An air pressure transient approach looks like an answer to a common health concern.

The appliance trap seal has been the primary defence against drainage cross-contamination since its invention in the late 18th century and its introduction into ‘modern’ drainage design from the 1850s.

The Victorian era obsession with the prevention of odor ingress led to complex drainage venting schemes.

These aimed to ensure that local air pressure at the appliance trap was never sufficient to deplete the trap through applied suction pressure or to drive contaminated air though the trap into habitable space by the action of positive system pressures.

Such schemes invariably involved connecting the appliance waste downstream of the trap to vent pipework that eventually led to an open termination above roof level – thus the imposition of atmospheric pressure as a terminal boundary condition.

They were essentially passive in nature, as their operation did not depend on changes in local air pressure conditions.

The ‘if it exists vent it’ passive approach of the Victorian engineer could not survive the growing complexity of buildings, and there had to be modifications to the acceptable level of vent connection.

Design solutions

Current regulations now accept a range of design solutions, from the more conventional ‘one pipe’ system favoured in Europe and the United States to the single-stack approach pioneered in Britain 40 or more years ago but still not universally recognised as a suitable solution.

Technological developments have also added to the interest.

There have been long-running debates on the acceptability of local active control, including air admittance valves and variable volume containment devices being introduced to limit local air pressures, hence protecting the trap seal.

The operation of drainage vent systems is now fully understood and indeed is capable of simulation.

The unsteady flows involved have been recognised as part of a well-known set of flow conditions, including water hammer.

They are also dependent on air pressure transients propagated in the network by changes in the applied water flow following appliance discharge, and possible surcharges of the network due to excessive water flows.

However, it is also clear that an ‘engineering only’ approach to understanding the risks inherent in system operation is insufficient.

Just as important are the maintenance levels applied to the system, the controls possible on occupancy (hence loading), and the control and recording of system modifications that may lead to big changes in system usage – thus a heightened risk of trap seal depletion.

It is difficult, if not impossible, for the designer to allow for such loading changes in the future use of the system.

Although it seems quite reasonable to the Code committee in whatever home country to prescribe allowable maximum flow rates and air pressure excursions, the actuality of operation may be quite different.

The SARS connection

The prevention of contaminated air ingress into habitable space has been a central concern in the design of building drainage and vent systems since the 1850s.

However, failure to provide the necessary protection – primarily through appliance water trap seals – was a contributor in the SARS epidemic in Hong Kong in 2003.

This was particularly relevant to fatalities in the Amoy Gardens housing complex where World Health Organisation and local investigative reports identified dry trap seals as a primary cause.

The Amoy Gardens event was characterised by a lack of maintenance that allowed floor drains serving bathroom areas to become dry, providing a path for contaminated air to pass into habitable space.

This effect was exacerbated by the use of extractor fans in the bathrooms served.

It is important to avoid ‘preparing for the previous battle’, but it is clear that future prevention depends on good design and good maintenance, coupled to some degree with control on occupancy, usage and unauthorised system modifications.

Graphic evidence from Amoy Gardens shows trapless sinks connected to the drainage network. This feature was not directly implicated but it cannot be allowable.

Defects

A maintenance regime in a large and complex building network requires a degree of prior knowledge of possible defective appliance trap seal locations.

Assistance in determining likely areas of failure may stem from the application of fundamental air pressure transient theory.

This will allow the use of transient simulation and transient response measurement to identify – during periods of system non-use – the location of depleted trap seals.

The fundamental point here is that air pressure transients, once propagated in the network, will continue to be reflected and re-reflected at all system boundaries until they naturally die away due to frictional attenuation.

The reflective properties of various boundaries are well known from the wider water hammer subject area.

For example, closed ends result in a positive, or +1, reflection coefficient so that any incoming transient – positive or negative – is reflected as a pressure wave of equal magnitude and the same sign.

Conversely the reflection at an open termination, or any constant pressure zone, has the same magnitude as the incoming wave but reversed sign, effectively a -1 reflection coefficient.

An expected full trap that has dried out or has been depleted will display a quite different reflection coefficient and will thus be recognisable.

A low-amplitude air pressure transient propagated in a building drainage system therefore obeys all the mechanisms of transient propagation (travelling at the acoustic velocity in air) and system response (in terms of boundary reflection and transmission).

The propagation may be simulated by the proven method of characteristics solution of the St Venant equations.

Prediction

The changed response of a network with a dry trap to a low-amplitude applied pressure pulse – effectively a new open termination with an identifiably different reflection coefficient – may be predicted.

In practical terms it is possible to predict the arrival time of a reflection from a changed termination at any monitoring location in the network.

Predictions at two monitoring locations in the network would clearly identify the location of the dry trap.

To translate this into a practical method of identifying dry trap seals it would be necessary to subject the building drainage and vent system to a low-amplitude pulse – probably by activation of a fan or a simple purpose-built pressure propagation device.

This would be done during a quiescent period: a night-time short-duration automated pulse would be sufficient.

Activation at periods of non-flow in the network will enhance the probability of a successful identification of the dry trap seal.

It will also remove many of the operational difficulties encountered in similar methodology directed towards water supply network leakage identification.

In view of the established consequences of poor maintenance and trap seal depletion in complex building drainage networks, this approach seems timely and interesting.

Active control

Successful development would have implications for facilities management in complex buildings. However, in a sceptical world it clearly requires proof of concept.

Heriot Watt University has undertaken initial laboratory testing, and a two-year research program that commenced in January 2006 was agreed to with funding from the UK Engineering and Physical Sciences Research Council.

Identification of persistent trap seal depletion would allow local installation of active control devices to prevent further failures.

Air admittance valves (AAVs) could be used to limit trap seal loss due to negative transients, or variable volume containment devices installed to limit trap seal loss due to positive pressure transients.

Comparison of a defect-free simulation with the measured response of the network to an applied transient will identify the presence of a defect.

Simulation or measurement of the response at two monitoring stations will identify the defect location in terms of distance and travel time for a transient from the surge generator to the termination and back to the monitoring location.

The accompanying chart on page 83 illustrates a simple laboratory demonstration of the principle to be developed in this research, showing the defect-free system response with an AAV or an open end as the stack termination.

As the applied transient is positive, the AAV closes and becomes a closed end.

Depletion of one trap seal yields a quite different system response, with the reflection returning to the transducer earlier than previously, representing the shorter travel time at the acoustic velocity in air.

The way forward

The proven mathematical simulation of transients, together with the evidence provided by initial small-scale testing, indicates that this methodology has potential as a means of identifying defective system components.

Following further laboratory testing that will include the development of a suitable transient generator, extensive site testing will be undertaken using buildings made available by industrial research collaborators.

During this phase, detailed consultation with facilities management practitioners will ensure that the final methodology proposed meets their practical requirements and does not infringe any Code regulations on system components.

It will be necessary to ensure that the pressure monitoring equipment and transient generators do not themselves present any cross-contamination routes to habitable space.

Finally, suggested facilities management procedures for introducing transient identification will be presented for discussion and consultation through the relevant professional bodies, including the recently announced Consortium to Address the Role of Drainage in Infection Spread (CARDIS) working group of CIBW062.