Abstract: Vacuum pumping apparatus includes a scroll set having an inlet and an outlet. The scroll set includes a first stationary scroll blade and a second stationary scroll blade extending from a stationary plate and separated by a gap, and an orbiting scroll extending from an orbiting plate, wherein the first and second stationary scroll blades are intermeshed with the orbiting scroll blade to define one or more interblade pockets. The vacuum pumping apparatus further includes a relief port in the gap between the first and second stationary scroll blades and coupled through a relief passage to an exhaust, a relief valve in the relief passage, and a drive mechanism operatively coupled to the orbiting scroll element for producing orbiting motion of the orbiting scroll blade relative to the first and second stationary scroll blades.