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 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.

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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.

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