Air lock

An air lock is a restriction of, or complete stoppage of liquid flow caused by vapour trapped in a high point of a liquid-filled pipe system.

Flushing the system with high flow or pressures can help move the gas away from the highest point.

Air lock problems often occur when one is trying to recommission a system after it has been deliberately (for servicing) or accidentally emptied.

Otherwise, air lock may cause waterfall flow where the loss of hydraulic head is equal to the height of airlock; If the hydraulic grade line drops below the output of the pipe, the flow through that part of the circuit would stop completely.

Long pipelines built across fairly level, but undulating land are bound to have many such high and low points.

Gravity filling of an empty distribution pipe with undulating ground. The pressure on the trapped air is either H2 m of water (WG = water gauge) or H1, whichever is less.
'S' trap inlet to drain [ further explanation needed ]