Non-Code Diaphragm Type Tanks
This style tank is divided in half by a butyl rubber diaphragm, one side being the water side, and the other being the air side. The water side is connected to the pipe work in the system, and the air side is maintained using a simple car-tire type valve, also known as a Schrader valve. As pressure descreases in the system, the diaphragm is pushed against the inlet, and as the water pressure increases, the diaphragm moves away from the inlet and compresses the air on the other side.
One disadvantage of a diaphragm is it cannot be replaced if the tank fails. The advantage of this style tank is the low initial cost. For ease of replacement, many people find themselves resorting to bladder type expansion tanks, which also helps keep replacement costs lower. Information on bladder tanks can be found under ASME tanks & accessories.