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Side stream filtration will keep closed systems from failure, says Mick Prat.

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05 August 2019 | Mick Pratt

Closed systems such as heating and cooling circuits rarely fail because of corrosion and leakage; most failures are caused by an accumulation of sediment causing blockages, poor heat distribution or dissipation, and boiler and valve failures.

These systems are susceptible to three main problems:

  •  Biofouling – bacterial build-up within the system;
  •  corrosion; and
  •  scale.


These problems reduce the performance of the system, waste energy, increase maintenance costs and contribute to early failure of plant and components.

According to BSRIA BG50: Side stream filtration can be used to help maintain a low concentration of suspended solids in the circulating water following pre-commission cleaning (and also reduce the risk of differential aeration corrosion in areas of low flow). In addition to the longer-term benefits of early installation of side stream filtration, it can also help to maintain water quality during the commissioning period.

Common bacterial contamination:

  • Pseudomonas: slime forming and produces biomass;
  • Sulphate reducing: produces corrosive hydrogen sulphide/sulphuric acid;
  • Nitrite reducing: breaks down corrosion inhibitors and produces ammonia; and
  • Methanogens: feed on organic matter producing methane gas.


Without appropriate maintenance and treatment regimes, these problems can soon escalate and lead to system failures.


Available treatment options

  • Corrosion and scale inhibitors: These can be dosed proportionally based on system volume and monitored during the life of the system. There are various types of inhibitors but these should be carefully selected based on system metallurgy.

    Installation of automatic dosing equipment provides a fast, effective way of introducing the correct volume of treatment into the system as required. This also includes glycol, which enables a preset blend ratio to be dosed into all make-up water.

  • Biocidal treatments: The appropriate product or products must be selected based on many variables including bacterial contamination, metallurgy and temperature.

  • Filtration: To remove the dirt, debris and contamination. This dirt and debris absorbs/obstructs the chemical treatment programme, preventing it from working at its optimum. It provides essential nutrients for bacterial growth.


What FMs should be looking for

Systems should be monitored and evaluated to prevent them deteriorating. Analysing waterside conditions with a random snapshot of recirculatory water is antiquated when conditions differ dramatically throughout the system. 

A structured approach includes a considered valve-exercising programme coupled with a sampling regime, as suggested in BS 8552 and greatly enhances the chances of early diagnosis for deteriorating waterside conditions. 

Monitoring trends is an essential component of a comprehensive closed circuit preventative maintenance programme – with a high emphasis on preventative.

Chemical and microbiological results should be reviewed and trends graphed so the best treatment programme can be put in place and amended (if required) for optimum performance for many years. 

Unless they are regularly monitored – we recommend that monitoring should be carried out quarterly as a minimum – any problems that develop could contribute to irreversible damage through erosion, corrosion or biofouling. 


Mick Pratt is commercial director for the water equipment and consultancy business at SOCOTEC