Author(s): Wu Chun Ping; Onyx Wai Wing Hong; Li Chi Wai; Chan Wai Lam
Linked Author(s): Onyx W.H. Wai, Chi Wai Li
Keywords: Physical Modelling; Pumping Station; Flow Splitter; Swirl; Sedimentation
Abstract: The design of the Main Pumping Station 2 in Stonecutters Islands Treatment Works (SCIMPS2) is unusual. It consists of two elongated rectangular wet wells of 40. 5 m by 6. 75 m with operating water depth varying from 8. 45 m to 26. 6 m that favours sedimentation. The compact design replaces inlet well by manifold chamber that leads to complicated inflow conditions. Both the capacity of the station (32 m3/s) and the peak pump rate (4 m3/s) are large. The energy conservation philosophy requires high water levels under low inflow conditions but lower water levels under high inflow conditions. Physical modelling is therefore required to investigate the swirl and flow conditions in the pump sump and to minimize sedimentation for the finalization of the pump sump design. Following the Froude law in dynamic similarity between prototype and model, a model with an undistorted scale of 1: 12 was constructed. It met the requirements in ANSI/HI 9. 8-1998. Over thirty preliminary tests were carried to formulate the modifications to reduce swirl and sedimentation. Replaceable 4-blade flow splitter slanted at 45° was proposed at each suction pipe entrance to suppress swirl. The maximum and average swirl angles were significantly reduced from 13. 8° and 8. 1° to 5. 2° and 3. 6°, respectively. Large chamfers at bottom edges of wet wells were proposed to enhance flow circulation and to minimize sedimentation. Under peak inflow with four pumps operated at peak rate at low water level, the horizontal velocities at nearly most locations above the bottom were greater than 0. 23 m/s, which is the critical scour velocity for 0. 2 mm inorganic sticky grit based on Shield’s formula. It was believed that most of the particles in the influent would be resuspended and sucked away by the pumps. Over eighty tests under different inflow rates and water levels were carried out for the study.