Analysis of pumping systems for the cooling of underground transmission lines

Abstract

Various pumping arrangements and their pressure profile control for forced cooling of long pipe type transmission lines were investigated. In order to overcome the extensive friction head losses and provide ample cable cooling, a number of pump stations has to be used. Since the inner line segments cannot be provided with pressure control head tanks, line blockages, flow resistance changes, flow rate changes, pump shutdowns, or other imbalances in one segment can alter the pressure profile along the entire line, and, when two head tanks are used, create transverse flow. Using experimental and analytical methods, it was determined that the pump - relief valve combination operating as a constant flow source is superior to the pump - relief valve combination operating as a constant pressure source, and that the configuration consisting of an even number of loops, each loop having the opposite flow direction from its neighbor's, is the best solution when operated with only one pressure control head tank. The simplest, and yet effective, line pressure profile control appears to be the pump bypass, which could be easily implemented on existing installations. The head tank pressure adjustment, however, is the most effective line pressure profile control scheme, and should be considered when a new system is being designed. From the analysis performed on an electric analogy model it was found, that the head tank pressure adjustment or the pump bypass would be sufficient to mainain the line pressure profile within its working limits for all practical imbalance sizes, and that, to extend the range of either of these line pressure profile controls, the emergency pump shutdown and the pump bypass itself should be based on the pump discharge, rather than the pump inlet pressure.