Thermal expansion tanks are designed for pressure control in a potable water system. The building's water heater or water heating system typically creates the thermally expanded water. A properly sized thermal expansion tank will accommodate this additional volume of water created through expansion during the heating cycle, and control the system pressure increases, keeping pressures from reaching critical limits. The tank uses compressed air to maintain system pressures by accepting and expelling the changing volume of water as it heats and cools. These tanks are designed utilizing a flexing butyl barrier that separates the stored water from the captured pressure control air cushion. This barrier, a replaceable bladder or fixed diaphragm, allows the water to be contained within the bladder preventing corrosion and potential water logging. The thermal expansion tanks are engineered to meet ASME standards, and are also available in non-code designs. Wessels' thermal expansion tanks are offered in industry's broadest range from 2 to 4,000 gallons and up to 250 psi.
Pressurized thermal expansion tanks differ from plain steel expansion tanks in that a flexible bladder or diaphragm separates the air cushion from the system fluid. The air side pre- charge of the tank must be field adjusted to equal the system supply pressure. As the system water expands, the bladder expands open, pushing against the air cushion to accept the expanded water.