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Self purging is not the same as self-draining

There seems to be a belief that the terms ‘drain-down’ and 'self-draining' are synonymous with 'self-purging' when, in fact, they are entirely different processes.

HTM 04-01 Part B, page 24 para 7.88 states: ‘It is important to note the distinction between self purging and self-draining showers. Self purging showers can be an effective Legionella control procedure, while self-draining showers can support the proliferation of Legionella’.

However, we often meet customers who consider that a valve that does not drain down after usage is a bad thing, but this is classic misuse of the term. A drain-down or self-draining valve will, on closure of the tap or shower, empty some of its water volume from the tap or shower outlet. In doing so air, which could be contaminated, is drawn into the outlet from the local environment and has the potential to colonise the outlet with harmful bacteria. In almost direct contrast, a self-purging valve will, either by thermostatic or electronic (time delay) means, purge itself of stagnant water by replacing it with fresh water.

Swan neck taps in healthcare have recently been deemed unsuitable because they have been found to yield higher than normal pathogen counts. On closure of the outlet the fitting retains a volume of water in the upstanding part of the spout. However, there is also a partial exchange of water for local air, which can contaminate the remaining water – already at a temperature that is ideal for the proliferation of harmful bacteria. Beware too that many sensor operated outlets will drain down after use and are therefore susceptible to the same local contamination.

There needs to be a system approach to the prevention of bacterial proliferation within hospital water systems. Firstly, the whole pipe system needs to be appropriately sized for the expected demand, bearing in mind that for environmental reasons flow rates are continually being reduced and an increase in the use of hand gels to decontaminate hands can also have a negative impact on the turnover of tap water. Secondly, Dead-Legs should be designed out and, where possible, outlets doubled up to ensure greater turnover – e.g. a single tap outlet for delivering mixed water and dedicated cold water or a shower offering bi-modal functionality. Thirdly, the temperature regime needs to be rigourously controlled and regularly audited to ensure that temperatures throughout the hot supply remain above 55°C at all outlets and not below 50°C on the return to the calorifier. Cold water temperatures need to be maintained below 20°C by suitable turnover of water and proper insulation. If the facility allows, raising the temperature of the calorifier to 70°C or above can also have a beneficial effect on cold water usage as more cold water will be required to mix the hot water down to a safe and comfortable temperature for handwashing. Don’t be fooled into thinking that raising the temperature of the calorifier will waste energy – the energy input will match the demand for warm water irrespective of the hot water supply temperature. Fourthly, we recommend periodic high velocity flushing of the upstream pipe system. The change in flow rate compared to normal usage helps to destabilise and shear excess biofilm (host for pathogens) from the upstream pipe-work and remove it to drain. This process may also help to reduce biofouling of the basin trap, which could be a potential source of contamination to the local environment as previously discussed. Finally, in the case of the Optitherm thermostatic tap, the facility to thermally disinfect the tap’s internal surfaces will help to sanitise the fitting and help to maintain water quality. This can be done in-situ using the Optitherm’s Thermal Disinfection Adaptor, fully illustrated in our narrated animation, Enhancing Legionella Control (and compliance). If the system will not allow for in-situ thermal disinfection, perhaps due to limited maximum system temperatures, then the Optitherm can also be removed from its spigot and bathed in boiling water for a period of time.

View narrated animations about the Optitherm Thermostatic Tap.

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