Bell & Gossett diaphragm expansion tanks contain a butyl rubber diaphragm attached to an internal polypropylene tank liner. The heavy-duty butyl diaphragm separates system water from the air in the tank preventing water logging problems. Diaphragms cannot be replaced independently. The entire tank would need to be replaced if there was a problem with the diaphragm. However, the diaphragm is made of thick, high-quality rubber that will last for years.