Safeguarding a 119-path 'pigging' system

Friday, 13 October, 2006

A silicone pig sweeps through the lines at Wolf Blass, entering a 'key station' where winery staff must choose one of 119 paths with the precision of an air traffic controller.

Made to form a tight fit within the pipes, the pig is a flexible 75 mm diameter amalgam of food-grade silicone materials pushed along by propellant gases. Rather than using water to flush pipes, the pig literally sweeps wine from the hundreds of metres of piping that extends around the winery to buffer tanks and the bottling plant.

The benefits over flushing with water are compelling. Because there is no mixing of water and the 2000 litres of wine that remain in the line at the end of each run, Wolf Blass conserves every drop of its production, eliminates the risk of contamination, reduces water consumption and makes significant time savings.

Used both for cleaning pipelines with caustic solutions and the changeover from one wine variety to another, pigging takes place on average eight times a day.

While the process sounds straightforward, its design was not. At the heart of the system is a key station where pipelines from around the plant converge and connections are made to direct the pig from one area to the next.

For each run, three large U-shaped manifolds join pairs of lines to form a path for the pig. The permutations and combinations are staggering, resulting in a total of 119 possible connections.

Such complexity presented a challenge for the designer, Wolf Blass project engineer, Ross Wilkinson.

"We did a risk assessment and found that without a good safety system, the risks caused by a bend not being in place would be much too high," he said. "There were also risks if a caustic solution was being used for cleaning."

Wolf Blass determined the system needed controls to meet Category 3 under Australian Standard 4024.1 and called in Sage Automation's Rob McDonald and Steve Hack for expert assistance with the safety system's design.

"The pig is propelled with 3 bar of pressure," Steve said. "It was clear we needed devices sensing whether the connections matched the path. To achieve Category 3, the performance of these devices had to be monitored and, if there were any failures, the system had to be able to shut the process down immediately."

The solution was elegantly simple. Each of the bends incorporates a central metal tang and when the bend is correctly connected, the tang is positioned over a Pilz PSEN 2.2 non-contact safety switch, which senses its presence. Unlike proximity switches, which can be falsely triggered by metals in the surrounding environment and fail to 'on', the Pilz PSEN 2.2 safety switch is well protected against defeat.

The bank of safety switches is monitored by a Pilz Programmable Safety System (PSS), or 'safety PLC'. Once a connection is made, the PSS and an operations programmable logic controller (PLC) interact to confirm the path is safe and the pig is deployed. If the connection is removed while the pig is in motion, an alarm is activated and the propellant shut off immediately, de-energising the system.

The status of each line is easily checked by operators. An indicator light paired with every switch glows green when the line is depressurised, amber when a wine order is in transit and red during clean in place (CIP) procedures. The entire system is also presented graphically on a screen or 'human machine interface' attached to the PLC.

Seven months on from installation, Ross reports operators are pleased with the pigging system and its controls.

"Operators have responded extremely well," he said. "When they're ready to pig, they just hit the button and it does its thing. In other plants, someone has to walk the lines to make sure all settings are correct and then radio back for someone to instigate the flushing.

"It's a great system and if we were building a similar facility, we'd do it all again," he said. "We give it a fair hiding and it just runs."

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