A vacuum pump with a pressure relief valve

A vacuum pump comprises at least two rotors each comprising at least one lobe 10. At least one of the lobes 10 comprises an inertial pressure relief valve, which comprises a valve body member 20 mounted to move between a closed position in which it obstructs a fluid flow pathway 30 between leading and trailing sides of the rotor and an open position in which the fluid flow pathway is not obstructed. The body 20 is moved between the open and closed positions by centrifugal force, i.e. in response to the rotational velocity of the rotor. An increased pressure at the inlet will result in increased flow rate and loading, which reduces the speed, and allows the pressure relief valve to open. The valve may be defined by a tip member which is biased away from the stator wall, but is urged into sealing contact by centrifugal force at normal operating speeds.