PUMP EFFICIENCY Ragging at Levenhall should become a thing of the past
VSD trial beats ragging blockages
A problematic pumping station was adopted for trial of Emerson Control Technique’s variable speed drives. Scottish Water’s senior project manager for innovation & technology, Graeme Moore explains the project
S
cottish Water identified that Levenhall Sewage Pumping Station (SPS) was its worst site for blockages in its south-east region. Issues with ragging needed to be dealt with two to three times a week and it was not unusual for pumps to be lifted at least once a month.
Levenhall was a very labour intensive site and is a significant draw on operational resources. It was therefore decided to try out a low-cost solution for pump blockage detection and control, Intelligent Pump Control (IPC) using a variable speed drive (VSD) from Emerson’s Control Techniques division. Levenhall is a low-lift station with a consented pump forward flow of 675l/s and an average static head of about 7.2m. The rising main from the PS is about 17.15m and due to its short length, none of the pumps have non- return valves and there is no interconnecting pipework between the pumps’ discharge. This means they discharge individually into a gravity sewer, not into a rising main.
It was identified that all significant pumping inefficiencies can only be a direct result of blockages and ragging. The blockage problem subsides during dry spells, as the rags appear to be settling out in the catchment during periods of lower flow. However, there is no clear pattern of blockages and these can be equally frequent during dry or wet periods.
A build-up of rags in the wet well is also a problem and these rags are often cleared manually, once or twice a week. When Levenhall SPS fails, the upstream PSs are inhibited and this
18 Water & Wastewater Treatment June 2011
results in a storm discharge at Prestonpans SPS, with the screened sewage being discharged into the Forth Estuary. Raw sewage discharge has issues for bathing water quality and Scottish Water’s reputation.
Pump system
Levenhall SPS has four foul pumps rated at 43kW each along with three storm screw pumps. The screw pumps operate without any significant issues, are not a consideration in these trials.
Pump motor full load current (FLC) is about 70A per phase and at the theoretical pump duty point the pump motors should operate at about 35A per phase. Levenhall uses about £28,000 per annum of electricity and requires about £15,000 per annum of operational interventions to deal with ragging, blockages and pump trips, but there are additional hidden costs, such as the knock-on effects due to resources being diverted to deal with problems at Levenhall SPS. An alternative pump had been trialled, but still experienced blockages. All the existing pumps are generally in good working order, and had recently been refurbished. Replacement pumps were estimated at £120,000-£140,000, compared to the drives which were around £7,000 each.
Pilot trial
The pilot project began on 10 June 2010 and initially saw the installation of a single VSD with the IPC system, on the first of the four pumps, Pump 1. The aim was to investigate if the
energy usage and the occurrence of blockages on that pump could be significantly reduced. Within two days, on 12 June, with the exception of Pump No 1, all the pumps in the station had become blocked and had to be lifted to clear blockages. On the 16 June, Pump 2 and 3 were again blocked or partially blocked and Pump 1 was clear of blockages.
It was noted at this stage that the average running current of Pump 1 was now between 15-30% less than the others. Some minor teething problems with VSD parameter settings resulted in premature tripping, which was quickly resolved.
Inspection of pump impellers revealed cavitation damage on the surface of Pump 2’s impeller, due to blockages. Because none of the pumps have non-return valves, a blockage in one pump can be flushed back into the wet well when the pump stops or trips. From here they can be sucked into the other pumps, making them more likely to trip.
July results
The first month of operation proved that inlet for Pump 1 remained free of ragging during daily operation. However, the number of pump trips in Pumps 2, 3 and 4, due to rag balls, has remained an issue and rag balls are still being removed from the wet well two or three times a week.
There was one reported choke in the suction of Pump 1 – this appears to have occurred when the other pumps had also tripped. Patterns of tripping and blockages from the operational
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