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Investigation into pump reliability problems at sewage treatment works

Challenge
A UK water company experienced significant reductions in dry well sewage pump reliability at one of its main sewage treatment works. Reliability had decreased following a number of changes implemented at the pumping station including changing the inlet screens allowing coarser solids to enter the well, altering the pump sealing technology from packed glands to mechanical seals and changing the maintenance contractor.

ESR Technology was commissioned to determine the cause of the reduced pump reliability, perform on-site and laboratory investigations and work with the water company to deliver a long-term solution.

Solution
Following detailed investigations by ESR Technology, the main causes of pump failure were identified as shaft fatigue and blockage of the pump by gross solids. The shaft fatigue was caused by a combination of poor maintenance practices and increased shaft loading due to the change in sealing technology, while the increased frequency of blockages were caused by the change in screening methodology.

ESR Technology recommended a series of improved maintenance procedures to reduce the susceptibility of the pump shaft to fatigue and a change to the wear ring design to make the pump less susceptible to blockages.

Benefits
These steps resulted in a significant improvement in reliability of the pumps at the sewage treatment works leading to improved performance and much reduced maintenance costs.

Investigation into excessive noise and vibration at sewage pumping station

Challenge
Management at a sewage pumping station in Scotland became increasingly concerned at the excessive noise and vibration levels that occurred within the pumpset, pipework and non-return valve (NRV) infrastructure. These problems caused mounting bracket failures and vibration alarms from the pump casing to go off leading to numerous site visits from management.

Investigations found that the noise and vibration were attributable to a number of factors. With single pumpset operation there was insufficient net positive suction head (NPSH) available to prevent cavitation occurring at high pump operating speeds. This cavitation caused excessive noise and vibration. Resonance of the pumpset, pipework and NRV body was also observed. This appeared to be driven by impeller vane passing frequency effects.

Solution
ESR Technology was contracted to analyse the cause of the excessive pumpset, pipework and NRV vibration and perform on-site investigations as required to support the project. The aim was to propose detail design modifications to alleviate the problems. A key part of the project involved the ESR Technology team supporting the asset maintenance provider in technical discussions with the asset owner and working with both parties to deliver a long term solution.

ESR Technology’s solution included a number of recommendations including:  

  • Modification of the control system so that pump speed during single pump operation was restricted to a maximum of 75% of the maximum operating speed, thereby avoiding cavitation and the associated noise and vibration
  • Modification of the control system to give a ‘soft stop’ during pump changeover to reduce dynamic loads on the NRV
  • Stiffening of the pumpset, pipework and NRV support structures to increase their natural frequencies and thus avoid resonances due to impeller vane passing frequency effects

Benefits
These measures resulted in a significant decrease in the noise and vibration levels of the pumpset, pipework and non-return valve (NRV) equipment. Consequently failures to the mounting brackets was reduced and unnecessary visits to the station resulting from vibration alarms ended.

Risk assessments for major utilities company

Challenge
ESR Technology was commissioned by a major Electricity Utility company to provide a risk assessment model for 11kV Substations with oil filled circuit breakers.

The client was concerned that a number of Substations with a make and/or break duty of  100% was increasing.  This was due to additional loading and generation from third parties into the 11kV distribution system.  A model was required that could be used by the client to help prioritise the Substations in terms of overall risk and feed into the asset management programme.

Solution
The approach involved close collaboration with the client to provide a workable solution that involved:

• ESR Technology’s facilitation of a workshop to identify major substation risk issues
• Production of a  risk assessment of substation causing most concern
• Production of a generic model based on the risk assessment
• Testing the model against further “live” examples
• Production of a “user guide” for the model
• Providing support in training client personnel in the use of the model

Benefits
• Important input into the Asset Management Planning and decision making process
• Guidance on access arrangements for maintenance and operational personnel
• Assessment of risk to public/third parties where this was a potential issue
• A simple, transparent, risk model was developed that could be easily understood by client personnel
• The risk model could be adapted easily for other types of substation

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