Making safety plumbing safe

In the first of two articles in this issue on thermostatic mixing valves (TMVs), Dave Huckett explains why such products should be viewed not in isolation but within an appropriate plumbing context.

The duty of care to avoid scalding people in hospitals and other healthcare premises is backed by legal sanctions which, besides pain or even death inflicted on the victims, can involve serious financial penalties or imprisonment for those responsible. Risk assessment is vital and remedial plumbing work may be necessary.

NHS Estates guidelines are no more than that, and they need to be applied with care to ensure safety. Simply choosing products like thermostatic safety valves may be no guarantee of safety if they are viewed in isolation. The plumbing system as a whole needs to be considered.

Looking at water usage, there could appear to be a contradiction between two requirements. On the one hand, to prevent Legionella infection, hot water temperatures need to be high enough to scald. On the other hand, safe washing, showering and bathing temperatures need to be much lower.

The contradiction is resolved by differentiating between temperatures generated at source and those at the points of use. It is at points of use that both British Standards and the NHS D08 specification are meant to apply, and this is where devices like thermostatic mixing valves come into their own.

Consider the possible consequences if hot water generation temperatures were kept low enough to be safe in use. Legionella could multiply. Hot water and energy wastage would rise as users ran taps for longer periods. Baths might not be hot enough, and lukewarm water would not cut grease when washing up in kitchens.

`EXCEPT THE KITCHEN SINK'

Kitchen sinks are not covered by NHS advice, but they also need to be looked at in terms of user safety. Where staff are the sole users of such facilities, an ordinary domestic hot water temperature of 60C could be considered, although 50C is perhaps more advisable.

Where kitchen facilities are used under staff supervision by patients, perhaps after a long stay and prior to their return to the community, the NHS safe hot water guidance can similarly be considered - ie the use of thermostatic mixing valves to bring temperatures down to 60C or 50C at points of use.

However, where kitchens used by unsupervised patients have facilities purely for washing hands, or for washing up such items as cups and saucers, hot water temperatures need to be restricted to 41C, as they do in bathrooms for handwashing or showering, although 44C would be acceptable for baths.

PRESSURE-REDUCING VALVES

Looking at thermostatic mixing valves in a wider plumbing context, the use of pressure-reducing valves could be crucial to their correct functioning and therefore to safety. UK mains water pressures vary considerably, from barely acceptable to nearly 20 bar, and some 20% of installations need pressure control).

Pressure-reducing valves are the answer to high or variable mains pressures, both for watersaving purposes and stable flow conditions. When TMVs are used with stored hot water supplies and cold from the mains, pressure-reducing valves can control the pressure differential as necessary.

DO8 thermostatic mixing valves require a maximum dynamic pressure differential of 10:1 which translates to an approximate static pressure differential of 5:1, depending on system layout. Pressure-reducing valves can provide stable pressures, both under flow-rates conditions, and flow rates acceptable for any applications.

Where an existing installation is being updated, the cold mains distribution pipework will be sized to the mains pressure, so fitting one pressure-reducing valve at the inlet could cause water starvation problems, with a resultant threat to safety at the various points of use.

On an existing installation it is often better to install pressure-reducing valves local to groups of thermostatic mixing valves. Because these can operate on wide pressure differentials, it is seldom necessary to fit pressure-reducing valves to them individually.

Design Variations

While thermostatic mixing valves are the best method of safe temperature control, they do vary. There are pre-set valves for fitting to bath and basin or sink supplies, and adjustable valves for use with showers. The correct type of valve needs to be selected for each application.

Designs vary a great deal in terms of flow-rate, ability to run on unbalanced hot and cold water supplies (all should but some do not), reaction time, and the temperature differential at which they will operate - this being the difference between the set temperature and the actual hot water temperature.

Simpler valves which act like temperature-actuated 'fuses' to prevent scalding have more restricted applications, while the simplest option is mixers with a maximum temperature stop. These work well as long as hot and cold water supplies remain constant both in temperature and pressure.

Dave Huckett is technical manager at Reliance Water Controls.

Copyright Wilmington Publishing Ltd. Apr 1998
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