Oxygen alone can also cause corrosion with or without bacteria. Remove the oxygen and the problem is usually solved. Steven Booth gives his five-point plan for eliminating the real culprit in corrosion.
04 June 2018 | Steven Booth
When we think of water quality problems in HVAC systems, bacteria is the issue that usually springs to mind and is commonly associated with corrosion as well as threats to human health such as Legionella.
While bacteria and its associated biofilm can ultimately cause degradation in pipework, it is actually oxygen that is creating the conditions for bacteria to thrive.
This is because oxygen ingress can take place at any stage of a 'closed' water system's life, with potential problems arising from the moment of construction.
By understanding what these issues are, FMs can take preventative steps against corrosive conditions.
1. Don't over-flush
In the early stages, preventing oxygen ingress may be out of the FM's control. Pre-commissioning cleaning, for example, designed to ensure that a water system is handed over to the FM team in the best possible shape, can in itself cause problems.
Over-flushing water and chemicals can result in pitting in the pipework, potentially shortening usable life. If you get involved at this stage, take an interest in flushing and, where possible, keep it to a minimum to reduce system aeration.
We use the innovative Hydrosphere solution, which dramatically reduces water volume without compromising the effectiveness of the process.
2. Check your pipes
What are your pipes made of? Low temperature hot water (LTHW) and chilled water systems are often made from stainless steel because it is corrosion-resistant. This doesn't mean there is no corrosion. In stainless steel systems we commonly see problems with the fitment components because these are usually made from copper, brass or mild steel, which are not corrosion-resistant.
These components then become the focus of degradation if oxygen is allowed into the system, potentially falling foul of more damage than if the whole system were at risk. Fitting stainless steel and then just accepting that it won't suffer corrosion is a mistake.
3. Beware of change
Any change in the system has the potential to alter the water conditions and allow oxygen into the system. This can be at the fit-out stage, when maintenance and repairs take place further down the line, or where different zones are integrated.
FMs should follow the protocols as advised in BSRIA Guidance BG29 on pre-commission cleaning. But every system is different and, even if these guidelines are followed to the letter, problems of water loss and oxygen ingress can cause disruption to the base build water quality.
During maintenance, care must be taken to minimise the amount of oxygen re-entering the system. Making sure that pressure settings are at the correct levels is crucial. If they exceed pressure relief valve (PRV) settings, this can lead to water losses, which means more aerated water will need to be added.
Under-pressurisation will result in air being sucked-in through air vents and dissolved oxygen rising to dangerous levels.
4. Don't rely on sampling
Although sampling is important in understanding water quality and a key element in meeting the BSRIA guidelines, it has a number of flaws and should not be the sole source of information.
Laboratory results from a sample can take days or weeks to return by which time conditions may have changed. Bacteria is most damaging in the sessile stage at which point it has created a biofilm on the wall of the pipework and may not even be detectable by a sample. Most importantly, sampling does not normally provide accurate checking of oxygen levels.
5. Monitor continuously
The best way to understand the condition of a water system at all stages of its life - from construction to maintenance - is through continuous monitoring. 24/7, real-time monitoring of important system parameters, such as dissolved oxygen, pressure, conductivity and corrosion rates, will provide a more accurate picture, allowing FMs to take immediate action on small snags before they become big ones.
This approach may also lead to better maintenance practices whereby problems are fixed rather than covered up. We see the overuse of inhibitors as a common reaction to high oxygen levels, which doesn't actually deal with the root cause.
The best defence against corrosion is one that focuses on oxygen elimination, and the best way to keep track of oxygen levels and understand the effects of different processes on a water system is through monitoring.
Steven Booth is managing director at Guardian Water Treatment