A modular liquid ring pump has a liquid ring overload protection system including a passage from a working chamber directly to the pump discharge passage and a mechanical relief valve configured to release liquid from the working chamber during compressor overload. The liquid ring pump, when configured to have two stages, has an inter-stage by-pass system that includes an opening in an inter-stage passage and a pressure sensitive mechanical valve that allows the discharge of a first stage compressor to flow directly to the pump discharge at start up or during low pressure operation. The liquid ring pump's modular construction may be easily configured from a single stage pump to a two-stage pump and vice versa by using the same bearings, head, and drive system and only changing the body, cone, and rotor.