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

Scottish 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 nonreturn 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 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 log are not particularly clear, but the number of manual interventions due to rag balls appears similar to previous months.

A log indicates that Pump 1 is operating at much lower average running currents than the other drives, indicating that it is able to operate much more efficiently than the others. It appears that the average energy consumption at the site could be reduced by up to 15% per annum, if all pumps had the VSDs with the IPC system fitted.

August results
A VSD was installed on Pump 2 in August, to check that the impact on pump operating efficiency could be repeated. The graph shows that the change in running current is quite significant. However, rag balling in the wet well remains a significant issue, resulting in pump trips and blockages in Pump 3 and 4. Investigation into the frequent tripping of Pump 2 revealed a faulty relay.

Only two trips have been reported on Pump 1 since the VSD was installed and the pump has not been reported as blocked or choked. Emerson had highlighted that rag balling from uncontrolled pumps could be a risk at the onset of the trials, as they had had similar experiences on other sites with mixed controlled and uncontrolled pumps.

With VSDs on two pumps, ragging continues to be a significant problem, resulting in high energy costs and excessive manual interventions to keep the station operational. However, average running current and peak operating currents for Pumps 1 and 2 have been significantly reduced, indicating a significant impact on the operating efficiency of the pump.

Results are showing that pumping efficiency has been seen to improve by up to 15%. Other operational expenditure (Opex) associated with blockages could reduce by up to £15,000 per annum, not including that associated with the knock-on effects of problems at the site, such as worker overtime.

October results
VSD's were installed on Pump 3 and 4 in October 2010 and it was agreed with operations that this was the only reliable way 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, with all pumps blocked or partially blocked and significant rag balls forming in the wet well and running currents on all pumps were significantly higher than expected. An investigation into the problem revealed that the blockage detection and control features of the VSD's had been disabled and 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 allow the pumps to be unchoked. It served to emphasise how effective the equipment had been when operating to control blockages.

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

November results
Rag balling in the well during the first two weeks of November saw a significant decline, indicating that they are either no longer forming in the wet well, or that the cleaning cycle is breaking them up. Operations believes that the rag material is being transported forwards before it gets a chance to consolidate.

Performance
The Levenhall trial proves that pump blockage detection and control is achievable using Emerson Control Techniques' drives. Regular pump blockages at the station should now become a thing of the past and the pumps should operate more efficiently as a result.

The team leader indicated that there has been a noticeable reduction in overtime and 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 Levenhall.

Where pumps are operating significantly above their theoretical running currents due to pump ragging and blockage, pump blockage detection and control techniques can be utilised to reduce running to nearer the theoretical value.

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