Pancake style safety tee was opened to remove vacuum from a hose being used to dust but clean the upper building of a silo. Cover plate did not have a locking mechanism to hold open. Employee 1 was cutting hose while employee 2 was dragging hose back in. Hose hit safety tee closing plate 3/4 engaging vacuum. Employee 1 had arm caught in hose approximately 45 seconds to one minute.