What was found
(CAN) RT ENGINE ANTI-ICE VALVE REPLACED, INVESTIGATION SHOWS THAT INNER HALF OF LOWER COWLING FASTENERS NOT SECURED. AIRCRAFT DEPARTED YVR, CREW NOTICED A 'BANG' AND THEN A CONSTANT VIBRATION, GOING THROUGH 8,000 FT, CLIMBING OUT. CREW CONTINUED TO CLIMB THROUGH 15,000 FT, BEFORE DECLARING AN EMERGENCY AND RETURNING TO YVR AND COMPLETING A NORMAL LANDING ON RUNWAY 12. INVESTIGATION SHOWED RT LOWER ENGINE COWLING PARTIALLY DETACHED FROM AIRCRAFT AND SKIN KINKED AND BENT, AND FORWARD OUTER FASTENER HOLES TORN.