Switched to Safety

The European Union’s Machinery Directive spurs bottling line maker Krones to revise its safety engineering model all the way down to the safety position switches.

Krones AG, Neutraubling, Germany is the world’s leading maker of bottling lines. With 12,000 employees worldwide and 2012 sales of $3.44 billion, it’s also a company that puts a very high value on safety: “Delivering functionally safe production equipment is vital to growing our worldwide customer base,” stresses Thomas Kellhammer, a member of the Krones research and development team.

When Krones decided to comply with the European Union’s Machinery Directive 2006/42/EC, it chose to refresh its entire safety engineering model. The standards’ new methods for assessing failure probability in components encouraged the change, Kellhammer says.

Today Krones bases safety engineering for the majority of its machines on the AS-Interface Safety at Work (ASIsafe, http://awgo.to/033) network standard. ASIsafe is based on the AS-Interface that’s designed to connect simple field I/O devices (e.g. binary On/Off devices such as actuators, sensors, rotary encoders and valve position sensors) using a single two-conductor cable. ASIsafe provides the flexibility needed to easily integrate other safety-related components.

Particularly important to Krones in this regard were position switches required for protective doors and hoods. Krones selected Sirius 3SE5/3SF1 safety position switches from Siemens (www.usa.siemens.com/safety), which would also help Krones comply with the EN ISO 13849 and IEC 62061 machine safety standards.

Kellhammer wanted to ensure that finely tuned adjustments could be made to the safety position switches, so it was essential that the safety characteristics required for defining the performance level or SIL were available from the outset.

Krones opted to use the “safety position switches with tumbler,” available in a plastic version with a locking force of 1,300 N, although a metal version with a locking force of as much as 2,600 N is available. The reason: these sensors are highly resistant to the cleaning agents the food industry uses. What’s more, notes Reinhard Wagner, a member of the Krones R&D team, "The position switches also provide IP66 and IP67 protection."

Safety switches with tumblers operate in accordance with the quiescent current principle. This means they ensure safe locking even in a power failure, which protects plants against unauthorized access.

Krones plants use two-channel fail-safe sensors. "In theory, it’s only possible to attain Performance Level c (PLc) or SIL1 using this equipment," Wagner explains. “But, by combining it with separate tumbler querying plus sophisticated switch diagnostics, we have actually attained PLd/SIL2 with the safety position switches with tumbler." Kellhammer adds that the diagnostic abilities of these safety position switches are important; for example, the AS-i master can detect whether the electromechanical contacts are working properly.

Additionally, without ASIsafe, even the process of wiring all possible potential signals would alone need the use of an appropriately thick 12-wire line with wires measuring 0.5 mm2 in thickness. The AS-i concept, however, only requires a T-connection to be fastened to the yellow trapezoidal cable and a spur line with an M12 screw connector to the Siemens position switches with integrated bus node.

Normally, with the highly dynamic processes that are typical of the beverage industry, plants would either have to use additional sensors in order to achieve a higher safety performance level, or the protection areas would have to be expanded significantly. "”Now, a second fail-safe position switch would only have to be added if PLe were required,” says Kellhammer.

>> Video: Krones’ EvoLite packaging machine for plastic bottles produces strapped packs with integrated handles that do not use shrinkwrap as secondary packaging. Click http://awgo.to/034

Safety position switches have several mechanical advantages that also contribute to safety. First, they’re designed with hygiene in mind—what the food industry calls "clean design." This makes them well suited to aseptic areas, where sterile bottling systems operate.

Second, the switches have a symmetrical drilling pattern for mounting. This lets their head drives be rotated easily by 90 degrees with each turn, which ultimately increases the flexibility of the structures. Also, by having a lateral direction of actuation, the safety position switches can be operated via the front.

Third, the switches feature a narrow design, which Wagner calls "extremely practical."

Another safety component that has proven its worth in practice is the tried-and-tested universal radius actuator, which can be individually adjusted to rotation motions. In addition, Krones installs extra mechanical guide pins on large protective doors to make sure the doors are guided correctly. Kellhammer and Wagner needed an actuator that was highly resistant to dirt—a feature that also contributes to both high switching reliability and high plant availability.

"We conducted comprehensive reliability tests to ensure that the actuators would be inserted in the switches properly,” Kellhammer says, “We can exclude certain faults within the scope of risk assessments and thus attain PLd/SIL2 using just one safety position switch with tumbler."

Krones uses fail-safe position switches with tumblers that are available with an auxiliary release as well as an escape release.  If a person is still present in the protected area, for example, the door can be opened from the inside simply by activating the escape-release function. It’s even possible for someone to open the door from the outside, as the escape-release feature can be attached to the front or rear of the switch.

In either case, the Krones plant would be switched to a safe state instantly and the relevant switch could then be manually activated to restart plant operation. To supplement this function, the safety position switches are available with a conventional auxiliary release feature and an auxiliary release feature with lock. 

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