A new approach of sizing a drainage stack for residential buildings in Hong Kong

In order to design an effective drainage system for buildings, discharge loads at a drainage stack must be determined. A ‘fixture unit’ approach has been used to determine such loads, with the assumptions of simultaneous operation of all connected appliances and no attenuation effect of flow rate along the drainage pipe that would overestimate the drainage loads and hence the stack size. In this study, empirical expressions are proposed to determine the probable drainage loads for a stack serving a number of domestic washrooms of high-rise buildings in Hong Kong, taking the factors of the attenuation effects of an assigned gradient 1/100 in a horizontal drainage pipe as well as the discharge patterns and simultaneous use of appliances in a typical domestic washroom into account. With reported probable demand patterns of domestic appliances from laboratory testing results, on-site measurements and survey studies, the probable discharge flow patterns in the stack are estimated with MonteCarlo simulations for the probable maximum discharge flow rates from a number of domestic washrooms at a limiting failure rate. It is shown that, comparing with the estimation method in current practice, a reduction of predicted discharge flow rate at the stack serving a number of typical domestic washrooms would be up to 13% by taking account of the above two factors.