Discussion of “Analysis of Geometrical Relationships and Friction Losses in Small-Diameter Lay-Flat Polyethylene Pipes” by Giuseppe Provenzano, Vincenzo Alagna, Dario Autovino, Juan Manzano Juarez, and Giovanni Rallo

For a lay-flat drip line with an inner nominal diameter dn = 16 mm, characterized by three different pipe wall thicknesses, WT, 6, 8 and 10 mil, the Authors of the original paper very accurately investigated the geometrical expansion of the cross-sectional diameter as a function of different pressure head values, ranging in between 0.84-17.55 m. Results obtained by the Authors, for pressure heads not exceeding a threshold pressure value, showed significant d variations, in a very narrow range of pressure heads, which however is seldom considered in practice (less than 3 m). The Authors experimentally evaluated a modified Blasius’ law friction coefficient, c, equal to 0.285, corresponding to 10 % lower than that of the well-accepted Blasius’ coefficient (0.316). The discusser is of the opinion that an experimental lower c value should be very carefully estimated and verified before being adopted in practice, especially given that an erroneous friction coefficient underestimation in lateral designs leads to reductions in operating flow rates. The procedure allowing the Authors to proceed from the experimental friction loss observations to the friction coefficient can be improved as described here. To study the law of resistance, and the effect the associated pressure head variations (which diminishes along the pipe flow) on the inside diameter variation, the latter could be considered according to Blasius’ friction factor. This assumption is equivalent to approximating the non-uniform (accelerating) pipe flow process actually occurring in a “converging” pipe, by considering it as a continuous sequence of uniform states. In conclusion, for the considered lay-flat Polyethylene pipes (dn = 16 mm), when experimentally evaluating the friction coefficient c of the Blasius’ law, if one wishes to investigate a range of inside diameter variation where a very low pressure has been experimentally observed, it is advisable to use the extended formulation, especially for pressure heads not exceeding a threshold pressure value, for which relative errors in hf, which reflect in c estimation, can be up to -30 %, three times greater than the Authors’ correction.