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This corrosion

Sodium nitrite corrosion inhibitors, although well-proven and reliable, 
often represent a false economy, while nitrite-free inhibitors can more 
than repay their higher initial cost, argues Chris Shelton

For the end-user, undertaking a programme of professional water treatment has a direct impact on chiller efficiency, saving energy and yielding additional economic dividends that can boost competitive advantage.

However, despite the attractive benefits on offer, which also includes extending the life of chilled water systems considerably, as well and reducing maintenance, many people still view their chillers as ‘fit and forget’ units.

Regardless of a system’s design, corrosion is always a likely cause of problems in chilled water circuits simply because circulating water or a water-based coolant is in contact with system metals. Ferrous metals are particularly vulnerable.

Furthermore, when corrosion is not controlled properly, it leads to deposits that have several undesirable and costly effects. Most obviously, heat transfer becomes inhibited, which consequently degrades cooling efficiency.

Indeed, it is possible for corrosion deposits to cause heat exchanger leaks or the complete loss of system integrity.

Such problems can lead to a host of problems, from unplanned downtime, to reduced system longevity, to increased energy consumption and reduced productivity. In short, corrosion is bad news for business continuity and profits.

The answer to this headache is quite simple: deploy a chilled water corrosion inhibitor.

Traditionally, this role has been fulfilled by sodium nitrite based corrosion inhibitors, which are proven, effective and relatively inexpensive.

However, as most plant engineers will be aware, this type of inhibitor often comes with a downside – nitrites are a nutritional source for bacteria, so bacterial contamination (aka bio-fouling) of chilled water systems can arise under certain operating conditions.

Specific operating conditions such as oxygenation, the presence of glycol or water loss can exacerbate biological contamination in addition to corrosion.

Bacterial growth is harmful in chilled water systems and results in a number of evils from complete component failure and impaired flow to poor heat transfer and blocked control valves. Once again, it will give rise to unplanned downtime and increased energy consumption, unplanned repairs and will require increased maintenance (for example, regular system cleaning and filter replacement). Taking this into account, use of sodium nitrite based corrosion inhibitors often represents a false economy, we believe.

It is essential to take whole life costs when considering the optimum treatment programmes that are cost-effective in terms of energy, water, and chemical usage, as well as manpower and maintenance.

This ultimately means considering inhibitors that deliver superior corrosion prevention such as sodium nitrite based inhibitors, and factoring in the price premium.

So how can engineers ensure that the right solution is deployed?

Unfortunately, there are no industry standards for measuring the corrosion and bio-fouling prevention capabilities of inhibitors, and only one that tests for corrosion prevention, BuildCert, and this is only to minimal standards.

As such, engineers should take good care to not to overlook the details.

A high quality inhibitor should be a blend of several corrosion control technologies developed specifically for closed circuits that deliver optimum protection for all metal types – ferrous and non-ferrous – while providing an ongoing maintenance concentration for long term security.

Care should also be taken that the inhibitor is compatible with all typical of plastics, rubbers, and fibre seals and washers, as well as the adhesives or sealants that may be found in closed systems, in order to ensure complete system integrity.

Furthermore, the inhibitor should also restrict limescale formation (in closed circuit heating systems), system noise and hydrogen gassing, while stabilising and maintaining a prevailing, near-neutral water pH.

Characteristics such as being non-hazardous, non-foaming, non-flammable, non-volatile, non-toxic, biodegradable and non-polluting should also all be considered standard.

Of course, as discussed, an additional feature should be that the inhibitor does not encourage bacterial contamination (bio-fouling) in chilled water systems.

To achieve all this, engineers should take care to seek out high quality nitrite-free corrosion inhibitors. Inhibitors that meet such specifications are likely to fall into the ‘premium’ category and so require higher initial investment.

However, when those whole life costs have been taken into account, these premium inhibitors should provide a greater return on investment by ensuring optimum system protection and performance, improved cooling efficiency, and simpler maintenance.

Effective system protection can only be fully realised if system cleaning is undertaken prior to inhibitor dosing.

This will restore flow and heat transfer to an optimum state, and remove any debris from the system that may otherwise prevent the inhibitor from working effectively.

Basically, there is little point in adopting any form of corrosion protection unless the system itself is initially clean, otherwise the positive effects of the inhibitor will be reduced and the benefits will never be achieved.

Ultimately, only a correctly applied programme of water treatment, involving advanced cleaning chemicals and inhibitors, can ensure that chilled water systems operate at optimum efficiency and performance, and thereby reap long-term cost benefits.

A leading international plastic packaging manufacturer recently switched from a traditional corrosion inhibitor to a premium product at its UK plant after it gave rise to significant bio-fouling.

The chilled water system comprises a chiller, cooling rollers, return water tank, open cold water tank and pumps.

Historically, the 10,000-litre system, which contains a 20 per cent glycol concentration, was being treated with a basic sodium nitrite solution – a chemical treatment that is used commonly in industrial applications owing to its good corrosion prevention properties.

While the system maintained acceptably low levels of corrosion, quarterly water quality analyses began to show increasing levels of bacterial growth in the system water.

The culprit was the sodium nitrite, which is a nutritional source for bacteria.

Bacterial activity started causing fouling to a point where the system surfaces and tank were visibly dirty from pseudomonas slime-forming bacteria.

The main operational consequences were repeated filter blockages, requiring additional maintenance as well as the cost of regular filter replacement. Additionally, the fouling was impacting heat transfer, reducing energy efficiency.

As a result, the contractor tasked with evaluating the water treatment programme moved away from the traditional sodium nitrite based corrosion inhibitor, replacing it with our product Sentinel X100.

The chilled water system is now not only free of bacterial contamination, but the X100 solution has demonstrated superior corrosion prevention for all system metals, including mild steel, copper and aluminium.

As a result, the plant is benefiting from prolonged life, improved efficiency, enhanced product quality, and significantly reduced maintenance.

Chris Shelton is sales director at Sentinel Commercial


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