A modular steam injection line, for use in SAGD operations for delivery of an equal steam mass flow along a length of the apparatus, incorporates steam splitter modules fluidly connected for forming the steam injection line. Each of the modular steam splitters is fit with interchangeable nozzles for delivering steam to the formation. The interchangeable nozzles have orifices of different sizes and the nozzle orifice size required for each individual module to deliver an equal mass flow of steam from each module along the entire length of the steam injection line is calculated based on steam conditions at each module so that each module can be fit with nozzles having the appropriate sized orifices.