How to Estimate Flow Area Reduction and Excessive Roughness Effects in Aged Pipelines

Abstract

Accurate hydraulic analysis and head loss calculations are essential for conveyance pipeline design. Flow modeling tools significantly expedite the hydraulic analysis steps, but estimating nominal pipe characteristics may result in considerably erroneous results. This paper presents critical design requirements to estimate flow area reduction and quantify excessive roughness effects in aged pipelines. Correction factors shall be applied to the Hazen-Williams equation for turbulent flow to reflect the actual flow area and corrected roughness coefficients. Two scenarios of an actual design will be presented to illustrate the order of magnitude of erroneous results when the design is based solely on nominal flow area and basic roughness factors. In addition to defining extreme conditions, this paper describes experimental methods, boundary conditions, and field measurements required to develop hydraulic characteristics specific to aged systems. This paper also presents general corrective actions required to improve hydraulic performance of existing systems. DESIGN FUNDAMENTALS AND OBJECTIVES For systems with high dynamic losses (e.g. long length), analysis and modeling should be based on the actual pipe inner diameter and roughness. For example, the Hazen-Williams equation is a popular method for calculating friction losses using either hand calculations or hydraulic model developments. Traditional equations (Streeter 1985) define “d” as pipe diameter without correction factors for actual conditions. The Hazen-Williams equation for turbulent flow is: