N:dlzc D:aol T:com (dlzc) wrote:
>
> But you are not going to compress the gas to 800 psi, are you? A
> word of caution on how much pressure to apply would be in order.
> Otherwise his next of kin will come looking for you.
No, 800 PSI would be a bit much in the air pressure department. I
figured the next of kin would spare me for two reasons. First, I don't
expect many people to have that kind of air pressure just lying around,
waiting to be plugged in to the first inflatable toy that happens to
become available. Second, I DID recommend a company that knows how to
do this, and that has the skill and experience to fill in the details
that I didn't include (and plenty that I don't even know.)
The thing about air leak testing is that it doesn't NEED the same
pressure as the application will endure when it's in use. If you're
looking for leaks (and NOT burst-strength results, or other nasty stuff
like that), then all you really need to know is whether gasses or fluids
can get out of a container. In most cases, if they can get out at high
pressure, they can also get out at lower pressures, though in smaller
volumes. Using air instead of fluids often increases the rate of volume
loss through any given leak point, since air will frequently go through
holes that fluids won't. That makes it possible to produce detectable
rates of loss without gigantic pressures.
Higher pressures can cause some special problems, of course, that the
design of low pressure tests needs to account for. If the workpiece
under test, or it's seals, or whatever, are prone to deformation at high
pressures, then leaks might exist only when the pressures are
dangerously high, and might not be detectable by safer and saner
methods. In that case, this is a truly demanding situation, and will
need some very specialized equipment and procedures.
Even at relatively high pressures, however, air seems to me like a
better and safer medium for leak testing. If air leaks out of a
workpiece at, say, 200 or 300 PSI, that can be a hazzard. But
reasonable caution, and maybe a lexan shield or two, can save the next
of kin from needing to dust off their black suits and dresses. However,
that same leak, at the same pressure, could cut holes in a brick wall,
if the fine, narrow, high-velocity steam of leakage happens to be made
of something like hydraulic oil. Equally important, gas can be
compressed. You can pressurize a test subject, then turn off the air
source and wait to see what happens. If a leak results, it only lasts
as long as the contained pressure does, which can be a very short time
if the internal volume is small. Hydraulic systems can't just be
pressurized and then isolated. If the source is turned off, then
pressure goes to zero immediately, with or without leaks. This means
that if a leak starts, it won't just be a short and diminishing burst of
danger, but a continuous stream that lasts until either the pressure
source is turned off, or until all the fluid is gone. Not good. (An
accumulator can maintain pressure when the pump is turned off; but
that's one of those details that's determined by good test design, and
which is way outside the scope of a newsgroup discussion.)
So, to answer your question directly: I HOPE nobody is busy hooking
up 800 PSI air lines, this morning. I certainly didn't advocate that
explicitly. And since gas testing offers some advantages at low
pressures, I'm hoping that I've merely pointed the author of the
original post in a direction that will keep him out of trouble, rather
than getting him into any.
I think maybe I'm gonna grow a beard and change my phone number
though. Just in case. You can never be too safe.
And thanks for jumping in with your concerns about safety. I hope
your question was unnecessary; but it was absolutely appropriate.
KG
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