Abstract: A tank formed using two half shells may be used as an expansion tank, expansion equalizer, pressurized fluid buffer tank or the like. Two half shells are provided with a projecting flared profile having a first section angled outwardly from the wall and a flange directed perpendicularly outwardly. The tank halves are joined by abutting the flanges, and the angularly offset sections adjacent the flanges form a bearing area or seat for sealing with an internal diaphragm. The diaphragm separates the tank into sections and is arranged at its end to receive and positively hold a ring of a profile complementary to the internal profile defined by the outwardly flaring sections adjacent the flanges of the tank. The flanges can be folded over one another and against the wall of the tank to physically attach the half shells.