SIL pressure switch: Safety in tyre manufacturing

Numerous industrial processes have an inherently risky potential. Special pressure switches support the safety of people and the environment. Example: the vulcanisation of tyres for motor vehicles. To monitor this process, leading manufacturers utilize the model MW switch from WIKA with SIL 2 rating.
Why this type of pressure switch is necessary for safety in this application is shown by a consider the process: In a steam-heated press, the raw tyre is first filled up with warm water to adapt it to the inner form of the press. Then, under the influence of pressures of around 24 bar and temperatures of 150 �C to 170 �C the vulcanisation itself occurs. In the process, with the help of sulphur, the tyre is given its elastic and robust final shape. However, when there is no appropriate monitoring, overpressure can occur in the curing press unexpectedly. If this happens, the danger of boiling hot steam escaping is high. To rule out this occurrence, tyre manufacturers generally use pressure switches to guarantee the safety of the procedure.
The model MW pressure switch with SIL 2 rating has been developed for safety-relevant applications, such as for example tyre vulcanisation.
Control of overpressure and interlocking
The instruments fulfil two functions simultaneously: On the main one hand, they intervene in case of overpressure, and alternatively, they control the inner interlocking of the curing press to avoid it from opening accidentally before vulcanisation is completed. The process is finished as soon as a preset pressure value is reached. Then the pressure switch lifts the lock. This mechanism is constantly in motion in high-throughput automatic tyre production. As a result, the switch is continually being loaded. Consequently, it will need to have a long service life as of this critical point. Premature wear would otherwise lead to an unscheduled shutdown of the plant.
Lowest failure probability
Tyre manufacturers therefore require pressure switches with distinct robustness and safety. For this reason, WIKA is rolling out the model MW pressure switch for safety-relevant applications such as vulcanisation. As a mechanical device, the model MW will switch without dependence on any energy supply, a significant safety aspect. A SIL 2 rating in accordance with the IEC 61508 standard certifies the functional safety of the pressure switch. Its operational safety can be SIL 2 certified. Thus, the expected service life is about 90,000 pressure cycles and switching operations. The chance assessment of the model MW relative to EN ISO 13849-1 by an external certification led to the best performance level ?PL e? (lowest failure probability).
Microswitches, also with argon filling
All model MW instruments have microswitches that switch electrical loads up to 250V AC, 20 A directly. For certain applications, microswitch versions with argon filling offer an additional boost to safety and service life. The gas reduces oxidation, protects against corrosion and prevents sparking. For low switching powers, for example in PLC applications, there exists a variant with gold-plated contacts.
Retrofitting existing plants
The model MW safety pressure switches from WIKA are also suitable for retrofitting. In Captivating of tyre vulcanisation, they are able to therefore help adapt existing installations to the specifications of DIN EN 16474, that was amended in 2015. This standard obliges tyre manufacturers to operate new machines with improved safety technology. The regulation is supposed to avoid risks of danger in the long term.
Note
Further information on the model MW pressure switches and their safety features can be found on the WIKA website. In case you have any questions, your contact will gladly help you.
Also read our articles
SIL-certified: Functional safety with mechanical switches
Mechanical pressure switches: How does one set the switch point?

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