A derrickman opened the relief valve to bleed off the mud pump then made repairs to stop the end cap leakage. After repairing the mud pump, the derrickman instructed an employee to ensure the valve to the main discharge line was completely open. The employee operated the valve. Meanwhile, the derrickman called the driller to start pumping with both pumps. The derrickman noticed returns from the pop off relief line and instructed a second employee to close the bleed-off valve. Upon closing the bleed-off valve the pop off valve blew causing a piping “T” after the pop off valve to split, releasing pressure/mud and the employee near the “T” fell backwards. The employee received a lacerated hand.