Shut off A to remove or work on pump. Should also be a valve on other side of pump (right side) to shut off also allowing you to work on or remove pump without back flow. Im assuming a lot here, that being a circulating o booster pump? And flow is left to right? Be sure power is off to pump so it is not running when water is off. Never shut off a valve when you have a pump supplying water unless you secure the power to pump or a pressure switch will turn off pump.