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Author Topic: Flash steam plant control.  (Read 93303 times)

rathikrishna

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Re: Flash steam plant control.
« Reply #150 on: December 05, 2010, 06:25:22 AM »

Good morning..and Sir..yes sure..all you are much fortunate...if you have any idea to do..all needed stuffs with you, as i buy 1 kg sugar from my nearest shop...but here if i want o do anything...must start from scratch...yes its more satisfying...but i do not have any ample muscle power to do big things...and fun..people always make fun on me as mad, as i wander through all junkyards and scrapyards to collect some usefull stuffs...and i always wonder while reading many posts here in MBMH , as each and every one is far beyond imagination and rate of work...what for more.. now we are submerged a stuff like these...now i am in my workshop to make a coiled engine by seeing that video in youtube yesterday...and i have a detailed study of this stuff ,because its too amazing and sure work of a lifetime...ahhh....
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ooyah/2

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Re: Flash steam plant control.
« Reply #151 on: December 05, 2010, 06:56:06 PM »

Hi Ian,
Just having another look at your coil, you seam to have more tube in this one than the old coil.
At 13.5 m (43 ft) is quite a length of coil. how does the new one at 250 sq" heating surface compare with the old one and if it's greater are you trying for more steam ?
Do you think that the coil being as tightly coiled you will get convected heat up through the coil with so little spacing and the plumbers type burner that you are using.
In my early experiments with a st/st coil wound as per Benson& Raymand   Experimental Flash Steam with the outer coil wound around the inner which left very little space between the coils I found that I couldn't get enough heat through the coil which was about 22 ft long and using a roaring type burner pressure fed from the engine and it had a .018 " jet I just couldn't get the boat to perform.
I then cut out the inner coil and rewound the outer coil and welded it to the inner giving a straight through coil which instantly produced a 20 mph boat.
My empirical deduction was that the flame was now getting all around the coil producing more heat.
My new coil is 32ft long wound around a tapered mandrel and with the three burners produces volumes of High pressure High temp flash steam, there is at least 1/4" space between the coils

Now I know that this isn't what you are trying to achieve but don't you think that with a shorter coil and more space between the coils and using a ring burner that you would get a better performance from your coil and still retaining the O/All  dimensions of your boiler.

Ian I hope that you don't think that I am trying to tell you what to do, it's just some thoughts from my early experiments.

Another thing I found was that I opened up the exhaust outlet and again when the gases were allowed a bigger escape rout achieved a better burn.
The exhaust outlet on the pic is the old one before I made the outlet much bigger.
Regards
George.
 



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flashtwo

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Re: Flash steam plant control.
« Reply #152 on: December 05, 2010, 08:15:05 PM »

Hi George,

I really appreciate your experience in the matter of coil "density".

I agree with you that there may not be enough circulation of the combustion gases and the combustion chamber may get pressurised, such that we have blow-back of the flame - I'll find out during the tests. I'll order some more copper tube and use half the length (I would loose 75sq.in.) for additional tests, - it all adds to the empirical database!

As the boiler is more of a monotube rather than a high performance flash, I might (hope!) get away with a more gentle application of heat - did you ever estimate the combustion exhaust temperature of your flash boiler; with my horizontal boiler it was about 220degC, but it came down to 170degC with the economiser fitted.

On the horizontal boiler, I started with a 1 inch dia. stack and couldn't maintain combustion for any length of time, but when I increased the stack to 2 inch dia. every thing settled down with the increased ventilation. I started with 84 sq. in. and gradually increased the area to 200, principally because I met a chap at the Guildford Show who had 164 sq.in. conventional boiler steaming his D10. I did find that this made a great difference in performance.

The 250 is the sum of the 200 in the boiler plus 50 in the feed heater.

The external boiler dimensions are to the scale that I need for the Edwardian steam launch I'm planning to build, so that part of it is effectively fixed.

So, that's my starting point with the vertical boiler and I will "unwind" the coil as necessary.

