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Mechanical Seal Systems

How to Select the Right Seal for Mixing Applications

| Author / Editor: Bernd Reichert / Dr. Jörg Kempf

Pressure maintenance: Alternatives for pressure maintenance are continuous flow systems or pressure compensator arrangements.

Cooling: The physical processes taking place on the seal faces are very sensitive to high temperatures. If critical values are exceeded, this may lead to localized areas of drying, resulting in “hot spots” and greater shear stresses on the surface of the seal rings. The sealing function is compromised as soon as the surface structure has been destroyed (blistering). Therefore, the heat conducted to the seal from the vessel and that caused by friction in the seal interface must be continuously removed. Continuously operating cooling systems (Fig. 3) are extremely important for reliable operation.

Flushing: In many processes, corrosive or erosive substances contaminate the surfaces of the seal rings. To protect the rings against these substances, they are flushed with a liquid that is compatible with the process taking place in the vessel and the product-wetted mechanical seal components.

Seal liquid with automatic refilling system: An outstanding characteristic of mechanical seals is their very small leakage rate, even at elevated vessel pressures. A leakage rate of only 20-50 ml/day can be expected during normal agitator operation at vessel pressures up to 70 bar. Nonetheless, it is still recommended to monitor the leakage and refill automatically when necessary, in particular under conditions of continuous operation. An automatic refilling system can be seen in Fig. 2 that is controlled by the level indicator of a pressure compensator.

Seal liquid — emergency supply: In the event of an unexpected increase in the leakage rate due to damaged seal rings, the amount of escaping seal liquid may no longer be sufficiently replenished. To maintain the positive pressure difference between the mechanical seal and the vessel, and thus maintain the lubricating function, the seal liquid (often water) is circulated through the mechanical seal with a higher flow rate. This allows the reactor to continue operating for a certain time after a leakage has occurred. Fig. 4 shows an overview of the application criteria for modules of seal liquid supply systems.

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