RELIABILITY OF MECHANICAL BACKFLOW PROTECTION DEVICES
Report No DWI0687
A review was carried out of the regulations, standards, installation and maintenance practices and reliability in service of all known mechanical backflow preventers in use in the major countries of Europe, North America and Australia. This was undertaken as part of a Department of the Environment research contract managed by the Drinking Water Inspectorate. In particular, the reduced pressure zone principle device (RPZ) and the double check valve assembly (DCVA) which are used in countries other than the UK were examined.
We studied 11 countries or groups of countries (European Union/Countries represented in CEN, UK, Germany, France, Italy, Spain, Netherlands, Scandinavia [Sweden, Norway, Denmark, Finland and Iceland], USA, Canada and Australia).
The significance and status of standards and "Codes of Practice" varied between countries. In most parts of Australia, Canada and the USA as well as several member states of the European Union standards and codes of practice are regulatory documents enforced by regional or local government or water undertakings.
European standards for backflow protection devices are in preparation to specify products for conformity to the Essential Requirements of the Construction Products Directive. These will be transposed into national standards replacing existing standards. A general application standard, prEN 1717 has recently been issued by CEN for public comment, and the family of specialised product standards which contain the performance requirements, are in preparation. Our examination of prEN 1717 found it to be severely lacking in a number of respects, notably in being hard to understand and in being incomplete. Major rewriting is needed before this could become acceptable.
The risk categories used in the countries studied are quite varied, ranging in number from none (Spain) to five (France and CEN). Some countries use "Health" and "Nonhealth" categories, some use the potential (or not) of causing death, while the UK uses a high or low probability of contaminants being present. CEN is the only one that attempts to use a quantitative toxic level, but this is not self consistent nor sufficient, being based on the LD50 (Lethal Dose) method. The LD50 method is widely considered to be obsolete, inexact and inhumane. An alternative measure such as the Fixed Dose Method is being suggested.
Usage of RPZ valves is widespread, throughout the world, and this type of device is described in standards in Australia, Canada, France, Italy, the Netherlands and the USA. For similar applications the more expensive and complicated mechanical pipe disconnectors were only found to be covered by standards in Germany.
The performance requirements for the individual devices are spelled out in a series of laboratory tests contained in the various national standards. It is not possible to summarise many of these in quantitative terms without referring to the particular test apparatus specified in the standards. However, there does not seem to be a huge difference in the required performance characteristics of the same device over several countries.
Some countries require the use of in-line strainers for RPZ and DCVA devices. Some only recommend them, and many do not mention them. The need for periodic inspection and maintenance, especially of RPZ and DCVA devices is well apparent. Most countries that permit the use of these devices also require stringent inspection and test by qualified inspectors at annual intervals. The devices themselves are required to be equipped with test cocks for this purpose. It is therefore surprising that the CEN standard includes "non-testable" versions of such devices, which are seen by many experienced users as being dangerously unreliable.
The reasoning behind the selection of an annual inspection interval is unclear. We were completely unable to obtain any information on field testing for reliability of any devices in any country. We established that some such work is going on in the UK under commercial confidentiality, and we were told that the reliability of some manufacturers' devices was "poor". It seems that any CEN standard covering such devices must be demonstrated to exclude those showing such "poor" characteristics.
We studied national and industry standards from 10 countries. These were found to vary greatly in their quality, ease of use, and coverage. The British standards were last updated in 1988 and cover only a few devices. Of all the standards we looked at those from Australia were found to be far superior in all the above respects. They are of recent issue, (1992) and are even now being revised again. We recommend their close study as a model upon which to base future UK regulatory requirements, and if possible, as a model for CEN to learn from.Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.