Scottish water unplugged

Scottish Water has trialled new technology based on the use of variable speed drives to prevent pump blockages at its Levenhall Sewage Pumping Station (SPS), which had been the worst site for blockages in its south-east operating area.

Staff at the East Lothian site had the unenviable task of dealing with blockages and ragging two to three times each week - a dirty job and a major draw on operational time and resources at the site.

At Levenhall SPS, there was no clear pattern of blockages, which could be equally frequent during dry or wet periods. Operators often had to deal with ragging issues every other day of the week, with the pumps being lifted at least once a month.

The station has four foul pumps, rated at 43kW each, and three storm screw pumps. Pump motor FLC (full load current) is around 70 Amps per phase and at the theoretical pump duty point for the pump motors is about 35 Amps per phase.

Due to the ongoing blockages, Scottish Water trialled an alternative pump design, but even this unit also experienced blockages. Replacement pumps were then considered, but were estimated as being likely to cost some £120,000 to £140,000, compared to drives, which would cost around £7,000.

The company, therefore, eventually opted for a variable speed drive (VSD) solution put forward by Emerson company Control Techniques.

Employing Intelligent Pump Control (IPC) software, which is integral to the VSD, the unit monitors active current to determine variations in torque. The live data is compared to measured values taken during commissioning, plus expected pump profile characteristics. Any ’out of profile’ performance feedback triggers a reversing cycle to break up rags as they begin to form on the impeller.

A trial of the IPC technology started at Levenhall last June, with a single VSD installed on pump 1. During the first full month of operation, the pump 1 inlet was free of ragging during daily operation. However, the number of trips in pumps 2, 3 and 4 due to rag balls remained an issue, reported Graeme Moore, senior project manager, innovation & technology, at Scottish Water.

And, although, ragging in the suction of pumps 2, 3 and 4 was reported, there was also one reported choke in the suction of pump 1, which occurred when pumps 2, 3 and 4 had also tripped.

Patterns of tripping and blockages from the operational log were not particularly clear, said Moore, though he noted that the number of manual interventions due to rag balls were similar to previous months.

A log of operating currents (figure 1) indicated that pump 1 was operating at much lower average running currents than the other drives - indicating that it was running more efficiently than pumps, 2, 3 and 4. The pumps cycle duty1 / duty2 / duty3 / standby, so only three units operate simultaneously during a pumping sequence.

Very high peak currents at starting also indicated that a blockage had formed. These spikes were regular occurrences: up to one in six operating cycles had very high peak currents, which often resulted in pump trips. However, as pump 1 had the VSD, high starting currents were no longer seen on this drive, said Moore.

The average running current and peak operating currents for pump 1 were also significantly reduced and this had a significant impact on the operating efficiency of the pump. Running currents were up to 40% higher on the pumps that did not have any blockage detection and control associated with the pump, although operating currents between 10-25% are more typical.

Compacted rag balls up to 600mm in diameter were observed during the early stage of the trial. These, continued Moore, were too big for any pump to deal with, resulting in blockages, or trips, when they were pulled into the pump suction. The formation of these large compacted rag balls was a continuous problem, he said.

By October 2010, VSDs had been installed on all four pumps. This enabled the Scottish Water team to determine if the rag balling in the wet wells could be managed, or prevented from forming.

Within the first week of operation, the rag balling and pump blockages were as bad as they had been at the start of the project. All pumps became blocked or partially blocked and significant rag balls formed in the wet well. Running currents on all pumps were also much higher than expected.

An investigation into the problem revealed that the blockage detection and control features of the VSDs had been disabled, so the pumps were not going into their cleaning cycles when a blockage developed.

This was a significant inconvenience to Scottish Water as a crane had to be hired to allow the pumps to be unchoked, noted Moore. However, he added, it also served to emphasise just how effective the Emerson equipment had been when operating to control blockages.

The blockage detection and control was enabled on all four drives in the week commencing 25 October. The pump blockages stopped immediately. The rag balling issues in the wet well appeared to decline over the first week of operation and running currents on all drives returned to the lower values previously observed, between 38-43 Amps.

Rag balling in the well during the first two weeks of November declined significantly, operational reports indicating a significant change in the number of rag balls evident in the wet well.

Early indications, said Moore, suggested that rag balls were either no longer forming in the wet well, or the cleaning cycle was breaking up rag balls, which flow into the wet well, to allow rags to be pumped forward without blocking the pumps.

Operations believed that the rag material was being transported forwards before it got a chance to form larger rag balls in the wet well, he also reported.

Early results, said Moore, indicated an improvement in pumping efficiency of up to 15%, as well as annual operation savings of up to £15,000 (see table below). This calculation also excludes any relevant knock-on savings, such as on staff overtime in order to maintain scheduled work and routines that used to be regularly impacted by problems at Levenhall. three a week to none at all reported in the last month of operation.

No crane hires were required to remove any pump fitted with the pump blockage detection and control system since the trial began.

Rag balling in the wet well and the need for manual intervention to remove rag balls and partial chokes have also reduced significantly, with no reported blockages since all drives were put into operation.

Levenhall staff are now better able to keep on top of routine activities without being diverted to deal with persistent blockages and rag balling problems at the site, added the Scottish Water engineer.

They were, he noted, also able to help the local wastewater treatment operatives deal with issues associated with recent severe winter weather, which would not have been possible previously.

“The Levenhall trial proves that pump blockage detection and control is achievable using Emerson’s Control Techniques [variable speed] drives,” concluded Moore. “Regular pump blockages at the station should now become a thing of the past and the pumps should operate more efficiently as a result.”