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The wet sludge at around 3% dry solids (ds) is first dewatered by centrifuges which bring the solids content to around 26%. The sludge is then dried in two thin film driers using steam generated by the waste heat boiler which brings the solids up to around 36%. This allows autothermic combustion in the incinerator without having to add any supplementary fuel.

The sludge cake is pumped from the dewatering plant to the drier feed silos using positive displacement pumps operating at up to 100bar. The amount of pressure required to move the filter cake along the pipelines is very much dependent on the moisture content of the cake. Good performance of the dewatering centrifuges produces drier cake but this is more difficult to pump so greater pressure is required. Also, the abrasive nature of the cake can quickly wear the pipelines. In order to ease the flow a thin film of liquid is introduced to the pipeline through an annular ring at high pressure to lubricate the internal surface of the pipeline – this is termed boundary layer injection.

Cat Pumps’ 3CP high pressure triplex positive displacement pumps have been installed as part of the expansion contract to provide the boundary layer injection on two pipelines feeding the new incinerator. Introduced into the pipeline as close to the filter cake pumps as possible, the boundary layer helps to reduce friction loss and wear in the pipeline and reduces pumping pressures. Reducing the operating pressure makes it possible to increase the pumping distance while at the same time reducing the operation and maintenance costs. The Cat Pumps introduce and even a small volume of water at constant flow at pressures up to 100 bar in the same direction as filter cake. This flow maximises the lubricating effect without significantly affecting the dryness of the filter cake.

Two sets of Cat Pumps are provided along the length of the pipeline to first provide the boundary layer as close as possible to the pump and then, dependant on pressures, to re-establish the layer at a point further along the pipeline.  The pumps used for the boundary layer system in the sludge dewatering plant do not operate continuously. They are activated only if the pressure rises to 85 bar and will continue running until the pipeline pressure drops back to 60 bar.

Cat Pumps’ 3CP triplex plunger pump is well suited to providing the liquid input for the boundary layer system as it delivers the smooth liquid flow that is necessary for it to be effective. Containing three synchronised plungers, the compact 3CP produces a low-pulsation flow that is sufficiently smooth for most applications. The flow is highly stable and predictable because the flow rate is determined almost exactly by the running speed of the pump and is virtually unaffected by back-pressure, liquid viscosity or specific gravity.

The decision to introduce and install additional boundary layer injection system in the Shell Green plant was made on the grounds that there was a significant risk that pumping pressures would otherwise be too high to allow the rate of transfer required to feed the new incinerator, or that cake dryness would have to be sacrificed at the cost of losing the ability to burn autothermically.

Currently the second set of pumps have not been required to operate, but it is anticipated that when a new treatment process comes on line at one of the feeder sites at Davyhulme the centrifuges will produce a drier cake of up to 38% which will make it more difficult to transport the dewatered sludge cake along the pipelines. The second set of pumps installed at Shell Green will then provide some insurance that the feed rates can still be achieved.

The pumps’ contribution to keeping the dewatered sludge flowing into the incinerator plant in the multi-million pound S3JV project should not be underestimated. Their specification demonstrated Cat Pumps’ ability to work to Water Industry Standards, such as WIMES. The operating efficiency of the plant relies very much on the continuous availability of the dewatered sludge and in providing effective boundary layers in the incinerator feed pipelines, the Cat Pumps are making a valuable contribution. The contribution that Cat Pumps’ appreciation and understanding of the application and ability to provide a highly effective solution has not gone unnoticed by United Utilities who have commended us on our attention to detail.