HEAT NETWORKS | WATER QUALITY n Lack of remote water-quality monitoring on networks. On-going management n System leaks leading to ingress of oxygen and dissolved solid precipitation from the mains water supply feed n Incorrect water-treatment choices for metallurgy of system n Lack of monitoring and maintenance of filtration equipment nLack of monitoring and maintenance for water treatment after practical completion nBacterial proliferation in stagnant areas nOver dosing/under dosing of chemistry nLack of live monitoring systems to flagupissues. Detail # Systems Comment No unbudgeted maintenance required over 24 months 157 1 had system loss because of burst district heating pipe Loss of system water not corrosion related 4 1 system drained in error by contractor Full loss of system water corrosion/scaling related 4 Considerations at design stage n Minimise areas of potential stagnation n Specify suitable side-stream filtration for all district energy systems n Specify that fill water is deaerated n Specify that district heating systems have continuous monitoring for pH and conductivity as a minimum. The system should also be able to report remotely either via the BMS, or email/SMS to indicate failure of the control levels to suitable stakeholders n Water treatment should be specified to be automatic in nature n Water treatment, fill water quality and method and pressurisation should be considered at design stage, based on system volumes and metallurgy. If secondary and tertiary systems are to be filled from the primary network, consideration must be given to ensuring no conflicts between the water-treatment regimes of both systems. 3 systems had poor, incorrect water-treatment dosing, which lead to incorrect pH for the aluminium heatexchange 1 system had a leak for more than three months, which led to scaling and blocking of the boiler tubes Full loss of system water bacteria related 2 Conclusions Currently, there are gaps in the design, operation and guidance for water-treatment in the UK district energy sector. The following should be borne in mind: 2 had system loss because of failed bellows 1 system had failure because of a failed de-aeration unit, causing stagnation in the pipework, which led to underdeposit bacterial pitting 1 system had failure because of poor pre-commission flushing in the horizontal sections before terminal units, which led to under-deposit bacterial pitting 1 system handed over with sections and buffer vesselsisolated Poor system water quality at handover 12 8 systems handed over with insufficient/no inhibition/ general water-quality issues 3 systems handed over with high metals and suspended solids count Systems handed over with disused plant still connected to system 6 Total 185 Issues occurring within 185 systems monitored for the UK heat network water-quality study 4 CHP systems not in use and not decommissioned correctly 2 biomass boilers not in use and not decommissioned correctly stagnant areas should have full velocity and heat flushed through for a minimum of two minutes every four days n Legacy equipment no longer in use should be decommissioned and disconnected from the system waters, leaving no dead-legs n Side-stream filtration should be monitored and managed on a proactive basis, to ensure suspended solids are removed as required n Continuous monitoring and automatic dosing should be installed on all district heating networks to minimise the risk of corrosion and associated water treatment issues. Improved water-quality treatment in UK heating networks will prevent non-budgeted capital expenditure, minimise disruption for end users and extend the life of the system. CJ JON GREAVES is national technical manager at Hydro-X Water Treatment Consideration for pre-commission stage n For pipework >150mm diameter, alternate flushing methodology to BSRIA BG29:2012 should be sought for example, ice pigging and traditional pigging n Improve record-keeping and audit processes during precommissioning works and implement a daily log of works on site n Have an independent audit of the works before handover n Consider existing water treatment when connecting new systems to old networks. Ongoing considerations n Stagnation should be avoided any Sludge and biofilm removal from a district heating system Filters showing the effects of progressive levels of cleaning 44 August 2019 www.cibsejournal.com CIBSE Aug19 pp43-44 Water quality.indd 44 19/07/2019 14:42