The Steamboating Forum

Every time I hear mention of frictional losses I think about all of the slide valve engines I've owned that had no valve balance at all. The 5A among them, of course.

It would be nice to know approximately the friction budget of our launch engines. Given the wear and breakage issues of many such valve gears, I suspect that slide valves are well up the list. That said, some of the solutions might well be worse than the problem. I've seen attempts at balancing a slide valve that wore out in hours. Or just dis-assembled themselves.

My current Strath Warrego engine uses piston valves with inside admission and I marvel that I didn't base designs around this long ago. Low friction and easy on the packing gland. A single adjustment for centering the travel. Years between maintenance.

Oops! Off topic a mile. Sorry.

Mike

Well, piston valves do have their downsides:
* w/o rings, they are quite sensitive to wear and become leaky easily (Otter's engine is like this).
* with rings, machining the ported valve sleeve is a bit tricky as is fitting and pressing it in.
* no automatic safety valve like a slide valve; if your boiler primes you can easily break something in the engine due to hydraulic lock, whereas the slide valve will lift from the face of the ports...

But they sure do save the valve gears; Otter's new (15 years ago) engine is a piston valve and has much easier working than the old engine that had a slide valve.
Balanced slide valves are definitely more involved, though.

- Bart

The old adage:

Slide valves wear in
Piston valves wear out

A piston valve in good fit is far more efficient though. A slide valve with 5 square inches of face at 250 psi has 1250 pounds of force exerting on it, quite a loss in horsepower.

DetroiTug wrote: ↑Mon Mar 21, 2022 6:13 pm The old adage:

Slide valves wear in
Piston valves wear out

A piston valve in good fit is far more efficient though. A slide valve with 5 square inches of face at 250 psi has 1250 pounds of force exerting on it, quite a loss in horsepower.

I did some looking for coefficients of friction measured on slide valves, and ended up with numbers all over the place, anywhere from .02 to .25 or more*. As a demonstration, I'm choosing .1.
We'll assume valve travel of 1.5" inches. speed of 400 rpm, and calculate force X distance and use rpm to convert to hp.

1250 lbs x .1 lbs/lbs x 1.5" travel * 1'/12" * 400 revs/min * 2 (valve strokes/rev) * 1 min/60 seconds * 1 hp second/550 ft-lbs = .37 hp - not inconsequential.

The high valve forces also result in high eccentric loadings, which leads to further losses.

This is a good illustration of why balanced slide valves or piston valves are much preferred as steam pressures climb.

* Some experiments that attempted to measure the forces in the valve rods found that as engine speeds climbed, the measured friction forces dropped. This was often determined to be due to restrictions in the steam piping and valve chests reducing the pressure on the valves rather than any change in the dynamic coefficient of friction.

- Bart

Bart, Good synopsis.