What is the difference between a bellows and an expansion joint?

The terms can be confusing since there is no universally accepted meaning. We usually use the following definitions, but the terms “bellows” and “expansion joint” are generally considered interchangeable:

• Expansion joint: An expansion joint is a complete unit with metal, rubber or fabric bellows and end connections and accessories, as well as optionally, internal linings, outer covers, etc … Other terms used: compensator, flexible.
• Metal bellows: it is the flexible element of metal expansion joint.
• Rubber bellows: it is the flexible element of rubber expansion joint.
• Internal lining: the internal lining goes inside the expansion compensator, underneath the metal bellows or the rubber bellows to prevent the circulating fluid from falling directly on the bellows. This reduces flow disturbance and allows the bellows temperature to be controlled in the design.