Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

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Default Welding on a compressor tank :-(

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.

..

===========

!

Here the "=" signs denote the weld, and "|" signs denote the crack.

In any case, when I saw this happen, I was quite scared and
immediately relieved the compressor of pressure.

I am not yet sure what was the cause of this crack in the first
place. I will try to find what I can.

My question, obviously, concerns my repair options. I can hardly think
of any welding where more is at stake than here, due to pressure. So,
I see the following options:

1) Take off motor and pump, cut holes in the tank and throw it away,
look for another tank.

2) Repair the tank by welding and hydrotest.

Considering option 2, the first question concerns welding. How would
you weld? How do you identify where the crack ends? Would you drill
relief holes at ends of the crack?

The second question is about hydrotesting. I was thinking about
something simple, such as replace tha gauge with a 400 PSI gauge,
close off all openings besides one, fill tank with water, connect to a
hydraulic pump or grease gun, and bring pressure to 400 PSI (the tank
supports 200 PSI) and look for leaks.

This is very time consuming and I would like to know how likely would
it be that I would make some very bad mistake.

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Default Welding on a compressor tank :-(

I was thinking overnight. If the issue is fatigue from vibration, what
if I take the motor and pump off, set them up somewhere else, weld and
test the tank, and use it as air receiver tank at a slightly lower
pressure (say, 120-150 PSI).

Then further fatigue will not be an issue, as there will not be
further vibration.
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Default Welding on a compressor tank :-(


"David Lesher" wrote in message
...
"SteveB" writes:


They aren't that big, and they don't have that much pressure. Plus, it's
only air. Danger is relative to distance, and the danger falls off
exponentially with distance.


Only air????

There's a reason the pressure test is done with water.....and never air.


I guess you missed the part with the point.

Steve


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Default Welding on a compressor tank :-(

Ignoramus335 wrote:
I was thinking overnight. If the issue is fatigue from vibration, what
if I take the motor and pump off, set them up somewhere else, weld and
test the tank, and use it as air receiver tank at a slightly lower
pressure (say, 120-150 PSI).

Then further fatigue will not be an issue, as there will not be
further vibration.

It's not fatigue from the pump vibration, it is
fatigue from the expansion of the tank under air
pressure. Every time you pump up the tank, it
expands a few thousandths of an inch. Any
weakness or corrosion concentrates that stress.

Jon
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Default Welding on a compressor tank :-(

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 --


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Default Welding on a compressor tank :-(


"Jon Elson" wrote in message
...
Ignoramus335 wrote:
I was thinking overnight. If the issue is fatigue from vibration, what
if I take the motor and pump off, set them up somewhere else, weld and
test the tank, and use it as air receiver tank at a slightly lower
pressure (say, 120-150 PSI).

Then further fatigue will not be an issue, as there will not be
further vibration.

It's not fatigue from the pump vibration, it is fatigue from the expansion
of the tank under air pressure. Every time you pump up the tank, it
expands a few thousandths of an inch. Any weakness or corrosion
concentrates that stress.

Jon


I disagree with that, Jon, based on data from the AWS Pressure Vessel
welding reports I've seen over the years. The expansion/contraction due to
pressure increases and decreases is a minute fraction of the fatigue loads
imposed by vibration.

True fatigue, as I said, is a very strange phenomenon. There are all sorts
of failures we commonly call "fatigue," but true fatigue often behaves very
differently from what many people expect. It's the product of number of load
cycles imposed at some large fraction of the elastic limit, and it varies
widely by material type. For aluminum, for example, that
expansion/contraction due to pressure changes, as in a jet airliner, can
indeed fatigue the material to failure. In steel, that isn't likely to
happen. Steel is vastly more resistant to fatigue than aluminum is. Fatigue
cycles to failure for steel usually are measured in the tens of millions of
cycles. That's a lot of expansion and contraction, but maybe only a few
years of vibration cycles, given the normal uptime on a compressor in
commercial service.

--
Ed Huntress

--
Ed Huntress


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Default Welding on a compressor tank :-(

On Jan 24, 8:52*pm, Ignoramus11135 ignoramus11...@NOSPAM.
11135.invalid 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.

.

===========

!

Here the "=" signs denote the weld, and "|" signs denote the crack.

In any case, when I saw this happen, I was quite scared and
immediately relieved the compressor of pressure.

I am not yet sure what was the cause of this crack in the first
place. I will try to find what I can.

My question, obviously, concerns my repair options. I can hardly think
of any welding where more is at stake than here, due to pressure. So,
I see the following options:

1) Take off motor and pump, cut holes in the tank and throw it away,
look for another tank.

2) Repair the tank by welding and hydrotest.

Considering option 2, the first question concerns welding. How would
you weld? How do you identify where the crack ends? Would you drill
relief holes at ends of the crack?

The second question is about hydrotesting. I was thinking about
something simple, such as replace tha gauge with a 400 PSI gauge,
close off all openings besides one, fill tank with water, connect to a
hydraulic pump or grease gun, and bring pressure to 400 PSI (the tank
supports 200 PSI) and look for leaks.

This is very time consuming and I would like to know how likely would
it be that I would make some very bad mistake.

--
* *Due to extreme spam originating from Google Groups, and their inattention
* * * to spammers, I and many others block all articles originating
* * * *from Google Groups. If you want your postings to be seen by
* * * * *more readers you will need to find a different means of
* * * * * * * * * * * *posting on Usenet.
* * * * * * * * * *http://improve-usenet.org/


Ig...replace the tank.

I repeat...replace the tank.

The replacement is cheap...make sure you get a ASME rated tank.

Not worth the time, hassle or liability messing with it as a pressure
vessel.

And yes air compressor tanks do go *BOOM*.

Use the old tank for your favorite HSM project.

Be aware that where the water sits it will likely be rusting through.

Let us know what you build with it.

Good luck.

TMT
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Default Welding on a compressor tank :-(

Too_Many_Tools fired this volley in
:

Use the old tank for your favorite HSM project.


That's spelled, "Build a _real_ barbeque" G.

LLoyd
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Default Welding on a compressor tank :-(

But then will have to call you

pIggy :-)

--.- Dave

OK, thanks to all. I will not use the tank for compressed air. Maybe
for pig roasting or some such.



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