Understanding the adjustment of a compound

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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froya66
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Re: Understanding the ajustment of a compound

Post by froya66 » Wed Apr 07, 2010 6:02 pm

Hi All
farmerden wrote:It's interesting that my engine [a compound]starts,reverses,with no assistance from me .It has stevenson link and appears that there is no way for the steam to bypass from the HP to the LP.Den
In Funnel no. 124 page 56 Sam Wilkinson is talking about piston (spool) valves and there clearance.

He suggests a clearance of approx 0.005" (0.013 mm) for a simple, and 0.0015" (0.038 mm) for a HP valve on a compound. The latter in order to give a small leakage to ease starting and reversing without the need for a simpling valve.

Den:

I am not so familiar with balanced slide valves. Is there no possibility at all that they are leaking just a tiny bit due to the minor backpressure?

Best regards
Jørgen
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Re: Understanding the ajustment of a compound

Post by farmerden » Wed Apr 07, 2010 6:57 pm

Good thought Jorgen I just had a look at the spare engine I have and yes because the whole steam chest is under pressure it is quite possible that there is a certain amount of blowby allowed in this system .As a pressurized steam chest limits the amount of wear on the slide valve it would be right to assume that a certain amount of steam would leak out and put pressure to theLP . It is interesting that the Semple 354 "V" I have has a valve built in the crossover tube to place steam to the LP .It also has slide valves. I'm not too good on theory but if I can touch it, feel it,see it-I can understand it!! And yes I never read instructions but my wife does!!! :lol: Den
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Re: Understanding the ajustment of a compound

Post by mcandrew1894 » Thu Apr 08, 2010 2:06 am

"...In Funnel no. 124 page 56 Sam Wilkinson is talking about piston (spool) valves and there clearance.

He suggests a clearance of approx 0.005" (0.013 mm) for a simple, and 0.0015" (0.038 mm) for a HP valve on a compound. The latter in order to give a small leakage to ease starting and reversing without the need for a simpling valve...."


YUP
Another way is to cycle the gear from astern to ahead and back to astern quickly.

That will move the valves enough to put a puff of steam in the receiver and get the HP of dead center.

Of course...if you've got a big slug of water in your LP....it won't do squat.. :D

I built my valves with .001" clearance on my HP spool and .0007" clearance on my LP spool, so she's pretty tight...maybe too tight...but she runs nice at 160psi!

But that may explain the lock up sometimes. It happens when I am coming into the slip, I have some headway on and I stop the engine and coast, expecting to back to a stop. I think in the 10-15 seconds the engine sits there stopped, I get the LP cool and she condenses, not only the steam in the cylinder, but the steam in the feedwater heater and exhaust pipe as well. If I had a seperate wet air pump running as an independent auxillary, I think this problem would not happen. The condensate would work its way out to the condenser. Having a tight fitting piston valve just makes this worse......I guess I will have to make sure I open the drains on the LP when I am coming into the slip.....and keep the speed down a bit....NO SHOWING OFF! :D

Dave
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Re: Understanding the ajustment of a compound

Post by mcandrew1894 » Thu Apr 08, 2010 2:11 am

Oh and Den. That valve, if its small is called a starting valve. If its BIG they sometimes call it a simpling valve!....

Here's Sabino's thottle, bypass and starting valve
Image

The bypass is on the top left, the starting valve is on the bottom left, and the large one that is left is the throttle.

Dave
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Re: Understanding the ajustment of a compound

Post by piet schuurs » Thu Apr 08, 2010 6:28 pm

Hi guys.
It took me some time to see that the treat is going on over here.
A lot of information, give me some time to figger out what you all are saying.
But again, as Dave asked, has anyone changed his engine from slip to Stephenson reverse?

The engine I have now runs well, so the timing must be oke.
So there I don't want to change anything. Just copy the eccentrics, dubble them, make the Stephenson and set the timing again.
The lenght of the connecting rod between the two valve rods ( the Stephenson?) is the only thing I can play whit, Yes? and timing of eccentrics. Whitout rebuilding the engine.
Is that the right way to start whit?

Regard Piet
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Re: Understanding the ajustment of a compound

Post by Edward » Thu Apr 08, 2010 10:47 pm

Dear Piet ,
I am not any sort of an expert on this , but I believe that to get the best results the geometry of the complete arrangement needs to be correct . It will almost certainly work with considerable errors but would be a bit rough and probably noisier and more prone to wear .

I think that the most expert advice is likely to come from Steamboatjack . He did quite a lot of research into the best theoretical relationships between the wayshaft ( or weighshaft or layshaft , I've seen it called all three names ) links , crankshaft ,block and drag links when he modified the given design on the Elliot Bay Triple Expansion Engine which was featured in this forum . He is also redesigning the Stephenson Link valve gear on a Taylor twin cylinder engine .

A help in his research was an engineer named Don Ashton who has written a detailed and quite technical pamphlet "Design Procedures For Walschaerts' and Stephensons' Valve Gears ." This is available from Camden Miniature Steam Services , a link to their website is given in the " Recommended Reading " thread of " FAQs ."
I was shown Steamboatjacks' copy of this pamphlet ; it is definitely not gentle bed-time reading but is probably the best technical description/analysis available .

I think it would be worth researching this thoroughly , as I wrote earlier an inaccurate design might work , but your beautiful boat and venerable engine deserve only the best .

Regards Edward
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Re: Understanding the ajustment of a compound

Post by mcandrew1894 » Fri Apr 09, 2010 12:44 am

Edward, I would agree. The more data points the better...

I'll work on digging up some of my references too Piet.

Dave
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Re: Understanding the ajustment of a compound

Post by artemis » Sat Apr 10, 2010 7:43 pm

Probably one of the most authoritative books on the subject is:
Valve-Gears for Steam-Engines
Cecil H. Peabody (Prof. of Marine Eng., M.I.T.)
John WIley & Sons
New York
(Any edition is OK)

Definitely not light reading, but covers all you need to know (and a lot you don't). Still available in online used book stores at reasonable prices. And a 1906-8 edition is in Google Books Online. Including the foldout charts and graphs. Free is always a good price for books.
Ron Fossum
Steamboating Magazine Editor
http://www.steamboating.org
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Re: Understanding the ajustment of a compound

Post by mcandrew1894 » Sat Apr 10, 2010 11:01 pm

Artemis,

Agreed. I have recommended that Piet document the events he has now. This documentation will be vital as he lays out the links, way shaft and reach rods.

In this case though, if trying to maintain the existing timing and reuse the valve, some limitations may be unavoidable. Because he is using a slip eccentric, obviously link slip is currently non existant, as well as link eccentricity. A box link might work well here as it puts the arc of the link in line with the eccentric rod ends. There will be some noodling for sure.

I think long links, with a long horizontal reach rod such that the way shaft is in an unobtrusive location, and minimum eccentricity of the link will be some of the design parameters to work around.

Should be interesting....

Dave
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Re: Understanding the adjustment of a compound

Post by piet schuurs » Wed Apr 21, 2010 7:25 pm

Sorry guys for my late reaction, I had a long trip to sail, two weeks.

Interested in the ship I work on? http://www.nieuwemaze.com/Midlife

Its in Dutch but the pics are talkink for them selfs.

Dave will/can you specify whitch numbers / measurements you require?

I wil take them the coming weekend before firering up the boiler.

Dave what is a box link?

Piet
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