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Good engines are provided with a plug or flap valve.

A flap valve is provided between the two chambers to prevent the gasoline in the lower one from being sucked back into the upper one.

The churned-up soil is removed by a sand pump, which is a hollow tube with a flap valve at the lower end opening inwards and a hook on the upper end.

A light flap valve to keep out the cold in winter is also a desirable feature for the overflow pipe.

Where directly sewer-connected, they must be cut off from the rest of the building and plumbing system by a brass flap valve on the inlet to the catch basin and the trap on the drain from the catch basin must be water-supplied.