Identification of depleted appliance trap seals within the building drainage and ventilation system – A transient based technique

Foul air from the drainage network is prevented from re-entering the occupied zone bythe barrier created by the appliance trap seal. The loss of this defence, following trapseal depletion, can provide a path for cross-contamination and infection spread; a causalfactor in the SARS epidemic in 2003. Current means of locating open appliance trapseals rely on the difficult task of visual inspection. This paper introduces a remote andnon-invasive technique to identify depleted appliance trap seals through analysis of thesystem response to an applied low-amplitude sinusoidal pressure wave. As the appliedpressure wave propagates throughout the drainage system it will be reflected andtransmitted by each encountered system boundary. If one of these boundaries ischanged, e.g. the depletion of a trap seal, the reflection coefficient of that boundary willchange, thus altering the system response. The theoretical basis of this technique will bediscussed and accompanied by results from laboratory and field tests and a Method ofCharacteristic (MoC) based numerical model which confirm the validity and practicalityof this technique.