The Relevance of Time Domain Effects for the Design and Stability of Hydraulic Transport Pipelines
Type:
Presented during:
CEDA Dredging Days 2021
Authors:
E. de Hoog, A.M. Talmon, and C. van Rhee
Abstract
Traditionally, the design of a hydraulic transport system entails analyzing energy sinks and sources of a pipeline system. More specifically, the energy characteristics of the pump and drivetrain are weighted against the frictional energy losses of the slurry in the pipeline. From this follows an operating velocity for the system, which should be within safety limits to ensure flow: the critical velocity and the vacuum limit. The designer assumes that any variations in mixture velocity and density, in time and in space, do not affect the aforementioned methodology. However such steady flows are only possible in laboratory circuits.
Furthermore, the empirical models of the pump characteristics and frictional losses are attained under steady conditions, where any type of slurry dynamics are damped out over time. Under field conditions, in dredging- and mining pipelines, slurry dynamics are caused by fluctuating loads on the centrifugal pump, due to temporal variations of the mixture density flowing through the pump. These dynamic loads, if not corrected for, result in dynamic driving forces, expressed as mixture velocity variations. This specific interaction with the slurry and centrifugal pump(drive) can cause instabilities in the form of growing density waves, especially for long pipelines, which further increases dynamic loads on pumps and makes the system unstable. These growing density waves have been observed in pipelines even when operating far above the critical velocity. Analyzing these time domain effects, gives the designer the ability to investigate whether a system is at risk to become unstable in this manner, and to design pump drives and feedback control algorithms to deal with high dynamic loads.
This article discusses which time domain effects are relevant for such a temporal design methodology, and discusses methods to study these effects with a 1D CFD model, to maintain flow assurance in the pipeline.
Keywords: Hydraulic Transport, Slurries, Flow Assurance, Density Waves, Transient Analysis.