If washing/cleaning of tanker with chemical is prerequisite of your manufacturing process and you also documented it, then, you may avail the CENVAT credit on such chemical otherwise not. Please make a process flow chart of your manufacturing process by clearly depicting that where this washing/clearing process fits in to substantiate your claim that you are eligible for avail the CENVAT credit on Chemical used in cleaning of tanker etc. Get this process verified from your technical head/Management and if possible, then, from Chartered Engineer.
Further, you would also be required to follow of the provisions of CENVAT credit Rules, 2004 as amended from time to time.
Team YAGAY & SUN