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Old 04-01-2013, 09:54 AM   #1
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Default Piston Valve Vs Slide Valve

Good Morning all,
I'm in the finishing stage of my first steam engine, a Stuart S50 scratch built from stock material (with the exception of the bed and flywheel) and am now planning my next project, a twin victoria.

Being a larger model i want the Twin Victoria to be capable of "doing work" most likely driving a water pump although i'm open to suggestions.

My question is; are there any benefit's to be found in replacing the slide valve with a piston valve? Also, could i get away with simply redesigning the steam chest or would i have to redesign the entire cylinder valve face, steam chest and associated linkages?
Ill be machining the entire block from stock material so a complete redesign isn't the end of the world if it will result in a better performing engine.

Your thoughts appreciated,
Dom


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Old 04-01-2013, 12:10 PM   #2
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I know it's possible to make a steam chest with piston valves having the same appearance as the slide valve and no change to the cylinder. Some locomotives in the past had their slide valves changed to piston in this manner. In full size equipment there is an advantage to piston valves, but in a model I'd probably stick to slide valves as they are both easier to fabricate and easier to time. YMMV.


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Old 04-01-2013, 12:31 PM   #3
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Slide valves are forgiving of condensate (if you are running on steam) and if the piston hydraulic locks it will simply lift the slide valve - not so with piston types.

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Old 04-01-2013, 12:38 PM   #4
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If it was me I would stay with slide valves. The old saying is, "Slide valves wear in, piston valves wear out," and although both have their advantages the slide valve has more advantages (or less disadvantages) for small, low pressure, engines than piston valves.

Another consideration is this, piston valves are "inside admission" meaning that the steam supply is admitted to a central cavity on the valve spool - the cavity on the spool IS the steam chest . Slide valves on the other hand are "outside admission," meaning the steam is admitted outside the valve body, into the steam chest proper. The potential problem is that steam passages for piston valves are not exactly the same as those for slide valves, so if your cylinders are cored or drilled for slide valves they may not be able to be reworked for piston valves without a great deal of time and effort. The question would be why make that work for yourself, when functionally there would be virtually nothing to gain?

There is such a thing as "faux" piston valves in some small engines, mostly the manufactured engines, but also in some kit designs. I don't know much about these valves, which are usually a round rod or bar with milled flats which cover and uncover steam passages as the valve moves. This valve type doesn't appear to produce much power.
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Old 04-01-2013, 01:50 PM   #5
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It has been awhile since I have read up on the valve configuration. What GWRdriver said about the inside verses outside steam admission is a good point. The other point that goes with that is the valve gearing needs changing depending on inside or outside admission. If you are going to change to piston valve you may want to study valve gearing.

Daniel
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Old 04-01-2013, 03:28 PM   #6
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The main advantage of a piston valve is that you can reverse the engine by simply reversing the air flow.

The biggest disadvante in my experience is getting the piston valves to fit without an air leak. I've never been able to make a piston valve engine that didn't have that annoying hiss of air (or steam) leaking.

Chuck
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Old 04-04-2013, 07:20 AM   #7
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I can't comment on the performance benefits if any of using piston valves over slide valves. However, piston valve engines are easier to make. You don't have the problem of drilling long steam inlet passages at a carefully set up angle or do you need to fabricate the valve chest and cover with often numerous fixing holes required to affix it to the cylinder. Because the valve rod is only exposed to exhaust steam and not HP inlet steam there is no need for a gland on the valve rod. Very careful turning of the piston valve cylinder and piston valve is required. But that's all done in the lathe.

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Old 04-04-2013, 03:37 PM   #8
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Dom have you incorporated a Walschaerts valve gearing system into your first design?
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Old 04-04-2013, 03:38 PM   #9
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Anyone have a rough design of a slide valve?
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Old 04-04-2013, 03:45 PM   #10
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I used a piston valve on my first steam engine. It worked well! I'm sure it will withstand full steam pressure. However I am thinking about a slide valve to save brass and increase efficiency


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