Mechanical seals finally under control

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Technnical Paper by Lars Meisenbach

Damaged mechanical seals are the most common cause of pump failures. Seal failure means substantial damage to a plant: the production process is halted and broken seal parts may damage pump components, for example the impeller. Moreover, removing leaked pumping media is a costly special task. According to operators, broken seals are by far the most frequent cause of pump failures.

Fig. 1 BeMoS sensor element for smart seal monitoring

However, a smart pump designed with predictive maintenance in mind can prevent this. Mechanical seals often break within a few seconds under unfavorable operating conditions. In such cases, temperature and structure-borne noise measurements would raise the alarm much too late. To solve this problem, sensor specialist BestSens AG and Metax GmbH, specializing in sealing technology, have developed a new active monitoring system based on a fundamentally different approach.

The measuring system from BestSens AG continuously probes the lubricating film by means of ultrasound and detects any film crack within milliseconds. For that purpose, the sensor module (Fig. 1) is glued on the static ring of the seal.

Fig. 2 Smart seal with sensors (source: BestSens AG)

As a seal specialist, Metax  offers expert solutions when it comes to integrating smart seals into pumps, agitators, extruders and the like (Fig. 2). The sensor module from BestSens AG generates surface waves on the sealing ring and a fraction of this sound energy propagates directly across the sliding surface. If a lubricating film is being formed, these surface waves interact with the lubricating medium. By means of sophisticated signal analysis it is then possible to gauge the running behavior of the seal and categorize its state (Fig. 3)

Fig.3 Dashboard showing the running behavior of the mechanical seal until its failure (source: BestSens AG)

Up to now, a large n umber of seals have been equipped with sensors from BestSens and tested in the test laboratory of Metax (Fig. 4) under  extremely varying  operating conditions (temperatures, pressures, speeds etc.), including trial runs that eventually led to (intentional) dry running of the seals and subsequent destruction of the sliding surfaces. Meanwhile, more and more pump manufacturers inquire about smart seals that would enable the plant operators to maintain safe operating conditions without surprising seal failures.

Fig. 4: Test room with test rig for mechanical seals (source: Metax Dichtungstechnik GmbH)

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