Metrology and hazard analysis: Metrological investigation of the drinking water installation in a hospital

Drinking water is water intended for human consumption.  It is by far the most important food and body care product and must therefore meet high quality requirements. In Germany, these are defined by the Drinking Water Ordinance. It requires that

"Drinking water must be such that its consumption or use does not cause damage to human health, in particular through pathogens. It must be pure and fit for human consumption. For this purpose,] at least the generally recognised rules of technology must be observed during water treatment and water distribution" (TrinkwV § 4 Absatz 1, 2001).

The ordinance thus influences the planning, construction and operation of a drinking water installation in a building.

Construction and operational problems within the drinking water installation have an influence on the comfort at the tapping points: The user perceives the hot water as not warm, the cold water as not cold or is annoyed by disturbing fluctuations in the water pressure. More serious, however, are the consequences of these shortcomings in the microbiological and chemical areas: If a drinking water installation is incorrectly dimensioned, bacteria and germs can spread. Particularly in facilities of the hospital and care system, therefore, increased hygiene requirements come into play.

Technical procedure for hazard analysis

Measuring-technical examinations of the drinking water installation according to the generally recognised rules of technology make it possible to determine structural and operational deficiencies. Such investigations take place, for example, in hazard analyses which are prescribed by the Drinking Water Ordinance if the technical measure value for Legionella is exceeded within the framework of routine drinking water investigations. Here, they make a significant contribution to clarifying the causes. Our laboratory division is successfully active in this field at home and abroad.

The basis for our examination in a hospital in North Rhine-Westphalia is formed by the standards and regulations in their respectively valid version. The scope of the examination includes, among others,

• the determination of stagnation areas
• the checking of pipe dimensions
• checking the dimensioning of DHW cylinder heaters and the DHW cylinder charging system
• checking the temperatures in the cold water, hot water and circulation systems
• the inspection of the system hydraulics
• the determination of the hot water demand

Based on construction plans, we check every shaft, every pipe, every washbasin and every other extraction point connected to the drinking water installation in this clinic complex in order to create a complete real picture and subsequently to be able to give recommendations for renovation. The latest ultrasound, temperature and pressure measurement technology is used (see illustrations).

Abstract values become plastic

Often many individual deficiencies lead to problems in drinking water hygiene. Due to a reduced water withdrawal, the pipe volume is no longer suitable, the volume of a storage tank is not completely replaced for too long or a faulty hydraulic balancing causes pressure fluctuations.

Thus the metrological investigation of a copper pipeline with DN 100 showed, for example, that the volume removed was only 110 litres per day. Due to this insufficient water withdrawal, the cold water temperature is permanently at the ambient level, which means that the operating conditions for the water withdrawal and thus the intended operation of the pipeline on which the planning is based are not met.

The metrological investigation of a storage water heater with a volume of 400 litres showed that the permissible temperature of 60 °C at the drinking water heater was systematically undercut. Almost independently of the amount of water removed, the temperature drops to less than 55 °C in the course of the day and even cools to less than 35 °C at night. The reason for this is inadequate heat transfer of the storage charging system. As a result, the storage volume is not completely heated and a hygienically questionable temperature stratification takes place in the storage tank. Due to the insufficient heating, the required minimum temperature in the drinking water heater can no longer be maintained when the drinking water is drawn off. The storage charging system erroneously drives into the night setback mode during the night hours, i.e. no heating takes place at this time. Heat losses of the circulating hot water system including the storage tank as well as hot water withdrawals lead to hygienically questionable temperature ranges both in the storage tank and in the entire drinking water installation.

Measurement tests at tapping points, for example at a washbasin, allow conclusions to be drawn about the hydraulic balancing within the drinking water installation. Our investigation showed an initial temperature of less than 25 °C at a required temperature of 55 °C at such a tapping point. The latter was only reached after three minutes, after 15 litres of water had drained off. This corresponds to a stagnating pipe length of more than 45 metres for a pipe with DN 20. The cause research showed that the circulation pipe did not flow through and thus the hydraulic adjustment of the circulation system was not carried out.