Help with Stuart Triple Steam Engine

[oldengineguy]

This may be long winded, please stay with me. I'm building a Stuart triple and the drawings call for the eccentrics to be made in one piece for each cylinder with the lobes offset from the centre line of the crankshaft by 15 degrees for the low and intermediate cylinders and 30 degrees for the high. I have made the eccentrics as drawn. That's not the problem. The question is why are they designed this way. I e-mailed Stuart with this question.

This is their reply." Good question , I don't have a definitive answer for you, but basically it's down to the way a Triple Expansion engine uses the steam expansion properties to power the engine. The eccentric offset on a reversing engine is determined by the direction of engine travel. with the engine rotating in one direction the offset will be set at just after top or bottom dead centre in degrees and when you reverse the engine the degrees will be the same but in the other rotation".

This does not explain why they are offset or why high pressure cyl. is different than the other two. Can anyone tell me why they are to be made this way.
Thanks Colin

 

[ironman]

Just because, I guess.

 

[Mechanicboy]

Timing of eccentrics and overlap in slide valves is determined by making the best use of the expanding property of steam without wasting as much of the steam amount from the boiler. I has Stuart triple expansion steam engine. To start up the steam engine, you need simpling valve, also lead steam from main steam pipe before high pressure cylinder to low pressure cylinder via simpling valve or we call it for "start valve".

 

[Charles Lamont]

In order to make expansive use of the steam the valve must close to inlet steam before the end of the stroke. This is done by adding 'lap' to the valve so that the slide valve is longer than the distance between the outside edges of the steam ports. The eccentric then has to be advanced in the running direction so that the valve opens to steam at (or a smidgen before) dead centre. For a reversing engine the reverse eccentric has to be advanced in the opposite direction to the forward one.

All the valves on the Stuart triple have the same dimensions, with 5/16" travel and 1/16" lap, and nominally that would require an advance angle of 24° for all cylinders. I don't know why the drawing has them different. It means that HP valve events are a bit early while IP & LP are a bit late.

 

[Andy Munns]

Marine steam engineers avoid the term "simpling valve" as it doesn't make the engine a simple, rather the term "impulse valve" is favoured. The impulse starting valve does not give the engine more power, just used to kick over the IP or LP if the HP is stuck on a centre. Most engineers will work the links first to get vacuum and chest pressures to roll HP off dead centre before using the impulse.
On angles of advance, if valve has no lap will be zero degrees. When lap added, angle of advance must increase. Note even with inside admission piston valves, we still call the trailing angle as "angle of advance" ref. Young son. Need to see link to drawings to advise in Stuart valve events, but it might be result of volumes of steam.

 

[Steamchick]

Hi Colin -"Old-engine guy". I have just picked-up on this thread and saw your comments from Stuart.
My guess. (As Compound valve timing is new to me!).
The HP cylinder gets steam from the boiler - so it is "always available". Hence the valve timing will be suitable for the optimum running speed with a continuous available steam supply.
However, the later expansion cylinders will be timed to suit the exhaust timing of the steam released when the upstream cylinder opens the exhaust valve.
I may be totally wrong in my guess, but if you can send me a bit more detail of the valve timing from the Stuart drawing, I may be able to fathom it out properly.
Any use?.
K2

 

[Steamchick]

P.S. I'm pretty sure Stuart timing will be correct, but the reason "Why?" may be lost with generations of lost engineering....
K2

 

[Charles Lamont]

However, the later expansion cylinders will be timed to suit the exhaust timing of the steam released when the upstream cylinder opens the exhaust valve.

They aren't. The transfer pipe is expected to act as a receiver.

 

[Steamchick]

Ok.
In that case, The low pressure in the transfer chamber may just be better with later timing... possibly to avoid any back-pressure remain in the cylinder just exhausted? Just another guess...
I'll do some research in my father's and Grandfather's text books and See what they say.
As far as I know, the last multi-expansion engine running daily in a ship was scrapped in the early 1970s... London based, 40 plus years old.... so the Engineers who designed these will have similarly been retired well before then. But their text books remain.... and we - their grandchildren - are fortunate to have their legacy.
K2

 

[Andy Munns]

Textbook you need is Youngson, which is a definitive work on marine compounds and triples. I will search for link. There are many compounds and triples still running in Europe, US, UK and Europe, but progressively fewer engineers who understand exactly the mathematics of steam distribution and effect of receiver pressures and vacuum.

 

[Andy Munns]

See https://www.amazon.com/Slide-valves...r+Youngson&s=books&sr=1-5&text=Peter+Youngson
Price is interesting. There is a download available somewhere...

 

[Charles Lamont]

In that case, The low pressure in the transfer chamber may just be better with later timing... possibly to avoid any back-pressure remain in the cylinder just exhausted? Just another guess...

In a triple expansion engine there is supposed to be considerable back pressure on the HP.

Here is one of many videos of SS Shieldhall:

And here PS Waverley's engine running flat out:


Waverley is currently getting her second set of new boilers in preservation, and normally runs daily during the summer. In 1990 We did Ayr to Brodick and back, except we had to go back to Largs instead as it was by then a force 6, and too rough to get back in at Ayr.

 

[Andy Munns]

Yes, if there is less back pressure on the HP --> less pressure to drive the IP and then even less pressure to drive the LP.

 

[Steamchick]

But am I right in thinking the LP is often acting at near atmospheric pressure, and expanding to "near-vacuum" in the condenser? - I'E. using the latent heat of vaporisation (or condensation) ? Or does it still exhaust above Atmospheric zero?
K2

 

[Steamchick]

I admit I know nowt, so I'll stop guessing.
K2

 

[Charles Lamont]

That would help , but questions are good.
The LP receives steam at somewhere not far from atmospheric pressure and exhausts into a (partial) vacuum in the condenser, but it is still (mostly) steam - even though it is below atmospheric pressure (which is nothing special, thermodynamically). The latent heat is given up in the condenser.

 

[Steamchick]

Thanks for the videos Charles L. (Showed me what my Father and both Grandfathers did!).
Was Shieldhall the last Recip steam engine ship in commercial use - before becoming a jolly boat?

 

[Andy Munns]

2 steam ships I engineer see condenser vacuum to be 26-27" and LP chest at around 5-10" vacuum. If there is an air pump or circ pump malfunction the main engine slows dramtically.

 

[Mechanicboy]

till exhaust above Atmospheric zero?

There is the vacuum pump (airpump) to remove pressure in the condenser then the LP cylinder get high vacuum then the piston are on way to exhaust the used steam then the steam engine get increased power when the steam is cooled down to water where it increase vacuum to withdraw the piston to TDC under outblowing the used steam under period.

 

[Mechanicboy]

If there is air leakage in the condenser, airpump or not enough cold water into the condenser Loss of vacuum in LP cylinder.