When there was a lack of computing power, several approximate formulas (Weymouth, Panhandle, AGA, etc.) were developed to obtain pressure drop in gas pipelines, which are in many instances still being used. As it is well-known, they can be sometimes grossly inaccurate, necessitating the addition of an arbitrary parameter ("pipe efficiency") for each case. The right answer, now that we have computers and numerical integration methods, is to perform integration of the mechanical energy balance at least, if not both the mechanical and the overall energy balance when possible. While we advocate the numerical integration to obtain pressure drop, sometimes hydraulic calculations are embedded in several application procedures (pipe design, leak detection, compressor station operating optimization, etc.), and they require algebraic expressions to be used. We investigate the use of existing approximate formulas, a procedure to adjust the "pipe efficiency" for a given set of conditions, the building of new nonlinear surrogate models, and a Quadratic Metamodel amenable to nonconvex optimization procedures, as opposed to the current rational formulations.