Nearly everyone uses the sewage treatment plant facility every day: 99% of households in Germany are connected to a sewage treatment plant. These plants purify the waste water to such an extent that it can be returned to rivers and waterways safely, and, thus put back into the water circulation. The plants carry out this task non-stop, 24 hours a day, seven days a week. The basic precondition for this is that all waste water purification processes work smoothly. In the Stadtwerke Rotenburg an der Fulda’s waste water treatment plant, however, the problem of spiral pump gearbox failures has occurred repeatedly in the sludge recirculation pumping station.
Failures due to design
Stadtwerke Rotenburg a. d. Fulda’s sewage treatment plant is designed for a load of 34,000 population equivalent, and it actually caters to around 20,000 people at present. Three Archimedes pumps are connected to the secondary settlement basin at the sludge recirculation pumping station. They ensure that biological sludge from the secondary settlement basin does not flow into the Fulda, but back into the aeration basins, where the micro-organisms break down dissolved and fine particulate organic pollutants in the waste water. “The sludge from the biological treatment should, under no circumstances, enter the Fulda, because the micro-organisms would lead to environmental problems. We have to make sure that the pump drives in the sludge recirculation pumping station are always running perfectly. In an emergency, we can make do with submersible pumps to maintain the flow rate,” explains Antonio Genovese, Manager - Waste Water Treatment Plant, Rotenburg.
Due to a design fault, each of the three pumps failed within a period of about a year. The maximum service life of a gearbox was about two and a half years. At the time the failure occurred, only mechanical damage could be found as due to bearing damage, the gear block was literally torn apart and had to be completely replaced, which was a costly affair.
Monitoring solution implemented quickly
Stadtwerke Rotenburg has been working with Willich since 1992. The system integrator relies on automation solutions by Mitsubishi Electric. This means that various generations of Mitsubishi Electric inverters, control systems and I/O modules, as well as operating units, can be found in Rotenburg. To solve the problems in the sludge recirculation pumping station, Willich and Mitsubishi Electric proposed to the Stadtwerke the condition monitoring solution with FAG SmartChecks, which they intended to test in use in the field as a reference project within this context. Michael Böttner, a programmer at Willich, describes the idea and says, “By monitoring with the vibration sensors, we obtain information early regarding any deviations from normal values, which could lead to damage to the machine. The exact cause can be determined depending on the type of deviation, and the problem can quickly be targeted and corrected. This means that a time and cost intensive total breakdown can be avoided, with a positive effect on operational safety and overall operating costs.”
A few weeks passed from the original idea until monitoring was implemented – the system layout was defined in an onsite meeting, pre-wired and installed and commissioned within one day. The first pre-alarm came after nine months. After another four months, the gearbox failed completely. Genovese explains, “The documentation to date makes it clear as to how early the initial warning points to a deviation from the normal, which can presumably be corrected easily, but can lead to a total failure about three months later. With the assistance of condition monitoring, it is possible to target the fault and correct it. Depending on the severity of the problem, it is also 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 report alarm
Generally, a MELSEC controller monitors mechanical parts with the FAG SmartCheck, as in the case of Rotenburg, or also electrical components. The communication protocol SLMP, implemented specially for the Mitsubishi MELSEC controllers of the L and Q series, allows direct transmission of the characteristic values. An intelligent vibration sensor is 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 in case damage occurs is detected by the sensor through a rise in temperature difference, and thus corroborates the error message. “For example, if a gearwheel is faulty, vibrations occur on the drive, which are initially imperceptible to humans. Over time, further gearwheels become wedged into one another. It is not until a few months later that noises are heard as well, and then it is only a matter of a few weeks until the motor temperature rises and total breakdown finally occurs within a few days. So, vibration changes are the first sign that something isn’t right,” explains Böttner.
After installation, the system determines the standard values in normal operation by autotuning. 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, to which certain measures are allocated. Genovese elaborates, “The system differentiates between no alarm, and the pre-alarm and main alarm, i.e. a slight or massive deviation from the normal values. Then it’s our responsibility as operators to judge whether there is an immediate need for action or whether maintenance within the next 24 hours will suffice. An LED lamp directly on the sensor also displays the maintenance status – green means no alarm, and yellow pre-alarm, which means maintenance is needed. Red indicates a main alarm and immediate action is called for.”
