by Dr Pamela Simpson
Pre-commission cleaning of closed circuit pipework systems and the subsequent monitoring of water quality are essential in any building. The implications of getting these wrong can be catastrophic. The resulting problems include disruption to occupants whilst systems are re-cleaned or, in the worst cases, complete closure of buildings whilst entire systems are ripped out and replaced due to early failure. But it seems that although the risks associated with open systems (where the circulating water might come into contact with humans) are generally appreciated, there is less awareness of the problems that can affect closed systems
A closed re-circulating pipework system is one which, as the name implies, is closed i.e. the water in them is not exposed to the atmosphere and is not significantly depleted due to evaporation or draw-off. The water is permanently enclosed and typically spends all of its time being heated, cooled and re-circulated in the process of delivering heating or cooling. All systems serving terminal devices from radiators to fan coil units or chilled beams are examples of closed systems.
The potential problems start during construction. In large buildings, heating and cooling circuits can include pipes that are over a metre in diameter. In an ideal world, these pipes would be installed in a clean, debris free condition but in practice, nothing can be ruled out.
Hard, hats, coke cans, plastic bags and even dead foxes have all been found inside systems. If left undetected, when the pumps are switched on, items such as these can cause major damage to expensive boilers, chillers and pumps. The smaller particles can be just as bad. Some modern control valves have clearances of less than half a millimetre. This means that sand, grit, jointing material or welding slag can cause blockages and consequent heating or cooling dead spots.
Danger not ended
All of this debris should therefore be removed by dynamic flushing of the system during pre-commission cleaning. But successful removal of these items does not end the danger.
Most closed re-circulating systems are constructed, predominantly from carbon steel pipe. Carbon steel has the significant advantage of being both strong and cheap. However, as we should all remember from our school science days, in the presence of oxygen and water it will corrode rapidly i.e. within hours. Our high strength steel is replaced by low strength semi soluble particles of soft brown rust or, if the supply of oxygen is limited, black magnetite. Thick walled steel pipe has some tolerance built into it and can survive for a while. Thin walled steel has less.
Dynamic flushing of pipework involves circulating highly oxygenated water through the pipes at high velocity. Hence, as we’re removing the problem of system debris we are potentially encouraging corrosion. As a result, following the dynamic flush, some form of chemical clean is usually essential to remove corrosion products from the surfaces of steel pipes.
Corrosion process potentially controllable
In theory, once the system is put into operation, the corrosion process should be controllable. If there is no replacement of the water in the system, the oxygen in the water should gradually become depleted thereby stifling the corrosion. Furthermore, corrosion inhibitor chemicals can be added to further reduce the rate of corrosion. However, corrosion protection regimes can go wrong and water quality monitoring is therefore essential.
For, example whenever water is lost from a system, whether due to system modification or to replace a component, fresh oxygenated water is drawn in whilst water containing valuable corrosion inhibitor is lost. This combination can be sufficient to initiate a burst of corrosion.
Furthermore, inhibitor levels can drop even without water being taken out of the system. The active ingredients of inhibitors can be used up in developing protective layers on pipes or reacting with oxygen in the water. But, even more shocking is the realisation that some inhibitors can provide a food source for bacteria - and not just any bacteria. The bacteria we can find in closed systems can initiate catastrophic damage on a scale equal to or worse than that cause by simple debris or oxygen induced corrosion.
Many types of bacteria present
All natural sources of water (including mains water) contain many different types of bacteria, some of which may multiply and lead to problems within closed systems if they encounter suitable conditions for growth. Mild steel, stainless steel and copper are thought to be particularly prone to microbial influenced corrosion (MIC). For MIC to occur, it is necessary for some types of bacterial species to colonise the metal surface. The extracellular material produced by rapidly multiplying aerobic bacteria species eg Pseudomonas spp develops into a biofilm (i.e. slime) which produces both aerobic and anaerobic zones.
The anaerobic conditions enable anaerobic bacteria such as sulphate reducing bacteria (SRB) to multiply and a potential difference is established between different areas of the metal surface. SRB metabolise naturally occurring sulphate in the water to produce sulphuric acid under bacterial clumps. This results in accelerated, localised pitting corrosion and eventual perforation of the pipe. Corrosion by SRB can cause significant damage to surfaces, in particular where pipework may have bends, uneven surfaces, abrasions, or joints and welds.
Increasing electrolytic corrosion risk
Other bacteria of concern are nitrate/nitrite reducing (NRB) and nitrite oxidising bacteria. These bacteria can cause rapid loss of nitrite-based corrosion inhibitor from the system and so increase the risk of electrolytic corrosion. Also, the ammonia produced by some of these bacteria when metabolising nitrite can lead to stress corrosion cracking of brass fittings if present at significant levels.
During the pre-commission cleaning stage of any new build, it is important to prevent microbial presence wherever possible and to avoid areas of low flow rate or dead legs where bacteria can multiply, settle and develop biofilms unhindered by circulating biocide chemicals.
For many systems the precaution of a “biocide wash” is included as part of the pre-commission cleaning process. This involves circulating a biofilm disrupting chemical through the system to destroy any biofilms that may have developed during the construction process.
Bacteria related problems and their potential to incur major costs on the system owner should never be forgotten or under-estimated. As an over-riding principle it should be remembered that it is much, much easier and cheaper to maintain microbiological control within a closed heating and cooling system than to clean up a badly fouled system containing biofilm.
Microbiological control can be achieved by:
• Ensuring the system is free of suspended solids and debris which may be utilised by bacteria as an energy source.
• Carefully managing biocide dosing and maintaining records of the treatment efficacy.
• Regularly monitoring and sampling the system water content in a correct fashion (refer to BS 8552 water sampling of closed systems).
• Maintaining good flow around the system to ensure that biocide treatments are properly circulated.
Using and appropriately qualified contractor
It can be seen that pre-commission cleaning and on-going monitoring of water quality incur too many potential pitfalls to be left to installing contractors or building maintenance contractors. It is usually essential that a properly qualified pre-commission cleaning or water treatment specialist contractor is involved in these activities.
BSRIA Guides BG29/2012
Pre-commission cleaning of water systems and BG 50/2013 Water treatment for closed heating and cooling systems provide an explanation of the procedures, tests and monitoring regimes that need to be adopted. However, proper implementation of this guidance requires an organisation that can draw on expertise across a range of specialisms including pipe system design, pre-commission cleaning, water treatment chemistry, corrosion and microbiology.
About the author Dr Pamela Simpson
Dr Pamela Simpson is a Chartered Fellow of the Society of Biology. She established Whitewater Technologies in 1998, before which she spent over eight years working in the speciality chemicals industry, initially as a technical and European director of the Industrial Biocides Division of a major chemicals manufacturing and processing company.
She has developed a broad knowledge of the application of microbial control techniques in product preservation and antimicrobial surface protection, process water control, and microbial issues within hot and cold closed systems for both healthcare new-builds and commercial premises. She is also an approved trainer for Legionella awareness courses for water treatment engineers.
Her recent work involved expert work for microbially-influenced corrosion in a range of commercial and healthcare buildings of hot and cold closed systems. She was on the Steering Group for the writing of BSRIA BG50/2013: Water treatment for closed heating and cooling systems.
I worked with Pam on a large healthcare project where specialist input was required to identify potential microbial problems in the heating & chilled water systems. She is very knowledgeable in her field, conscientious, pleasant and approachable. I would highly recommend Pam for all aspects of microbiological matters.
Adam Sands | Senior Project Engineer Vaughan Engineering Services Ltd