It is a valve used to prevent the machine to operate with capacities lower than the minimum one.

If the machine operate in vacuum the valve shall be installed before the suction mouth

If the capacity is lower than the admissible limit, the valve opens and takes air from outside.

If the machine works as turbo blowers the valve shall be installed after the delivery mouth.

If  the delivery pressure exceed the limit, the valve open and allows to discharge the fluid outside.