Chapter 6 - Fundamentals of Hydraulic Transients

Abstract

This chapter provides an overview of the problems caused by hydraulic transients and insight into the circumstances that make a more thorough analysis necessary. Hydraulic transients are the time-varying phenomena that follow when the equilibrium of steady flow in a system is disturbed by a change of flow that occurs over a relatively short time period. Some of the primary causes of transients are valve movements (often), flow demand changes (rarely), controlled pump shutdown (rarely), pump failure (often), pump start-up (rarely), air venting from lines (often), failure of flow or pressure regulators (rarely), and pipe rupture (rarely). The analysis of unsteady flow in pipe systems is generally divided into two major categories that include rigid water column theory (surge theory) and elastic theory (water hammer). Methods of analyzing pipelines for the effects of hydraulic transients, with or without various means of controlling them or reducing the severity, include graphical, arithmetic, algebraic, method of characteristics, finite element, and implicit differentiation. Water hammer could occur in a pipeline flowing full when the flow is increased or decreased, such as when the setting on a valve in the line is changed. When a valve in a pipeline is closed rapidly, the pressure on the upstream side of the valve increases, and the pulse of increased pressure travels upstream at the elastic wave speed. This pulse (called an “upsurge”) decreases the velocity of flow. Downstream from an inline valve, the pressure is reduced and the wave of decreased pressure travels downstream, also at the elastic wave speed.