Articles & Case Studies

Cat Pumps provide boundary layer lubrication for dewatered sludge pumping

Posted: Friday 25th February 2011

United Utilities’ digested sludge recovery centre at Shell Green, Widnes is a critical element in the safe and efficient disposal of waste water sludges from the Manchester and Liverpool area. The existing plant, completed around 1998, is currently approaching the end of a major extension programme, which will allow greater volumes of sludge to be treated. At the heart of the investment programme is a major upgrade to the existing sludge dewatering process involving the replacement of the existing filter plate pressing technology with the installation of high performance centrifuges and the provision of a third incinerator to increase incinerator capacity.

The original plant employed filter presses which were designed to dewater the sludge to a level of dryness which allowed the incinerator to operate autothermically. However, the desired cake dry solids proved difficult to achieve and required significant amounts of coagulant and polymer dosing. The presses also required high levels of operator and maintenance attendance and were run on a batch system. Variances in cake quality from the presses also led to occasional sludge cake handling problems. A key objective of the expansion project was to improve reliability, optimise the performance of the dewatering plant and achieve autothermic operation of the new incinerator.

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 which is then more difficult to pump and greater pressure is required. Also, the abrasive nature of the cake can quickly wear the pipe through which it is being transferred. In order to ease the flow through the pipeline a thin film of liquid is introduced through an annular ring at high pressure to lubricate the internal surface of the pipeline – this is termed boundary layer injection.

At the Mersey Valley Processing Centre, Cat Pumps’ 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. The thin film of water that creates the boundary layer helps to separate the viscous and potentially aggressive materials being pumped from the inner wall of the pipelines. 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. By evenly introducing the water at a small constant flow and at a sufficiently high pressure, up to 100 bar in the case of the Cat Pumps 3CP, in the same direction as filter cake flow maximises the lubricating effect without significantly affecting the dryness of the filter cake.

Two sets of 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 Cat Pumps used for the boundary layer system in the sludge dewatering plant do not operate continuously; the new pumps are called to operate 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. Triplex plunger pumps are often the first consideration when high pressure and high operating efficiency is required. Where the application pressure exceeds 50bar the loss of efficiency for other types of pumps falls rapidly, which is not the case with Cat Pumps triplex models. In fact, these pumps are actually increasingly efficient at higher pressures.

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.

In the overall scope of the multi-million pound S3JV project, the Cat Pumps are a very small item. However, their contribution to keeping the dewatered sludge flowing into the incinerator plant should not be underestimated. 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.




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