On Sat, 24 Jan 2009 20:52:44 -0600, Ignoramus11135
wrote:

reposting to sci.engr.joining.welding

I am sure glad that I chose not to sell this 80 gallon Speedaire
compressor.

Based upon first impressions, the motor and pump run fine. However,
when bringing the tank up to pressure, I heard a hissing noise at
about 140 PSI, and realized that there is a hairline crack in the
tank, near a 2" long weld attaching the engine/pump mounting platform
to the tank.

The crack does NOT track the heat affected zone of the original weld,
in fact it is perpendicular to it.

You (meaning the average "dauber" welder) DO NOT EVEN THINK ABOUT
welding on a pressure vessel. Anyone who does it for you (especially
for cash money) has his ass hanging out a mile and a half for
liability - this is a specialized field with lots of pitfalls.

If you really want it fixed, you need a certified boiler and
pressure vessel welder to do the work. They do exist, these are the
same people who weld in new crown sheets and repair patches on old
steam engine boilers and large stationary boilers.

They have to match the type and gauge steel used in the tank, roll
the patch to the proper curves, use the proper welding wire or rod,
pre and post heat treat as needed, repair any staybolts or attachments
as needed, X-ray or otherwise check their work (and for any other
latent problems they might have missed) and then hydrotest.

But a brand new factory built tank is probably cheaper than paying
someone to repair and recertify the one you have. And this time, mount
the compressor remotely.

The reason they spend a lot of money to repair those historic boilers
is repair is still cheaper than building a new boiler to the exact old
dimensions from scratch - and then rebuilding the entire vehicle
around it. Or in the case of a huge power plant, wrecking the old and
erecting an all-new boiler in the same space - they don't ship well
when you get up in the million pound range...

Or they don't want to destroy any 'historic value' of a steam
engine with an all new duplicate boiler, which is silly if they
duplicate it faithfully (well, except for the lap-seams and rivets...)
and they reuse the chassis and all the old running gear.

There are times when safety trumps perfect historical accuracy, and
this is one of them - the aftermath of tanks and boilers going BOOM!!
is not pretty.

-- Bruce --