The Steamboating Forum

My thoughts, if I were to build another boiler, would be to operate it between 150 and 175 psi, with pop-off at 175 psi.
This assumes I am running a compound with 3" and 6" bores.

After studying the Stanley boiler, I got curious about how they operated at 500 psi.
One trick was to wind the boiler shell with several layers of piano wire.
I have never felt comfortable standing around a boiler operating at 500 psi, but another thought was the rating of everything else, like fittings, pipes, gauges, sight glass, injector, etc.

A quick study of various steel pipe and copper tubes does show definite operating ranges, and 500 psi seems to get rather close to some of the safety ranges of the materials.

You can purchase standard off-the-shelf safety valves for compressed air systems up to 175 psi, so 175 psi does not seem to be an excessive amount, although operating at that pressure would take careful consideration, moreso than say 100 psi.

I was intending on forcing my boiler with a foundry burner, not that I should but that nobody has told me "don't do that" (yet). I would run a VFT boiler, with welded steel tubes. I would say using a forced boiler is not for everyone since it would not be tolerant of low water or casual neglect. Some people just don't pay attention to what they are doing, I have seen boilers filled to the very top before someone says "Wow, the engine quit running, and the sight glass seems to be either full or empty".

I did operate a large horizontal boiler for a while for a lumber kiln drying operation, and the fact that I am still around is a testament to the fact that I watched the water level at all times.

The discussion about valve gear above is interesting, and this is the first time I have had an interest in compounds, and so the first time I have paid attention to such a discussion. I am a little confused about the statement "the valve action gets unusual at early cutoff" (not the exact quote, I will go back and read again).
The valve action should be perfectly predictable at all cutoffs (not a statement of fact, but just my guessing outloud, correct me if I am wrong). The only thing I am aware of at a short cutoff is wire drawing (drop in the pressure entering the hp cylinder due to the restriction at the small port opening when the valve has very little travel).

The statement "don't worry about cutoff on the lp cylinder" makes perfect sense.

I can see the relationship between the operating pressure, the pressure drop across the hp cylinder, and the resulting pressure left to operate the lp cylinder, and also know that the forces acting on the hp piston need to be approximately the same as the forces acting on the lp piston (incoming pressure x area of hp piston = secondary pressure x area of lp piston).

I have heard discussions of a pressure throttling valve to control an engine, vs using full pressure without a throttling valve, and using the valve gear hooked up, such as a Stephenson's link, to adjust cutoff, and thus control the engine speed and power.

Supposedly, using the valve gear to control engine speed/power is better, since using a throttling valve to reduce pressure would be the same as wire drawing, and would not allow any expansion of the steam.

I would guess that when operating a compound as mentioned above, one would want the hp to cutoff at 70-80%, since you could dial in an earlier cutoff using the Stepenson link. It does not seem to make sense to design the hp for an early cutoff, since that would negate hooking up the valve gear for an early cutoff.

I don't know props, but have seen my dad change from a 3 blade to a 4 blade, I think the same diameter, don't know the pitch, and he liked the 4-blade better.

Just from a common sense standpoint, and from what others have said above, too small a prop will just slip without transferring power to the water, and too large a prop will not let the engine reach rated rpm and rated power. I guess you could err on the larger diameter prop when using a steam engine, since a steam engine produces a lot of torque at a low rpm.

An I on the right track with these lines of thought?

A couple of thoughts. Others with more information and experience will comment as well.

I would be very careful and do a lot of research before using air safety valves in steam service. The one on my shop compressor is not a 'pop' valve and honestly don't know if it is either rated for steam service or would work in that application.

Thoughts on valve events. Almost all of our engines have rigidly defined but non symmetrical timing. With connecting rods, the piston does not move in a sinusoidal way. The piston spends more time at bottom center than it does at top center. With an infinitely long connecting rod it would be. In some applications we can use a 'scotch yoke' which is symmetrical. The old beam engines were great this way. Nice big long connecting rods.

