Ejectors use water as the pumping mechanism which creates a vacuum by recirculating water at a high pressure through a nozzle and venturi within the ejector body. An ejector system is typically used in soils such as Silts, silty Sands or laminated Clays which have a low permeability. They are well suited for deep excavations were the depth is beyond the scope of a vacuum wellpoint system and where well yields are below the effective use of a deep well pumping system.
Ejectors wells are installed at relatively close spacing (typically 3m-6m centres) and drilled using a rotary or cable percussive drilling rig (typically 200mm bore) into which a well liner is placed (typically 100mm diameter) with a surrounding filter annulus. To assist in generating flow inducing vacuum within the well a bentonite or grout seal is installed. For most projects vertical wells are installed, however where access is an issue, wells can be installed at an inclined angle.
After airlift development of the wells to increase efficiency and reduce the risk of pumping fines. A twin pipe ejector is installed on supply (25/32mm) and return (32/40mm) riser pipes that in turn are connected to two parallel header mains. One header main is a high-pressure supply line and the other header is low pressure return line. Both lines run to a recirculating central surface pump station which supplies pressurised water (typically 5-7 Bar) to the ejector units, which then returns water to a tank, to feed the pump and were any excess (or induced groundwater) is discharged.
Typically, a single pump can operate 100 lin.m system (30no-15no ejectors) although this varies considerably on the volumes of water to be pumped, the lift or supply flow and pressure required. The volumes of water which can be pumped by each system are generally low, typically less than 5 m³ per hour.
Ejectors are predominantly used for temporary construction dewatering. However, they can be used for permanent dewatering schemes and as a means of abstraction for irrigation or private water supply systems.
