Internal liners smooth the flow through the expansion joint. The smooth flow reduces pressure drop and prevents flow-induced vibration of the bellows. Liners are usually recommended when the flow velocity exceeds 1.3 ft./sec. as a minimum and are recommended when the flow velocity exceeds about 25 ft./sec. Consult the manufacturers catalog for additional information. Heavy gage liners should be used in high velocity or turbulent flow systems. Also, heavy liners should be used when the media is abrasive.