Optimising Your Chilled Water / LPHW Loops

Many processes within your plant depend on effective chilled water and low-pressure hot-water (LPHW) systems. These are typically constructed from carbon steel pipework and circulate water / glycol with the addition of a corrosion inhibitor.

One of the most commonly used corrosion inhibitors is sodium nitrite. The inhibitor is dosed into the circulating water and works by supressing the anodic reaction (metal dissolution) by forming a passive film on the pipe surface.

Nitrite inhibitors work best at a concentration of 900-1500 ppm, pH 9-10.5. However, as the concentration and/or pH lowers, the pipework is more susceptible to corrosion. This can occur due to a variety of reasons such as the presence of oxygen, chlorine and/or bacteria.

Denitrifying bacteria is usually found in sections of your system that have low flow or no flow. Denitrification is a process in which nitrite ions are broken down into molecular nitrogen, forming various by-products on this journey – one of which is nitric acid. Nitric acid is very corrosive, and will aggressively attack carbon steel. Systems that have been exposed to this will see a build up in dissolved iron as the pipe wall is corroded away. Subsequent corrections to the inhibitor that bring the pH levels back up can create a knock-on effect of precipitating the dissolved iron out of solution, creating sludge that will reduce the efficiency of the system or even create blockages in small bore pipework.

If you are experiencing a loss in efficiency within your cooling water and/or LPHW systems, there is a good chance that at some point during its life cycle that it has suffered from a chemical imbalance and you have a build-up of iron-rich sludge. To restore the system to its original design parameters, you must purge the system and disinfect before reinstating the corrosion inhibitor.

INOX Passivation use a multi-stage process to achieve this.

• Systematically flush all lines/legs at high velocities to remove -sludge and loose debris (in accordance with BSRIA BG-29)

• Chemically clean the system to remove rust/scale (using specially formulated inhibited acids to protect the carbon steel, stainless steel and copper alloys, etc.)

• Sanitise to remove bacteria

• Finish by re-dosing the system with fresh inhibitor and biocide

Get in touch if you require any assistance with your systems.

Photos show debris captured on chilled water system, flow meter during high velocity flushing and dissolved iron readings during final rinsing.