Because steam can behave as a liquid and as a gas and everything inbetween, the way steam flows through a pipe can change through long straight runs. Two phase flow regimes are very complicated and often require specialty programs to determine how the fluid is expected to flow. For example, the liquid in very wet steam will move slower than its vapor counterpart. High flowing velocites in the vapor space might cause liquid slugging that might damage equipment or piping. If a wet steam line is flowing uphill, there could be two directional flow in the pipe as the vapor travels uphill and the liquid travels downhill. This may cause large wetspots in the line which use energy to overcome. The ideal flow regime for steam is called annular flow where the liquid is moving along the entire ID of the pipe and the vapor travels down the middle. During this flow, the flow it is very predictible and easy to operate.
Tags: Equations for Steam