Ever since the landmark NSW regulatory changes in 2018 that mandated a risk management approach to managing cooling towers, there has been some uncertainty as to what techniques satisfy the requirement for corrosion monitoring in cooling tower systems.
NSW Health has sought to offset any further uncertainty recently by directing the industry back to the standard they must comply to, AS/NZS 3666.3 Air-handling and water systems of buildings—Microbial control, Part 3: Performance-based maintenance of cooling water systems.
The Department has confirmed that when complying to this standard, corrosion monitoring is mandatory. Building owners, property managers and tenants are responsible for ensuring it is undertaken. Whilst documented visual inspections are permitted, on their own, this action cannot evaluate the enclosed plant and pipework. Additionally, visual assessments do not quantify the rate of corrosion to industry standards making it impossible to assess whether good corrosion control is being maintained and they may fail to identify corrosion until it has reached a catastrophic tipping point.
Corrosion monitoring in water systems goes well beyond asset protection. When the internal surfaces of a cooling water system become corroded, contaminants such as iron can be released which is a major growth factor for harmful Legionella bacteria. Corrosion formations can also harbor Legionella bacteria away from biocides where colonies can multiply without control.
Corrosion monitoring can also protect your investment and reduce the life-cycle costs by helping to provide:
- An early warning system to alert you before a corrosion-induced failure occurs.
- An evaluation of the effectiveness of corrosion prevention measures, including the corrosion inhibitor and the chemistry of the system water
- Accurate, verified data which can be used by management as input into decision making and planning.
The most common test method for monitoring corrosion is ASTM D2688. This standard requires the installation of monitoring equipment within your system and defines requirements such as the flow rate, rack dimensions and the method for processing and coupon analysis.
Customers wanting real time corrosion monitoring can use Linear Polarisation Resistance (LPR) technology. This electrochemical type measurement is an accepted method and affords continuous monitoring of corrosion. As there is no test piece to be removed from the system for analysis, this type of measurement is often utilised in crisis situations where the causes for excessive corrosion need to be identified and addressed quickly. The remedial steps can be assessed for effectiveness almost immediately rather than waiting three months to test a corrosion coupon.
The LPR method applies a low voltage (10-20 mV) to a set of electrodes in the system and measures the resulting current. The intensity of the current can be linked to the rate of corrosion.
Additionally, dissolved iron and copper testing is useful and can provide a possible pointer towards corrosion in the system. To accurately determine the source of these metals however, it is important to test the incoming make-up water. Whilst testing may indicate that metals in a circuit are corroding, they may in fact be unblemished. Dissolved metals are sometimes simply introduced into the system from the mains water supply and then concentrated due to the evaporative effect of the cooling tower.
All Risk Management Plans (RMPs) in NSW ought to have included an action to undertake routine corrosion monitoring. This is generally done quarterly by the corrosion coupon method.
NSW Health have signaled a sharper focus on auditing compliance in that state. It is important to ensure corrosion monitoring is part and parcel of the maintenance and testing program for all relevant assets.