With my steam jet engine the coil was wound even tighter than this vertical boiler, I used all the steam output through a nozzle and blower, which that drew a tremendous combustion gas flow through the coils - it did melt the brass components though!

I did try using a blower on the horizontal boiler - it did increase the performance, but caused the steam temperature to cycle.

As with the horizontal boiler, I can see myself winding many more coils (I ought to unwind some of the old ones!).

Thanks for your interest George and sorry to hear about your shattered part of the D10.

Ian.
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ooyah/2

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Re: Flash steam plant control.
« Reply #153 on: December 06, 2010, 09:13:28 PM »

Hi Ian,
I have been thinking about your Monotube today, I have had plenty of time as we had 8" of snow dumped on us in 3 hrs this morning and that's on top of the 18" we already had and it's too cold for the workshop.
Unfortunately you are in an area similar to myself when I started out on my experiments, you to a certain extent are doing what very few have done before you so you plow a lonely furrow.
I can only comment as before that I think that your coil is too tightly wound but as you have already made it why not experiment further and see what happens, so I will watch with anticipation to see how you get on before I start.
You mention that you have melted s/solder joints, in my early experiments with copper tube I tried to keep any joints away from direct heat and wound stripe of ceramic wool  boiler cladding ( kaowool ) around the joint and then bound it with strips of .015 " stainless steel ( peddle bins are .015" )

If I hadn't been committed and started to build the D10 I could have been tempted to make a low pressure coil to power a 10V as I am quite sure that very high pressure and high temp steam would ruin a slide valve engine, in fact if I was to whack the flash steam from my present set up into a slide valve engine it would melt the valve before the pressure seized the engine.

Regarding Temperatures, I was never really concerned about that, I was more intent in getting the plant to run and as there was nobody up here doing this sort of thing I plowed my lonely furrow with the help of telephone calls to Bob Kirtley.
I can tell you that when the engine fired on the bench and I let it run for about 1-min looking through the burner tubes the St/St coil was white hot 2" from the front  and at the exhaust end the tail of the coil in the cold end (L.O.L ) was bright red with about 12 ' of flame shooting up so it wasn't a thing to be trifled with and as I didn't like to run it off load out of the water 1 min was enough, needless to say all of this was done outside of the workshop.
I think I mentioned that I have retired the boat at present as it's going too fast and with some times a radio glich it's very dangerous.

I am intrigued with your Steam Jet experiments, I wonder if a high pressure Flash set up would work. Keep up the experiments.

George.
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SteamboatPhil

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Re: Flash steam plant control.
« Reply #154 on: December 07, 2010, 11:46:40 AM »

Hi Ian & George

I also had trouble with combustion on my plant, and had wrapped the s/s liner to tight around the coil, although the plant ran, it was no great steamer. Redoing the inner wrapper masde a huge difference, and then I played with the exhust outlet (its now huge) and I'm now getting a resonable performance (its a V4 straight runner with piston valves)
With regard to high pressure on a slide valve engine, this should not be to much of a problem (as long as you are not going to compete with Bob (Kirtley) and his Hydroplane.
I am building a flash steam (radio this time) boat  with a slide valve engine (ala Mr Rayman's High speed engine) but using electric driven pumps (ala Ian), so not sure how it will all come together, but watch this space...........
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ooyah/2

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Re: Flash steam plant control.
« Reply #155 on: December 07, 2010, 09:51:12 PM »

Hi Phil,
This is what happened to me in my very early experiments, I made the coil and casing as A.Rs design in Exp.flash steam and found the it didn't work.
The double coil was too tight in the casing and the exhaust escape was too small, so as the pic that you already have I changed the coil to a straight one with plenty of space between and around the coil which gave great improvement, I also changed the burner to a tapered cone as R.Ks and this gave further improvement.

