The following report by Graeme Moore, Senior Project Manager, Innovation & Technology for Scottish Water, details the pump blockage problems and their approach to its solution.
Scottish Water recently identified that its Levenhall Sewage Pumping Station had significant issues with pump blockages and was considered the worst site for blockages in its south-east operating area. Often blockages and ragging needed to be dealt with two to three times each week. It was therefore an ideal site to try out a low-cost innovative solution for pump blockage detection and control, ‘Intelligent Pump Control (IPC) using an ‘off the shelf’ Control Techniques - Emerson variable-speed drive.
Pump blockages and rags are a significant barrier to realising energy and operational efficiencies in waste-water pumping throughout the world.
A number of new approaches to the problem of pump blockage detection and control have been developed in recent years and these are offering to improve reliability, whilst delivering improved efficiency.
Pump blockage problems are a significant problem in Scottish Water; dealing with blocked pumps is a significant draw on operational time and resources. Implementing solutions at the most problematic sites would undoubtedly reduce call-outs, disruption, health & safety risks, travel time and both customer and environmental impact. The true scale of the problem is, however, unclear, as, often, the problem appears to be accepted as a routine maintenance task for some pumping stations.
Pump Blockage Detection and Control
A frequently occurring problem, with almost all types of impellers, is that fibres and other particles tend to build up on the impeller vanes. Generally, this build-up of rags causes a decrease in the pump’s hydraulic efficiency, increasing power consumption. This build-up of rags can then cause pump blockages if it continues to build up.
Pump blockages and rags are a significant barrier to realising energy and operational efficiencies in wastewater pumping. Further, an estimated three quarters of all pumps are over-sized by more than 20% to provide a ‘factor of safety’ in the design. The use of variable speed drives can be used to reduce the energy used by over-sized pumps, and reduce hydraulic system losses, but the increased frequency of pump blockages in wastewater pumping has always been an issue. Reliability is always more important than efficiency when it comes to meeting regulatory consents and delivering the high standard of service that our customers expect. Previously, the only solution to pump blockages was to try different types of sewage pumps. Pump designs to deal with rags has improved over the years, but no centrifugal pump currently on the market can be guaranteed to deal with all rags under all conditions, and increased frequency of blockage has been a persistent problem for pumps with variable speed drives, (VSDs).
The use of VSDs can play a substantial role in energy reduction by bringing the pump in line with the consented flow and reducing friction (energy) losses in the system. The ‘soft start’ functions can also increase the life of components, such as bearings and seals. There is significant value in energy savings and the resources associated with pump maintenance and repair. There are also the H&S issues that need to be managed every time a pump has to be lifted and maintained. It’s a dirty job and far from the ideal working environment, which requires a high degree of staff diligence.
Where VSDs are used, a small reduction in pump speed can reduce energy costs, especially where the system has significant friction losses. While process control and energy management are the two primary reasons for VSDs, there can be associated problems. A greater incidence of blockages at a wastewater pumping station can soon offset the energy-saving benefits associated with the VSD. Pump blockages can also result in premature spills/overflows, causing environmental issues with water courses, localised flooding and damage to our reputation, especially where blockages are regular and frequent. Because the increased risk of blockages can be a significant factor, when using VSDs for (unscreened) wastewater pumping, the ‘tried and tested’ fixed speed pumping solution has remained the favoured design approach over the years. Good design and pump selection generally ensures reliable operation with few blockages. Hence, blockage avoidance through a traditional design approach, and selection of blockage resistant pumps, has generally been considered to be more important than energy savings. Trouble-free operation is often seen to be more important than energy efficiency, as operating costs associated with problematic sites can often out-weigh what might be relatively marginal benefits on energy efficiency.
Some pumping systems, whether they are fixed speed or controlled by VSDs, can be prone to blockage simply due to poor hydraulic design, poor pump selection or an unmanageably high volume of rags in the system.
A number of new approaches to the problem of pump blockage detection and control have been developed in recent years and these are promising to put an end to frequent pump blockages, even at the most problematic sites.
At Scottish Water, we have tried PLC-based solutions, with success. However, this type of bespoke control package has to be provided over and above the VSDs, a costly solution that is only viable where it is not cost-effective to replace the drives.
One interesting alternative has been put forward by the VSD manufacturer Control Techniques – Emerson – their Intelligent Pump Control (IPC) software. This alternative solution is integral to the VSD, so doesn’t require any bespoke control package and can be provided as a standard VSD option. There is no additional cost to the purchase price of the VSD and its integral programmable controller. The Emerson VSD drive is also unique in that it monitors active current to determine variations in torque, which then triggers a reversing cycle to break up rags as they begin to form on the impeller. The active current monitoring allows very small changes in operating torque to be monitored. In short, the Emerson VSD solution appears to be a relatively low-cost solution to pump blockage detection and control. Furthermore, the VSD can be provided with a programmable module to integrate complex control logic into the VSD, so that separate programmable controllers are not required.
Levenhall SPS – Trial site for VSD Blockage Detection and Control using Emerson VSDs
Levenhall SPS is a low-lift station, part of the East Lothian coastal chain of pumping stations. The pumping station has a consented pump forward flow of 675l/s and an average static head of circa 7.2m. The rising main from the pumping station is circa 17.15m. Due to the short length of rising-main, none of the pumps have non-return valves and there is no interconnecting pipework between the discharge of the pumps, i.e. the pumps discharge individually into a gravity sewer, not into a rising-main.
The friction losses in the discharge of each pump are almost negligible, i.e. there is no real opportunity for efficiency gains through a reduction in hydraulic losses, through the use of VSDs. Therefore, all significant pumping inefficiencies can only be a direct result of blockages and ragging. This makes this an ideal pumping station to study the effects of ragging on pump efficiency, as normally VSDs are used to reduce hydraulic losses in the system. Since there are no significant hydraulic losses in the Levenhall system, energy gains can only be associated with better pumping efficiencies.>>>>>>>>>
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