Piston Valve Vs Slide Valve

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Dom

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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
 

kvom

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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.
 

Ken I

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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.

Ken
 

GWRdriver

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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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cncjunior

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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
 

cfellows

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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
 

Runner

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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.

Brian
 

robcas631

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Dom have you incorporated a Walschaerts valve gearing system into your first design?
 

robcas631

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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
 

Tin Falcon

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Looks like the slide valve wins here.

IMHO if you are making a stuart engine or a copy of a stuart engine stick to the original classic design. And like others said if you are running on steam they are more forgiving on condensate of course you can install drain cocks to deal with that issue as well. from what I remember seeing most full size steam engines have either slide valves or corliss valves.

It is good to have a variety of designs in models so you may want to build a piston valve engine at some point as well.


robcast look at the elers engines designs many of his engines have the classic slide valve in several orientations. the prints show the pistion and all the valve assembly parts.
Tin
 

Dom

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Thankyou for all the advice,
i guess ill stick with the slide valves on this build and move onto piston valves and possibly Walschaerts gearing next time. Could anybody suggest a nice model to build? I want to stick to stationary or vertical engines but experiment with piston valves and Walschaerts gearing. I know these would normally only be found on loco's but for me this is about the learning curve.

Regards Dom
 

kvom

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The "paddleducks launch engine" that BogStandard designed and published here and on other sites uses piston valves, and is relatively easy to build. Bogs did a great writeup explaining everything as you go along. There are several build threads on this site. I built mine in 1.5x. Plans are metric, so for imperial it's just a matter of multiplying dimensions.
 

Tin Falcon

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A simple but nice horizontal engine that uses a piston valve is the PM Research model 3 IIRC the kit is about $77 cast iron and bronze. Rudy Kouhoupt also had a couple piston designs that were basic.
Tin
 

Septic

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I agree totally that slide valves at smaller scale are much more forgiving, cheaper and easier to build, but the challenge of producing an efficient piston valve delivers a nice sense of achievement when it's right....
 

robcas631

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I have made three engines that work rather well utilizing the cylinder valve steam chest system. When I obtain a proper mill I will attempt a slide valve. Being busy I will upload pictures of current projects, I have not had time to do so.
 

Ryker Carruthers

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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.
Not always, Alot of piston valves were made for outside admission so you still have the steam chest. Traction engines from the factory were all made for slide valves but companies sold branchhouses kits for piston valves. The steam chest cover is changed usually, Makes it extend out for more room. and the valve and cage. They use the same valve face, The cage is held against the seat by two bolts from the steam chest. In all cases there is much less wear on the valve gear, on a traction engine take 4x9 (about the size of the valve) times 150 or 175 for boiler pressuer... 5400 lbs plus. On a piston valve there is darn near nothing.
Ryker
 

kvom

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For a stationary engine a Stephenson linkage is easier than Walschaerts and works just as well. A major reason for locomotives abandoning the Stephenson linkage was that the mechanism is between the frames under the boiler, so maintenance access was difficult.
 
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