Microbiological monitoring is seldom needed in hot water loops that operate continuously above 140°F (60°C) or in chilled systems where the glycol concentration is above 20%. However, when these conditions are not met, bacterial monitoring should be part of the testing program, especially with readily biodegradable treatments such as nitrite. Testing can either be via plate counts/dip slides (giving results in colony forming units (CFUs) per mL) or ATP (adenosine triphosphate) assay, which gives a measure of all types of micro-organisms and provides results within minutes. The ATP test result is in ng of ATP/mL, although some companies use the less accurate and not consistent units of RLUs (relative light units: light output relative to a particular instrument and a particular batch of reagents).
Any monitoring program is only as good as the frequency with which it is carried out. Corrosion coupons and corrosion product determination (in the system fluid) are most effective at illustrating a trend rather than providing absolute values. A regular program of sampling will allow one to verify the level of protection that is being achieved and to determine if it is changing over time.
Monitoring the product concentration is normally a routine part of the treatment program. It can be done either by plant personnel or the chemical supplier. The objective of the chemical testing is to confirm that the corrosion inhibitor is present in an adequate amount and that the pH is buffered to the right level. In the case of glycol loops, verifying that the glycol concentration is more than 20% is critical. At levels less than this (for chilled and out of service hot loops), rapid biological degradation of either ethylene or propylene glycol to an assortment of organic acids and intermediate products, will take place.
Considering what can happen if a system is not treated to a reasonable standard, closed systems must be treated to give the lowest possible corrosion rate and to control microbiological activity.
Using effective inhibitor packages, it is possible to obtain corrosion rates of less than 0.2 mpy on mild steel and less than 0.1 mpy on copper and copper alloys. With these corrosion rates and schedule 40 piping, a system life of more than 50 years is a realistic expectation. While the overall, or general corrosion rate is important, a successful program must also control pitting or localized corrosion.
In systems where low flows or stagnant conditions can exist for extended periods, protection from localized corrosion is equally important. The amount of iron and copper present in the system fluid (whether water or water/glycol mixtures) should be quite low. In well maintained systems, it is common to find iron and copper concentrations at or below, 0.2 mg/L and 0.1 mg/L, respectively.
Microbiological activity will vary from zero in an operating hot water circuit to being present in chilled water loops treated with nitrite. In chilled water systems, it is generally agreed that <103>
At a time when the need for new, innovative approaches to treat closed loops appears to be growing, little new technology is coming to the marketplace that can match the performance of 20-year-old technologies.
As restrictions on metal-based and nitrite programs continue to grow, the demand for effective “green” alternatives will expand. Newer chemistries, such as neutralized dibasic acids, phosphonates, triazines, etc., either do not provide the level of protection of traditional programs or are not applicable to a broad range of applications.
At a minimum, treatment programs for the next millennium will need to provide:
- Low environmental impact, with good long-term biodegradability;
- Biological stability (not readily biodegradable) under use conditions;
- Mild steel and copper corrosion rates of less than 0.5 mpy and 0.1 mpy, respectively; and
- Easy testing by plant personnel.
The company that can develop and market a product with this performance profile will be well positioned to capture a low portion of this market.