In a wastewater treatment plant, spiral pump gearbox failures occurred repeatedly in the sludge recirculation pumping station, causing the site to use submersible pumps to maintain the flow.  The introduction of vibration monitoring was key to preventing further wastewater pump failures.

The maximum service life of a gearbox is about two and a half years.  Following the failure of three pumps in twelve months due mechanical damage with bearing damage causing the gear block was destroyed. To solve the problems in the sludge recirculation pumping station, the local integrator and Mitsubishi Electric proposed using a condition monitoring system.

Early warning of problems

By monitoring with the vibration and temperature sensors the user obtained early information on any deviations from normal values which could lead to damage to the machine.  Depending on the type of deviation the exact problem can be determined and the problem can removed and a costly total breakdown avoided.

Depending on the severity of the problem, it is possible to keep using the motor until the end of its service life, but order a replacement in good time thanks to the advance warning. Replacement can therefore be carried out without any time delay.

Intelligent sensors

An intelligent vibration sensor was attached to each of the three pump drives and the system provides early information on vibration changes which are above the defined threshold values of the normal range. The rise in temperature associated with this if damage occurs is detected by the sensor through a rise in temperature difference, and thus corroborates the error message.

After installation, the system determines the standard values in normal operation by auto-tuning. Certain alarm ranges are established on this basis depending on the type, strength and origin of the vibrations. The warning messages, sent via the controller to the control centre or remote maintenance unit, differentiate in clear text between different breakdown types such as bearing damage, imbalance, incorrect axis alignment or lack of lubricant. The deviations are translated into specific error codes.

Generally, a Mitsubishi MELSEC Q Series PLC monitors mechanical parts with the intelligent condition monitoring device.  The PLC differentiates between no alarm, and the pre-alarm and main alarm, i.e. a slight or massive deviation from the normal values. It is then for operators to judge whether there is an immediate need for action or whether maintenance within the next 24 hours will suffice.

A SLMP communication protocol allows direct transmission of the characteristic values to any Mitsubishi MELSEC controller from the L or Q series.  The sensors store all the values so they can be viewed via the integrated web server through the MELSEC controller for analysis.

Inverter drive monitoring

The values from the frequency inverters controlling the pumps with variable speeds to adjust to the relevant flow rate are also monitored. This means that the measuring system recognises possible damage to the machine reliably at an early stage, as a function of the parameters. It contributes to preventing unplanned downtimes and stopping expensive consequential damage.

The monitoring solution contributes to the overall plant efficiency and is part of the OEE Control Packs by Mitsubishi Electric (OEE = Overall Equipment Effectiveness). It ensures high plant availability through automatic system monitoring and the Vibration monitoring is key in preventing wastewater pump failures.

According to the end user, “the vibration sensor can guarantee efficient condition monitoring wherever mechanical, rotating parts lead to wear, or where changes occur because of other influences. In the Mitsubishi Electric solution the sensor measures condition changes which are compared with an established normal value. If this value is exceeded, an error message is generated – and promptly, some months before total failure occurs.

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