PUMP EFFICIENCY
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
Running currents VSD on pump 1
90 80 70 60 50 40 30 20 10 0
1 2 3 4 5 6 7 8 9 Operating cycles
Running currents VSD on pumps 1 and 2
90 80 70 60 50 40 30 20 10 0
1 2 3 4 5 6 7 8 9 Operating cycles 20 Water & Wastewater Treatment June 2011 10 11 12 13 14 10 11 12 13 14
Levenhall impact Opex savings
Energy consumption Pump maintenance
£4,200pa £15,000pa
Capex savings and expenditure Pump replacement savings
Investment for four new 45kW drives
£150,000 £39,000
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
Pump 1 Pump 2 Pump 3 Pump 4
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
Pump 1 Pump 2 Pump 3 Pump 4
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. nnn
Current A
Current A
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