Slide Valve Operation

[Alan Worland 1Participant @alanworland1]

Just a thought!

Am I correct in thinking that steam entering the valve chest of a steam engine (Stuart No 4) acts upon the ‘top’ of the valve? ie steam pressure will push it onto its valve face?

If the above is correct I don’t understand why the inlet port is larger in diameter than the exhaust port?

Surely after the steam has been used, ie ‘expanded’ it will occupy more volume and therefore require a larger exhaust port for escape? Hence larger low pressure cylinders on compounds and triple expansion engines.

 

Alan

[Alan Worland 1Participant @alanworland1]

[John BaguleyParticipant @johnbaguley78655]

You are correct in thinking that the steam pressure holds the valve onto the port face and it is usual for the exhaust to be a bigger diameter than the inlet. Is the inlet to the valve chest a larger diameter to take a displacement type lubricator that screws diectly into the valve chest?

 

John

[60019Participant @60019]

Hope you don’t mind me adding a supplementary question, it is related.

 

My understanding is that slide valves operating in this way fulfill an important supplementary function. Any condensed water in the (double acting) cyclinder will be forced out by the piston and is able to ‘push back’ against the valve face and lift it to escape. Which is why some designs e.g. Metro have no cyclinder drain cocks.

 

Correct?

 

Mitch

[mgjParticipant @mgj]

Alan – do you mean a larger inlet port diameter into the steam chest, or into the cylinders.

 

Mostly the ports into the cylinders are smaller than the exhaust out of the cylinder, in part because the exhaust is carrying twice the mass and much more by volume than are the inlet slots (which also double as exhausts). The port into the steam chest may be fairly large to ensure a reasonable reservoir of steam under pressure to fill the cylinder x2 because its double acting, and in the event of a sudden increase in demand. The size of the hole into the chest doesn’t matter so long as it is is large enough to cope with maximim mass flow because it plays no part in valve timing. – think of the steam chest as an extension of the boiler.

 

Aslo of course pressure velocity and temperature are to some degree interchangeable, , so within reason, like any fluid, when it arrives at a smaller hole, it just speeds up, and when it gets into a bigger pipe it slows down. Thats not strictly true, because there are limits and penalties in the exchanges, but its good enough for our purposes.

[Alan Worland 1Participant @alanworland1]

John, you are correct – the exhaust port in the cylinder under the slide valve is larger than the inlet, which is another reason to expect the exhaust from this port (in the cylinder casting) also to be larger than the inlet (to the valve chest)

60019, Interesting point regarding drain cocks, but the cylinder does have positions for the normal tapered plug type drain.

mgj, the feed into the valve chest is 9/32 and the exhaust from a 1/4 inch hole drilled in the cylinder casting which connects the larger port face in the valve chest.

I can understand using the increased capacity of a larger feed pipe so more steam is available upon demand.

My this can get confusing! but to recap – feeding the engine I have a larger inlet pipe than exhaust pipe, and on the cylinder valve face a larger exhaust port than inlet port!

[John BaguleyParticipant @johnbaguley78655]

Alan – just noticed that you have a photo of the offending item! Yes, it does seem odd!

 

60019 – I think the drain cocks are left off in some designs just to keep things simple. The majority of the condensate then exits via the chimney and showers you and/or any passengers! There’s no problem not fitting drain cocks to slide valve cylinders for the reason you state but it can be a bit dodgy with piston valve cylinders where there is no easy escape for any trapped water. Hence the reason for many full size cylinders being fitted with spring loaded relief valves on the cylinder end covers.

 

John

 

Just found this photo on Steam-engines-for-sale which shows the pipe connections:

 

Stuart No.4

 

The inlet can obviously be fitted with this bolt on flange which takes the lubricator and the steam pipe. You can no doubt use any size pipe you like with the appropriate union nut and nipple. Don’t know if this is a standard Stuart fitting or a ‘one off’ ?

Edited By John Baguley on 27/05/2011 01:04:22

[Sub MandrelParticipant @submandrel]

I recall that the resistance a pipe offers to the flow of a fluid increases 16 times if the diameter is halved.

 

This means:

 

1 The exhaust doesn’t need to be much bigger than the inlet even though it takes a much greater volume of steam.

 

2 A rather short, narrow section of pipe is likely to be the limiting factor on flow even on a longer run of larger pipe. (think how a VERY short throttle can conrols flow down a VERY long pipe).

 

Neil

 

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