The present invention relates to a pressure independent commissioning unit for connection to for example a heat exchanger comprising a H-body bypass arrangement having primary and secondary inlets (12A; 10A) and outlets (12B; 10B), forming the sides of the H-body and communication between them via a bypass (13) forming the centre of the H-body; containing: - a bonnet mechanism (18) connecting the primary inlet (12A) and the bypass (13), - a pressure independent valve connecting the secondary inlet (10A) to the bypass (13), wherein the flow path through the outlets (12B; 10B) and inlets (12A; 10A) can be controlled by rotation of the bonnet mechanism (18) and closing the pressure independent valve.