ONAN Transformer Radiator Oil Leak

Many years ago I came across an outdoor mineral oil filled transformer that had been patched and sealed with some sort of epoxy. It did not leak. I really can’t remember whether a fin or a header, I’m tending towards a fin, a small one, like 3” across.

I think that there is a fix for you that will be less cost than draining the transformer and welding up the leak.

The repair is a 2 step process, where a temporary fix involving a proprietary Belzona compound is used, making the area suitable for a permanent repair.

However, based on your photo, access to the leak could be very difficult. Perhaps a consultation with a Belzona application engineer could give you a plan.

Yeah okay cheers I'll investigate.

This transformer isn't ours so we have been contacted to see if we can fix it. Given the nature of the previous attempt, they've asked us to do a 24h pressure test as well as certify that it will not leak for X amount of time eg 12 months. Which would be tough to do if we expoxied.

Im wondering if its even worth it to fix or do we just just chop the fins off, and block the holes. The only question i have is, would that derate the transformer at all?, Its just two fins.

I think the temporary fix gets the oil away from the repair zone, so it can be welded?

Removing any cooling surface would certainly reduce the cooling capacity. Whether that is enough to reduce the life of the transformer due to elevated temperature is unlikely, especially if the unit is not operated at 100% of its rating 24/7. The manufacturing design allows for hot spot winding temperature, a 40C peak and 30C average 24 hour temperature, for example. The tolerance likely engineered into the original design should provide expected equipment life. Oil filled transformers are generally known to far exceed their design life.
If I owned the business served by the transformer and had to pay for the repair, I wouldn’t think twice about cutting off a couple of fins, (assuming they are less than 5% of the total cooling surface).

However, in your case, a welding repair made on clean metal in a fin completely full of oil should not be a problem. No oxygen, and from one source, a 55DegC hot spot during the welding inside the tank should not cause any problems.

Regardless of how the leaks are stopped, the cooling of this transformer will be compromised due to the reduction in coolant flow through the compressed fins. Consider the possibility of including some temperature monitoring in the repair.

I have welded tank and cooling fin leaks before in a past life.

Apply a Vacuum to the tank, this moves the oil away from damage hole.

Weld it up, either TIG or mig with the vacuum pump running, taking care not to get oil into your vacuum pump.

On the nameplate there may be a max vacuum and pressure allowed for the tank, do not exceed and test away. Locking the vacuum on the tank and watching for bleed down will point to any weld porosity, then do the same with the pressure.

Happy melting!