WATER HYGIENE AND SAFETY
In most expansion vessels, the air/gas and water are separated by a rubber sheet – a diaphragm – or by an expandable rubber bladder that holds the water within the gas.
When a hot water system is running as usual, an expansion vessel will routinely partially empty and fill, exchanging water from and to the wider system at regular intervals.
However, expansion vessels have been identified by the HSE as specific points at risk of Legionella, as they can suffer from low flow rates or stagnant water – particularly if the system pressure and temperature remain steady over a period of time. This is especially true of large commercial properties, as well as public buildings such as hospitals and other healthcare facilities. In the current climate this is particularly prevalent, as many schools, office buildings, and entertainment venues are closed, or operating at a far lower capacity, and therefore running the risk of reduced flow in their water systems.
Risk of introducing colonisation Expansion vessels in any hot water system run the risk of introducing microbial colonisation (including Legionella) to a system as ‘plantroom temperatures usually exceed that of the incoming water’ (HTM 04-01), and in a pressurised system, there may be long periods where there is no water flowing through the vessel, causing the residue water to become stagnant.
The HTM provides guidance to address this risk, thus: “Expansion vessels should be appropriately sized, installed and operated in a manner that prevents the accumulation of debris, pockets
An ACV Smart Cylinder with expansion vessel.
of stagnating water, and increases in temperature within the vessels”.11 WRAS advocates the use of flow- through type expansion vessels ‘in pressurised heating/hot water systems to avoid stagnation or low turnover of stored water. Connecting pipework should be kept as short as possible, and should rise continuously’.12 HSE HSG27413
guidance also
recommends that, where it is possible, expansion vessels should be located in
cool areas, and on cold pipes, as well as being located as close to the mains water supply as possible. The vertical mount should cut down on debris being trapped, and an isolation valve should also be fitted. The placement and selection of the expansion vessel should also minimise the amount of water held inside the vessel. and stimulate the flow of water.
Benefits of a flow-through expansion vessel HTM 04-0110
states that ‘vessels with a
flow-through design should provide less opportunity for water to stagnate and become contaminated’. Unlike the traditional bladder and diaphragm expansion vessels, the flow-through vessel has been specifically developed to reduce the risk of stagnation, or contamination from slow-moving water, by keeping water moving through the expansion vessel.
Flow-through vessels are designed with synthetic rubber membranes, as opposed to standard expansion vessels that often have an EPDM (Ethylene Propylene Diene Monomer) membrane, which may support the growth of Legionella, and are therefore often unsuitable for healthcare premises, and, consequently, are not permitted in many NHS projects. For more reassurance an expansion vessel with a butyl rubber compound diaphragm should be selected. Although both butyl and EPDM are synthetic rubber products, butyl is less porous than EPDM, and, as well as being less prone to supporting the development of Legionella, does not add any colour, odour, or taste, to the water. A butyl membrane is therefore a lower risk and a safer option, particularly in healthcare premises.
PRACTICAL CITY&GUILDS ACCREDITED COURSES
April 2021 Health Estate Journal 47
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