PIPEWORK CLEANING
is recognised in the new BSRIA guide for pre-commission cleaning (BG29/2011), which details the recommended level of monitoring and provides guidelines for water quality during each stage of construction, starting with analysis of the fill water. The system is then periodically monitored between pressure testing and pre-commission cleaning, and more frequently through the commissioning period to practical completion. Cleaning and water treatment specialists should be able to correct any imbalance in water treatment, whether due to site activity or unexpected microbiological action, as soon as it occurs.
The BSRIA guide will be backed up by
a new British Standard (BS8552, currently being drafted), which details how, where and when system water samples should be taken for closed water systems in buildings and how they should be analysed. This is intended to improve the level of
agreement between results for samples collected by different parties and analysed by different laboratories, and ensure that guidelines can be applied in a consistent manner.
l Reginald BRown is head of energy and environment at building services research body BSRIA.
www.bsria.co.uk
Metal guru Ductile iron is best for water and sewer pipe applications, argues manufacturer David Smoker
iron because it can’t ‘overcome’ the perceived problems of iron pipes. There is a commonly-held view that welded polyethylene pipelines are less prone to leaks than jointed systems such as ductile iron. Butt fusion, properly undertaken, delivers leak-free joints, however, the same cannot be said of electrofusion jointing, whose failure and leakage issues are well- documented. The high performance of ductile
P
iron was confirmed in a recent report published by UK Water Industry Research, which stated that, on trunk mains, ductile iron needed only 30 repairs per 10,000 km annually, compared with 68 for polyethylene. The report continued that there were ‘higher repair frequencies observed on plastic mains than on ferrous or concrete mains’. This conclusion is consistent with other European studies. Polyethylene pipe is frequently supplied on ‘easy-to-use’ coils,
olyethylene (PE) is often perceived as the modern alternative to ductile
particularly in smaller diameters, ideal for slip lining and similar applications. Apart from the safety issues associated with unstrapping coils, for open-cut installation the fact that the pipe has been coiled means it may have to be held in place in the
This flexibility, coupled with the phenomenon of visco- elastic creep, requires far more stringent engineering design
trench, while its ovality makes it harder to weld or joint. Ductile iron pipes, being semi-rigid, can be reliably positioned. This ‘installability’ is key, with the emphasis in the UK now very much on open-cut pipe laying rather than slip lining. The flexibility of PE is often
touted as one of its great merits. However, this flexibility, coupled with the phenomenon of visco-elastic creep, requires far more stringent engineering design. For example, how many pipeline engineers fully understand Poisson’s ratio and
its role in producing extremely high end loads in medium and large diameter welded PE systems? With ductile iron, since its diameter does not expand significantly under pressure (and with a lower Poisson’s ratio) the resultant longitudinal stresses are easily accommodated by the variety of restrained joint systems developed by the major ductile iron manufacturers. Ductile iron’s inherent strength makes it less reliant on backfilling, unlike flexible polyethylene systems, which require aggregate to achieve the necessary compaction to resist side stresses. Installation standards also require flexible pipes to use a wider trench than ductile iron pipes of the same external diameter. Coupled with the potential to ‘downsize’ – the internal diameter of a ductile iron pipe is often equivalent to that of a larger external diameter PE pipe – this means that installation and environmental benefits can be maximised. Ductile iron wins the sustainability debate hands down. The process to extract and convert crude oil into polyethylene pipes is highly energy-intensive
and depletes a valuable natural resource. However, ductile iron is manufactured from 97% recycled material (scrap), which is fully recyclable and has a residual value at the end of its life. While polyethylene systems may claim recyclability, thermoplastic products degrade each time they are processed, meaning they can only be made into low-grade products. Meanwhile, the propensity of polyethylene to retain chemicals in the pipe wall mean it cannot be remanufactured into water pipes. Where short-term cost is the
major consideration and risks from system failure are negligible, polyethylene systems will be more frequently specified. However, the criticality of the pipeline, along with the direct and indirect costs in the event of failure, must be part of any pipeline material decision. In terms of a cost-in-use analysis, ease of installation, longevity, and inherent performance benefits, ductile iron is a highly sustainable option.
l dR david SmokeR is the business development director, Saint- Gobain PAM UK
58
CIBSE Journal November 2011
www.cibsejournal.com
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