A construction worker was rushed to Hackensack University Medical Center yesterday after being seriously injured by a valve cap while working at Ramapo College in Mahwah. The Occupational Safety and Health Administration said what was originally characterized as an explosion turned out to be a blown gasket, the worker was struck in the head and shoulder. Published reports said workers were conducting a pressure test on a cooling line at the Peter P. Mercer Learning Commons Library when the incident occurred yesterday morning. OSHA is investigating the incident. No word on the worker’s condition.