Once prime has been achieved, what should be done before opening the discharge to supply the attack line?

Once the prime has been achieved in a pumping system, it is crucial to close the primer valve before opening the discharge to supply the attack line. The primer valve is used to create the vacuum necessary for priming the pump by removing air from the pump and suction line. If the primer valve is left open, it can allow air to re-enter the system, which could compromise the priming and lead to pump cavitation or loss of suction.

Closing the primer valve ensures that a proper seal is in place and that the system maintains the vacuum created during the priming process. This action contributes to maintaining optimal performance when the pump is put into operation and helps ensure a steady flow of water to the attack line without interruptions.

Waiting for 5 minutes, quickly opening the discharge, or maintaining pressure in the system do not adequately address the necessity of securing the priming process before engaging the discharge. These alternatives could cause inefficiencies or complications in the operation of the pump and the overall firefighting efforts.