I have 1/2" copper tubing between the pump and my tank on my compressor. I replaced it recently with all new fittings and the copper tubing broke within a few weeks. The point of failure was just outside the compression nut just where the pipe goes into the nut. I replaced and it broke again in the same spot in a week. Any suggestions. 

I thought to replace with a flexible hydraulic hose but there is a compression fitting on the tank that cannot be replaced with a npt fitting. 

Any thoughts? Thanks

Did you use off the shelf parts or did you order replacement parts from the OEM?

compression fittings from plumbing supply. It appeared to be exactly as what was originally there......

Sounds like the pipe is vibrating, causing the connection to fail. Any way to brace the pipe or put a coil in it? A picture of the setup may help diagnose the problem.

Possibly vibration.  Is the pump tight to the tank body?

Also did you try replacing the copper tubing as well?

Find some adapter fittings so you can use flex hose.  Look for "compression to NPT threads".  I found some on mscdirect but am not allowed to post a commercial link.  Almost any plumbing supply house should have them as well.

I did replace the copper tube and added a loop to the copper. I will try the flex hose. Thanks.

That copper tubing may have been at a different work hardness level and subject to metal fatigue.

Coiled copper tubing should always be soft at least when new. 

Copper does "age" harden though so a quick anneal will ensure softness. 

To anneal copper heat pretty dang hot and quench in water.

Copper work hardens fairly fast. It sounds like there is something loos that is allowing it to vibrate. also only run the compression fitting down enough to seal it, over tightening can cause a crimp failure point. I see a lot of lines broken right at the ferrule.

Another option would be to do a flare nut fitting instead of a compression.

Hi Steve. 

I'm assuming we are talking about a piston compressor here? The vibration from the pump plus the pulse from the air will often break plumbing grade copper pipe, the manufacturer will typically use thicker wall stuff. 

Anealed pipe should be fine, don't go mad on tightening the compression fitting though, a very compressed olive will create a weak spot in the pipe. 

Becareful using hose, again assuming it's a piston compressor and your taking the pipe direct from the cylinder head to the tank depending on how hard you run the compressor the pipe could get up to 150C to 200C. Most hydraulic hose isn't rated for that hot.