iron content in chilled water loop

[Callan Dyke]

Testing on our chill water loop has indicated an iron level of 3ppm to 5ppm for several years; not increasing, this is a constant total. Recently, we've had coil failures at several locations, and the HVAC lead is citing the iron content as corroding the coils. Molybdenum is steady around 110ppm, azole is around 20, conductivity is ~910 uS/cm, pH is 9. Chemistry levels vary somewhat, as the addition is a manual process, and not fully automated. Testing and sampling is performed on a weekly basis.

My question is, with total iron being constant at 3 - 5 ppm, should this be cause for concern? Water samples in the chill water plant and from the headers are crystal clear, with no discoloration. If this small amount of iron really is deteriorating the coils and piping, why wouldn't this show on corrosion coupons, and why does the ppm remain constant, and not steadily increase?

 

[bimr]

Iron is a corrosion product, not a source of corrosion. Oxygen is the parameter that causes corrosion in cooling loops. Since your cooling loop is closed to the atmosphere, then the source of corrosion is greatly minimized. You may see some evidence of corrosion when the system is initially filled, but the corrosion will not continue as the oxygen is quickly depleted.