You said some time ago that you were going to try and achieve 40 mph from a double acting slide valve engine.
The engine that you are intending to use I don't think that this is possible, I say this as I have in the past made one of A.Rs engines and it seized under high pressure/ temp's.
If you lap the piston to the bore the tolerances are too tight and the valve wares tremendously under pressure which caused the engine seizure, this was with the boiler designed by A.R. working at 150 psi also with the shaft at 1/4 " dia with a groove for oiling the big end in my opinion is it's liable to sheer under the torque required for 40 mph.
It was after my experiments with this set up that I contacted R.K. and with his help and his drawings I made the 3/4 bore poppet valve engine with the twin burners which I eventually achieved 25 mph, but had to rebuild every 6 months.
The latest engine is a copy of R.Ks but only 1.125" bore ( R.Ks is 1.187 " bore ) with three burners same length of coil and the valve is made from a Cummins engine exhaust valve and with this set up have achieved 40 mph. and the engine peaking at between 10,000-- 15,000 rpm. the pumps are geared 6/1 from the engine all as my posting on the flash steam OOYAH.

40 mph is a bit of a handful on radio and very dangerous and plenty of space is required. In all off my early researches  90% of the old flash steam racing exponents used piston valve engines once they were dealing with flash steam as they found that double acting slide valve engines  couldn't produce the power.
Now Phil I am not  suggesting that you abandon your build, we can only learn by what others in the flash steam brigade do, there's not many about, it's only my opinion based on what I have already done, I hope you prove me wrong so I shall watch this space.
You will have to have some very fast electric pumps to feed the burners and boiler.

Regards
George.


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Circlip

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Re: Flash steam plant control.
« Reply #156 on: December 08, 2010, 11:01:59 AM »

Good to note that the spirit of experimentation is still alive and well and carried on by those who know what they're doing. That's not to say we all have to start somewhere so we shouldn't dampen the spirit of discovery, but, there's a difference between progressive tests and the "I've just designed" brigade who have naff all experience of even the Basic principles of steam operation from a safety point of view.

  Given the depth of knowledge exhibited by the present posters,(and that's a compliment), I did flinch at the thought of a "Free Running High speed flasher" though and have to agree with George (WHAT, a Yorkshireman agreeing with a Scot??????) in Phil's case. In the case of a "Glitch" it IS a free runner.

  Given that the worst case scenario is to mix Water, Electricity and a Woman, (Washing Machine) the second must be to mix High pressure steam and electronics to an untethered fast mover? Yes, the gas turbine people have done it on both toy aircraft and boats but even a toy glider has been responsible for at least two fatalities to my knowledge. Strange how the "Go faster cos I can" brigade don't have a little forethough on non "Organised" events when the uneducated bystander thinks "Oh, I've a pot of spare money, I'LL have a bash at that", and YES it does happen in ALL paths of the toy hobbies.

  Bricks on a string will always look impressive (strong string hopefully) and the present speed runs shown look good, but note how they hide BEHIND a cage when they've let go.

  Regards  Ian.
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flashtwo

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Re: Flash steam plant control.
« Reply #157 on: December 09, 2010, 12:28:55 PM »

Hi,

Thanks for the compliment Circlip, we all appreciate it.

I've just had the first steaming trial with the new vertical boiler, just one leaky connector going to the engine, but otherwise ok.

I had the prop spinning in air up to about 350RPM with the steam pressure at about 6psi; I won't be able to run it properly until its in the water.

A few figures were obtained though: the boiler feed inlet temperature from the feed heater was 93degC and the steam outlet was 109degC. The stack exhaust temperature was 96degC showing that most of the heat was being transfered to the water from the combustion gases.

There was no burner blowback, but saying that, the gas cylinder pressure was down at 7psi, due to the very cold weather, whereas, during the summer it is at 35psi.

The boiler case insulation needs to be increased as the outside was quite hot.

I've brought the gas cylinders indoors to warm up overnight and then I'll do some more test runs.

Ian.
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gondolier88

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Re: Flash steam plant control.
« Reply #158 on: December 09, 2010, 04:19:03 PM »

A few figures were obtained though: the boiler feed inlet temperature from the feed heater was 93degC and the steam outlet was 109degC. The stack exhaust temperature was 96degC showing that most of the heat was being transfered to the water from the combustion gases.
...Ian.

