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 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 this type of pressure switch is necessary 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 warm water to adapt it to the inner shape of the press. Then, consuming pressures of around 24 bar and temperatures of 150 �C to 170 �C the vulcanisation itself occurs. Along the way, with the help of sulphur, the tyre is given its elastic and robust final shape. However, if you have no appropriate monitoring, overpressure can occur in the curing press unexpectedly. If this happens, the danger of boiling hot steam escaping is high. To eliminate such an occurrence, tyre manufacturers generally use pressure switches to guarantee the safety of the process.
Vulnerable 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 at once: On the main one hand, they intervene in the event of overpressure, and on the other hand, 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 your 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 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. That is why, 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 risk 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 provide 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 may also be suitable for retrofitting. In the case of tyre vulcanisation, they can therefore help adapt existing installations to the specifications of DIN EN 16474, that was amended in 2015. This standard obliges tyre manufacturers to use all 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 are available on the WIKA website. Should 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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