The cracker is installed inside a vertical tank at the inlet and outlet and serves for loading and unloading oil products and for additional protection against possible leakage of oil products from the vertical tank in case of a faulty pipeline. The clapperboard consists of a body with an oblique cut and a tightly fitting lid connected to the body by a lever mechanism. When filling a vertical tank, a stream of oil product by force of pressure lifts the lid of the cracker. When pumping is stopped, the lid of the cracker, under its own weight, falls into place, closing the pipe. When dispensing oil from a vertical tank, the lid of the cracker opens forcibly by means of a rotating drum with a rope winding around it.