The microprocessor integrated in the FAG SmartCheck not only creates the error messages, but stores all the values over the long term, so that they can also be viewed retrospectively and evaluated externally via the integrated web server through the MELSEC controller.
Minor hurdles in the development process
The monitoring solution integrated in the existing system consists mainly of three FAG SmartChecks by Schaeffler FAG, an eWon Router of the 4005CD type and a Mitsubishi Electric MELSEC System Q control system. Mitsubishi Electric and Schaeffler FAG have been working together since 2010 within the framework of the e-F@ctory Alliance; eWon has been an e-F@ctory Alliance partner since 2011.
The compact FAG SmartCheck monitors the vibrations in the pump drive, but is also able to record other machine and process parameters such as temperature, speed or pressure, which the MELSEC controller transmits, in correlation with the vibration. 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 compact, intelligent system monitoring is easy to operate and allows permanent decentralised online monitoring in real time.
For an accurate analysis of the vibrations, the FAG SmartCheck needs the values from the frequency inverters, which control the pumps with variable speeds to adjust to the relevant flow rate. The frequency inverters are not attached directly to the pumps in the case of Rotenburg, however, but are located in a switch cabinet about 50 metres away, and are controlled by a MELSEC control system installed there. “What now?” the installers wondered. There was no fieldbus network cable present, but there was an unused seven-core signal cable. The integration experts used this signal cable to connect the MELSEC controller with the process control system to which the three frequency inverters are connected. The connection problem was solved.
“To determine the speeds for the different flow rates, the MELSEC System Q had to access the values from the frequency inverters. Direct access to the inverters’ network was not possible, though. For this reason, we transferred the values to an existing process control system and created an analogue connection to the System Q via the seven-core signal cable,” says Böttner. “The Mitsubishi controller converted the analogue values into frequency values and passed these on to the FAG SmartChecks. This meant that we could use the existing control infrastructure. Additional network cables weren’t needed.” In this case, the controller does not fulfil a control task, but handles all the information for the FAG SmartChecks. It transmits the speed values from the inverters to the sensors for comparison, receives a status report if deviations are detected and passes this on to the higher level control system.
When the project was implemented, signal transfer to the PC at control level did not prove altogether unproblematic without an existing network connection. With the special remote control router 4005CD by eWon, however, the transfer of information could soon be achieved. The attached access point on the router passes all the information from the control system to the control centre via a wireless Ethernet connection.
For external access, eWon offers the Talk2M function with an integrated VPN (Virtual Private Network), through which the system integrator can configure and display the MELSEC System Q and the FAG SmartChecks. The information is exported as a web interface in Internet Explorer. System integrator Willich therefore has rapid, flexible remote access at any time.
Improved overall plant efficiency
The FAG SmartCheck makes an essential contribution to increasing overall plant efficiency and is part of the OEE Control Packs by Mitsubishi Electric. This scalable, real-time condition monitoring system assures high plant availability, whether in production or in sewage works as in Rotenburg, where it is not a matter of maximising productivity or profit, but ensuring continuous, disruption free operation. Overall plant efficiency and energy efficiency are closely connected with one another here, because predictive maintenance minimises life cycle costs – also by reducing energy consumption, thanks to early recognition of wear. In combination with an energy management system such as the Mitsubishi Electric Energy Control Pack (ECP), the OEE Control Pack can also use electrical current values for condition monitoring, which means energy costs can be reduced.
The OEE solution is suitable for various areas of use. In the Rotenburg sewage works, for instance, the condition monitoring system could be used on the inlet pumps, in the aeration basin, on larger drive systems and also on the cogeneration unit. As well as its use in waste water, solids, liquid, vacuum or heat pumps, the FAG SmartCheck can also be used, for example, in fans, ventilation units, compressors, CNC machines, separators or centrifuges. “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. This means that servicing can be planned better,” summed up Böttner. “With good results of the test run in Rotenburg, we can make the benefits of the system even clearer when talking to our customers,” he added.
Not least, the Mitsubishi Electric condition monitoring solution also brings with it some ecological benefits, in that it guarantees fault-free process sequences. For example, in the case of Rotenburg, it ensures that the sewage sludge stays where it belongs, namely in the sewage works and not polluting the Fulda.