the electric motor. On the crank arm is a small 2mm rare-earth magnet which is detected by the Hall effect chip located on the bearing block (see 2nd photo for detail).

The Hall effect chip sends a once-per-rev pulse to the micro-controller PIC where the software program converts the signal into cc/min feed flow, knowing the effective pump displacement. The feed flow signal then goes to the feed pump control PIC, which completes the control loop by controlling the feed pump's electric motor via an ESC.

The whole reason for doing this is to make the feed flow independant of the boiler pressure - if the boiler pressure goes up then the pump pumps harder (not faster) to maintain the desired flow.

Hope this is the detail that you wanted.

Ian.
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flashtwo

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Re: Flash steam plant control.
« Reply #21 on: March 15, 2009, 04:04:38 pm »

Here's the one with the coin before it was spent on sticky sweets!

Ian
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nick_75au

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Re: Flash steam plant control.
« Reply #22 on: March 16, 2009, 10:29:12 am »

Hi Ian
Thats was exactly what I was looking for, I inferred from the first group of photos that it was a gear type pump, obviously not :-)
I don't quite understand this statement?

"if the boiler pressure goes up then the pump pumps harder (not faster) to maintain the desired flow."

you don't increase speed of motor at all? or is that tied to the rpm desired perameter, is there a servo operated bypass that varies the output of the pump? am I getting two things confused?

RPM desired tied to cc per minute I guess?

Nick
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flashtwo

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Re: Flash steam plant control.
« Reply #23 on: March 16, 2009, 02:48:20 pm »

Hi Nick,

The pump pumping harder not faster has the analogy of driving a car from the level then up a hill. You can maintain the exact same speed (i.e. pumps strokes/min) ,if, when you start to climb the hill, you put your foot on the throttle and the engine's power output increases, but not it revs ( assuming you stay in the same gear).

With the electric motor, the electrical current increases, creating a greater magnetic force in the coils, which ultimately opposes the boiler pressure.

The steam engine speed depends on the steam production, which depends on the feed water flow. Obviously, if the engine is required to work harder (and not faster!) the steam pressure has to go up, which according to the steam tables, gives you less volume of steam to play with and results in more feed water having to be supplied.

The other control loop, which looks after the engine speed, is the one that measures the engine speed (the same method as measuring the pump RPM), compares it with the desired speed (transmitted from the shore) and works out if there is a discrepancy in the feed water being pumped. If the engine is running slow, then the feed water demand is increased and the pump speeds up - more steam is then created to speed up the engine, and vice-versa.

The is no by-pass valve or throttle valve in the system - only a gas valve and pump non-return (clack) valves.

The pump's  effective measured displacement is 87%  of that calculated from the ram diameter and stroke - measured against a nominal boiler pressure of 2 Bar.

Ian
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nick_75au

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Re: Flash steam plant control.
« Reply #24 on: March 17, 2009, 05:34:14 am »

I thought that's what you meant, couldn't get my head around it.
 I think I understand it now. Pump speed determines engine speed. at steady state rpm the control loop measures pump rpm and attempts to maintain fixed rpm, Slowing (increased pressure) increases and speeding up (reduced pressure) decreases the speed command to the ESC, in effect only changing the current draw of the pump.
Am I correct?
Nick
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