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Name: Jeff
Status: other
Grade: other
Location: OH
Country: USA
Date: Winter 2013-14


Question:
We winterize outdoor irrigation systems by draining the water and evacuating the remaining water by blowing compressed air through the system. Great results and yet at times we experience a brass backflow device - that has been drained of water and blown out - that develops a crack. Does the effect of Midwestern winters - cold, freezing weather, a lot of moisture, and occasional warm up periods - have an effect on the brass metal?



Replies:
It is possible that the brass device is connected to an iron pipe. The ratio of the coefficients of thermal expansion for brass and iron is about 1.6. I can imagine that if the brass fitting is installed on a hot summer day when the brass has expanded considerably more than the iron it might get tightened down enough that in cold weather, when it contracts more than the iron it then splits.

Greg Bradburn


Hi Jeff,

Up here in the Okanagan Valley (south-central BC, Canada) we are quite familiar with blowing out our underground sprinkler pipes. I have mine done every fall.

There is nothing about brass itself that causes it to crack in extreme cold. The problem you are having is likely caused by water trapped in the back flow device and not being blown out properly. This trapped water freezes, and is undoubtedly what causes the brass fitting to crack.

Regards, Bob Wilson


Hi Jeff,

Thanks for the question. Brass plumbing pieces (as well as other metals) can and do crack under the thermal cycling experienced in temperate climates. Plastic is even susceptible too as I found out a couple of weeks ago. The backflow preventer (or check valve) typically contains another piece of metal inside. This piece has to be in tight contact with the housing for proper operation. If this piece of metal does not contract at the same rate as the brass housing does, then a stress will be applied to the brass housing and the brass will crack. If this becomes a major problem, I would consider purchasing a backflow preventer with a brass piece inside.

I hope this helps. Please let me know if you have more questions. Thanks Jeff Grell


Brass suffers from "dezincification" -- this is where zinc is leached from the brass, leaving behind weakened metal. The process is accelerated if the valve is exposed to highly chlorinated, oxygenated, salty, or stagnant water. Water service brass fittings should be made of types of brass (often called 'red', meaning low zinc content) that limit this effect, but inexpensive parts with poor manufacturing can be more susceptible. A white powder residue is often seen in dezincified brass fittings. The freeze-thaw cycle may introduce microcracks in the metal, which could accelerate dezincification, but that's a secondary factor. If you do not completely blow the valves (or if they are of a design that does not blow clean), or if the fitting is in a highly fluid soil (e.g. muddy or water), then the freeze-thaw cycles, the direct mechanical stress from the freeze-thaw cycle is likely the bigger problem than dezincification.

Hope this helps, Burr Zimmerman


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