The foam extinguishing agent of the plant fire department of a large mineral oil company is stored in underground tanks. It has to be circulated once a month in order to ensure constant quality. This is done by means of the Börger rotary lobe pump. The Börger pump fills the foam extinguishing agent tanks of the fire trucks in case of fire. The Börger rotary lobe pump is equipped with a Variocap. It offers efficient overpressure protection regardless of the flow direction of the pump.