Figure: Schematic of Yuen Long transmission main

The transmission main is located in New Territories West (NTW) running along Castle Peak Road, Yuen Long, HK. The NTW pipeline is made of a mild steel pipeline 700 mm in diameter. Three access locations to the pipe are a 150mm DAV1 along Castle Peak Road (Yuen Long) bridge, a fire hydrant (PH 1295) and an air valve chamber DAV2, 167m upstream from DAV1 (see figure above). The line is fed from a pump station at the upstream end which also feeds a distribution system upstream of the area of interest. The main outflows into Au Tau Fresh Water reservoir. A total length of 826 meters spans between the DAV1 and the downstream reservoir. The complexities and uncertainties from consumer connections into a distribution line are greatly simplified in a transmission line. The access points are fitted with appurtenances that accommodate pressure sensors and valves. The transmission main neighbors a multilane rural trunk road (Castle Peak Rd.) with large traffic (approx. 850 vehicles, one-way flow AM peak hour) from 2019 Transport Department census. The access locations at DAVs are adjacent to the transmission main (as shown in the figure below), the pipe wall will be mounted with acceleration sensors. Triaxial piezoelectric accelerometers with high sensitivities of 10mV/g (IPC 352A60 PCB accelerometer) and 316pC/ms^–2 (B&K 4326-A, charge accelerometer). will be used to sense pulsations and vibrations expected from traffic and human-related activities coming from mechanical shops, construction works, gas stations, and others.

This facility is instrumental for sensing high-order modes due to its large diameter. The cut-off frequency for plane wave is of the order of 1 kHz for this main. Since the access monitoring stations are remote from each other, the synchronization between transducers will be achieved using an in-house GPS synchronization system with accuracy below 0.1ms.