A system for injecting water into a well boosts pressure in a flowline leading to the injection well. A bypass flowline is connected into and forms part of the main flowline. A bypass valve is located in the bypass flowline for opening and closing the bypass flowline. A housing has an inlet connected to the bypass flowline upstream of the bypass valve in an outlet connected to the bypass flowline downstream in the bypass valve. Inlet and outlet valves selectively open and close the inlet and outlet of the housing. A centrifugal pump assembly is mounted in the housing, the pump having an intake in communication with the inlet of the housing. The pump has a discharge tube that communicates with the inlet of the housing. An isolation member separates discharge pressure of the discharge tube from the intake pressure at the intake. A removable end cap on an end of the housing enables the pump assembly to be inserted and withdrawn from the housing.