General design characteristics

Fig. 5 shows a typical mechanical seal design for an agitator. