The Home Machinist!

Were check valves in full size sight glass shutoffs common practice?

The idea is that if the glass breaks, a sudden high flow of pressure will slap the valve shut (usually a ball check) to prevent massive amounts of steam from escaping into the cab.

The problem comes if you blow down the glass exuberantly, the ball can become stuck closed, and if you don't know that there is a ball check in there choking off the line, you'll think you have a glass full of water. This happened on a live steam engine the other day that a driver was unfamiliar with and there was almost a dry firing event as a result. This is the second time this has happened with this engine. I don't think that in live steam the advantages of having a check valve in the upper glass shutoff offsets the disadvantage of potentially getting a false reading. Perhaps if there were a weak spring in the check it would prevent this situation. On the shutoff on this engine there is not a spring, it is a free floating ball.

So, design flaw in that particular shutoff check, or bad idea in general for live steam? This is the only engine I have personal experience with that has one.

--Mike

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Mike

In theory the ball will float under normal conditions because the pressure is equal on both sides of the ball.
BTW the chamber the ball is in is larger in diameter than the seat for the shutoff and again under normal conditions the ball rolls back and away from the seat.

When a large differential pressure occurs i.e. the gauge glass breaks, the high velosity steam rushing past the ball forces it against the low pressure opening in the seat. This can happen with a gauge glass blowdown if the valve is opened too fast.

It sounds to me like the seat may be too large allowing the ball to stick in the opening or the ball too small. If everything is working correctly the ball should drop back to an open position when the pressure is equal again.

Ball checks in gauge glasses are required on ASME boilers. I have them on my Mogul and used them on full sized engine I ran.

John B.

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http://www.ngshay.com/
Shay drawings and castings

John, where does it say that? I have seen sight glasses with them, but not on a locomotive. I wouldn't want them. It is too easy for them to scale up and stick. They then give you a false reading like Mike described. How do you remove them to rod out during a boiler wash?

Marty

Section 1 PG-60.1.6 Automatic shutoff valves, if permitted to be used shall conform to the requirements given in A-18.

Section 1 Appendix A-18 1 thru 6 give details. I guess like many of the sections of the code the "IF PERMITTED" means you don't have to adhear to this one. When I built the boiler on my mogul the Hartford man wanted a code glass which was no big deal. That gage glass is still on it. I guess if you don't permit it you don't need to use it.

John B.

_________________
http://www.ngshay.com/
Shay drawings and castings

in the UK ball checks are required.
if a ball sticks in the seat there is something sized wrong as John stated.

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Fred V
Pensacola, Fl.
home track:
http://www.waleswest.com

Huh.

I do not believe the 12" to the foot Santa Fe 940 has them, but I will go look.

Brian

Sorry to contradict you Fred but I've owned and operated loco's both full-size and live steam since the 60's and have never heard of gauge glass ball shut offs being compulsory in the U.K. Sure I've seen and used them on some full size locos but most times they tended to stick open not shut.I've yet to see one on a model and there is nothing about them in the Blue Book (of boiler regs).
Regards Ray.

i stand corrected.

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Fred V
Pensacola, Fl.
home track:
http://www.waleswest.com