I purchased a compressor recently. It is a Speedaire, 80 gallon compressor with 7.5 HP motor and 80 gallon tank.

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 HAZ, and "\" signs denote the crack.

In any case, when I saw this happen, I almost dirtied my pants 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 do weld and have some experience with both TIG, as well as with 7018.

But, 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.