Pumps account for an estimated 4 to 5 percent of the world’s total electricity consumption. Potential for energy conservation exists through reduced speed operation, utilising Variable Speed Drives (VSDs). However, operating sewage pumps at reduced speed increases potential for blockages, reducing machine reliability. This article explores reliability issues experienced when VSDs are used for speed sewage pumping, principally the problem of excessive pump blockages and includes information about a solution now on the market.


Pump Blockages
The solids-handling capability of a pump is greatest when it is operating close to its maximum speed. Consequently, when VSDs are used to control sewage pumps over a range of speeds the pumps pick up debris, which affects performance and efficiency.

Detecting Pump Blockages
Through continuous monitoring of pump drive motor operating characteristics, and comparisons with stored benchmark data, potential pump blockages can be detected. The successful detection of blockages relies on the ability to differentiate between genuine fluctuations due to changes in pump speed and level and abnormal fluctuations due to a potential blockage.

Pump Operating Profile
The stored benchmark data is an operating profile for each pump. Each profile provides an understanding of the how the pump should operate at each level, at each pump speed, and any combination of the two. Any deviation from the stored values may be indicative of a potential pump blockage.

Correcting Pump Blockages
When a pump blockage is detected corrective action is automatically initiated, which consists of operating the pump in the reverse direction at a pre-determined speed for a pre-determined duration.

Implementation
The Retroflo RPC_2000 was specifically designed to control pumping stations as a whole, rather than just the individual components within. The new system requires no mechanical work aspects and may be implemented in the form of a retrofit software solution for existing installations.

Case Study 1
The RPC_2000 is installed at a site where VSDs exist and excessive pump blockages have been experienced. The original pumps and infrastructure remain; additional new software was simply installed alongside the existing Programmable Logic Controller (PLC). Calculated energy, cost and CO2 reductions indicate minimum savings of 11.63%, 0.6 pence and 0.0272Kg CO2 per m3 pumped were achieved at selected test points as a result of the trial. For all possible calculable test points an average 10.9% reduction per m3 pumped is calculated. Potential overall energy savings, calculable on a station-by-station basis, are immense.

Case Study 2
Scottish Water’s Charlotte Street Pumping Station in Fife is a good example of Retroflo technology 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, causing overflows that have encroached public areas and threatened the water quality of surrounding blue flag beaches. The Retroflo RPC_2000 was installed at Scottish Water’s Charlotte Street Pumping Station on June 1st 2009 and in its first three months of operation performed 47successful Pre-Blockage Detection routines, all of which returned the pumps to optimum performance. During this period there were no recorded blockages and therefore no maintenance call-outs.

UK-based manufacturer Retroflo has received a prestigious international award for its innovative pump control system. Selected as one of five finalists from a global field of industry entrants, the Retroflo RPC_2000 placed second overall in the US-based Pumps and Systems inaugural Product Innovation of the Year Award.

Developed in 2008 in the North of England, Retroflo’s RPC_2000 has garnered two innovation awards in the UK, with the Pumps and Systems award marking its first overseas accolade. The new pump control system has been making waves across the water industry, eliminating persistent blockage problems and saving energy costs at pumping stations operated by the UK’s largest water companies.

Pumps and Systems is considered the voice of the pump and rotating equipment industry in the US and provides news and technical information to over 40,000 industry subscribers. The Product Innovation Awards’ Advisory Board studied each nominated product to ensure it met the criteria of being a significant improvement over current technology. Despite Retroflo’s recent introduction to the US market, the judges were impressed by the potential of the RPC_2000 to have a major impact on the water sector.

The new system was extensively trialled and commissioned at several Northumbrian Water sites in 2008. The success of these sites led to take-up across the UK, with the RPC_2000 now in operation at Scottish Water, United Utilities and South West Water pumping stations.

The new control system has delivered on its pledge to virtually eliminate blockages, which is a very common and costly problem at wastewater pumping stations. Partial Blockages cause pumps to operate inefficiently, wasting energy and once choked require time-consuming and costly call-outs to clear. Of the greatest concern is the environmental damage that can occur when failed pumps result in spillage. Post RPC_2000 installations have recorded a vast reduction in blockages, meaning enhanced environmental protection, fewer maintenance call-outs, reduced operating costs and much lower carbon emissions.

