Dear all,
could you please someone tell me what is the differences between pressure vessel and drum? when we call a pressure vessel as a drum? on the other hand what is the exact definition of drum?
thanks.

My Marks Handbook for Mech Engineers doesn't include the term in its index or table of contents, so I wouldn't worry too much about an "official" definition. In PV's I've seen the term most oftenused for the horizontal pressure vessels joining the hundreds (thousands) of pipes lining a boiler. The upper drum is the steam drum: about half-filled with saturated water and steam, each pipe coming into the lower half of the steam drum. (The steam leaves through the upper half to the supperheater and steam separators.) At the bottom, the lower water drum connects all the lower ends of the pipes, receiving high-pressure make up water from the boiler feed pump.

A definition? I would say you could use "a separate, integrally connected pressure vessel receiving the process fluid (liquid or gas) from one or more non-isoable pipes leading directly to the primary pressure vessel or reactor."