Pros/Cons Piston Valve and Balanced Slide Valve

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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cyberbadger
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Pros/Cons Piston Valve and Balanced Slide Valve

Post by cyberbadger » Thu Nov 09, 2017 4:39 am

I recently learned about Balanced Slide Valves, and I'd like to know more about them in regards to comparison with Piston Valves.

Wikipedia says:
The D slide valve, or more specifically Long D slide valve, is a form of slide valve, invented by William Murdoch and patented in 1799
...

The balanced slide valve was invented by the Scottish engineer Alexander Allan. It was not much used in the UK but, at one time, had great popularity in the United States. It gave some of the advantages of a piston valve to a slide valve by relieving the pressure on the back of the valve, thus reducing friction and wear.
...
In the 20th century, slide valves were gradually superseded by piston valves, particularly in engines using superheated steam. There were two reasons for this:
  1. With piston valves, the steam passages can be made shorter. This reduces resistance to the flow of steam and improves efficiency.
  2. It is difficult to lubricate slide valves adequately in the presence of superheated steam.
a) Regarding point (2) on "piston valves" - How does lubrication fair on balanced slide valves?
b) How does the wear on the o-ring seal I see in some photos of balanced slide valves between the valve and the steam chest compare to piston valve wear?
c) Feel free to correct, augment, share your opinions

I'm totally not biased in any way.... :lol: ;)
-CB
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Re: Pros/Cons Piston Valve and Balanced Slide Valve

Post by Lionel Connell » Thu Nov 09, 2017 5:31 am

Ok so firstly there is really no such thing as a truly balanced slide valve, because the slide valve can only be partially balanced. There is a compression phase in the valve events after the exhaust is closed, if the slide valve were completely balanced it would be forced off it's seat during the compression phase. In answer to your question regarding the life of the O ring on the back of a balance piston fitted to a slide valve: In order to prolong the life of an O ring used in this situation it would be beneficial that the steam chest cover be polished and corrosion proof, so very well plated or made of bronze or fitted with a bronze insert would be ideal. The smallest amount of rust on the surface of the plate will tear the O ring to nothing in just a few strokes.

Piston valves that are of inside steam admission are the best as far as wear control is concerned, the oil is fed to the inlet steam just before the valve and the steam pressure is continuously forcing the oil into the rings on the valve. Modern practice is to use many thin rings on the piston valve, the ring types used on 2 stroke motor cycles are suitable, these rings are designed for use where the rings are skipping over the open ports in the cylinder, or in this case the ports in the valve sleeve.

A piston valve is truly balanced, in good design the two pistons on the valve are different sizes, the piston on the end of the valve opposite the piston rod is slightly larger and this provides to counter the weight of the valve and rod in a vertical engine, it also counters the imbalance that is created by the piston rod exiting the valve chest on one side of the valve. So the piston valve can be considered to be balanced in all directions if properly designed. All of this serves to greatly reduce the force required to move the valve.

The length of shut of edge on a piston valve is almost 3 time that of a slide valve where the slide valve width matches the diameter of the piston valve. This greatly reduces the amount of wire-drawing of the steam as the valve is opened and improves efficiency.

People will make the ancient argument that slide valves wear in and piston valves wear out, but modern rings will last forever in an engine that is used part time. Look at how long rings in a modern car last.

In a modern piston valve design the spool itself does not touch the bore of the cylinder, it is machined under size a small amount and only the rings touch the bore. There are often 4 or more rings per end on the valve, I have seen as many as 8 rings per end in railway designs. The gap between the rings becomes filled with oil and there is very little or no steam leakage.
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Re: Pros/Cons Piston Valve and Balanced Slide Valve

Post by RGSP » Thu Nov 09, 2017 12:26 pm

Piston valves CAN wear out, and need new pistons, but with a couple of rings per end, and ports restricted in circumferential width (or bridged) so that the rings don't catch, they should last comfortably long enough for any steamboater.

There are many variations on balanced slide valves, some of them good AND reliable, some of them remarkably poor on life and reliability.

Balanced slide valves can sometimes be fitted to an existing conventional slide valve engine, when piston valves would need more radical engine modification to the point of it being easier to machine up a new cylinder & valve block.

Except for that special case, piston valves are less critical in their machining, and work marginally better than balanced slides, so when there are no over-riding constraints, go for piston valves.

In full-sized locomotive design, the extra port area of piston valves usually makes for a better and more versatile engine, but with small launch engines the square-cube law of area v volume makes this much less important, especially at the speeds we normally run them.
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Re: Pros/Cons Piston Valve and Balanced Slide Valve

Post by DetroiTug » Thu Nov 09, 2017 1:12 pm

Quote: "the oil is fed to the inlet steam just before the valve and the steam pressure is continuously forcing the oil into the rings on the valve."

And there's the problem, most engines are ran with no or only occasional oil in a condensing system. So far, no one has came up with an inexpensive or easy to make infallible oil separator for our small boats or small plants in general, and oil in the boiler is not an option. Slide valves can tolerate this condition much better than a piston valve from all indications.

Another huge advantage with a slide valve is in a carry over event, it can lift off the face and relieve the pressure from the water in the cylinder, a piston valve cannot. If the port is covered it's bye bye cylinder head. I've seen more than one piston valve engine badly damaged when they were ran on a monotube boiler.

I agree with everything you wrote in your post, but the above are two points that need to be considered when choosing between the two.

