District Energy - 2008 Second Quarter

obvious cost reasons they are not used as such. As a side note, that is exactly why the most noble metals are also used in jewelry due to the corrosive nature of the body’s salts and oils. They are resistant to that as well.

In addition, table 1 indicates why magnesium is often used as a sacrificial anode for buried piping, since it is less noble than steel and will give up the electrons first to the soil. In above-ground installations, corrosion issues typically occur in small, threaded connections from steel to copper or brass connections. The accompanying photo shows what happens to a steel pipe when a brass fitting for a pressure gauge is connected in the absence of an isolating dielectric union. Not a good thing. Definitely some purpling going on there! (Where is the chaperone?)

Although fresh water used in heating and cooling systems is not as highly conductive as seawater, it still has impurities that assist in the galvanic process as a conductor or electrolyte, and the higher the mineral content within the water (hard water), the more the conductivity and galvanic action is increased. Hence, nothing is safe, and there must be constant vigilance in observing this process in your piping system.

So how can you mitigate this natural immigration of electrons? With the use of dielectric fittings or unions that have a

nonconductive or inert material (ceramics or plastics) to isolate the two dissimilar metals. (Technically the electrical circuit is still completed since water is still in the piping, but the process will be slowed.) Even though dielectric fittings are extremely affordable – a half-inch fitting is less than $5, and a 1-inch is less than $15 – and are cheap insurance to install in piping systems, many times they are not present due to installation errors or oversights, etc.

Don’t forget the importance of
such a small component as the
dielectric fittings or let someone
talk you out of them for the
reason of costs.

The galvanic corrosion shown in the photo, on a 3-inch steel pipe to a brass/ bronze fitting tap, developed after less than two years of operation. In this case, the contractor had neglected to add dielectric fittings, resulting in rapid corrosion. Galvanic corrosion is similarly the reason behind the recent renovation of the Statue of Liberty where the copper skin of Lady Liberty was connected to a wrought-iron structural frame. Originally, an asbestos isolator existed between the

steel and copper materials, but it became saturated with moisture from the salt air and thus created a conductor of electrons. After the isolator became useless, the steel started corroding.

Interestingly enough, you can get some galvanic action by joining new, clean steel with older steel. New steel is slightly anodic when connected to old steel; hence, a dielectric isolating flange should be used in expanding or retrofitting systems.

Therefore, don’t forget the importance of such a small component as the dielectric fittings or let someone talk you out of them for the reason of costs. Also, be sure to walk down the visible piping systems every now and then to visually inspect for discolored insulation and piping. This is one of the simplest and greatest tools in determining the condition of your piping system.

So, with my oldest daughter coming of dating age (probably not 30, like I tell her), I am hoping that I can find the perfect dielectric isolator to keep those bad boy minuses away from my angelic plus!

If you would like to learn more about corrosion in piping systems, how to design systems to avoid it and techniques to mitigate it, excellent resources can be found online at www.nace.org as well as www. wbdg.org/ccb/DOD/UFC/ufc_ 3_570_06.pdf .

Based in Madison, Wis., Steve Tredinnick, PE, , is vice president of energy services for Syska Hennessy Group, which has more than 16 locations across the U.S. He has more than 25 years’ experience related to building heat- ing, ventilation and air-conditioning systems. The past 14 years of his work have been focused on district energy systems. Tredinnick is a graduate of Pennsylvania State University with a degree in architectural engineering. He is a member of IDEA and ASHRAE and is currently chair of ASHRAE TC 6. 2 District Energy. Tredinnick currently serves on IDEA’s board of directors. He may be reached at stredinnick@syska.com.