Pump Control Pays Off For Problematic SPS

Since the installation of Retroflo’s RPC_2000 Pump Control System in June 2009 the problem of blockages, flooding and odours has been resolved without need for a single reactive maintenance call-out. The station is also on target to achieve an annual OPEX saving of £30,000, equating to a CAPEX payback inside the first year.

Understanding that every pumping station is different goes some way to explaining the philosophy behind the Retroflo pump control system. Scottish Water’s Charlotte Street Sewage Pumping Station (SPS) in Fife is a good example of the RPC_2000 solving the issues of a particularly problematic station. Situated as the last pumping station before the treatment works in Kirkcaldy’s sewerage network, the station handles large flows and is subject to frequent blockages.

Willie Burnside, an Asset Planner for Scottish Water, explains: “As soon as it rains in Kirkcaldy, resultant flows hit Charlotte Street like a cascade. Network contents previously settled in the low/flat sewer along The Promenade caused blockages and lots of ‘reputational’ internal flooding as a direct result. Eight skips worth of ragging are removed from the Kirkcaldy treatment works on a weekly basis, and as such Charlotte Street handles the bulk of this waste.”

At Charlotte Street SPS frequent blockages involved the need lift the pumps from the station for de-ragging using a crane in a public car park. As a result, operational expenditure and was extremely high and the frequent maintenance meant new pumps had to be installed three t¬imes in four years. “Over the last few years the blockages in the pumping station have resulted in repeated replacement of pumps at about £200,000 a time,” says Willie Burnside.

Scope of Work

The Retroflo RPC_2000 Pump Control System was successfully installed and commissioned at Charlotte Street SPS in the first week of June 2009. During installation the pumps and wet well were thoroughly cleaned to determine the ideal operating criteria of the pumps. This data was then used as the base line for monitoring all control conditions associated with the station.

The RPC_2000 was integrated with the station’s existing PLC - full functionality of the station was maintained during installation, ensuring pass forward flow to Kirkcaldy waste water treatment works. The RPC_2000 interfaces with the upstream treatment works enabling onward flow to be reduced to 80% or stopped entirely when upstream conditions dictate. Signals from the RPC_2000 are transmitted to the Scottish Water Open Enterprise telemetry monitoring network.

Constant level pumping is the mode determined most suitable for this SPS. Each time a pump is called to run a short maximum boost cycle is initiated, ensuring any debris that may have settled in a pump or its pipe-work is flushed clear. This flushing feature is also repeated at each pump stop. Under normal conditions Cycling Pump Duty is selected and the operation rotates around the two pumps based on an integral run-timer, currently set for 60 minutes, helping to alleviate blockages and therefore reduce pump wear.

In its first six months of operation the RPC_2000 performed over 100 successful Pre-Blockage Detection and Self-Cleanse routines, all of which returned the pumps to optimum performance. During this period there were no recorded blockages and therefore no need for reactive maintenance call-outs.

The RPC_2000 pre-empts a blockage situation by comparing real-time data against data benchmarked to actual performance of the pump in the well rather than to the flow curves. The system continuously monitors motor power, current, torque and speed, together with wet well level and volumetric flow, and compares this with up to 10,000 pre-determined reference points per pump to identify and correct operational and efficiency issues.

On detection of debris build-up on the pump impellor the RPC_2000 initiates a self-cleansing routine. The pump is stopped and reversed within site defined parameters, a process repeated until operating efficiency is re-established. This ensures that the impeller remains debris free and maintains optimum pump performance.

Another principal cause of blockages at Charlotte Street was the lack of self-cleansing velocities being generated in the flat rising main, resulting in a build-up of sediment and solids. The RPC_2000’s Intelligent Flushing Cycle uses historical operating data to predict the incoming flow rate, allowing the well to fill to a pre-set level before initiating a rapid well-emptying sequence.

The RPC_2000 performs this function on a daily basis. If the conditions are acceptable – i.e. not in high flow – the well is allowed to fill to a level higher than normal operating levels. The pumps are then operated to rapidly empty the well to a low level. The benefits of this process is to scour the wet well to remove settlement, whilst achieving appropriate settlement dilution ratios and generating rising main self-cleansing velocities.

The condition of the Charlotte Street wet well was inspected by a Scottish Water team three months after the Retroflo installation. The results, say Willie Burnside, were much better than expected: “You wouldn’t have thought it was a wastewater wet well at all. There were no scum lines, no ragging, no grease, not a hint. It’s the cleanest I’ve ever seen a wet well.”

After almost a year in operation Willie Burnside is still enthusiastic about the effect the RPC_2000 has had at the SPS: “The benefits of the installation speak for themselves. Since Retroflo was installed there have been no operational call-outs to the pumping station, no cranes required to lift out the pumps and, most importantly, no flooding as a result of a failure in the pumping station. It also keeps the rising main clear which helps reduce the odour problems.”

The RPC_2000 has delivered operational and maintenance efficiencies and improved the overall energy performance. “[OPEX] equates to a minimum of £30,000 annual savings,” says Willie Burnside, adding that larger savings can also be derived through longer asset life: “Retroflo has the potential to minimise the replacement frequency which is a significant saving. There may also be opportunity to offset some of the planned capital investment for Charlotte Street as well.”

A reduction in blockages also dramatically reduces energy costs. By initiating pump reversal cycles on detection of partial blockages and returning the pump to optimum operating conditions, the improved pump performance equates to significant decrease in power during normal operation. These benefits have not gone unnoticed by Scottish Water R&D Manager George Ponton: “We are currently developing a business plan that could generate up to £500,000 annual OPEX savings, based on the work at Charlotte Street.”

The RPC_2000 has also been successfully put into operation at pumping stations operated by Northumbrian Water, United Utilities and South West Water and has lived up to its billing by delivering a vast reduction in blockages and energy consumption costs.