Investigation into fine non-cohesive sediment removal by Swirl/Vortex settling basins at small river abstraction works

Kiringu, Kuria (2020-03)

Thesis (PhD)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: The often fine non-cohesive and cohesive nature of sediments in South African rivers makes sediment management at river abstraction works rather complex. Sediment removal at river abstraction works is essential for the protection of the pumps and pipelines. A wide range of sediment control design guidelines for large abstraction works are available, but these are not applicable for small abstraction works with a duty pump capacity of less than 100 l/s (7.2 Ml/d at 20 h/d), which is typical for rural potable water schemes in Africa. For sediment removal of fine non-cohesive sediment at small abstraction works, Vortex settling basins (VSBs) offer a promising alternative to conventional sediment settling structures such as sand traps, settlers, hoppers with jet pumps or primary settling tanks. VSBs have a small footprint, no moving parts, require no chemical dosing and continuously flush sediment back to the river. This study seeks to furnish the hydraulic designers with parameters for an optimized design of a VSB. Numerous computational fluid dynamics (CFD) model simulations were carried out using the software package ANSYS FLUENT and validated against two physical VSB models: 0.48 m diameter and 0.7 m high, as well as 0.68 m diameter and 1.0 m high. These tests indicated that non-cohesive sediment removal in a VSB is mainly gravity driven and centrifugal forces play an essential role in keeping particles in suspension near the outer wall, thus increasing residence time. The inlet velocity, the diameter and height of the VSB, underflow, deflectors, sediment size and concentration, the location and type of outlet structure all play important roles in controlling the sediment trap efficiency. The cone angle and the angle of inlet effects are minimal. The following design ratios are recommended: Underflow(Qu) Inflow(Qi) = 0.05-0.10, position of inlet(Hi) cylinder height (Ht) = 0.50-0.88, Cylinder height (Ht) cylinder diameter (D) >0.5, Cylinder diameter(D) Inlet diameter (Di) = 8.2 and inlet velocity of 0.26 m/s. Deflectors of length = Di extending 180° clockwise and 70° anticlockwise, inclined at an angle 1:2 (H:V), just above the inlet were found to give maximum efficiency combined with a rectangular central outlet length =1.28 Di, width = Di and height = Di, located at 180° opposite the inlet. With these findings two VSB designs are proposed: (a) for an inflow of 5 l/s with 5% water loss at a 99% trapping efficiency for sediment particles as small as 75µm in diameter and maximum inflow sediment concentration of 10,000 mg/l, and (b) for an inflow of 10 l/s with 8% water loss at a 91% trapping efficiency for sediment particles 75 µm in diameter and maximum inflow sediment concentration of 10,000 mg/l. A possible river abstraction works layout incorporating VSBs is suggested for abstraction discharges smaller than 100 l/s for use by rural small local authorities for potable use.

AFRIKAANSE OPSOMMING: Die aard van fyn nie-kohesiewe en kohesiewe sediment in Suid Afrikaanse riviere maak sediment bestuur vir water onttrekking uit riviere gekompliseerd. Verwydering van sediment vir wateronttrekking uit riviere is belangrik vir die beskerming van pompe en pyplyne. ‘n Wye reeks sedimentbeheer riglyne vir groot wateronttrekking skemas uit riviere is beskikbaar, maar dit is nie die geval vir riglyne vir klein wateronttrekkingskemas (kleiner as 100 l/s of 7.2 Ml/d) nie – soos vir tipiese wateronttrekking hoeveelhede in die platteland in Afrika. “Vortex” besinkings-bakke (VSB’s) is ‘n aantreklike alternatief vir konvensionele sediment besinking-strukture soos “sand traps”, “settlers”, “hoppers” met “jet pumps”, of primêre sedimenttenke. VSBs het ‘n klein omgewingsvoetspoor, het geen bewegende dele nie, benodig geen chemikalieë nie en spoel voortdurend sediment terug na die rivier. Hierdie studie poog om ontwerpriglyne vir VSB’s daar te stel. Verskeie “CFD” model simulasies is in hierdie studie uigevoer met die numeriese model “ANSYS FLUENT” en gekontroleer teen twee fisiese VSB modelle: 0.48 m diameter met ‘n hoogte van 0.7 m, asook 0.68 m diameter met ‘n hoogte van 1.0 m. Die resultate van die toetse het aangedui dat nie-kohesiewe sediment verwydering in ‘n VSB is hoofsaaklik gravitasie gedrewe en sentrifugale kragte speel ‘n belangrike rol om die partikels in suspensie te hou naby die buitewand van die VSB om so die “residence” tyd van sediment te verleng. Die inlaat snelheid, diameter en hoogte van die VSB, onderuitlaat, deflektors, sedimentgroottes en konsentrasie, posisie en tipe uitlaat struktuur speel ‘n belangrike rol in die sediment besinkings- effektiwiteit. Die invloed van die keëlhoek en die hoek van die inlaat is minimaal. Die volgende verhoudings word voorgestel: onderloop(Qu) inlaat vloei(Qi) = 0.05-0.10, posisie van inlaat(Hi) silinder hoogte (Ht) = 0.50-0.88, silinder hoogte (Ht) silinder diameter (D) >0.5, silinder diameter (D) inlaat diameter (Di) = 8.2 en ‘n inlaat snelheid van 0.26 m/s.Die studie het getoon dat deflektor lengte = Di wat 180° kloksgewys en 70° antikloksgewys verleng is, met ‘n helling van 1:2 (H:V) net bo die inlaat, die maksimum effektiwiteit gee as dit gekombineer word met ‘n reghoekige sentrale uitlaatlengte van 1.28 Di, wydte = Di and hoogte = Di, geplaas 180° oorkant die inlaatVanuit die studie se bevindinge word twee VSB ontwerpe voorgestel: (a) met ‘n inlaat vloeitempo van 5 l/s met 5 % waterverliese met ‘n 99 % sediment besinkings-effektiwiteit vir sedimentpartikels tot so klein as 75µm in diameter en maksimum inlaatvloei sediment konsentrasie van 10,000 mg/l, en (b) met ‘n inlaat vloeitempo van 10 l/s met 8% waterverliese met 91 % sediment besinkings-effektiwiteit vir sedimentpartikels 75 µm in diameter en maksimum inlaat vloei sediment konsentrasie van 10,000 mg/l. ‘n Potensiële VSB geïnkorporeerde uitleg word voorgestel vir wateronttrekking uit riviere vir vloeitempo’s kleiner as 100 l/s.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107751
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