Tech focus: Flushing district cooling systems

Flushing is seen as an effective way of removing contaminants from a system's pipelines.
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The sixth and final stage is passivation. This is when chemicals –called passivators – are added to the system.

They render the ‘active’ surface of the pipe work ‘passive’, or put simply, less prone to further corrosion. This gives the system its own corrosion protection and compatibility with the chilled water plant system. Samples are sent to the lab for analysis with a sodium nitrite control level of less than 800ppm.

This closed loop flushing method ensures there is no need to dump water, saving a valuable resource and eliminating the problems of disposal. Once the cleaning, chemical pre-cleaning and passivation is completed, the system is handed over to the client.

“The butterfly valves from the suction and discharge line will be closed, then all the looping hoses from the pump and filtration skid will be disconnected, without loosing much water,” said Ajith Kumar, Hydrolink’s Bahrain branch manager.

“All the temporary looping installed in the network, for the closed loop flushing within the valve chambers, will be removed and all the take-offs will be blinded. Thus the system will be secured with the passivated water inside the loop.”

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The same system will work with treated sewage effluent, or seawater, should they be employed as the process fluid for the district cooling plant. Although the challenges of each system are different, the system can be easily adapted to suit.

“The use of biocides is restricted, as the system generates sufficient heat to destroy bacteria, fungus and microbiological factors,” said Kumar. “Recent tests at Bahrain Financial Harbour (where Hydrolink has deployed the system) show water analysis results comfortably within the control limits.”

“Every district cooling pipe network is unique. We have to see whether all the main and branch lines can be looped to create a closed loop system for flushing. If not we have to suggest to the client to add the required temporary arrangements or ‘take offs’ in the network.

“Since we use a hot flushing system, we can achieve the desired results, even for bigger diameter pipelines, say up to 1200mm in size.

“The customer has peace of mind since they are thoroughly supported by us during project execution, the submission of daily, weekly, monthly activity schedules and reports, including the preparation of chemical analysis reports from various stages of flushing.”

Retaining the water within the system solves the obvious disposal problem and sets the right precedent for the sensible use of a scarce resource, something the district cooling industry as a whole needs to look to do more often.

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