EMPLOYEE WAS OPERATING THE WATER TRUCK, SPRAY NOZZEL NEEDED TO BE ADJUSTED, THE DRAINED THE AIR LINE BY THE CAB, SHUT OFF THE TRUCK, AND PROCEEDED TO TURN TO SPRAY NOZZLE, DID NOT BLEED OFF TH E AIR LINE AT THE NOZZLE AND SO WHEN ADJUSTING THE NOZZLE THE METAL CAP ON THE END OF NOZZLE, CAPPING AIRLINE AND WATER, FLEW OFF HITTING EMPLOYEE IN THE NOSE.