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Closed Systems Importance of Testing and Treatment

Risks of a Poorly Treated Closed System

Any solids within a closed system can lead to blockages and under deposit corrosion, furthermore, low treatment reserves leading to pitting corrosion. Microbiological presence can also cause direct and indirect corrosion, as detailed below:

  • High microbiological levels can be caused by lack of circulation, increasing the risk of stagnation which can cause an increase in sedimentation and formation of biofilms, leading to Microbiologically Influenced Corrosion (MIC)
  • Solids in a system can also increase microbiological levels as they can reduce effectiveness of biocides and provide areas of local stagnation. They also become part of biofilms and provide nutrients to aid bacterial growth
  • Biofilms form quickly on pipe surfaces in untreated systems, where anaerobic & aerobic bacteria can thrive
  • High levels of pseudomonads can lead to issues such as blockages and sludge formation
  • Nitrite Reducing Bacteria (NRBs) can rapidly deplete nitrite inhibitor levels and therefore increase the risk of electrolytic corrosion. They can also produce ammonia which increases the corrosion rate of brass and copper

How To Control Corrosion in a Closed System

  1. Choice of materials
  • Presence of certain bacteria can increase corrosion rates in certain metals
  • Some metals like stainless steel and cooper can be more prone to MIC
  • Some materials are not suitable for use in closed systems e.g. Carbon Steel which is susceptible to rapid corrosion and failure
  • Aluminium systems must be kept at a pH of 7-8.5
  1. Flow rates (circulate daily)
  • Circulate the system for at least 1 hour daily to reduce the risk of stagnation
  • Correct velocity of water to minimise risk of under-deposit corrosion & MIC
  1. Minimise dissolved Oxygen
  • Ensuring there are no leaks in the system
  • Design and commissioning should be at a good standard
  • Use of physical devices e.g. vacuum deaerators
  • Use of inhibitors to passivate metal surface
  1. Fill, flush & clean before use
  2. Chemical & physical treatment
  • Addition of biocide and inhibitor, making sure to use an appropriate chemical for the system e.g. ensuring buffer is used in aluminium systems to maintain a pH between 7 and 8.5
  1. Monitoring & maintenance
  • Take regular samples from the system to monitor levels of chemical, microbiological activity and metal levels
  • Disinfection and leadleg removal to minimise biofilm formation
  • Installation of a sidestream filtration unit to minimise the level of solids in the system

Checking for bacterial growth and monitoring the chemistry of the water on regular basis is essential to ensure the system operates effectively and is kept under control.

Control levels

Below is more information surrounding parameters and control limits.

ParameterControl Limits
Suspended Solids<30mg/l
Settled Solids<60mg/l in pipework extremes

<90mg/l in terminal units not previously sampled

Dissolved Iron<3mg/l
Total Iron<15mg/l
Total Copper<1mg/l
pH (aluminium system)7-8.5
TVC at 22°C<100,000 cfu and no increasing trend
Pseudomonads at 30°C<10,000 cfu/100ml and no increasing trend

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