P&M gets its teeth into a gritty problem for Southern

The screw centrifugal pumps had failed at a Southern Water site due to rag and grit in the sludge they were handling. But chopper pumps solved the problem, writes Matt Harvey, director of P&M Pumps

Southern Water has boosted the throughput of sludge and minimised reactive maintenance at one of its sites in Hampshire by installing Vaughan chopper pumps and mixing systems.

The site, a key element in Southern Water's sludge strategy, serves a catchment of about 600,000 people and uses mesophilic digestion and a thermal drying plant, which needs to operate constantly.

The key processes at the site are split into two main functions:
  • For the wastewater process the stages are primary settlement, biological treatment and final settlement
  • For sludge recycling, the stages are digestion, gas recycling, drying, recycling the drying granules to agriculture
Pumps play a key role in the smooth running of these processes and Southern Water was experiencing problems prior to the installation of the chopper pumps. The site has three 2,400m3 mesophilic digesters, which are fed 24 hours a day with raw sludge containing a dry solids content of 6-8%.

The digesters are heated using spiral heat exchangers with a primary heat source of around 60°C. Pumps then recirculate the sludge through the heat exchangers to maintain a constant heat within the digesters and this temperature must be maintained at 35°C ± 2°C to ensure Hazard Analysis and Critical Control Point compliance.

The previously installed screw centrifugal pumps frequently failed due to their inability to handle the amount of rag and grit in the sludge. In addition, seal failure was common with one pump needing to be replaced on average every four to six weeks.

The rag also caused blockages within the heat exchangers, which meant they needed to be cleaned weekly to remove trapped rag from within. This led to operational process issues such as expensive maintenance to sustain the required throughput of the site constantly.

For example, if the temperature on any digester dropped below the critical control point, this would inhibit digester feed, reducing the site throughput by as much as one-third. The financial consequences were monthly costs on the pumps of around £2,000 each, plus the labour costs for contractors and call out of Southern Water staff.

P&M Pumps was invited to look at a possible solution and the recommendation was to install a HF4P6 15kW Vaughan Chopper Pump to ensure the required flow was maintained and which would be able to cope with the rag and grit within the system.

The first pump was installed and the operators reported how quiet the Vaughan Chopper pump was in comparison with the other pumps.

As part of the maintenance process, operators continued to open up the heat exchangers, although they found that the build-up of rag was almost zero. It was also found that the digester temperatures consistently stabilised and remained so thanks to the ability of the heat exchangers to transfer the heat more efficiently without blockages or pump failures.

Consequently, heat exchanger operational maintenance now consists of an annual inspection and there are no issues with rag build-up within the units. A further two pumps have subsequently been installed at the site and have been running for more than three years with no reported blockages and minimal reactive maintenance.

Commenting on the performance of the Vaughan Chopper pumps, Mark Catlin, Southern Water's senior process engineer, said: "Installing the Vaughan equipment has made a significant improvement to asset availability and a large reduction in this site's operational costs."

The conclusions underlined by the subsequent elimination of the downtime problems occurring at the Southern Water site are for operators to have a clear understanding of the potential operating costs when specifying equipment. Life costs are a significant factor in today's increasingly regulated environment as are the incessant demands for improved efficiency. All these factors need to be included in the initial specifying / buying process where comparisons of initial purchase price with probable on-going maintenance costs should be taken into account.

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