The improved performance from pumps operating efficiently has also resulted in a minimum 12 percent reduction in energy consumption costs at each individual pumping station. If you consider that Northumbrian water alone operated nearly 600 wastewater pumping station, the potential for savings across the industry is enormous. The RPC_2000 has low installation costs and can be retro-fitted to existing pumping stations without the need for major refurbishment works. This means that utility companies can expect a return on capital investment within 12 months of installing the system.

Understanding that every pumping station is different goes some way to explaining the philosophy behind the Retroflo system. The RPC_2000 was specifically designed to control pumping stations as a whole, rather than just the individual components within. It is this systematic approach that has brought rich rewards to a British innovation.

The Retroflo RPC_2000 Pump Control System has been winning plaudits across the UK water industry by resolving the traditional blockage problems and the resultant energy efficiency issues that plague wastewater pumping stations.

The RPC_2000 has been successfully put into operation at pumping stations operated by Northumbrian Water, Scottish Water, United Utilities and South West Water and has lived up to its billing by delivering a vast reduction in blockages while guaranteeing a minimum 12 percent reduction in energy consumption costs. This combined with almost no reactive maintenance call-outs, means that utility companies can expect a return on CAPEX within 12 months of installing the system. As an example of efficiencies, the recent installation of the RPC_2000 at South West Water’s Ilsham Valley Pumping Station has recorded a substantial reduction in absorbed power through reduced blockages, cleaner wells and the optimization of pump performance. In terms of labour costs alone the station saved 20k on the 2008 expenditure for reactive maintenance call-outs to unblock pumps.

Understanding that every pumping station is different goes some way to explaining the philosophy behind the Retroflo system. The RPC_2000 was specifically designed to control pumping stations as a whole, rather than just the individual components within. Through continuous dynamic monitoring of the pump characteristics over a range of wet well levels and pump speeds, the system is able to utilise the full range of data available to optimise pump performance.

Scottish Water’s Charlotte Street Pumping Station in Fife is a good example of Retroflo technology 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 rising main 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.”

Blockages are a common and costly problem at wastewater pumping stations due to the gradual build-up of debris on pump impellers. During this gradual build-up pumps operate inefficiently and this eventually leads to a blockage, resulting in time consuming and costly call-outs. Retroflo’s RPC_2000 utilises its patented Pre-Blockage Detection function to detect debris build-up and remedy the problem by initiating a rapid pump reversal before it becomes a blockage.

At Charlotte Street frequent blockages involved the need lift the pumps from the station for de-ragging using a crane in a public car park. This proved extremely problematic, both logistically and financially. The Retroflo RPC_2000 was installed at Charlotte Street Pumping Station on June 1st 2009 and in its first three months of operation performed 47successful Pre-Blockage Detection routines, all of which returned the pumps to optimum performance. During this period there were no recorded blockages and therefore no need for expensive reactive maintenance call-outs.

Another principal cause of blockages at Charlotte Street was due to a 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 are 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 two months after the Retroflo installation and the results, says 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.”

The introduction of the Retroflo pump control system not only maintains consents through reduction of blockages but can also dramatically reduce 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. At South West Water’s Ilsham Valley Pumping Station the introduction of Retroflo technology has resulted in a vast reduction in blockages and power usage. The station had a history of pump blockage problems, added to which pumps often ran for extended periods whilst partially blocked, leading to extremely high electrical energy costs.

In an attempt to alleviate blockage problems, two of the station’s four channel type pumps were replaced with better solid-handling pumps, which though less prone to blockage, are less efficient. With the installation of the RPC_2000 the system was able to identify and eliminate pump blockage issues, allowing the more efficient channel type pumps to be utilised.

Since the RPC_2000 came online the number of pump blockages and the resultant costly call-outs have been virtually eliminated, which has led to a substantial reduction in the station’s OPEX costs.

For more information about Retroflo call 0191 4972840