INDUSTRIAL AND COMMERCIAL HEATING PIPEWORK
If the system is predominantly constructed
of non-corrodible materials, with only a small proportion of corrodible material, then the corrosion process is not self-limiting, as it will not significantly reduce the concentration of oxygen in the circulating water. The corrodible materials will therefore be vulnerable to continuing high rates of corrosion if they are not protected by chemical inhibitors and/ or the use of oxygen reduction technologies, such as vacuum deaeration. Particular care must be taken with the specification of plastic pipework, which – while not affected by corrosion itself – may allow oxygen permeation to adversely affect other components. Another feature of modern commercial buildings is the increased use of two-port valve control of terminal units, combined with variable speed pumping. There is no doubt that this strategy can result in a significant reduction of pumping energy and lower return temperatures for heating (higher for cooling) that improves plant efficiency. The downside can be branch pipework
with very low flow rates and out of season stagnation. Low flow rates and stagnation lead to deposition of suspended solids, reducing the effectiveness of corrosion inhibitors and increasing the risk of microbiological problems. The effects of stagnation can be easily reduced by incorporating a daily circulation routine in the control strategy – for example, one hour per day (out of normal operating hours), with automatic control valves fully open. The issue of low flow rates in distribution pipework, particularly to fan coils, can only be addressed at the design stage by selecting smaller diameter pipe and control valves to maintain a reasonable flow velocity.
Managing water quality The BSRIA guide to Pre-commission cleaning of pipework systems (BG29/2011) includes new recommendations for the management of system water quality between the pre- commission cleaning and practical completion stages. In particular, the guide recommends that the pre-commission cleaning or water treatment specialist be contracted to monitor and maintain the system, with two-weekly sampling until practical completion, and to ensure that water treatment is not neglected during this critical period. In parallel with the development of BG29,
BSRIA worked with industry representatives and BSI to produce a new British Standard BS 8552:2012: ‘Sampling and monitoring of water from building services closed systems – Code of practice’, which was published in
54 CIBSE Journal February 2013
The inner surface of a piece of 67mm thin wall carbon steel pipe from a heating system that perforated less than two years after installation due to severe oxygen pitting corrosion
November 2012. In essence, BS 8552 explains the sampling and monitoring process in terms of where, when and how to sample, while BSRIA BG29 (slightly revised in 2012) provides practical guidelines for interpretation of the results, up to and including practical completion. Taken together, BG29 and BS 8552
provide a framework and guidance for the management of closed system water quality during construction projects, and should help to reduce the risk of premature failure of pipework systems due to corrosion. However BS 8552 does not stop at practical completion, but provides detailed recommendations for water sampling and monitoring through the entire life of the building until demolition. So, where are the guidelines for water
quality in use? Unfortunately, the simple answer is that such guidelines are difficult – if not impossible – to provide in absolute terms, as many of the parameters used to judge the water quality and the condition of the system depend on the specific water treatment strategy being employed. Therefore, the best source of water quality guidelines for a specific system is the water treatment specialist looking after it and their chemical supplier. Nevertheless, BSRIA is currently working with the Water Management Society (WMS), British Association of Chemical Specialities (BACS), Institute of Corrosion (ICorr) and Commissioning Specialist Association (CSA) to produce a new guide to water treatment that will explain the principles and processes involved, to be published in 2013. CJ
REGINALD BROWN is head of energy and environment at BSRIA.
www.bsria.co.uk
Why do corrosion failures seem to be increasing, sometimes even before the building is handed over?
www.cibsejournal.com
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