WATER SYSTEMS ‘‘


Legionella pneumophila, the bacterium that causes Legionnaire’s disease, grows in water systems where water is stored at between 20 and 60 °C.


reducing the opportunity for clusters to form.


In a closed hot water circuit, the water cannot be compressed, so any increase in volume, created by an increase in temperature, has to be accommodated by an expansion vessel. When water is cold, the pre-charge pressure forces the bladder to collapse until the pump is started when the bladder starts to inflate (see diagram, right). As the temperature in the system


increases, with the associated increase in pressure and volume, the expanded water enters the bladder, creating additional volume and lowering the pressure. When the temperature decreases, the pre- charge pressure forces the water from the bladder and back into the main water circuit.


Thermostatic mixing valves Thermostatic mixing valves (or ‘TMVs’ for short) keep water temperatures safe by blending hot and cold water and maintaining it at a constant level. The recommended temperature range for delivery of safe hot water where a TMV is installed is 38-44 °C. To prevent the growth of Legionella bacteria, water should be stored at below 20 °C or above 60 °C. As TMVs allow for high temperature water storage, the risk of bacterial growth is reduced, but not eliminated. In order to store water at temperatures that will thermally disinfect the water system, TMVs must be fitted at outlets. In addition, they must adhere to the strict testing regime of the TMV3 / NHS D08 regulatory standard. The TMV3 approval scheme


provides assurance that a TMV is tested and deemed safe to use in an NHS setting. This scheme applies to TMVs for use in high-risk commercial healthcare applications within the UK. It is highly recommended that, once installed, TMV


In a closed hot water circuit, the water cannot be compressed, so any increase in volume, created by an increase in temperature, has to be accommodated by an expansion vessel


valves are checked and maintained regularly (once a year) to ensure that they are still providing the required and intended scald protection. The outlet temperature should be tested to check that there is no significant change (≤ 2 °C), and that the failsafe shut-off activates as intended. If the TMV does not meet these requirements, a full TMV service, plus recommissioning, or valve replacement, will be required.


Using components to prevent growth of Legionella and Pseudomonas The Health and Safety Executive’s3 Approved Code of Practice and Guidance on Regulations on controlling Legionella bacteria in water systems3


states that


operators should: ‘Consider the individual nature of the site and system as a whole, including deadlegs and parts of the system used intermittently. These should be included because they can create particular problems, as microbial growth can go unnoticed. When they are brought back online, they can cause heavy contamination, which could disrupt the efficacy of the water treatment regime’. For this reason, it is


recommended that: n TMVs are installed as close to the water outlet as possible, mixing water at the point of use where possible.


Flowjet


n Multiple TMVs should be used in the case of multiple outlets; use of a single TMV increases the risk of bacteria growth.


T-piece


Flow-through expansion vessels, when used with a Flowjet valve, are designed to keep potable water moving through a system to prevent stagnation or contamination.


56 Health Estate Journal April 2022


n The movement of water through the system should be controlled, preventing the flow of stagnant water.


n ‘Sentinel’ outlets must be identified and checked monthly, with hot water and cold water cylinders needing to be cleaned and checked for debris periodically.


Full thermal flushes of water


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