Telescopic bellmouths are used extensively in sewage purification works to draw off surface fluids and scum. They consist of a lower (outer) tube which is connected to a drain pipe, and an upper (inner) tube which slides up and down, so varying the level at which skimming is required to take place. Seals are fitted between the inner and outer tubes to prevent leakage.

Operation is usually manual by means of a handwheel and spindle connected to a bridge-piece fitted across the top of the sliding tube, and care must be taken to allow enough room between the bridge and the underside of the operating platform when the inner tube is fully extended.