That IS impressive, well done!
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SteamboatPhil

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Re: Flash steam plant control.
« Reply #159 on: December 10, 2010, 08:06:24 AM »

Oooh, thanks George, it seems I might be heading down the wrong track, did not realise you had tried it (I should have guessed) there's me thinking I had come up with something new, should have known there is nothing new. I will have to start a bit of a re-think, and maybe go for just a nice performance (say around 20mph). As you rightly say Circlip could be a bit hairy (me and radio----hummm then add HP steam)....so back to the drawing board and a little re-think.
Right just off to the M E exhibition where I shall be showing my stuff.......
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ooyah/2

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Re: Flash steam plant control.
« Reply #160 on: December 10, 2010, 09:03:28 PM »

Circlip,

THE END OF THE WORLD IS NIGH !!!!!!!!!  (WHAT, a Yorkshireman agreeing with a Scot??????)
Soon put a stop to that ,look out for Flying Haggis they will be fired from the border, by steam cannon, Bradford ain't that far.

Ian , you are so right , when this thing OOYAH went it's own merry little way with me twiddling the sticks like mad and now't happening is quite a frightening thing and as I have said it's mothballed at present until I consider what to do with it.
George.
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SteamboatPhil

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Re: Flash steam plant control.
« Reply #161 on: December 11, 2010, 11:19:33 PM »

It has got me thinking to George, having spent my life with boats going straight over 100yds, and radio control being still a dark art...........

Oh circlip I'm slowing down.......... :((

Back to going straight-----but fast  O0
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flashtwo

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Re: Flash steam plant control.
« Reply #162 on: December 12, 2010, 10:41:17 AM »

Hi,

Did some more tests on the vertical boiler yesterday with a warm gas cylinder with about 22psi gas pressure.

Not surprisingly, there was some flame blowback with the gas valve fully open, but it does heat up much quicker than the previous horizontal boiler.

When the gas cylinder cooled down, I noticed that the engine revs responded very quickly to an increase in gas flow when I warmed the gas cylinder with my hands; this is very promising, since it will improve the acceleration / deceleration of the boat.

I've ordered some more lengths of 5/32inch copper tube and will wind a smaller area coil of about 120 sq.in in place of the current 200 in order to reduce the flame blowback.  As the feed inlet temperature is so high (96degC), I may not need such a large area anyway.

Ian
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Circlip

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Re: Flash steam plant control.
« Reply #163 on: December 12, 2010, 11:34:22 AM »

Quote
,look out for Flying Haggis they will be fired from the border

  Re-hulled Ooyah to look like one George??

Quote
it's mothballed at present until I consider what to do with it.
George.

 Psss, wana buy a Mamod??????

 Don't let me harden your damper Phil, just be aware. Many will say that the "New" 2.4Gigglies is far less subject to interference, probably so but then we found out about sunspot activity and the dreaded CB brigade on the Moggy cycle bands. We all know that we have an unrestricted right to play with our waterbourne Exocets with impunity don't we?? Have you got any grandkids that could become damaged by someone elses "Right of passage"?

  Regards  Ian.
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TurboTyne

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Re: Flash steam plant control.
« Reply #164 on: January 14, 2011, 09:36:46 AM »

Hello Flashtwo

It was through searching of the internet  for information about the control of monotube boilers, that I discovered the ModelBoat Mayhem forum - because of your posts  about Vital Byte.  Now I have joined, I wonder if I can ask your advice about a few aspects of your very impressive project since I hope to attempt to follow your lead and make my own controlled boiler.

Could you please give some details of the motors that you used to drive the feed pumps and where they came from? Also, did you consider using stepper motors for this? I know little about this sort of thing but I gather they can be easily controlled from PIC microcontrollers.  I am wondering if a stepper would have any advantages or disadvantages for driving the pump compared to a simple DC motor?

Thanks

Mike
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flashtwo

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Re: Flash steam plant control.
« Reply #165 on: January 15, 2011, 05:22:52 PM »

Hi Mike,

The most important aspect of flash boiler control is to maintain feed flow independently of the boiler pressure. Another feed requirement is to be able to set the feed to anywhere between zero and say 100cc/min, since this governs the rate of steam production and eventually the boat's speed.

