A separator arrangement in a multiphase production system. A closed separator container includes a top and a bottom, an inlet for production fluids at an inlet level and at least one outlet for at least partly separated liquid at a level lower than the inlet level. The container outlet is connectable to a liquid pump arranged downstream of the container. A gas accumulator is arranged for by-passing the pump. The gas accumulator includes an inlet end connectable to a top region of the container and an outlet end connectable to a discharge side of the pump for flow communication between the container and the discharge side of the pump. A region near the inlet end the gas accumulator is connected to a supply of injection liquid that is controllable via a valve to be injected to the discharge side of the pump via the gas accumulator. A method for gas by-pass of a liquid pump in a multiphase production system.