Long period analysis of transient pressure signals for in-line valve checking.

In this paper the results of laboratory tests and 1-D numerical model concerning transients in a high density polyethylene pipe, in which a partially closed in-line valve is installed, are discussed. Experimental tests are carried out at the Water Engineering Laboratory of the University of Perugia, Italy. Transients are generated by a fast and complete closure of a valve placed at the downstream end section of the pipe. The pressure signals – i.e. the time-history of the pressure – are analyzed in the time domain. Notwithstanding the many practical applications of the interaction between a pressure wave and an in-line valve, such a phenomenon is poorly understood. The analysis of the pressure signal in the first phase of the transients (short period analysis) has already been presented by the authors [1]. In the present paper the analysis is extended beyond the first characteristic time of the pipe (long period analysis). Specifically, the aim of the paper is to show that the damping of pressure signal depends on the initial discharge and the opening degree of the partially closed in-line valve, for a given pipe. In order to evaluate the effect of unsteadiness and viscoelasticity as well as to point out the role played by local head losses during transients, a 1-D numerical model has been developed and tested. Firstly, to evaluate model parameters – particularly those describing the viscoelastic behaviour – the analysis has been focused on transients in a straight pipe. The second phase of the numerical modelling has been devoted to transients in pipes in which a partially closed in-line valve is installed. Such results are compared to the experimental data.