Analysis helps Corus avoid "bad vibrations"

The latest vibration monitoring technology is playing a vital role in helping Corus Strip Products UK cut long term operating costs, increase uptime and improve safety at a plant in South Wales.

The latest vibration monitoring technology, manufactured by Monitran, is playing a vital role in helping Corus Strip Products UK (CSP UK) cut long term operating costs, increase uptime and improve safety at its granulated coal processing plant in Port Talbot, South Wales.

The processing plant is used to prepare coal for injection into blast furnaces for iron production and is based around a large coal impacting (crushing) system, which is located in a special purpose building.

These crushers are constructed from two counter-rotating cages, located one within the other, and incorporating circumferential striking plates (hammers).

The cages each weigh approximately 0.75t and are suspended from two overhung shafts, driven via pulleys from electric motors.

The cages are continuously fed with coal, which is effectively disintegrated and reduced to fine particles; the granulated coal falls through the cages, via chutes and is transported away to the blast furnace area, to be used in the subsequent coal injection process.

The combined impact of the coal, especially when wet, plus the weight and rotation of the cages places considerable stress on the overhung shafts.

The Engineering Manager in charge of the Engineering Technology Groups Advanced Maintenance Techniques section (part of CSP UKs Central Mechanical Engineering) Michael Whittaker, explains: "As a result, bearing and seal wear within the shafts can be relatively high.

Until recently, components were failing as regularly as once a month leading to broken, bent and sheared shafts.

In each case we had to shut down the machine and take it off line while replacement parts were fitted; subsequent repairs could then take anything up to a week to complete".

The problems were exacerbated by the fact that the resonance profile of the steel-clad screen house in which the coal crushers operate matches that of the machine itself at around 12Hz.

As Whittaker points out: "Under normal circumstances this had little effect, but as forcing frequencies (eg imbalances in the cages) developed so they were amplified by the natural resonance of the structure".

In an effort to extend the operating life of the shafts Michael Whittaker and his team developed a predictive maintenance regime, involving the collection of vibration data from the crushers.

The company initially used handheld vibration meters which, although relatively effective, were limited in their application.

"The difficulty was that the portable monitors had to be operated from a safe distance because of limited access to the shafts, thereby making it difficult to gather sufficient or consistent data", explains Whittaker.

"We realised that a more sophisticated approach was required and approached Monitran for assistance.

Working together, we carried out considerable analysis before developing a hard-wired solution based around Monitran's MTN/6000 19in multichannel rack unit, plus MTN/1100CM8 accelerometers, which are permanently mounted on the shafts and at other key locations around the machine.

Separate speed and temperature sensors are also fitted giving access to a total of 28 different monitoring points".

The rack unit also features preset alarms, a display module and a buffered AC output, to enable a data collector to be connected.

The new system, which went live in October 2001, now enables Corus to perform continuous or ad-hoc vibration analysis, impose acceptance standards for maintenance contractors and provide operators with a real time overview of current machine conditions.

"The new vibration analysis system", Whittaker notes, "provides a wealth of valuable data that allows us to carry out far more detailed analyses than was previously possible.

As a result, we now have a far better understanding of the factors affecting the performance of the crushers and the parameters that we need to consider when forward planning our routine maintenance schedules".

As the system allows the Corus engineers to analyse vibration on specific shafts and at other key points on the machine, providing advance warning of possible problems, downtime has been minimised, to the extent that the Port Talbot plant hasn't experienced a single shaft failure.

Potential bearing failures have been identified, allowing planned maintenance to be performed in advance of potential catastrophic failure.

In cost terms Corus has realised a 7:1 reduction in third party maintenance costs between 2001 and 2002.

Michael Whittaker summarises the key benefits of Monitran's equipment: "It's a very cost-effective system, which allows us to customise our requirements and connect to Scada channels in order to set up mimics and displays for operators.

The integrated MTN/1100 and MTN/6000 set up gives us a comprehensive tool which not only monitors the plant but also provides us with additional analysis data so we can understand failure modes and drive our maintenance activities accordingly.

Ultimately, it has provided clear evidence of the value of an effective condition monitoring regime".

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