Gas booster stations are critical strategic components in the UK’s gas supply network. At these facilities, motor-driven compressors are used to ensure the distribution of gas to where it is needed. Consequently, any reliability issues at these sites can adversely affect gas supplies if not resolved promptly.
Initially, National Grid identified an issue with two large 16 MW motors, one installed at a compressor site near Wormington in Gloucestershire and the other located close to Churchover near Rugby. The problem arose when resistance temperature detectors (RTDs) embedded inside the motor windings began to fail, compromising the ability of operators to gain accurate temperature readings and condition monitoring data. As the motors were Ex-rated, this also threatened the compliance of the installations.
The rare six-phase synchronous motors used at the sites weigh up to 51t each and can operate at a nominal speed of 5,200 rpm and an overspeed of 6,435 rpm. Controlled by variable speed drives (VSDs) and featuring complex rotors 6.5m in length, the bespoke motors are designed to ensure that increased demand for gas can be met by speeding up the compressor to push more through the network.
Rather than go to the motor OEM as it had done in the past, National Grid approached Sulzer, which currently holds a framework agreement with National Grid, to solve the issue.
Sulzer determined that replacing the components wasn’t possible due to the RTDs being embedded into the old windings. Instead, it suggested carrying out turnkey redesign and rewind projects to solve the issue. Supporting this assessment, National Grid tasked Sulzer with overhauling the motors.
REWIND
The design and installation of replacement RTDs necessitated a complete rewind, testing, reinstallation and commissioning. Changes to the winding would need to be carried out while preserving the Ex certification of the units. Additional RTDs also needed to be installed into the stator to offer better protection for the motors in future. The new RTDs embedded in the middle of the stator slots would help to reduce the risk of failures.
Work would be carried out at Sulzer’s Birmingham Service Center. As a six-phase motor consists of two three-phase windings working side by side, the project was a complex undertaking.
At the compressor sites, the motors were raised on hydraulic jacks before being manoeuvred on their skids from 2-3m in height (pictured below) on to the back of a low loader. Once delivered to the service centre, they were dismantled with the help of a high-capacity crane.
To ensure that the rotor of each motor could be removed safely during disassembly, Sulzer produced custom stub shafts that attached to the end of the large rotors. This allowed each to be moved without impacting or damaging the windings.
The stator redesign and rewind work was supported by Sulzer’s in-house coil manufacturing department, which holds a database of thousands of coil designs. This helped to expedite the project by eliminating delays to source spares and ensured consistent quality.
Mark Prestt, project manager for National Grid, said: “Due to the bespoke nature of the machine and the technical challenge of replacing the RTDs with a superior substitute, the project required a high level of engineering expertise. This was amply demonstrated by the Sulzer design team, which then fed into the rebuild and equipment procurement, efficiently smoothing the rebuild process.”
To ensure transparency and technical insights to the customer, Sulzer had an open-door policy for National Grid engineers, who could visit the facility and view any part of the process. This close relationship kept National Grid informed of latest project developments, providing additional assurance that all required standards were met during tests and that the repair was proceeding as per the agreed schedule.
Furthermore, as National Grid lacked build documentation for the motors, Sulzer produced custom manuals as part of the project. These will help expedite future maintenance work.
The motors have now been reinstalled and are supporting operations, providing consistent temperature data to support condition monitoring – preserving Ex compliance. This has ensured a reliable gas supply during a challenging period for the energy sector. Prestt added: “Sulzer, at all levels, consistently showed commitment, expertise and flexibility throughout the project.”
BOX: HOT WORK
Roller tables transport hot and heavy slabs of metal, with each roller having its own motor. A large-scale retrofit anywhere is never easy, but in a steel mill with extreme conditions and tough productivity demands, that’s especially true. Aperam, a manufacturer of steel and alloys, chose Bauer Gear Motor to carry out what became a three-year project at the Châtelet, Belgium factory.
There, the Aperam factory consists of a smelter and rolling mill, which produce steel and alloy slabs. During routine inspections, in-house maintenance technicians determined that the motors of the roller tables needed to be replaced. Due to the weight of the slabs, each roll is driven by its own motor (pictured).
A conventional retrofit of the engines was unacceptable as it would require extensive mechanical modifications of each unit. Aperam therefore approached Bauer Gear Motor – a brand of Altra Industrial Motion Corp. – for a plug-and-play solution.
Bauer’s retrofit solution consists of two main elements. First, a standard shaft connection with a flange or shaft coupling facilitates and speeds up installation. This is combined with an adapter base plate, which allows the old motor to be replaced one-to-one with a new one. This plug-and-play design requires no costly and time-consuming mechanical modifications, allowing the new motors to be installed with minimal downtime.
Andre Vanhaelen, regional sales leader, Northern Europe, said: “One of the main challenges was to deliver the required number of engines within the plant’s planned downtime, but our production capacity enabled us to do so. We have replaced all the motors of the roller table in three years so that the system meets the new efficiency standard, while maintenance and any retrofits will now run more smoothly.”
Bauer supplies special motors specifically for roller tables. By standardising the axle joints and base plates for Aperam, Bauer was able to streamline the entire initial retrofit project, which also had the benefit of facilitating any subsequent upgrades or replacements of the motors on the roller table in future.