PICC improves Profibus network stability at a biosolids recycling plant



The Melbourne Water Western Treatment Plant recycles biosolids from Barwon Water’s key water reclamation plants, drying and processing them into pellets, which can be used as a fossil fuel replacement or fertiliser.

Like most modern industrial facilities, the plant has a Profibus network as the backbone for its entire process operation. The networks at the plant, for example, allow Barwon Water to monitor its air discharge points to ensure that its odorous gas scrubbers and biofilters are working properly.

The network also enables the EcoWarrior energy management system to track the facility’s greenhouse gas emissions and power and gas usage in real-time, in order to optimise operations and loads and help track equipment performance over time.

Despite the plant only being a few years old, Barwon Water was facing reliability issues with its Profibus network. It brought in the Profibus International Competence Centre (PICC), a subsidiary of Pentair Actuation & Controls to assess the situation.

 

Solution:

According to PICC engineer Saiyaz Hasim, the original network was not designed according to Profibus specification, and this, coupled with a lack of onsite fieldbus expertise, experience and tools, was causing a lot of operational disruption.

“Devices were falling off the bus in a random fashion. Sometimes, in the night, the process would run very well, and all of a sudden some part of the network would fail,” explained Hasim.

The PICC audited the network and produced a report detailing the issues that were causing operations disruption. It found issues with the basic design of the network.

“All the power and signal cables were pushed into one cable tray. We found high voltage cables at 415 or 240V were laid out beside Profibus cables, which is against Profibus specification,” Hasim said.

PICC auditors also found that the network was running at 1.5Mb/s, a faster speed than required, which was causing stability issues. Additionally, critical devices were daisy-chained in the network. “If there was a problem with any of the devices, the other critical devices would fail too,” explained Hasim.

After Barwon Water analysed the report, it tasked the PICC with carrying out the recommended fixes.

The PICC reduced the network speed to a more manageable 500kb/s, and introduced ProfiHubs into the system, breaking up the daisy-chains and connecting critical devices to individual channels. “If any of the devices fail, technicians can now isolate the affected segment and work on the problem device,” said Hasim. “This avoids the costly alternative of having to shut down the entire plant to change one or two affected devices.”

The PICC also found there were also a number of inactive devices that were still connected to the network, adding unnecessary distance to the network.

“We removed all these devices no longer in operation and installed shorter cables alongside the ProfiHubs. This modification effectively reduced the length of the cable from 500 meters to almost half that length,” said Hasim.

In revising the network, the cables were re-laid to ensure compatibility with Profibus specifications. “For higher voltage cables, we used steel conduits or put it at a distance as required by the Profibus specification,” Hasim explained.

 

Conclusion

The PICC stabilised the network by moderating its baud rate, reducing the cable length, eliminating redundant devices and introducing ProfiHubs to make it more flexible.

As a result, Barwon Water’s network now functions as a very robust system. Upon completion, the PICC prepared another report and since then there have been no problems with the network.