corrosion of pipe fitting 1

[pgmmce]

I surveyed an apartment complex that has a water source heat pump system for HVAC with a condensing loop serving
all units.The condensing loop is steel but in each seperate apartments mechanical room, the pipe material changes to pvc. The last steel fitting before transitioning to pvc is severely corroded. The piping is in the mechanical room that serves as a return plenum but the dewpoint of the return air is above the pipe temperature.
Any ideas on why this is happening?

 

[Compositepro]

My guess would be that you have very slight leakage occuring at this connection. The wet condition leads to corrosion. Plastic and metal have large difference in CTE so with thermocycling it is not unusual for threaded connections to weap.

 

[kenvlach]

Re "the dewpoint of the return air is above the pipe temperature."

If so, better coat the steel pipe with something that protects it from corrosion.

 

[pgmmce]

Thank you compositepro. What does CTE stand for.This is happening at almost all apartments (200+)
Thanks
Thanks kenvlach.The dewpoint of return air is above pipe temperature, therefore condensation should not be happening.

 

[EdStainless]

CTE, coefficient of thermal expansion. The plastic expands more when warmed than the steel will, this will lead to loosening of the joint. This happens even if the plastic is the female part.

If the Dew Point is above the pipe temp, then when the air is cooled to the pipe temp you should get condensation. Do you mean to say that the pipe temp is above the dew point of the return air?

It is also possible that the PVC itself has something to do with this. A lot of leachable chloride is not good on metals.

These fittings should at least be galvanized, but that hasn't stopped the external corrosion on mine at home. I just let it go.

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[pgmmce]

Yes , I meant to say the pipe temp is above the return air dew point. I will specify galvanized fittings and thank you for the response.