Technical Bulletin # P-09

Recognizing The Hidden Threat To Every
Chill And Cold Water Piping System

THE PROBLEM:

     Throughout the course of most ultrasonic surveys, we find some degree of corrosion present on the outside surface of the chill and cold water piping.

     Often, the presence of condensation at the outer pipe surface produces the same corrosive effects typically associated with interior pipe corrosion. In extreme cases, the condensation of humidity will build up to the point where the insulation becomes completely saturated with water. Generalized corrosion, excess surface pitting, as well as the deterioration and failure of the insulation itself, usually result.

     While fiberglass and other forms of insulation serve to provide important protection against unwanted heat transfer, its equal or more important function for cold water systems is to prevent the condensation of moisture on the outer surface of the pipe. The thinner the insulation, the greater the transfer of both heat and moisture to the pipe surface, and therefore the greater the resulting corrosion problem. Even installing 2 in. or thicker fiberglass insulation, though excellent at reducing heat transfer losses, is often not alone sufficient to prevent the infiltration of moisture and condensation at a cold pipe surface.

     Generally, corrosion will be more severe at the supply side piping - simply due to the presence of lower pipe surface temperatures. Depending upon the relative humidity present, it is not unusual to find a 55 º F. chill water return line free of any surface corrosion while the 45 º F. supply side pipe has been severely deteriorated. Brine systems, and those operating at 40 º F. and below, are most susceptible to outer surface corrosion.

     Unlike interior pipe corrosion which can never be stopped, and which exists due to a complex variety of physical and chemical causes, piping failures due to the extreme outer corrosion of chill and cold water systems are totally preventable. Such failures are generally attributed to inadequate maintenance, or the failure to check for and recognize such corrosion problems.

     The below photograph dramatically illustrates the ultimate consequence of inadequate pipe insulation. This 1-1/2 in. Schedule 80 black pipe chill water supply line was identified as having an outer corrosion rate of approximately 4 times that of the water side, and an existing wall thickness well below minimum acceptable standards - at approximately 0.089 in. All evidence of threads, 0.073 in. deep for this size pipe, have been completely corroded away. Similar examples were found at the chill water piping throughout the building, and required replacement at tremendous cost.

     Most examples of severe outer corrosion are found at such smaller pipe sizes simply due to the many fixtures, elbows, and transitions, and consequently - difficult or ineffective insulating efforts. Given sufficient time and low enough water temperatures, however, large pipe can suffer just as well. ECI has documented the total destruction of insulated condenser water return piping, having a 95 º F surface temperature, and traveling through a leaking steam room having extremely high humidity conditions.

     In the below example, years of high humidity conditions effectively penetrated the insulation to cause a severe corrosion condition. Extensive rust removal, painting, and re-insulation was required. Surprisingly, sufficient remaining wall thickness yielded an acceptable remaining service life of 25 years.

     In another example, severe external corrosion actually produced a layer of rust product of approximately 1/2 in. thick. Again, a general ultrasonic piping evaluation discovered this condition, and it was repaired before destroying the entire chill water piping system.

     Chill water pipes are not the only victims of such outer surface corrosion. A common occurrence in domestic house tank investigations is to find extreme outer corrosion at the bottom or underside. Here, inaccessibility often prevents adequate coating or painting. For the same reason, normal maintenance to the side walls is rarely performed at the bottom surface. See Technical Bulletin # C-11 regarding special corrosion problems at house tanks.

THE SOLUTION:

     The first step toward protecting against outer pipe wall corrosion is to find out if it exists, and to what extent - a step requiring some investigative effort. Recommended prime locations to remove samples of insulation for inspection are high humidity areas such as steam rooms, mechanical rooms, open areas, and rooftop or other locations exposed to the elements.

     Supply side piping should be checked first. Hard shell older style insulation, or insulation which is hand formed of plaster or similar materials is usually very effective at holding back any moisture from penetrating. Standard unpainted paper or cloth covered fiberglass provides virtually no barrier to moisture, and is always a high priority location to check. Old style cork insulation, still in use from years ago, actually attracts moisture, and has been shown to cause the most severe cases of outer pipe corrosion we have seen. Painted insulation is effective at retarding moisture penetration depending upon its composition and mil thickness.

     While foam type piping insulation is convenient to apply, it too has been shown to fail in preventing the migration of moisture to the pipe surface. Painting foam insulation is generally not an option. In addition, a recognized chemical reaction between the components of the foam and the pipe itself has been cited in the literature as greatly accelerating pipe surface corrosion. ECI has documented such foam insulation failures in numerous inspection cases, one example of which is shown below.

     Broken, missing, crushed and other damaged sections of insulation will obviously lead to outer pipe corrosion problems, and should be repaired or replaced as soon as it is discovered. Missing insulation can be frequently found at transitions through walls and floors, or in areas of heavy traffic.

     A high amount of outer corrosion can be typically found at smaller diameter pipe simply due to the inherent difficulty of insulating its many elbows, tees, valves, strainers, and other small piping components. Unfortunately, it is also that smaller diameter piping which has a thinner initial wall, and which is substantially weakened by the amount of metal cut away during the threading process. It requires little outer corrosion at small diameter pipe, therefore, to create a failure condition. See Technical Bulletin # P-02 regarding threaded pipe problems.

     Our standard recommendation to ensure against moisture caused outer pipe corrosion is to install 2 in. or heavier fiberglass pipe insulation on all cold water pipe of 50 º F and below. In addition, a high solids paint, epoxy coating, or hard shell outer covering, sealed thoroughly at all seams, is critically important in order to resist moisture penetration.

     Because the surface of the pipe which is insulated is never seen, it is almost never painted. As a result, the moisture which is allowed to build up at the pipe surface is able to attack completely unprotected steel. As a precaution against such possibility, ECI recommends painting all cold water piping with a strong rust preventative coating prior to being insulated.

     The following list offers some good general recommendations for providing the maximum heat transfer efficiency and condensation protection of any insulated chill or cold water piping system:

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