SIL pressure switch: Safety in tyre manufacturing

Numerous industrial processes have an inherently high risk potential. Special pressure switches support the safety of individuals and the surroundings. Example: the vulcanisation of tyres for motor vehicles. To monitor this process, leading manufacturers use the model MW switch from WIKA with SIL 2 rating.
Why such a pressure switch is necessary for safety in this application is shown by way of a look at the process: In a steam-heated press, the raw tyre is first filled up with hot 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 addition of sulphur, the tyre is given its elastic and robust final shape. However, if there is no appropriate monitoring, overpressure may appear in the curing press without warning. If this happens, the chance of boiling hot steam escaping is high. To rule out such an occurrence, tyre manufacturers generally use pressure switches to guarantee the safety of the process.
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 one hand, they intervene in the event of overpressure, and on the other hand, they control the internal 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 your pressure switch lifts the lock. This mechanism is continually in motion in high-throughput automatic tyre production. Emerging to this fact, the switch is continually being loaded. Consequently, it will need to have an extended service life at this critical point. Premature wear would otherwise result in an unscheduled shutdown of the plant.
Lowest failure probability
Tyre manufacturers therefore require pressure switches with distinct robustness and safety. Because of this, WIKA has developed 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, an important safety aspect. A SIL 2 rating relative to the IEC 61508 standard certifies the functional safety of the pressure switch. Its operational safety is also SIL 2 certified. Thus, the expected service life is about 90,000 pressure cycles and switching operations. The chance assessment of the model MW in accordance with 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 around 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 instance in PLC applications, you will find a variant with gold-plated contacts.
Retrofitting existing plants
The model MW safety pressure switches from WIKA are also suitable for retrofitting. In the case of tyre vulcanisation, they are able to therefore help to adapt existing installations to the specifications of DIN EN 16474, which was amended in 2015. This standard obliges tyre manufacturers to operate new machines with improved safety technology. The regulation is intended 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. Should you have any questions, your contact will gladly assist you to.
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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