An employee was servicing equipment and had disconnected a 4-inch line. A proportional control air valve on the equipment actuated, releasing a large volume of air at 99 psi into the 4-inch braided stainless steel line hanging down from the valve. The sudden release of pressure caused the line to whip and strike the employee's face. He was hospitalized due to an orbital bone fracture requiring surgery. The equipment was not locked out/tagged out at the time.