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[Neil]

I hope Nick sees this post.
I was wondering what are the relevant benefits/disadvantages of Sliding valve timing as opposed to Piston valves.
I used to be a definite adherent to sliding valves, but the more I read, the less I like them. The main reason I read is they sap a "lot" of power, but the lap is easier to arrange than a piston valve, or so it seems. How one would ascertain the power loss, is beyond my comprehension.
I have made both types, and now I prefer the piston, as long as you use plenty of "slippery stuff", (oil), they work really well.
That's ONLY my opinion. It would be great to see a professional standpoint. It wouldn't be an opinion, would it? It would be fact, I suppose.
I would like someone who knows what he/she is talking about to "hop in here". I am definitely self taught, and do not have much in the way of engineering theory, (read none). If it weren't for books...... I'd have no idea of steam related topics. There are others on this forum that obviously have plenty, thankfully They stop us blowing ourselves up, do they not, or try to? 


Different topic. Does anyone know what happened to Helen Verall? Now there's a person who knows how to handle a gas torch, copper, and a match.

Neil

Best wishes for Easter to all.

[MONAHAN STEAM MODELS]

Gotta to run out the door to the shop this morning but I will try to answer this when I get back home tonight.

[Circlip]

Added to a post on boilers but was confused, THIS was the person I meant to endorse, It's the Chedder connection.

   :- http://www.westernsteam.co.uk/about.htm

  Regards  Ian

[oldiron]

 To begin with, piston valves present far less drag than do slide valves. One has to imagine boiler steam pressure on the surface of the valve to see the amount of pressure on the valve, hence an indication to the amount of drag presented to the engine and valve gear. For example, lets suppose your slide valve is one square inch at a boiler pressure of 100 psig, then the equivalent pressure (weight?) on the valve is 100 pounds. By comparison, there is no downward steam pressure on a piston valve. The amount of drag, in terms of energy, could be worked out using the coefficient of friction for the valve material and the lubricant between that and the valve surface.  This was one of the main reasons for doing away with slide valves as boiler pressures increased.
  That said, slide valves are much better at releasing water from the cylinder, as a result of carry over or condensation, than piston valves. Water can be trapped in a cylinder with piston valves resulting disastrous effects to the cylinder and rods. A slide valve, on the other hand, has a tendency to lift when water is pushed to it. The piston and rods will receive a good thumping, but the chances of serious damage are reduced. This is why one should always keep the cylinder cocks open on a piston valve engine until you are sure the cylinder is clear of condensation.
  Some slide valve assemblies were designed as balanced valves in an effort to reduce the amount of steam pressure on top of the valve hence reducing the drag and wear factor.
  A multi expansion steam engine such as a triple, will have slide valves on the low pressure cylinder. This cylinder operates under very low pressure to a vacuum and because of the greater chance of condensate in the LP, the chances of damage due to condensate build up are greatly reduced.
  That's it in a nut shell.
  As to lap, the calculations for lap on a piston valve are similar to that on a slide valve. Just remember slide valves are outside admission, most piston valves, but not all, are inside admission. Changes things a bit.

John

[MONAHAN STEAM MODELS]

John, You nailed it! I couldn't have explained it better myself.

[kiwimodeller]

I always thought that one of the main advantage of piston valves in a model situation was that they could easily be set up to allow for reversing and speed control in one action? I have a lovely little Saito V4 engine which runs like a clock on low steam pressure and can be reversed and throttled from the same servo. The valves are fitted along the tops of the cylinder banks so the engine is still quite compact. It does not have cylinder drain cocks and can sometimes need a bit of rocking by hand to get water out on the first run of the day but once run it is usually fine, self starts and has never had a hydraulic lock. Based only on my experience with this engine I would have said that piston valves were the best option on a model engine. Cheers, Ian.

[steamboatmodel]

Can anyone recommend some plans or drawings of Piston Valves?
Regards,
Gerald.

[oldiron]

Can anyone recommend some plans or drawings of Piston Valves?
Regards,
Gerald.

  There's lots of information available on piston and slide valves. It all depends on what size of engine you're looking at and what performance characteristics you're looking for.
  I see you're listed as in Toronto, where abouts?