Our simple valve gears use eccentrics which suffer from the same defect. Again, there are complicated gear designs which try to improve on this and other short comings of systems like Stevenson valve gear. None of them has stood the test of time. The Walschaerts gear displaced the Stevenson system for reasons of reduced friction, ease of maintenance and lack of room on more powerful locomotives for shaft eccentrics.

At longer cut offs, the problems have less effect. At extremely short cutoffs things get, as I quipped earlier, screwy. I mean by that phrase that the geometrical limitations begin to be significant.

Historically, the Corliss design, which controls the engine with variable cut off was the last gasp in the direction of trying to make reciprocating steam engines efficient. A few locomotives tried cam driven poppet valves. Too much work to keep them working and about that time the modern stink pot engines took over. The large multi-stage turbine killed all of them.

One of you with the desire and the time should sit down at your computer and model these issues with our most common valve gear. Stevenson. I have long suspected that matching the rod to stroke ratio of the engine with the rod to stroke ratio of the valve gear might be worth investigating.

In reality, out engines are so frightfully inefficient, die to heat loss, that all of this is only interesting rather than important. The fuel savings that we might gain from fiddling with properly adjusted valve gear are totally swamped my poorly insulated piping, inadequate boiler lagging and exposed cylinders.

If you want efficiency, you need a big boat with a turbine. If you want fun, don't sweat it. We are hopeless romantics or we wouldn't be running these things!

A few comments to your post, IMHO:

“I have never felt comfortable standing around a boiler operating at 500 psi, but another thought was the rating of everything else, like fittings, pipes, gauges, sight glass, injector, etc. A quick study of various steel pipe and copper tubes does show definite operating ranges, and 500 psi seems to get rather close to some of the safety ranges of the materials.”

I think the best guidance is what we have in the ASME Boiler and Pressure Vessel Code. It is based on long experience, and established technology, rather than what “seems” right.

For example, the ASME Code clearly prohibits the use of copper above 406F, which corresponds to a saturation pressure of 250 PSIG. At 500 PSIG steam pressure, copper and copper alloys are way past the acceptable standard (ASME Code) for any part of a boiler and its attachments that can reach saturation temperature.

On the other hand, the shell of the Margaret S. boiler is 16 inch OD x 3/8 inch wall thickness low carbon seamless steel pipe, and the ASME Code calculation would allow the operation of this boiler at over 750 PSIG. That boiler operates at or below 100 PSIG, but that does not mean that I can casually neglect the water level!

I work around boilers that carry 2800 PSIG or higher all the time, and I am more comfortable there than in the presence of a boiler built without knowledge and application of appropriate engineering technology.

“You can purchase standard off-the-shelf safety valves for compressed air systems up to 175 psi, so 175 psi does not seem to be an excessive amount, although operating at that pressure would take careful consideration, moreso than say 100 psi.”

Here I see that word “seem” again, and pressure boilers, there design, construction, and operation should not be based on how things “seem” . The point here is that, with proper design, construction, and operation, there is no real reason to be uncomfortable because a boiler operates at high pressures, be it 500 PSIG or 2800 PSIG. Conversely, a poorly designed, or deteriorated boiler , or a very good boiler operated with neglect, running at 60 PSIG might kill you in an instant. At least a reasonable annual hydrostatic test at a pressure well above the safety valve settings, plus proper inspection provides confidence that the boiler is probably OK.

”Just from a common sense standpoint, and from what others have said above, too small a prop will just slip without transferring power to the water, “

That is correct.

“and too large a prop will not let the engine reach rated rpm and rated power. I guess you could err on the larger diameter prop when using a steam engine, since a steam engine produces a lot of torque at a low rpm.”

The actual parameter here is propeller pitch, not diameter. Almost all steamboats (and low power boats) can easily tolerate propellers as large as the hull can take. For example my 3/4 HP electric launch 14 feet long, swings a 14 inch prop, this same prop would absorb more than 100 horsepower in a speedboat. I use such a big prop, at low RPM, for best efficiency.

Mike, Fred-

Very good points indeed.

Mike
I love this statement you made "We are hopeless romantics or we wouldn't be running these things!"
It is thought provoking to consider the complexities but not much will be applied to reality. We just want to run em!