Document Type

Report

Publication Date

January 1981

Abstract

The Fielding Ditch Company Pipeline is almost 3 miles long and supplies irrigation water under low pressure to adjacent fields through 33 turnouts along its length. The 24-inch non-reinforced concrete pipeline began to experience repeated structural failures soon after it was placed in operation. This study was done for the Soil Conservation Service by the Utah Water Research Laboratory to gather field data on the pipeline operating characteristics, to analyze the hydraulic transients in the pipeline with the help of a computer simulation model, and to suggest modification to protect the pipeline from future failures caused by transient pressures. Following a description of the pipeline system, the concepts and principles of unsteady flow in pipelines are summarized. Then the general equations for transient flow are presented followed by a summary of their solution using numerical methods. Under the field verification data collection program, instruments and recorders were set up at four locations along the pipeline. Pressure and flow measurements during both steady and unsteady flows were recorded to obtain data on the operating characteristics of the pipeline. These field data as well as preliminary analysis indicate that moderate closure times of valves could generate pressure waves which could overstress the non-reinforced concrete pipe. The field data also provided a way to verify that the computer simulation model could truly represent the behavior of the actual pipeline system. The field data also shoed the pressure wave speed to be about 1170 feet per second rather than 3640 feet per second predicted by the wave speed equations. This significant change in wave speed was attributed to the effect of free air trapped in pipe joints and high spots in the pipeline. Seven increasingly complex computer models were developed to represent the pipeline. The first was a simple basic water hammer program for a pipe with a reservoir upstream and a valve at the downstream end which could close instantly. Later programs added the effects of air pockets along the pipeline, damping or dissipation at the air pockets, gradual closure of the downstream valve, gradual closure of a valve at an interior point, simultaneous closure of two valves and provision for protective standpipes at any or all interior valve locations. Comparisons of the final programs with field data showed the system to be adequately represented. The computer programs were then used to compare the effectiveness of various proposed protective modifications to the pipeline. Modifications considered but not recommended included requiring a longer valve closure time (not fail safe), installation of pressure relief valves (not reliable), and installation of air chambers at each valve (not economical). The recommended pipeline modification was to install eleven 18-inch pressure relief standpipes at selected interior turnout locations and one 36-inch standpipe at the downstream end. The study showed that the spillage at the standpipes to an acceptable amount. Smaller 2- or 3-inch diameter standpipes should also be installed at all other turnouts to release trapped air and to serve as indicators (to nearby valve operators) or too rapid closure of the valves.