Avoid Bearing the Cost of Failure

A mineral processing company in the USA relies on many large motors to power essential equipment such as water pumps, pulverisers and vacuum pumps. To minimise the spares inventory, they use similar motors where possible. For example, seven pieces of equipment are powered by 700HP mediumvoltage motors that are now around 20 years old. Aside from the essential spare parts, a complete motor is used to replace assets that are removed for overhauls every three or four years. However, some of the maintenance routines are a little sporadic.

Sulzer has carried out regular planned maintenance and overhauls of rotating equipment for the minerals company, including the 700HP motors, which are interchangeable between different applications. When one of the motors failed unexpectedly, it was removed from service and sent to Sulzer’s Colton service centre for inspection and repair.

When the motor was disassembled, the inspection showed that the drive-end bearing had overheated and failed, allowing the rotor to contact the stator, damaging the windings in both the stator and the rotor and the bearing cap. The motor had been overhauled by Sulzer three years previously, so all the technical details of the asset were on record. This made ordering new windings much quicker, expediting the repair process.

REPEAT FAILURE

In the meantime, the rotor and stator were stripped out before checking for any defects in the laminations. In this case, some minor machining enabled any hot spots to be removed, and the core loss tests achieved satisfactory results. At the same time, a new bearing cap was manufactured in-house, ready for reassembly.

Gary Patton, Colton SC Manager, explained: “Under normal circumstances, the minerals company would have installed the spare motor, and production would have continued unaffected. However, in this case, the spare had recently been shipped to us for repairs following a premature bearing failure caused by a problem in the application.”

Unfortunately, in this case, the lamination stacks of the rotor and the stator could not be saved, so additional parts were required and this repair had a longer lead time. This dual outage significantly affected the production volumes of the minerals processed, which dropped by 25%. It was essential that the latest failure was repaired as quickly as possible.

Once the new coils, bearings, and seals were installed, the motor was reassembled and run on Sulzer’s inhouse testbed before being returned to the customer. The expected project time was five weeks, but Sulzer reduced this by a whole week by expediting the coils and working two shifts.

Gary concluded: “On this site, we found that the vacuum pumps are located beneath another process, which means that the motor cooling ducts can become blocked with debris. Unless this is cleared regularly, the motor and bearings can start to overheat, as happened in this case. A simple maintenance process could have prevented repeated failures of these motors and saved time and money for the business.”

Industrial applications such as this need reliable equipment, but repairs need to be of high quality and delivered quickly, especially when productivity is reduced due to equipment failure.

Sulzer has suggested adding a shelter for the motors to reduce debris build-up and enable them to operate reliably between planned overhauls. Bearing temperature sensors could also alert maintenance staff to the issue before the bearing fails.