Abstract
This article deals with the design of concrete coated steel aqueducts interconnected with aqueducts with bituminous coating. The galvanic current generated by interconnecting the two aqueducts with different coatings is shown in a specific case. The proper method for isolating these aqueducts is discussed as well as the results of trying to isolate the two pipe lines by installing insulating joints within a network of interconnecting pipes. Test data are given on forced drainage on these incorrectly placed insulating joints in order to reverse residual galvanic currents in the pipe network. The reason operating experience should not overrule recommendations based on electrolysis investigation is given. The AC method, developed for locating accidental short circuits in control piping, conduits and other metallic connection is given. A comparison based on soil resistivity is made between the Mokelumne Aqueducts and the Hetch Hetchy Aqueducts in galvanic currents generated by interconnection. Precautions in the form of a soil survey were taken by the Hetch Hetchy engineers. Insulating joints were installed in their first aqueduct at strategic points. Test data are given on these insulated joints after 17 years of service. Good results obtained by following recommendations based on electrolysis surveys for the pipe coating have been well justified. Accidental short circuit between the two Hetch Hetchy Aqueducts at the San Joaquin Valve House was discovered by the preliminary electrolysis survey after the installation of the Second Aqueduct. The misconception of some engineers that a welding machine grounded to one aqueduct may be used just as effectively on the adjacent aqueduct without ill effect has resulted in much disregard for the recommendations made by the electrolysis engineer. Recommended precautions are set forth in the conclusion of the paper.