The stepper motors are usually used for positional duties, e.g. telescope directional mounts, robotics, interfacing with hydraulic systems etc., and as you say these work very well with PICs. The required motor torque would have to calculated from the pump's ram diameter, operating crank length and the expected boiler pressure (and then double it!). Of course, if you have a ready supply of suitable stepper motors and interface I.C.s, there's no reason why they can't be used for pumping. One advantage of the stepper motor over an ordinary electric motor, is that it follows the pulse frequency of the PIC (as long as it doesn't pole-slip) and wouldn't need additional devices for measuring the pump speed and hence feed flow. You would have to weigh up the economics of which motor to use.

For pumping duties, I've been using an ordinary motor with a gearbox and a range of pressures and flows can be achieved by different combinations of gearbox and motor. A PIC can produce the necessary 1 to 2ms pulse to an ESC, which then powers the motor. It does however require a pickup (Hall effect device) to monitor the shaft speed and another PIC to convert the shaft pulses into RPM/feed flow in order that the PIC can amend the 1 to 2ms pulse if the flow is incorrect.

I've obtained my motor/gearboxes from http://www.mfacomodrills.com/ and are currently using a 385 motor with 100:1 gearbox driving a 1 inch stroke (1/2" crank), 1/4" ram. With this setup, I can achieve a flow of 75cc/min of up to 100psi with a 12volt motor supply. The actual required feed flow is set at the transmitter much as you would set the throttle (the flash boiler supplies steam directly to the engine and as such doesn't have a regulator / throttle).

The actual pump is of my own design and uses 3/16th ball valves, which are both accessable from above for cleaning without having to disturb any pipework. Since the boiler is using lake water (via a filter), I needed access to the valves for debris inspection, although any trouble is usually from the ram grease getting onto the ball valve

If you can build a controllable pump, then you are well on the way to controlling a flash boiler.

Ian.
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TurboTyne

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Re: Flash steam plant control.
« Reply #166 on: January 16, 2011, 07:22:11 PM »

Hi Ian

Thanks a lot for your very helpful reply.  I shall stick with your tried and tested use of a standard DC motor, especially now I’ve seen what is available at "mfacomodrills.com". My previous searching had failed to reveal any company selling anywhere near such a good selection of motors as theirs.

Can I please pester you with another question? This time it’s about the pressure sensors/transmitters that you used.  Can you please let me know the type(s) and from where you bought them. I have seen some quite pricey units at Farnell and have been wondering about trying to utilise parts from Maplin digital tyre pressure gauges. (They retail at just £3.99 and are rated for zero to 150psi apparently with a 1 psi resolution).

Thanks again

Mike
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flashtwo

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Re: Flash steam plant control.
« Reply #167 on: January 17, 2011, 11:53:01 AM »

Hi Mike,

I've been using what might be regarded as expensive (£85) pressure transmiters from RS Components, part no 455 4624. These are for the range 0 to 10 Bar (approx 0-150psi) and give a 4 to 20mA output, which can be put through a suitably scaled ammeter and/or into the analogue input of a PIC.

The advantage of using a transmitter is that the data, once "inside" the PIC, it can be used for control and also indication, either on a LCD and/or, as on "Vital Byte", a servo driven indicator that can easily be seen from the shore.

Frankly, I only tried using the pressure for control during the early experiments and now only use it for start-up and monitoring from the shore, in which case a 2 inch diameter (tyre pump gauge?),  traditional pressure gauge would suffice. The signal is also used to shut the gas valve if the pressure exceeds 60psi, but this has rarely been used in practice.

I see the Maplin’s digital gauge (A12JR) is +/-1psi accurate, which is more than adequate for our application for start-up and monitoring purposes, though it would be a bit small to see from the shore.

Whichever pressure device you use, you should ensure that a "swan-neck" (u-tube) is placed between the steam manifold and the gauge, so that a slug of cold water condenses and prevents the steam from over-heating the gauge/transmitter.

