Articles & Case Studies

Wemco grit and sludge pumps measure up in wastewater treatment

Posted: Tuesday 7th May 2013

Pump blockages resulting from large quantities of grit, debris and fibrous matter is a problem that operators of wastewater and effluent treatment plants face almost daily. For many it is a problem that is on the increase. The fouling of pump impellers is a longstanding problem that costs companies a lot of money in terms of plant downtime, pump repair and even complete pump replacement.

Where grit and sand are encountered, the pump can literally grind itself to pieces. Employing pumps that can withstand abrasive materials and deliver trouble-free performance over long periods should be the main consideration during the specification process. At United Utilities’ Sandon Dock wastewater treatment plant Wemco M4C torque-flow pumps have provided low maintenance, trouble-free performance in handling grit and transferring sludge for many years.

The current water treatment works at Sandon Dock, Liverpool were commissioned in 1991 and it is the second largest wastewater treatment plant in the North West of England. Receiving flows from the 29km Liverpool Interceptor Sewer, the plant is capable of treating up to 950million litres, or 11,000l/sec, of sewage a day.

The removal of grit at the wastewater intake stage is where the Wemco pumps are first employed. Here the pumps in an ‘operating and standby’ configuration run 24 hours a day, delivering a flowrate of 47m³/hr and head of up to15m in order to remove the settled grit from the four wastewater intake tanks. In spite of the considerable and aggressive solids content, United Utilities reports that the only maintenance that these grit pumps require is the occasional replacement of the mechanical seals.

Further along in the treatment process, the grit-free wastewater is transferred into six settling tanks where it is held for up to 10 hours to allow the agitator blades to move solids to the bottom where they settle as sludge. Each tank is equipped with three Wemco M4C pumps (Fig. 1) for the purpose of removing the sludge from the tanks and transferring it to the dewatering plant. These pumps are virtually identical to the models used for grit transfer and maintenance only involves routine oil changes, plus the occasional mechanical seal replacement. Maintenance is easy and can be undertaken in situ as the pumps are simple to strip down, due to the pump design which allows the casing to be removed from the front end and giving ready access to all internal components. In the decade that the sludge pumps have been operating, no refurbishment work has been performed on the impellers.

The Wemco Model C torque-flow pump has the ability to withstand the harshest of conditions due to its hydraulic vortex pump technique which is based on the principle of a whirlpool. The vortex is created by the rotating recessed cup-shaped impeller extending into the suction line, drawing the liquid / solids into the pump and then quickly through the discharge, whilst minimising the fluid / solids contact with the impeller and volute (Fig.2). This simple design allows solids to pass through the pump without choking the impeller, so considerably reducing pump wear and the potential for blockages.

“The fully recessed cup-shaped impeller and free passageways within the pump casing offer almost total freedom from blockages when handling rags and fibrous solids typically encountered in waste water treatment plants,” comments AxFlow’s Wemco pump manager Mark Redgrove. “The standard option of High Chrome Iron for the wet end, the hydraulic design and thickened wear sections provide a more than adequate combination to deal with grit and silica sands carried into the treatment works.”

With radial impeller design pumps, grit is discharged at high velocity against the pump casing or the wear ring. Unlike radial impeller vortex pumps, the cupped impeller action of the Wemco pump deflects material away from the case and back into the flow stream towards the extra-thick sacrificial suction piece. Behind the impeller is a High Chrome Iron wear plate which can withstand the grinding action of grit. To further maximise performance and minimise wear, the large diameter impeller design is the key to pump’s considerable durability and non-clogging performance.

Because the suction end of the pump is exposed to the greatest abrasive wear, Wemco designed the pump case in two parts. The replaceable suction piece is sacrificial, even though it is designed for long wear life. Replacing it is an easy operation involving breaking the suction flange, removing the slotted bolts and then installing the new suction piece. The job can be performed in just a few hours and is considerably cheaper than a completely new pump case.

“The WEMCO Torque-Flow range comprises various configurations, including close-coupled, horizontal or vertical mounting, cantilever designs and a small range of submersible units,” says Mark Redgrove. “As a result, we can satisfy many of the challenging wastewater and effluent pumping applications that face both the water utilities and companies which generate large volumes of solids-borne waste liquids.”

AxFlow has also recently introduced the Wemco WSP, a heavy duty self-priming pump that is equally at ease with clean and solids laden liquids. This new pump is seen as having great potential in the water and wastewater treatment sector where self -riming pumps are ideally suited to any duty with a suction lift where ease of access to the pump is necessary for maintenance purposes.

The Wemco WSP pump contains an open type, two vane impeller manufactured in a choice of cast iron or CD4MCu. The wear plate is also made from cast iron and the casings are in ductile iron. It is this build quality that enables the pump to handle both clean and aggressive fluids, the solids handling being excellent for this range with the maximum for a solid sphere being 38mm on the 50mm pump and 75mm on the 250mm pump.

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August 2021

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