Special Considerations for Protecting Very Long Transmission Mains from Hydraulic Transients

Abstract

Hydraulic transients are created during a change in system operation related to flow and pressure. It is typically a temporary unsteady state condition that every system experiences in between initial and final steady state conditions. Typical system operations such as pump starts/stops, valve opening/closing create unsteady state conditions creating pressure waves that propagate through the system. These created pressure waves are the mechanism that hydraulic systems utilize to go from a one hydraulic state to another. The propagating pressure waves create both high- and lowpressure events that can be detrimental to a transmission system. High pressures can create damage to pumps, valves, and other system appurtenances, while low pressures can lead to column separation, cavitation, and subsequent failure of pipelines. Most transients can be controlled by adhering to and implementing operational strategies that are geared towards transient control, such as utilizing procedures for controlled startup and shutdown during normal operation. But events that cannot be controlled by operational strategies, such as power failures or pump failures, need automatic devices that can control the magnitude of and minimize the impact resulting from such transient events. The control of hydraulic transients in very long transmission mains could require several additional considerations in analysis as well as in sizing devices. Even adhering to and implementing established operational strategies may also require several additional considerations and in-depth analyses. This paper will discuss two case studies related to analysis, design and modification of hydraulic transient controls for very long transmission mains with emphasis on operational modifications to existing components. CASE STUDY 1: Case Study 1 is for a proposed expansion of a raw water intake pump station located in the southern United States. The station will be expanded in stages from present capacity of 24-mgd to a capacity of 40-mgd and then to an ultimate capacity of 55mgd in the future. All capacity increases need to be handled by the existing 30-mile long, 48-inch diameter transmission main. The existing surge protection consists of