Sanitary Corrosion Failures in Cast Iron Pipe
For most of us older engineers, we were taught that cast iron piping was the preferred material for drainage systems. Durability, longevity, use in return air plenums and sound dampening were often used in describing the benefits. I can remember being told that it would last over a hundred years, but after looking at a source for this claim, I discovered that was for service water piping. The reality is sanitary waste is more aggressive than service water with respect to corrosion. While some might expect the first signs of failure to be dripping of liquids from horizontal pipe, more often than not it is the detection of sewer gas odors, a result of hydrogen sulfide corrosion. The decomposing waste in the pipe releases hydrogen sulfide gas, which is oxidized to sulfuric acid by bacteria living on the moist, non-submerged surfaces. As a result, the upper portion of horizontal piping tends to experience more significant corrosion than the bottom portion of the pipe.
The primary material alternative to cast iron has been PVC for quite some time. In most cases, cast iron was chosen if the building had return air plenums or the owner had design standards requiring the use of cast iron. The return air plenum is still an issue because PVC does not meet the code-required smoke development rating for use in return air plenums, though there are UL-listed wraps that can now be used to allow its use. Also, campus design standards for a lot of owners are now requiring the use of PVC, or at least allowing it. From a corrosion standpoint, PVC certainly performs better than cast iron. While the disadvantages of PVC haven’t really gone away, they do seem to be viewed less of a disadvantage today.
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