ENVIRONMENTAL
Flushing & Chemical Cleaning
CLOSED WATER SYSTEM HYGIENE
Newly installed closed systems, LTHW, MTHW, HTHW, Chilled Water and Condenser Water circuits should generally be pre-commission cleaned. The vast majority of these systems have small bore control valves, regulating valves etc, which readily become blocked with particulate matter and would then fail to operate.
It is therefore important to ensure the presence of acceptable water quality prior to putting the system into use. Debris and contamination that is present in a system as a result of the installation process needs to be removed.
GETTING THE BALANCE RIGHT
Cleaning systems prior to an initial balance is very important. Preceding the balance all valves can be fully opened and flushing flows maximised and manipulated to ensure that all possible debris is removed from the pipework.
It is rare that such an opportunity will present itself again since once a building is commissioned and occupied, regulating valves in the system will be at a set position with reduced orifice sizes to effect a system proportional balance. The use of chemical enhances the cleaning process by removing soluble contaminants.
OUR METHODS
BSRIA BG29/2012 ‘Pre-commission Cleaning of Pipework Systems’ outlines the methodology for cleaning new industrial and commercial systems (BS7593 2006 for domestic hot water central heating systems).
In addition this guidance document outlines the responsibilities of designers and installing contractors. These should be taken seriously since poorly designed systems cannot be adequately cleaned and systems installed without correct flushing facilities require additional installation work prior to flushing and cleaning.
THE IMPORTANCE OF REGULAR CLEANING
It is important to get systems properly cleaned in accordance with guidelines. In the event that the process fails subsequent cleaning becomes more difficult for the following reasons:
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Debris will have become trapped in small valve orifices, FCU coils, chillers etc. These areas are generally protected, by means of bypassing, until after the initial cleaning process.
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Debris can become baked onto the pipe walls and heat exchange surfaces with the passage of time.
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Microbiological problems can commence and become a serious problem in the presence of retained debris.
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We strongly recommend that pre-commission cleaning works are fully recorded and witnessed and that samples are taken to demonstrate cleanliness on completion. In addition we recommend that samples are taken at practical completion on a building site contract to demonstrate compliance with the specification for the new building and witnessing cleaning of a strainer after a pre-commission flush.
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Following pre- commission cleaning it is important that closed systems are carefully monitored and regularly maintained. This maintenance should continue throughout the life of the building and should involve sample taking at a maximum interval of 3 months.
WHAT WE TEST
Pseudomonas
Pseudomonas bacteria has the ability to cause corrosion to any open or closed water system.
Once the bacteria is established it forms a jelly like film which clogs valves and reduces heat transfer. The anaerobic conditions created between this film and pipe work provide a breeding environment for other bacteria including SRB’s and NRB’s. It also creates a barrier to debris preventing a successful chemical clean if not treated properly.
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As it is a living organism it will reappear if not completely destroyed.
The results of pseudomonas within the system are often shown by changing commissioning results, greasy films on strainers and often as muddy reddy brown colouring to the water. In simple terms these have been linked to corrosion.
Total Viable Count
The total amount of bacteria expressed in colony forming units. High levels are often an indication of a dirty system.
The dirt providing a hide out for the bacteria and ideal breeding conditions.
Sulphate Reducing Bacteria (SRB’s)
This is one of the most destructive groups of biological foulants because they cause corrosion resulting in severe localised pitting of pipework surfaces.
They have been implicated in severe corrosion problems for copper, iron, steel and aluminium pipe work.
As a result of their metabolism, they produce hydrogen sulphide which attacks iron and steel and forms the end of produce ferrous sulphide. Steel thus becomes pitted, and cast iron becomes ‘graphitised’.
Iron & pH
Iron
An important test for corrosion control in industry. BSRIA BG29/2011 states that total iron 7 days after pre-commission cleaning is <6mg/l and soluble iron <3.0mg/l.
Guidelines for system monitoring between pre-commission cleaning and practical completion states total iron <15mg/l and soluble iron <3mg/l
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pH
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Expresses the acid or alkalinity of a sample. This is an important test when known that certain installation & plant materials have been used.
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Nitrite Reducing Bacteria (NRB’s)
This bacteria gasses to produce ammonia. This poses a risk of stress corrosion cracking to brass/copper components such as valves and fittings. As with SRBs, these are anaerobic bacteria which tend to produce acid (corrosive) by-products. These, as again with SRBs, dwell beneath deposits or in systems with limited oxygen.
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N.B. Low/Medium NRB levels detected in isolation of any other Heterotrophic plate count is unlikely to be of concern and system(s) will be deemed under microbiological control.
Suspended Solids
Insoluble dispersed solids which may be separated from the water by filtration or sedimentation. These should be <30mg/l at circulating pump sets, which allows for the fact that corrosion inhibitor will cause an increase. At all other parts of the system these are expressed as ‘settled solids’ and should be <45mg/l on immediate completion of any water treatment works.