Severn Trent Water is the UK’s second biggest water company. It serves more than 3.7million homes and business customers in England and Wales. Its region stretches from mid-Wales to Rutland and from the Bristol Channel to the Humber. The company delivers almost 2billion litres of water every day through 46,000km of pipes. A further 91,000km of sewer pipes take waste water away to more than 1,000 sewage treatment works. The company has a formal advisory group, made up of internal and external personnel, to assess new approaches and new technologies for company and customer benefits.Renewal plansIn a recent asset renewal scheme, following an in-depth feasibility study, Severn Trent Water gave engineers from NMC Nomenca, one of its tier one design and build contractors, the green light to investigate new or not-widely-used technologies in the development of a new inlet works at Melton Mowbray sewage treatment works. The project involved replacing the current screenings, screens handling, and grit removal installation at its Lake Terrace terminal pumping station.Together, they investigated the feasibility of advanced, integrated motor control centres (MCCs) from Rockwell Automation. The approach also included the deployment of Form 2 construction; a proven, space-saving approach popular in other industries.The existing pumps at Lake Terrace will be retained, to transfer the raw sewage to Melton Mowbray, where the new inlet works will be constructed to treat the influent. The new inlet works will also be sized to accommodate future flows from nearby Asfordby.MCC choiceWith space limitations and Inherent hazards at the Lake Terrace site, the decision to relocate the screening infrastructure to Melton Mowbray gave engineers at Severn Trent Water and NMC Nomenca the opportunity to consider new technologies. Following an investigation by the ICA Community of Practice group and a competitive tender, MCC and associated technology from Rockwell Automation was chosen.“We wanted to do something a little different with the MCCs and the innovation-led approach we were able to take allowed us to do this,” explains Chris Webb, contracts engineer at NMC Nomenca and the company’s representative on the ICA Community of Practice group. “Severn Trent Water was looking for innovation in the new equipment and was also looking to save money (15% versus existing potential solutions). I had a few ideas based around the MCC, especially relating to the use of Form 2 construction, which is not very common in the UK water industry even though it is in the current standards. As well as using Form 2 to reduce the overall size of the MCC, I also wanted to try out some different, contemporary technologies, such as those offered by Rockwell Automation.”Form 2 defines the way devices are located and installed relative to each other. Using Form 4, the conventional installation method for the water industry, motors starters would be isolated from each other and installed in separate compartments. In a Form 2 MCC, just one compartment is required. The form factor defines the way the metalwork forming the MCC is constructed (isolation/segregation), with Form 4 being a mature design based on older standards and less-reliable equipment. Advances in equipment reliability now make Form 2 not only possible, but also more desirable when it comes to installations with restricted space.Webb presented his ideas around the Form 2 concept to the Severn Trent Water ICA COP, which granted permission to develop the concept for the scheme.Starter elementsOther significant elements of the design included the use of Allen-Bradley 141a busbars and MCS isolation modules. The use of 141a busbars and the modules allow the motor starters to be configured in a more modular fashion, so they can be installed side by side. Starters normally clip on to a top-hat rail, however the MCS modules – plastic bases that clip on to a bus bar – remove the need for wires by offering a direct, hard connection to the bus bar. As well as simplifying the design, starters can also be unclipped while the bus bar is live (hot swap) with no need to turn the whole MCC off.The complete installation of the intelligent MCC comprises Allen-Bradley CompactLogix programmable automation controller (PAC), PanelView 15in colour touchscreen HMI (human machine interface), three-component starters (MCB/Contactor/E3+ Overload Relay), 141a busbar system, MCS isolating modules c/w control plugs, PowerFlex 70 variable speed drives and the open DeviceNet network connected to the PAC using Ethernet/DeviceNet bridge modules.This system will be intelligent with all variable-speed drives connected to the PAC via Ethernet. All fixed-speed drives will be connected via twin DeviceNet networks (to allow duty standby separation), with the DeviceNet network connecting to the PAC via Ethernet/DeviceNet bridge modules negating the need to install scanner cards.BenefitsSevern Trent Water will see, in the first instance, that the inlet MCC Form 2 construction has a smaller footprint than a Form 4 construction, reducing the cost and size of the MCC, kiosk, and the base slab. A 23% CAPEX reduction has been estimated using Form 2 MCC construction, compared to standard Form 4 construction.The use of Ethernet communications via Ethernet/DeviceNet bridge modules also saves cost connecting to the PLC. The busbar system provides solid positioning for MCS-components, a planning tool for assemblies and type-tested assemblies; while the MCS system provides modularity, flexibility, quick exchange (low down time) and space saving benefits.Chris Webb explains. “I went to Rockwell Automation’s UK HQ for a presentation and I left with a head full of ideas.He adds: “The Ethernet connectivity deployed within the MCC is very useful and flexible and is almost certainly the way forward regarding control networks. Why put in a closed network when such a powerful open network is out there?
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