The paper presents the analysis of fluid–structure interaction occurring during hydraulic transients in pipe coils by means of a four-equation model. The purpose is to understand and describe the effect of the coil movement on the transient wave. The four-equation model couples the transient pipe flow with the axial movement of the pipe-wall neglecting radial inertia, flexure and torsion of the piping system. Three models have been developed: the first simplifying the coil to a straight pipe with a moving valve, the second model assuming independent axial deformation in each coil ring, and the third model that includes dry friction dissipation. The first allows an easier generalization of the method; however, it does not allow the complete description of the phenomenon; the second and third models fit better the experimental observations in the pipe coil facility, suggesting that its behaviour is dominated by the independent movement of the rings.