GLYCOL CONTAMINATION OF CHILLED WATER SYSTEMS

Ethylene glycol is often used to winterize coils that contain water to prevent them from freezing during the winter. These coils are isolated and filled with a glycol-water solution to provide the desired amount of freeze protection. When warmer weather returns, the coil is drained as much as possible and then put back on line. Since it is very difficult to drain all of the glycol from the coil, this allows a small amount of glycol to enter the main circulating water. At concentrations above 3%, ethylene glycol is toxic to most bacteria so that bacterial populations in properly winterized systems are very small. When trace amounts of glycol are found in the circulating water, however, the glycol can act as a food source for certain bacteria. This bacterial contamination does not happen all the time but it does occur often enough to be a problem.

Bacterial degradation of the glycol produces several organic acids and an indole. The acids include butyric acid, which has a very bad odor. Therefore, the breakdown of the glycol is first evidenced by a strong, foul odor in the circulating water. The next change is a drop in the pH as the organic acids concentrate. The pH of the water will rapidly drop to 4.0 to 5.0 pH units. This acid pH brings about the next noticeable change, which is a brown discoloration of the water, as the low pH causes the corrosion of the piping within the system. If a nitrite-based inhibitor is in use, the nitrite will rapidly oxidize to nitrate and lose its inhibitory properties. This will be seen as an increase in inhibitor use with a fairly rapid loss of nitrite from the system water. The conductivity of the water will increase as inhibitor is added to the system. Molybdate provides a certain degree of protection at this low pH but, at the normal treatment levels, some corrosion will take place. Higher than normal levels should be kept in the system as long as glycol is detected.

Trace amounts of glycol are difficult to measure as they are below what can be seen with the standard hydrometer or refractometer testing procedures. Test kits for trace amounts of glycol are available from the Hach Company, Loveland, CO.

PREVENTION

• Blow as much glycol out of the coils as possible to minimize contamination.

• Test chilled water systems periodically for inhibitor level, conductivity and pH. Changes in these parameters may indicate a problem.

If the water is fouled, the system should be drained, cleaned, rinsed and recharged. If it is not possible to do this, the system should be dumped, rinsed and recharged with a molybdate based inhibitor plus a quantity of Algaecide to keep down the bacterial population. The water should be checked periodically to determine if recontamination is taking place. If the pH becomes acid, an alkali may be added to raise the pH. However, if the water is brown (much dissolved iron) an alkaline pH will cause the dissolved metals to precipitate out of solution and foul the heat transfer surfaces. If this is possible, a side stream filter should be installed to clarify the water.

Feed a non-foaming biocide into the system to prevent the growth of troublesome bacteria. The total bacterial population should be monitored periodically with dip slide testers to be certain that large bacterial populations do not develop.

In systems, which are subject to periodic glycol contamination, a molybdate-based inhibitor should be used as it is not subject to bacterial degradation and provides some protection at low pH levels.