Proceedings

A study of the optimisation of the Fort Cumberland storm tank using Computational Fluid Dynamics

Egarr, D.A.1, Duda, A.1, Potter, S.2 and Heywood, S.3, 1MMI Engineering Ltd, UK, 24Delivery Limited, UK, 3MWH, UK

(free)

Abstract During periods of heavy rainfall, Eastney WPS diverts flow to the Fort Cumberland Storm Tanks which has a capacity of 40,000 m3. Currently, flow passing to the Storm Tanks passes through 6mm 2D band screens to remove solids. The screens have been subject to excessive solids loading caused by the first flush effect within Portsmouth’s relatively flat sewer.

The Storm Tank is divided in to four blind tanks connected by high level overflow weirs. Each tank is sub divided into a number of lanes by dwarf walls which were intended to channel the fluid from Vac-Flush chambers. The Vac Flush equipment has never been commissioned.

The tanks and equipment at Fort Cumberland are to be re-configured so that all flow passes through all of the tanks before it is able to pass through the band screens. All tank compartments are to be connected by creating low level openings in the inter tank walls. The aim is to pre-settle the storm sewage before it is presented to the screens, so reducing the peak solids loading that the screens need to handle.

Two alternative methods of clearing settled solids from the tank floor at the end of a fill / empty cycle were considered:

1. Hydro-ejectors, which pump flow around the tank as the tank is emptied &

2. Re-commissioned Vac Flush chambers, which would release stored volumes of water into the empty tanks from chambers at their upstream ends causing deluge flushing of the tank floor.

This paper describes a programme of work in which the proposed modifications at Fort Cumberland were analysed and optimised using Computational Fluid Dynamics. The main areas of work were:

1. Estimation of the solids distribution through the storm tank for different positions of the openings in the dividing walls in order to optimise the tank configuration.

2. Assessment of the effectiveness of hydro-ejectors at suspending settled solids.

3. Assessment of the effectiveness of the vac-flush systems at clearing settled solids from the tank floor at the end of a filling and emptying cycle.

Keywords: Computational Fluid Dynamics, Numerical Modelling, Storm Tank, Hydro-ejectors, Vac-Flush System

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