compund engine on air

[pcw]

hi all
small question. how well will triple expansion run on compressed air since it doesnt have the "left over pressure" in exhaust from high pressure cilinder as steam has. or will it be wiser to add bypass airlines to run on air and normal passages to run on steam? hope it makes sense.
pascal

 

[steamer]

Absolutely run a bypass for compressed air with a triple. On my compound, I actually end up with a partial vacuum in the LP that is noticable unless I open the drain cocks...

Of course, on steam she behaves completely differently

Dave

 

[pcw]

thank you, ill make a design change than so i can do both air and steam. ill bypass/re-route the airflow than when running on air.
Pascal

 

[steamer]

PCW,

On 12" to the foot stuff, there is usually a "bypass" between the HP and the IP/LP. It is used to get the HP off top or bottom dead center and is usually called a "starting valve". This is similar but not to be confused with a "simpling valve" which is MUCH bigger to provide boiler steam to the IP/LP for maximum intermittent power. Many early torpedo boat destroyers with very high strung recips had simpling valves for combat flank speed...you know the black gang was working hard then! Some of these 3000-4000 HP engines were turning over at over 400 rpm.....

Dave

 

[pcw]

PCW,

On 12" to the foot stuff, there is usually a "bypass" between the HP and the IP/LP. It is used to get the HP off top or bottom dead center and is usually called a "starting valve". This is similar but not to be confused with a "simpling valve" which is MUCH bigger to provide boiler steam to the IP/LP for maximum intermittent power. Many early torpedo boat destroyers with very high strung recips had simpling valves for combat flank speed...you know the black gang was working hard then! Some of these 3000-4000 HP engines were turning over at over 400 rpm.....

Dave

yeah, seen one of those huge triple expansion in action once what wonders me now is that if it would be a problem to put same pressure in the LP cilinder?

 

[steamer]

An ideal way to break cranks and connecting rods....

Dave

 

[Maryak]

yeah, seen one of those huge triple expansion in action once what wonders me now is that if it would be a problem to put same pressure in the LP cilinder?

The Impulse, (starting), valve has a small bore pipe so it is impossible to get full boiler pressure on either the IP or the LP piston. Once the engine moves off TDC/BDC the impulse valve is closed and the throttle valve is opened which continues the steam flow in the normal expansive way through the cylinders.

The best thing really is to not stop the engine on either HP dead centre.

Hope this helps

Best Regards
Bob

 

[steamer]

This is Sabino's 75 HP Payne compound (7 x 14 x 12)

The steam main is coming into the photo from the top right eventually to the large globe valve/handwheel tha is in line with it...that valve is the throttle

The first valve teeing off is the steam bypass. This just dumps steam to the condensor throught the feewater heater...just lets us keep the safety on it's seat which gets the riders nervous when it pops

The next valve below and slightly to the left of the steam main line/throttle valve is the starting valve. It dumps directly into the receiver volume between the piston valves...the heads of which are visible in the photograph.

Not to be confused with a simpling valve though..

When you come out of the slip, your running full throttle for the most part both astern, and ahead. If you pop the starting valve open when you get the backing bell...you should really try to remember to close it...if you don't, once your on the way down river and you throttle back to cruising speed, you will wonder why the LP cross head is thumping....and it runs very lumpy!....check the starting valve....the receiver pressure is ususally a dead give away as it will be up around 30 psi....WAY too high...even poped the reciever safety a couple of times.... :big:
Dave

 

[steamer]

"....The best thing really is to not stop the engine on either HP dead centre."


Absolutely correct Bob....and trick I used if I got caught there was to cycle the reverse lever up to ahead and back to astern quickly. This would put a puff of steam into the receiver and get the engine off TDC/BDC pretty reliably.....Once I learned that trick, I never even worried about getting the crank stopped at the "right" point....I knew I could get her going one way or the other....The other thing would be that even when you closed the throttle, when she coasted, the prop would turn the crank for a few turns....you never really knew where she was going to end up.

The reverse lever cycle trick works on my 2HP compound too!

A 2000 HP triple would be a different matter all together!

Dave

 

[steamer]

I don't think we ever found a starting (impulse) valve on this Seabury when we restored it.

It's a triple about 100HP

It has a bronze LP ring and the LP is about 11"

Dave

 

[steamer]

Here's the restored engine from the steam tug "Pegasus". The loop of pipe at the top of the photo is the steam line to a globe valve dumping steam into the receiver. The valve had a shaft leading down to the engineer's station in order to turn it open or closed...

Dave

PS it's about 19.5 x 39.5 x 30" compound of about 750BHP...the crank journals are 9".

 

[Maryak]

when she coasted, the prop would turn the crank for a few turns....you never really knew where she was going to end up.

My way is when you get your stop bell, rock the links. This does 2 things..........It ensures the reversing engine is working, ( there's nothing quite as exciting as getting an astern bell and the links refuse to budge). It goes a long way to stopping the engine rotating due to coasting, especially if you just crack the throttle a little, thus no HPDC's.

When I used to put on my other, (skippers), hat. After giving the stop bell before coming alongside, I always gave a slow astern about 200 yds out from the wharf, that ensured the engineers were awake and considerably reduced the pucker factor on my fundamental when the froth and bubble appeared under the stern.

A 2000 HP triple would be a different matter all together!
Dave

Nah! just a bigger bang if you stuff up.

Best Regards
Bob

 

[steamer]

"....Nah! just a bigger bang if you stuff up."

Rof}

I'll bet!

Dave

 

[steamer]

I suppose the other way Bob would be to bring her in on the link only...that'll stop the engine cold...can get exciting once the bring the link over though....

Dave

 

[JorgensenSteam]

I never though about it, but I guess you could put a lot of torsion on one side of the crank if you put full pressure to the LP cylinder.

 

[pcw]

I never though about it, but I guess you could put a lot of torsion on one side of the crank if you put full pressure to the LP cylinder.

yeah i was thinking that too. guess ill fake a triple than. just build it opticaly as triple, but use three pressure cilinders.

 

[steamer]

Not a bad idea PCW...unless you want to build a 180+ psi boiler...... ;D

Dave

 

[Maryak]

I suppose the other way Bob would be to bring her in on the link only...that'll stop the engine cold...can get exciting once the bring the link over though....

Dave

That's possible with "all round reversing gear" but not with direct acting oscillating gear. Unless of course you want pump the links over by hand and that takes quite some time and effort. The preceding applies to Stevenson's gear. I have no experience with Hackworth, Joy. Walsherts etc.

Best Regards
Bob