Effect of air mass fraction on transient pressure and structural integrity assessment in a relatively long cast iron pipe

A numerical model was developed to study transient flow in air–liquid mixtures in quasi-rigid pipes. Hydraulic transient was generated in a relatively long cast iron pipe by the fast closure of the downstream valve. The presented model is based on the homogeneous flow model approach which considers the two-phase flow as a mixture without the assumption of a non-interface exchange. The method of characteristics was used to solve the set of partial differential equations. To validate the numerical model, experimental data from literature were used. The pressure wave propagation was analyzed through the pipe to understand the behavior of the air–liquid mixture under pressure disturbances. The value of the air mass fraction was also varied to discuss its effect on transient pressure. Obtained results revealed that the increase of the air mass fraction in the operating liquid may contribute to decreasing the maximum transient pressure. Based on the thin-walled hollow cylinder assumptions, the maximum stresses in the pipes were calculated for different values of the air mass fraction. Obtained results were used to conduct a structural integrity analysis of the cast iron pipe with a semi-elliptical crack. A Failure Assessment Diagram (FAD) was obtained based on the SINTAP method to calculate the safety factor for different values of the air mass fraction. It has been observed that the safety factor varies with the presence of the gaseous phase in the operating fluid.