-Ron
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Re: Pros/Cons Piston Valve and Balanced Slide Valve

Post by cyberbadger » Thu Nov 09, 2017 2:58 pm

DetroiTug wrote:Quote: "the oil is fed to the inlet steam just before the valve and the steam pressure is continuously forcing the oil into the rings on the valve."
Are you talking about this plug that I may have just realized what it's for. :)

-CB
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Plug on lubrication cap
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Re: Pros/Cons Piston Valve and Balanced Slide Valve

Post by Lopez Mike » Thu Nov 09, 2017 6:45 pm

Two thoughts/observations:

I'm running my piston valve "dry" (only wet steam. No oil.) with no excessive wear. The maximum pressure is around 135 at the boiler so it's a foggy day in there.

I have a small relief valve at each end of the stroke on the cylinder. The object being to prevent hydrolock from carry over. So far I've been attentive enough to avoid such an event. My turn will come.
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Re: Pros/Cons Piston Valve and Balanced Slide Valve

Post by Lionel Connell » Fri Nov 10, 2017 3:52 am

I think that you need to look at the last of the small compound marine engines that were designed by professional steam engine manufacturers who had to provide their customers with efficient, long lasting engines. Look at the compounds and your going to see that all moved to piston valves on either the HP only, or both cylinders. As Mike has pointed out, the need for oil depends completely on the temperature of the steam. If you are using saturated steam then the many small rings on the piston valve will fill with water, both sealing and lubricating the valve. Many people do run oil in their launch engines, so there must be some reliable oil separating methods out there.
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Re: Pros/Cons Piston Valve and Balanced Slide Valve

Post by DetroiTug » Fri Nov 10, 2017 1:33 pm

Quote: "the need for oil depends completely on the temperature of the steam."

Water is a very poor lubricant. Between metal mating parts, water is somewhere in the neighborhood of 100 times less effective than proper oil. As I asked someone once, If steam engines don't need oil, then where did all these antique hydrostatic lubricators come from? Sometimes on Ebay, there are near a hundred for sale at any given time.

I have a foot in the steam car camp as well and have done quite a bit of research on them. Out of the hundred or so different manufacturers, not one left the factory without some form of administering oil in to the steam line either before or directly in to the engine. Superheated and saturated temperature levels.

This no oil philosophy seems to be isolated to the steam Boating hobby and nowhere else. Traction engine folks hydro-oil.

It's a bit of an old wives tale that Locomotives never used oil, from my research, all the mainline engines used cylinder lubrication in one form or another.

-Ron
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Re: Pros/Cons Piston Valve and Balanced Slide Valve

Post by cyberbadger » Fri Nov 10, 2017 3:56 pm

DetroiTug wrote:This no oil philosophy seems to be isolated to the steam Boating hobby and nowhere else. Traction engine folks hydro-oil.
Not exactly from what I have heard, but it is important to distinguish between a philosophy and what was done/can be done.

There always have been folks who neglect or forget oil. Some machines are more forgiving then others with this treatment. When you don't supply oil, almost anything that is available that has some aspects of lubrication goes a long way as a substitute, including say wet steam. Some steam engines were not really designed to last that long. I have heard the figure 7 years was the average life of a steam traction engine. Traction engines were designed as mobile power sources to really take a beating.

Also remember during the eras of steam engines in general you have a lot of variety. Lubrication did not really exist as we know it. So quality was often poor.

Even in steam boating - lets be honest now - how many miles/year does the average steam boater today travel under steam? I thought I had a great first season with Nyitra this summer, and I estimate ~30miles for the whole summer. If you don't do that many miles/year you can get a lot more years potentially. I noted that the Chautauqua Belle does hundreds if not thousands of miles a year. Different ball game. And we both use steam cylinder oil. But if you ask me, have you ever forgotten oil or ignored it for a while? - the answer is absolutely. Am I worried about this? A little, but not really. But I would never drive a modern i car with the oil light on - it's needs it much more.

Something is a lot better then nothing.

-CB
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Re: Pros/Cons Piston Valve and Balanced Slide Valve

Post by DetroiTug » Fri Nov 10, 2017 6:16 pm

Quote: "Some steam engines were not really designed to last that long. I have heard the figure 7 years was the average life of a steam traction engine."

That is not consistent with the level of workmanship that went in to these early engines, or anything I've ever read. It was anything but a throw-away society a hundred years ago. Look at how many of them exist until this day.

Quote: "Also remember during the eras of steam engines in general you have a lot of variety. Lubrication did not really exist as we know it. So quality was often poor."

Considering the majority of oil came from the Pennsylvania oil fields, regarded as oil of the highest quality, they had very good oil. I think you may be confusing Kerosene and Gasoline being low quality a hundred years ago when refining processes were not perfected.

Quote: "lets be honest now - how many miles/year does the average steam boater today travel under steam? I thought I had a great first season with Nyitra this summer, and I estimate ~30miles for the whole summer."

I have often traveled farther than that in one day, we did close to 50 miles in one day. In New York we did 63 miles in three days and that was going through several locks and finishing up early in the afternoon - and too going slower at times to keep the group together.

Quote: "And we both use steam cylinder oil. But if you ask me, have you ever forgotten oil or ignored it for a while? - the answer is absolutely."

The answer is absolutely not. I will not run any of my engines without oil. I've ran out a few times under way and the Swift oiler can be shut off and filled under way which I did.

Quote: "Am I worried about this? A little, but not really. But I would never drive a modern i car with the oil light on - it's needs it much more."

What's the difference? An I/C engine is certainly a much more demanding mechanical operation, but why do some sliding metal surfaces need oil lubrication and others do not? The steam engine can be in a very poor state which would render an I/C engine inoperable and still run seemingly well. My car and boat will both run on very low pressures, the boat will run on 10 psi. The car will pull itself on around 50 psi, which is very low for a car. Neither have shown any sort of degradation of performance over time.

-Ron
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