Double-acting mechanical seals

Double-acting mechanical seals are clearly dominant in agitator applications. Their two sealing faces enclose an annular chamber, which is filled with a buffer fluid compatible with the product. This fluid lubricates both pairs of seal rings. Some seals are designed for pressurization of the fluid, e.g. to improve the installation\'s safety. Depending on the spring configuration, one distinguishes between back-to-back seals (spring-loaded seal rings between the stationary seats), face-to-face seals (stationary seat between the spring-loaded rings), and tandem seals (springs acting in the same direction).

Double-acting mechanical seals are used

• wherever it is absolutely necessary to avoid product leakage (explosion hazard, environmental protection)

• in the case of gaseous media or media with high vapor pressure or poor lubricity, especially where the seal is installed vertically on top

• where the medium is under high pressure and at high temperature

• for many media that tend to polymerize or adhere to or build up on surfaces.

 
The advantages of standardized, cartridge-type seals have created an exceptionally broad application range for double-acting mechanical seals in agitator technology. The various types differ mainly in the pressure levels for which they are designed. Special designs exist for sterile and bottom-entry applications.

Typical mechanical seal for agitators
Seal ESD34G, which is shown in Fig. 8, has the typical characteristics of a mechanical seal built for sealing an agitator.

It consists of two identical seal ring pairs arranged back-to-back, which are supported hydraulically balanced by elastomer secondary seals in the form of O-rings. This configuration keeps the seal rings in exactly the same position even in the event of a pressure reversal. Because the O-rings have to withstand only the pressure applied by the springs, which is independent of the direction of rotation, they remain elastic´. This maintains the parallelism of the seal ring faces in spite of the high radial deflections often displayed by agitators. The duty limits of the ESD34 are listed in Table 3.

Sterile and bottom-entry seals have to meet special requirements. The gap between the shaft and the seal rings proximal to the vessel has to be kept free of contamination. If the seal rings are shifted into the vessel and are flushed from outside by the product, this gap is eliminated. this also opens the way to GMP-compliant design of the mechanical seal and provides optimal sterilization when the seal is used in a sterile installation. Type ESD44, shown in Fig. 6, is designed for these applications. these seals, which are also designed on the modular assembly principle, are normally adapted to the vessel or mounting flange as displacers.