I also use a pressure transmitter to monitor the fuel gas cylinder pressure. This signal again is fed into the PIC where it is used as a form of fuel-gauge (a switch is used to change-over from the servo indicating steam pressure to that of indicating gas pressure); this is very useful for extended runs (an hour) and you want to keep an eye on the available fuel supply. The gas pressure is also used to trim the steam temperature control loop so that, when a fresh cylinder is installed and the gas pressure is high, the gas valve doesn’t open too vigorously. This is a bit of icing on the cake regarding temperature control and I think you could initially get away without using it. A conventional 2 inch gauge (0 to 60 psi) indicating gas pressure would be very useful to have (no “swan-neck” required).

A couple of old foot pumps would be a good source of useful size gauges. Polly Model Engineering do a range of gauges and also Technobots on http://www.technobotsonline.com/pneumatics/pressure-gauges.html. do a very reasonably priced range of gauges.

Since the “flash boiler” is not legally considered a pressure vessel, it doesn’t require a pressure gauge nor a safety valve, since the amount of “stored energy” is so low.

Ian.
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gondolier88

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Re: Flash steam plant control.
« Reply #168 on: January 17, 2011, 07:57:58 PM »

I suppose with a flash boiler the use of a digital thermometer is out of the question a the boiler temperature would be far higher than the steam temperature?

Greg
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flashtwo

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Re: Flash steam plant control.
« Reply #169 on: January 17, 2011, 09:13:21 PM »

Hi Greg,

Are you suggesting measuring the steam temperature and inferring the pressure from the steam saturation table?

Ian
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Re: Flash steam plant control.
« Reply #170 on: January 17, 2011, 09:28:20 PM »

Hi Ian,

Yes, that's exactly what I was thinking, of course Temp./Press. being directly correlating- a question once given me- if ever I was in the situation where every pressure gauge on a ship was wrong and you had no means of getting an accurate pressure reading what would you do- answer use an accurate thermometer and test the temperature of the steam take off, checking it off on the steam table.

This might work at the steam take-off on a flash boiler, but I would think the coil would be too hot to give any kind of realistic reading of the steam temp.? Not knowing enough about electrics I would possibly suggest high tem. thermistors? (think I may have mentioned this before?)

Greg
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Re: Flash steam plant control.
« Reply #171 on: January 17, 2011, 11:55:55 PM »

Hi Greg,

As long as you haven't got any superheat, i.e. measuring the steam temperature at the drum, then you could use the temperature, as with the old Cheddar ABC system that used a thermistor.

With the flash boiler (no drum) it is very easy, in fact desirable, to generate superheated steam and I've obtained 710degC with my steam jet boat; obviously for the conventional engine a only few degrees is desirable for dry steaming. Therefore, you can't use the temperature as a pressure substitute. In power stations, to obtain a representative drum steam temperature, the pressure is actually measured and converted to temperature.

You're right about using the temperature as a substitute under certain conditions. I do find the steam saturation table very useful during experimenting, since a lot of the time it is handy to correlate the pressure and temperature measurements.

I've understand that thermistors, although now very accurate, are only suitable up to about 135degC, therefore thermocouples are used, which are suitable to above 1000 degC.

Ian
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Re: Flash steam plant control.
« Reply #172 on: January 18, 2011, 07:42:49 AM »

Hi Ian,

Of course, a higher reading in temperature due to superheat would definitely give a false pressure reading.

I think that the point about the thermistors is something you have told us before, forgive my bad memory. I'll have to check 135deg. off on my steam tables- im not reading steam tables while having breakfast, have to draw the line somewhere! I was thinking about using a thermistor on the boiler I'm constructing at the moment.

The thermocouples that you mention, is there anything 'special' about them or are they mechanical ones, like the ones old gas valves for flame failure sensing, or are they now an electrical 'sensor' that has replaced them?

Greg
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TurboTyne

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Re: Flash steam plant control.
« Reply #173 on: January 18, 2011, 08:08:04 AM »

Hi Ian

Thanks very much for the second lot of information and advice.

