Temperature and Brass Metal
Date: Winter 2013-14
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?
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.
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
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.
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,
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Update: November 2011