For a 40 story office property built in 1965 and having 1,000 or more feet of 12 in. condenser riser piping, the total loss of more than 5,000 lbs. of steel into the circulating system can be shown. See the below table for the weight losses of various pipe sizes and corrosion rates.

Often, a low to moderate corrosion rate will present no threat to the integrity of the pipe itself, yet may still damage the piping system after many years or decades of operation. At a constant 5 MPY corrosion rate, for example, 12 in. schedule 40 condenser water pipe will last well past 45 years before reaching 0.175 in. - its minimum acceptable wall thickness under most conditions. Yet, that same 5 MPY corrosion rate still presents a very serious threat.

Since the corrosion of steel produces a significantly greater volume of less dense iron oxide, serious secondary problems can be created unless such deposits are continuously removed. Interior deposits can produce even more serious problems at closed systems - where no blowdown exists and no indication is given. Corrosion problems are often localized, and a gradual build-up of deposits in the lower floor horizontal lines due to a low 1 MPY system wide corrosion rate can produce random but severe wall losses at rates well above 25 MPY - leading to premature pipe failure.

Correcting A Problem

Unquestionably, removing all existing deposits of iron oxide, which can total in the thousands of pounds for even a medium sized commercial building system, should become the most important focus in addressing any corrosion problem. Aside from lost heat transfer, clogged strainers, restricted flows, and other operating problems, interior surface deposits greatly accelerate pipe loss by preventing corrosion control chemicals from reaching the base steel, and by initiating various secondary corrosion mechanisms.

Due to the large volume of deposits typically produced by any corrosion condition, the addition of supplemental filtration is mandatory - with the option of filtering the greatest possible volume of water preferred over capturing the smallest micron particle.

Chemically removing the accumulated iron oxide by either dissolving it for blowdown or re-suspending it for filtration capture is typically employed, but often presents added threat to the piping and related system components depending upon the remaining integrity of the piping, cleaning agent, and cleanout procedure used. Greater maintenance demands in the form of punching heat exchanger tubes, cleaning strainers and tower pans, and added filter maintenance may be temporary, or in the case of a severe corrosion problem, may be a permanent addition to the operating schedule of the property.

Increased chemical inhibitor and biocide levels are generally required, as are supplemental chemical dispersing agents. For a microbiologically influenced corrosion (MIC) problem, repeated sterilization and cleaning of the system will be necessary. Contracting an outside consultant to oversee and advise the chemical treatment or cleaning program is often advised due to the complexity of various treatment options, conflicting claims and abilities of various chemical treatment contractors, and the potential threat to the piping system.

Under the most severe conditions of a well established under deposit corrosion or MIC condition, it may be impossible to save the piping system from premature failure. In such cases, effort usually focuses on minimizing damage and operating problems, replacing pipe as necessary, and extending its service life as best possible.

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