At 07:48 AM 5/31/02 -0600, you wrote:
>Roundhouse valves are pop valves, although they don't pop nearly to the
>level that Aster pop valves do.
Kevin,
Yes Aster has mastered the secret of the ball valve safety and there's
really no secret to it but in order to have a true "pop" safety valve there
must be more to it than just a ball on a seat. The theoretical
requirements are known and Aster has spent the money and made the effort to
develop a set of chamber geometry that satisfies those requirements for the
size safeties they make. No doubt it took some experimentation but once
the right combination is hit upon it you just crank 'em out.
Several years ago, after buying one of those hapless non-popping
Roundhouse safeties, I attempted to make a good pop valve myself using SS
balls as the valve based upon model Ross pop valve design information I had
at the time. A proper Ross valve of course uses a "disk" valve, which will
give much better geometry, but I had no desire to attempt a disk in this
size. The results were marginal, and ultimately not successful. One
version would pop cleanly open but fizzle shut. One would fizzle open but
pop shut. All of them allowed entirely too much of a drop in pressure
before closing. Of course what I wanted to achieve was a valve that would
pop open, reduce pressure by no more than 4psi, and pop shut. Frustration
led me to at one point consider making wax plugs of the chambers of Aster
safety valves and start knocking them off. But, in the words of Tricky
Dick, that would be wrong!
Without going into detail (even if I were capable of it), the
geometric relationship of the ball, seat, and chamber must follow certain
parameters and at that time I had not fully discovered what those were. I
still don't have a concrete set of parameters or a plug-in formula that
relates everything to say ball diameter but here are the high points. The
ball must sit in a stepped chamber or recess and the depth and diameter of
the recess relative to the ball diameter is critical. The diameter of the
steam hole, and the height at which the ball will rest realtive to the
recess chamber rim, is also critical. The steam hole also determines the
force of pressure that the ball valve will "see" and must resist. The area
within the upper chamber creates what is called clearance volume and is
also important as it has an effect on the popping action as does the area
of the escape passage, the holes, in the top of the valve. If any of these
relationships deviate from a narrow requirment one or more aspects of the
valve won't work, as was my experience. Some might ask "but what about the
spring?" Yes the spring is important but several combinations of spring
length, diameter, number of turns, and wire gauge will work but only one
chamber geometry will work.
I did not continue my safety valve experiments but I have continued
to collect design data for Ga1 (as well as a package of copying wax :-o)
and will give it another try before long. One encourging note to me is
that I have seen a number of home made safeties that operated and looked at
least as good as the Aster valves if not better. So it can be done.
Regards,
Harry
