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 technique, leading manufacturers utilize the model MW switch from WIKA with SIL 2 rating.
Why such a pressure switch is essential for safety in this application is shown by 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 shape 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, if 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 such an occurrence, tyre manufacturers generally use pressure switches to ensure the safety of the procedure.
The model MW pressure switch with SIL 2 rating has been developed for safety-relevant applications, such as 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 alternatively, they control the inner interlocking of the curing press to prevent it from opening accidentally before vulcanisation is completed. The procedure is finished the moment a preset pressure value is reached. Then the pressure switch lifts the lock. This mechanism is continually 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 reliance 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 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 relative to EN ISO 13849-1 by an external certification led to the highest 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 several 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, there is a variant with gold-plated contacts.
Retrofitting existing plants
The model MW safety pressure switches from WIKA may also be 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 all new machines with improved safety technology. Overnight is supposed to avoid risks of danger in the long run.
Note
Further information on the model MW pressure switches and their safety features can be found on the WIKA website. When 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: So how exactly does one set the switch point?

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