Water coil elbow cracking

Steve Bukosky_2

Attached is a picture of a water coil using in an air conditioning air handler. There are 16 apartments with this set up and just this unit has had this damage happen and to three coils so far.

Each apartment is on a monoflow loop and controlled by zone valves. Some takeoff's had flow problems so circulators were added to some zones. I have not visited the job yet but will be looking for entrapment of the coil, but no gas heat is present so I don't think that will be the issue. That leaves me to something to do with cavitation or flashing from pressure drop.

As I don't do much hydronics anymore and have not seen much coil damage, I'm asking you gents if something other than freezing could expand and crack these elbows as shown in the attachment?