Prerostal system succeeds where deep sumps fail

Edited by the Engineeringtalk editorial team Aug 9, 2000

After a problem at the Barry Bathing Waters Improvement Scheme, a solution was found in the form of a system of pumps that automatically match output with inflow, using a system known as Prerostal.

Engineers working on the Barry Bathing Waters Improvement Scheme in Wales came up against an unusual problem when they attempted to install a new pumping station.

Designed to house a conventional submersible axial flow pump, the Kymin pumping station on the promenade at Penarth, on the Severn estuary, was intended to pump foul and storm flows to Cog Moors waste water treatment plant.

However, engineers discovered soon after digging had begun that the depth of sump required would extend below the foundations of the sea wall.

The excavations also uncovered a secondary sea defence, covered by the new wall.

It was decided that tube mounted axial flow pumps would not be suitable, because of the possibility that the integrity of the sea wall would be compromised by a deep sump.

An alternative solution was found in the form of a system of pumps that automatically match output with inflow, using a system known as Prerostal.

A benefit of this type of pumping station is that shallower sumps are required; an important consideration for the engineers at Kymin.

Newbury, Berkshire company Hidrostal was commissioned to undertake the design, supply and installation of foul and storm pumps, as well as assuming the role of principal contractor for all the mechanical and electrical engineering.

This included the odour control system, storm screen, motor control centres and all electrical work associated with the control room.

At the heart of the Kymin Pumping Station are two 500mm Hidrostal solids handling storm pumps, each fitted with a screw centrifugal impeller capable of pumping large solids without clogging.

Each pump is rated to deliver 690 litres per second at a total head of 4.7 metres when pumping against the highest astronomical tide level.

The Prerostal basin, in which each pump is installed, is the key to the space saving aspect of the installation, which requires a smaller sump volume.

Because of the design of the basin and weir, prerotation is induced into the incoming flows.

Originally developed as a modern alternative to Archimedian screws, the Prerostal system has a number of benefits including; automatic matching of output with inflow, removal of floating solids and a reduced sump depth requirement.

It was this last attribute that became so important at Kymin, because of the restricted area in which the pumping station could be built.

By reducing the stored volume of water, the overall size of the plant could also be reduced.

The entire works at Kymin pumping station is situated on the main promenade, just east of Penarth pier.

Everything is underground except for an adjacent control room.

The sumps themselves are actually underneath a park bench and a flower bed.

Now that the pumps have been installed, foul water is pumped straight to nearby Cog Moors for treatment.

When storm conditions arise, the flows pass through a storm overflow screen and into the prerotation basins.

Here, the two Hidrostal pumps discharge to an adjacent chamber which feeds into the sea outfall.

Once storm conditions have passed, another Hidrostal pump, this time a submersible sump pump fitted to a lowering device, transfers the remains of the storm basin contents back over the weir and into the foul basin.

This pumps at 8 litres per second and is activated automatically at the end of each storm.