Actually I was wondering about trying to dismantle the Maplin pressure gauge so as to make use of the electrical output from the sensor that must be in there. Perhaps the whole thing will be so integrated that it is not possible, but at that price it seems worth a try – especially in view of the info summarised at end of this message.

Yes, the Farnell pressure transducer prices range from £50 up to £500, which fits with the £85 price ticket of your sensors. Over the years I have accumulated quite a collection of mechanical gauges and I was seeking an electrical output for pressure mainly because, in an early post about Vital Byte you said that the pressure data was used to decide the steam saturation temperature and I had assumed that was on a real-time basis. Is that the case? If so, do you use the pressure output in that role in addition to its forming a safety back-up?   

From your advice, I now see that I should concentrate initially on setting up a controlled feed pump and a servo-actuated gas valve that is controlled by a thermocouple via a PIC device.  Hmmmm - if I get that far, I shall be well pleased and may then feel more confident that an investment in a decent pressure sensor is worthwhile.

I became a little side-tracked on trying to find information on components in cheap tyre pressure gauges. This led me to a fascinating document describing in detail the really sophisticated design logic behind what looks very much like my simple Halfords digital pressure gauge. This describes how a sensor chip is connected via an op-amp to a PIC 16F873. The microcontroller employs cunning logic to optimise the pressure data before it is displayed.  In case you or anyone else is interested, this URL should take you to the document :  http://www.ece.msstate.edu/courses/design/ece4512/2003_fall/air_pressure/Design.pdf
Also, the following link gives a data sheet for the sensor integrated circuit that is referred to.  http://www.cdiweb.com/datasheets/icsensors/1451.pdf
Just for completeness, the following link shows a report by someone who utilised electronic components in a tyre pressure gauge (to control a pool filtration system).  http://misterhouse.wikispaces.com/1-Wire+Pressure+Gauge

I guess there must be something extra in a sensor from RS or Farnell that costs over £80, but maybe it will be interesting to see if these cheap sensors prove useful.
Regards
Mike
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flashtwo

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Re: Flash steam plant control.
« Reply #174 on: January 18, 2011, 10:15:50 AM »

Hi Mike,

That's some very interesting and useful research that you have done there.

Regarding the Maplin gauge, I think that if you can get to the op-amp output that goes to the PIC (assuming its not encased with epoxy), then just a pair of wires to your PIC would do the trick - as you say at the price its well worth the experiment.

When I was using the pressure signal for the sat. temp., I eventually noticed it was causing instability when I was operating the valve gear on the engine, i.e.  the pressure would fluctuate, changing the sat. temp. value, which eventually caused the gas valve to vary unnecessary. Keeping the pressure out of the control loop kept things stable; another nail in the coffin for needing a pressure signal.

Regarding the safety back-up of shutting the gas valve when the pressure gets too high(>60psi), I think the pressure signal has some value here since it prevents lifting the safety valve which is perhaps set at 100psi - safety valves don't always reseat very well after lifting and you end up with a irritating leak.

I did originally include software filtering for an erratic pressure signal, but removed it when I discovered the problem was with steam-induced static electricity.

Have you looked the PICAXE website. They sell systems which can be written in PBasic and they already have a suite of code for doing the maths. They don't handle negative numbers for some reason and have had problems with jittering servos, although I daresay that there are work-arounds. There is a very large user base.

I think a very useful start would be to couple an LCD to the PIC. This allows you "see" what is going on in the PIC and is extremely useful for diagnosing problems.

I see from the data sheets that the sensor's expected media is a dry, non-corrosive gas, so you might need to consider the effects of water on the device.

I suppose what you are paying for with the professional sensors, is that they come ready calibrated, have a guarateed spec. especially regard to temperature stability which is normally the killer for the cheaper instruments. And not to be overlooked is the substantial mechanical case and connector that it is packaged in and its suitability for  a range of media including water.

Anyway, I would certainly experiment with the Maplins gauge, it might benifit a lot of people looking for a less expensive solution - I might try one out of  interest.

Ian
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