An employee was monitoring the loading of a railcar when he noticed a vent was leaking. He tried to close the secondary valve to isolate the leak. He put his safety harness on and pulled a probe up to close the valve and isolate the leak. When he pulled the probe up, he heard a pop. The vent hose blew off, spraying him with acetic acid. He suffered first and second degree burns to his lower legs and feet, requiring hospitalization.