Author Topic: Developing a Small Steam Plant  (Read 14097 times)

Offline gary.a.ayres

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Re: Developing a Small Steam Plant
« Reply #105 on: January 14, 2020, 10:42:32 PM »
Thanks for this MJM. You have very clear knowledge of all of this, and the way you describe it makes the processes apparent where they previously weren't - at least to me. Sorry that my steam plant is invading your sleep though!

To be honest, I'm not totally sure that I was doing anything different with the throttle. I may have been, but I think I had it pretty wide open most of the time. I will play around with this next time. It's also possible that I'm imagining things - my impressions are very subjective after all, and it's been a while since the last run before this one.

There is however another variable not related to steam or heat, which is the tightness of the retaining spring holding the cylinder against the port block. I think I had it tighter this last time for most of the run, which could account for the slower speed, and - given your interesting point about the longer steam admission through the overlapping ports could - I guess - increase the torque too (though it would also be working against increased friction from the spring, so  that could mitigate the increase I suppose). So many parameters adding layers to the complexity in an apparently simple system!

One thing that has always happened with this plant is that I can let the pressure build up quite high initially (about 50 psi on the gauge, probably - as you say - much higher than the engine needs), then when I open the steam out valve it goes like a demon for about 30 seconds with clouds of wet steam around the engine, then the not very acccurate small gauge drops back down to the peg (i.e. a false reading of zero) but the engine runs for ages as described. Maybe I should try to control that by not opening the valve so wide at the start...

The displacement lubricator seems to work well. I'm not sure I had the needle valve open wide enough, but there was definitely some oil getting through. I can play around with that. It's all a learning curve...

As for the exhaust steam - certainly what steam there was was invisible, i.e. dry, so the separator must be doing its job. Very pleasing that that part of the build was a success. But what struck me was that I held a tiny scrap of tissue paper over the stack and it hardly fluttered at all. This was why I said that there seemed to be almost nothing coming out of the stack. I wasn't necessairly expecting visible wet steam, but I was expecting more of something  :). I'll look more closely at this next time, which will be after the pipes are all lagged, and I reckon I'll have a go with the pressurized meths burner.

But the thing is, it runs! None of the issues under discussion are worrying me, but yes it's interesting to learn a bit about the reasons why things happen the way they do. And your knowledge of them is making this thread into a resource which even others much more experienced than me will find informative.

 :ThumbsUp:

Offline MJM460

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Re: Developing a Small Steam Plant
« Reply #106 on: January 16, 2020, 04:27:13 AM »
Hi Gary, thank you.  I think you probably were right in the button remembering that you had tightened the spring.  That is a likely explanation for some difference in performance.

The spring holds the cylinder port face against the port face on the standard.  On one hand a tight spring seals against steam leakage across the port face to atmosphere, but on the other hand, a tighter adjustment increases friction as the cylinder oscillates. The steam pressure at the port in the standard is trying to push the cylinder away from the stand, so leakage will be high if the spring is loose.

Running unloaded, all the work produced by the engine is used purely to overcome friction.  So if you increase friction by tightening the spring, that increases the load on the engine, which will slow it down.  With luck and a well aligned port face with the cylinder pivot truly at right angles to the face, it will reduce that steam leakage, making more available to be converted to work by the engine, and preferably more than the extra power lost by the extra friction.  Room for experiment to find the best adjustment.

By the way, in trying to explain your observed increase in torque, I did overlook the very obvious.  The torque needed to overcome friction in the engine is relatively constant, regardless of speed.  So if the engine is overcoming friction, plus working against something rubbing on the flywheel, it is definitely producing more torque. 

If you did not adjust the burner, there is no extra power available, and as power equals torque times rotational speed, more torque, same power means less speed.  However, that also means that while the torque to overcome friction does not change much, the power lost to friction is less at lower speed.

Now the power is not necessarily exactly constant through all this as, with different steam velocity etc. the efficiency probably changes a bit.  But with no burner adjustment, it can never be more, and most likely less.

But at the end of the day we still have conservation of energy, and conservation of energy is one of the fundamental laws of physics.  It is neither created or destroyed, just changed to different forms.  Like your car keys, it does not vanish when you lose it, it just takes more effort to find out where it has gone.

