Influence of installation of S-shaped pipe offset on the water flow capacity of double stack drainage system in a high-rise building

In high-rise and ultra-high-rise residential buildings, the drainage systems usually adopt the S-shaped offset pipe joint to avoid collisions of pipelines with beams and columns. However, the flow direction changes at the offset point would significantly affect the drainage capacity of the systems. To better understand the influence of pipe offset joints on the drainage capacity, three different building drainage systems namely YZW 1, YZW 2, and YZW 3 systems, were built up and investigated. YZW 1 system is a standard double-stack drainage system with anti-reflux H-tube joints. YZW 2 and YZW 3 systems were developed from YZW 1 by adding S-shape offset pipe joints on the 4th floor. An extra anti-reflux H-tube joint added 1 m above the offset joint as a measure for improving the drainage capacity differentiates YZW 3 system from YZW 2 system. The pressure fluctuation and water seal losses were adopted as experimental parameters to determine the maximum discharge capacity according to Standard for Capacity Test of Vertical Pipe of the Domestic Residential Drainage System. The experimental results show that the discharge capacity limitation of the drainage system with the offset joint is 6.5 L/s, which is much smaller than 11.0 L/s of the system without the offset joint. In the experiments with the offset joint, the pressure fluctuated fiercely at the position where flow direction changed and caused severe water seal losses of the floor drain and P-shaped trap. An anti-reflux H-tube joint above the original offset joint was observed to relieve the pressure fluctuation as an improvement and the drainage capacity was improved to some extent. The relationships among the maximum discharge rate, pressure fluctuation transients, and water seal losses were discussed. Lastly, a nondimensional analysis was adopted to understand the relationship between water seal losses and pressure limit values under different discharge rates for current test facilities.