Corrosion red flags

Sampling alone will not help you tackle pipework corrosion in your HVAC system. You need a more robust strategy, says Steven Booth

Do you know if there is corrosion in your HVAC system? Even if you have had a recent sample, it only represents a snapshot. By the time it has been processed and analysed by a laboratory – which can take days or weeks – conditions may have changed.

Where sampling is the main source of checking water conditions, true problem solving and certainty of resolution is difficult. Without real-time data, flushing is overused as a remedial measure, causing unnecessary damage if interventions are uninformed.

Sampling does not detect dissolved oxygen (DO) – the precursor to virtually all types of corrosion in closed-loop water systems common to commercial HVAC plants; for example, low temperature hot water (LTHW) and chilled hot water (CHW).

With modern, energy-efficient systems installed with low-velocity water flow and leaner materials, it is important to monitor corrosion-influencing parameters and maintain the integrity of pipework and components in closed systems.

Looking behind the scenes

Our partner, Hevasure, studied eight years of data from its real-time monitoring systems in 80 buildings, including commercial spaces, hospitals, data centres and residential blocks connected to district heating schemes.

The study looked at how many times specific issues had triggered ‘alarms’, indicating early warning signs that could lead to serious corrosion damage. There were more than 1,800 alarms and over 50% of systems developed problems at some stage – mainly due to excessive DO – which would have been missed by intermittent water sampling.

Seven red flags

1 High DO: 443 alarms (25%) –closed systems need to be air-tight. The presence of DO indicates air ingress. For a typical closed system, DO should be around 0.1 to 0.2 mg/L.

2 Pressurisation: 402 alarms (22%) –to prevent air being drawn-in, positive (but not excessive) pressures must be maintained.

3 Conductivity/dosing: 403 alarms (22%) – if minimum thresholds of chemical concentrations are not maintained, inhibitors are ineffective and glycols offer insufficient freeze protection. Conversely, overdosing is an environmentally unacceptable and an expensive waste.

4 Temperature: 271 alarms (15%) – a key HVAC system parameter that needs to be constantly checked for efficient operation.

5 pH: 130 alarms (7%) – must be maintained within limits set by the chemical supplier to ensure metals are immune from corrosion or adequately passivated.

6 Crevice corrosion: 129 alarms (7%) – crevice corrosion can occur –even if general corrosion rates are low – in localised regions, such as weld seams, crimped joints or under debris due to differential aeration effects and is a frequent cause of pitting attack and pin-holing.

7 Galvanic currents: 34 alarms (2%) – the current that flows between different metals within a system; for example, steel and copper. Maintaining low galvanic currents indicates low oxygen levels and/or good inhibition of metallic surfaces.

How to prevent corrosion

Real-time HVAC monitoring systems are the only way to detect early signs of corrosive conditions. In comparison to sampling and corrosion coupons alone, these systems look for root cause indicators, identifying ‘red flags’ before damage occurs.

Measuring system behaviours, mechanical events and precursors to corrosion is the best form of prevention. Water sampling alone will only measure the results and effects of corrosion.

Monitoring units can be installed at any stage of a HVAC system’s life, either as a one-off ‘healthcheck’ or as an ongoing management tool. With real-time data to hand and alarms sent direct, FMs can save money long-term by making preventative and targeted service interventions, relying less on expensive laboratory sampling and reducing the risk of breakdown.