Lubricator are interesting things.  I often wonder if the needle valve does much to change to rate at which oil escapes to the engine, another area with room for more experimentation, but I suspect if you fill it before each run and drain the condensate afterwards, the engine will have enough oil, whether it is injected quickly or slowly.  Some oil is always better than no oil and all those little Mamod engines and similar had no lubricators, only what the owner dropped around the port face before and possibly during running.  Wet steam from a simple boiler probably helps a little, but if not oiled and a lot of running they probably showed a lot of wear.  So I think any oil will look after your engine, preferably try and find some proper steam oil, it sticks to your boots better, but use what you can find.

Longer admission time due to the engine running more slowly allows more steam to enter the cylinder while the ports are open just as far as they were when the engine ran faster.  More steam in the cylinder means more pressure at the piston face, which gives more torque.  That is why large valve ports and large steam passages in the block is important.

I mentioned that there are good reasons to run the boiler at higher pressure, even though you don’t need it to run the engine.  At higher pressure the volume of the steam produced is smaller, so it rises to the surface less vigorously, so it carries over less water to the steam outlet, thus delivering drier steam to the engine.  However, when you open the stop valve, just open it enough to run the engine at the speed you want and keep the boiler pressure higher.  The small volume of the high pressure steam increases as the pressure drops in passing through the only slightly open throttle valve.  Only a very small loss in boiler efficiency, as the higher boiling temperature means slightly less heat transfer from the flue gas.  The boiler exhaust gas will be a bit higher temperature.  But the chimney is a generous size so the stack velocity might be quite low.  Try if a longer ribbon of tissue taped to a bamboo stick to keep your fingers out of the hot gas is more sensitive.  And whether you can see a difference between when the engine is running and not.

So much to learn from such a deceptively simple looking plant.  But it all applies when you go on to build something more complex or larger.  And all part of the fun.

And above all, as you say, it runs.  Always an exciting achievement.

MJM460
The more I learn, the more I find that I still have to learn!

Offline gary.a.ayres

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Re: Developing a Small Steam Plant
« Reply #107 on: January 25, 2020, 03:34:37 PM »
Have been very busy with work...

MJM - yes, interesting. Increase the friction slowing down the speed which lets more steam in which counters the loss through friction... parameters working against each other means that there is a sweet spot somewhere in there waiting to be discovered.

Interesting that you suggest that I only let enough steam out of the boiler at pressure to run the engine. I had come to that conclusion myself, and will try it next time. I have up to now tended to let rip with the steam from the outset - probably why the pressure drops so quickly.

Another thing I'm going to do is put some graphite yarn in the groove in the piston. I machined the groove shortly after I made the piston but it has never been used. Definitely worth a try.

On lubrication - I do have steam oil - a big bottle of it that someone gave me with a lathe I bought. That's what I have been using. It did seem to me that not a great deal was getting through, but that said I did drain a reasonable amount of water from the lubricator post-run so clearly it was working. I'll try opening the needle valve right up and see if it makes a difference...

Offline MJM460

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Re: Developing a Small Steam Plant
« Reply #108 on: January 26, 2020, 11:01:38 AM »
Hi Gary,

Basically, if the boiler is operating at higher pressure and the steam outlet restricted by the throttle valve, then slowing the engine means less volume of steam to the engine.  The pressure drop cross that throttle valve will reduce at lower flow rate.  So the piston will see a higher effective pressure, so produce more torque.

If the increase in torque is enough, the engine keeps running at a slower speed where the torque produced is just enough to overcome the friction.  You might wonder if this will really produce enough extra torque.  Well it did, because you reported that the engine continued running at a lower speed.  If the torque increase was not enough, the engine would stall/stop.  The engine finds that sweet spot on its own unless you interfere by changing something.

It’s the rotational equivalent of Newton’s laws about a body continuing in a state of uniform motion unless acted on by an external force.

In rotational systems, a spinning body continues to spin at a uniform rate unless acted on by an external torque. 

In each case, the external force or torque is the resultant from adding all the torques or forces as vectors to find a single equivalent action.

Of course, having the engine run at constant speed is not the same as the whole plant being in equilibrium.  With the engine taking less steam, the boiler pressure will increase, starting another chain of actions.  The higher pressure means higher boiling temperature which reduces the temperature difference driving heat transfer from the flue gas, so the flue gas at the stack is hotter, taking the excess heat to the atmosphere.

If this is enough, the boiler finds a new steady pressure, if not, the pressure continues to rise until the safety valve lifts.  Or the operator turns down the burner a bit.

I hope that is not all too confusing.

MJM460
The more I learn, the more I find that I still have to learn!

