Reference diameter for determination of OHTC

The U-value of a pipe is often defined as above although the definition may differ from one engineering discipline to another. Insulation manufacturers calculate the U-value at the outer surface of the steel wall, excluding any contribution from internal convection, external convection, and conduction through steel. Pipeline designers typically use the inner diameter of the pipe to define the reference diameter and may exclude any contribution from convection.

The definition of the U-value also differs from one simulation tool to another. In the one-dimensional dynamic multiphase flow simulator OLGA the U-value is based on the inner diameter of the steel wall; user-specified U-values must include the contribution from internal convection. In the one-dimensional steady-state multiphase flow simulator PIPESIM, the U-value is based on the outer diameter of the steel wall; the inside film coefficient can be included in the specified U-value or calculated separately and added to the U-value.

For cables, heat sources and PAC/GIL, we consider $D_{ref}$ to be the diameter $D_{wall}$, same as the insulation manufacturers do.