John

[oldiron]

  There is a free software available for designing steam locomotive valve gear. You can plug in the various sizes and valves, then run the design to see how it works out. It may be useful for you. Its a free download:

http://www.tcsn.net/charlied/

John

[Neil]

Wow, that was a whole lot more in reply than I expected.

It backed up what I have read. What I have read is that in the latter stages of Steam Power, Slide Valves were replaced with Piston Valves.

I have a few piston valve engines, and sliders too. I can't find a way to compare them however. Perhaps a "Drag Race" would do it,(like the old days), or what about a "Tug Of War"?

I thank you all for your input.

Neil.

[gondolier88]

That is because as steam technology progressed the pressures got higher and higher and the cylinders got larger. This meant that a larger volume of steam at a higher temperature was needed to move the piston at the required speed.

Slide valves needed ever bigger faces- have a look at the face of a Herreshoff High Speed Navy Engine for example- the valves are huge- this added huge surface area and along with the higher latent heat made for a very innefficient valve- 100psi acting on a valve 2" square is a whole different thing to 250psi acting on a valve 6" square.

However piston valves- though more efficient- needed much more maintainence and aren't as forgiving- being a cylinder they had no automatic take-up as a slide valve does- this usually ended up with a situation as mentioned earlier where on a triple expansion engine with a WP of 250psi the first cylinder would be piston valved and the next two would be slide valves.

There is no better one than the other- it totally depends upon the situation in which you use them.

Greg

[kno3]

It's also interesting to note that piston valves, while actually difficult to make well and steam-tight, are nevertheless used in many of the cheapest toy steam engines (such as Wilesco, Jensen etc.). I guess that is so because it's easier to make a cylinder with a piston valve and the expectations aren't that high from this engine category. All Wilesco piston valves I've seen were loosing steam at the piston valves.

[oldiron]

It's also interesting to note that piston valves, while actually difficult to make well and steam-tight, are nevertheless used in many of the cheapest toy steam engines (such as Wilesco, Jensen etc.). I guess that is so because it's easier to make a cylinder with a piston valve and the expectations aren't that high from this engine category. All Wilesco piston valves I've seen were loosing steam at the piston valves.

  In this day and age of CNC machine tools, its easy to make something within consistent close tolerances. The piston valve and valve cage being circular are quick and easy to do on a production basis with those tools, hence their popularity for something like Wilesco. This, of course, doesn't say anything about the steam holding qualities of such valves, but they do work as you say.

John

[Neil]

Wow,

I opened the proverbial can of worms, did I not?

As an aside, the American company PM-R make a lot of piston valved engines (I can't remember if it's all of them), and the ones I have work very, very well.

I thank you all for your input.

Can you imagine 150 years ago, with the tolerances now achievable, how a huge beam engine with a 12 foot bore and who knows what stroke, (probably about double that), the power and torque that would be produced? WOW, it would pull Australia closer to New Zealand I reckon.

Just a thought.

Cheers Neil.

[gondolier88]

...Can you imagine 150 years ago, with the tolerances now achievable, how a huge beam engine with a 12 foot bore and who knows what stroke, (probably about double that), the power and torque that would be produced? WOW, it would pull Australia closer to New Zealand I reckon.

Just a thought.

Cheers Neil.

Hi Neil,

You slightly miss the point- the reason that beam engines of 180+ years ago had such massive cylinder sizes was because they had such low working pressures and had such slow piston speeds. And you be surprised at the accuracy of these engines too- not that it matters too much when you've got an 8ft piston- 1/8" isn't going to make that much difference.

The reason why they got smaller was because pressures got higher, piston speeds got faster, as you say tolerances did get tighter, and more efficient pumping/mill equipment were devised.

As an example- in the years between 1840 and 1870 the average amount of fuel needed to fire the boiler per HP of a steam engine went down by 200%!

Greg

[Neil]

That WAS my point, Gondolier.
Neil.

[gondolier88]

Can you imagine 150 years ago, with the tolerances now achievable, how a huge beam engine with a 12 foot bore and who knows what stroke, (probably about double that), the power and torque that would be produced? WOW, it would pull Australia closer to New Zealand I reckon.

Sorry, it looked to me like you meant if we built an engine to these dimensions today the power that would be produced would be phenominal- my point was that we can make engines of the same power as those monsters, but on a much smaller efficient scale.

Greg