Offline gary.a.ayres

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Re: Developing a Small Steam Plant
« Reply #109 on: January 26, 2020, 11:00:21 PM »
Well, I had to study it a bit but I got there. Yes, there's a heck of a lot going on, isn't there?

And the whole universe is like that...  :o

I will certainly try running it with much less steam from the outset next time.

Now, at the other end of the steam spectrum from hard science: I had a dream a couple of weeks ago that the base board for the plant was supported by four ball and claw feet, one at each corner, the claw being made of bronze and the ball of glass. I like to follow dreams (well, some of them anyway) so on to the internet I went and sure enough, ball and claw feet with a glass ball do exist. There were quite a few on ebay.com, and all appeared to be antiques from the USA for about $50 a set and the same again for postage to here in Britland. In other words, a bit more than I fancied spending on them.

However, on ebay.co.uk I found (and bought) these for a much lower price:



'Lion's paw' feet, possibly from a clock originally. Each one is 8cm long. They are chunky and quite heavy, appearing as they do to be made of cast iron with a copper-coloured plating on them. The seller thought they may be French.

Totally over the top! Going full Steamgoth...

 :pinkelephant:

Offline propforward

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Re: Developing a Small Steam Plant
« Reply #110 on: January 26, 2020, 11:16:15 PM »


Totally over the top! Going full Steamgoth...

 :pinkelephant:

I love it!
Stuart

Forging ahead regardless.

Offline gary.a.ayres

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Re: Developing a Small Steam Plant
« Reply #111 on: January 26, 2020, 11:33:38 PM »
 :ThumbsUp:

Offline zeeprogrammer

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Re: Developing a Small Steam Plant
« Reply #112 on: January 27, 2020, 12:51:16 AM »
Too cool!  :ThumbsUp:
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Offline Flyboy Jim

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Re: Developing a Small Steam Plant
« Reply #113 on: January 27, 2020, 02:59:55 AM »
I'm betting you're the first on on your street to have a set of these!  :thinking:

Jim
Sherline 4400 Lathe
Sherline 5400 Mill
"You can do small things on big machines, but you can do small things on small machines".

Offline gary.a.ayres

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Re: Developing a Small Steam Plant
« Reply #114 on: January 27, 2020, 11:16:08 PM »
Thanks Guys.

@ Jim - probably, but soon they'll all have them...

Offline gary.a.ayres

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Re: Developing a Small Steam Plant
« Reply #115 on: January 28, 2020, 11:48:48 PM »
My plan had been to hook up a tiny generating set and light to the plant as a finishing touch, even though I'm not quite ready for finishing touches. However, I ended up disheartened by unsuccessful tinkerings with toy motors and LED's. Electrics is not my thing.

So, instead, I have just taken a trip back to my childhood and ordered this:



It was the cheapest one available on ebay UK, despite it being in new condition. But what really drew me to it was the fact that it is not painted in the traditional Mamod pale green, but is of their more recent blue colour scheme. I like this because I plan to paint it to suit the plant, and to get rid of old-school Mamod green would be too excruciating for words, while removing that blue will pain me not at all.

So hopefully with this addition the plant will be able to do real, useful work, such as sharpening pins. Which I'll be able to use to jab myself with to make sure I'm not dreaming again...

See? A real machine - not a model at all...

 ;)

Offline propforward

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Re: Developing a Small Steam Plant
« Reply #116 on: January 29, 2020, 12:00:38 AM »
Great idea - I look forward to seeing that run. Fun little grinding wheel.
Stuart

Forging ahead regardless.

Offline zeeprogrammer

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Re: Developing a Small Steam Plant
« Reply #117 on: January 29, 2020, 12:34:04 AM »
 :Lol: Fun post.
Carl (aka Zee) Will sometimes respond to 'hey' but never 'hey you'.
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Offline gary.a.ayres

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Re: Developing a Small Steam Plant
« Reply #118 on: January 29, 2020, 11:03:46 PM »
Cheers guys!

 8) :ThumbsUp: :ThumbsUp:

Offline gary.a.ayres

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Re: Developing a Small Steam Plant
« Reply #119 on: February 02, 2020, 10:14:27 PM »
The feet are now screwed to the bottom corners of the base board, and a profiled trim of softwood beading has been cut and shaped ready to be attached to the board using brass slot-head screws. Since this picture was taken the trim has been removed and given a further tidy up and is currently being given a few coats of Danish oil which will bring out the figure and give it a nice golden sheen to contrast nicely with the dark varnish on the board:


 

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