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Tech focus: Flushing district cooling systems

Water, heat and pressure combine to clean out pipes in a closed loop

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Flushing is seen as an effective way of removing contaminants from a system's pipelines.
Flushing is seen as an effective way of removing contaminants from a system's pipelines.
A closed loop flushing method ensures there is no need to dump water.
A closed loop flushing method ensures there is no need to dump water.
Pipe flushing is essential in order to counter the threat of corrosion.
Pipe flushing is essential in order to counter the threat of corrosion.
Sludge colour indicates flushing progress.
Sludge colour indicates flushing progress.
Used filters demonstrate the effectiveness of the cleaning process.
Used filters demonstrate the effectiveness of the cleaning process.

The installation and assembly of a district cooling system is heavy duty work, which inevitably leaves behind a layer of debris and detritus.

Flushing is an effective way to remove contaminants from within the system’s pipelines. However, flushing means using water and as system sizes increase and district cooling continues its regional expansion, more water is now required. This water then needs to be safely disposed of, without having a negative impact on the environment.

The logistics of achieving this present a tricky challenge, but there are solutions that deal with the issues and fix the problem. Closed-loop circuits keep the water contained and then the cleaning can be done with a combination of heat, water pressure and filtration.

Engineers at Hydrolink, a fluid engineering company, have used their contamination control and system flushing experience to develop a way to safely retain the flushing water in the system. This acts to eliminate the environmental impact and reduces the logistic headaches of disposal.

Hydrolink has designed a pump and filtration skid that can be coupled to a district cooling system to create a closed loop flushing circuit. The unit is self contained, including its own power source and is small enough to be portable, using a crane or forklift with a five tonne load capacity.

Used at the district cooling system’s installation stage, the flushing engineers calculate the necessary water velocities and temperatures that will force grease and particle matter to be suspended, semi-dissolved and removed from the system. The pump and filtration units are set to generate the correct temperatures, velocities and flow rates to dissolve grease and preservatives in the system and carry any contamination to the filters, where any dissolved greases, sand, grit, loose carbon and rust particles are trapped in woven fibers.

During this process, the system ensures the iron content is maintained at less than three parts per million (ppm) and total suspended solids at less than 30 ppm. Visual checks of the filter elements allow operators to monitor the six stages of the process.

At the first stage, filter elements have a distinct red-black brown sludge, indicating grease and rust is being removed. At stage two they have a distinct black-grey sludge, as loose carbon deposits are removed from the system. Stage three is indicated by a distinct black-grey to light-grey sludge, which indicates that it is time to sample the system water.

The water velocities are then adjusted at stage four, to reduce temperatures and settle at an ambient level, to allow the addition of chemical inhibitors. At stage five the system undergoes chemical pre-cleaning, to remove any remaining contamination and render the metal surfaces fit for passivation. At the end of this process, water samples are taken and sent to the lab for analysis to ensure compliance with the system’s control limits.

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.”

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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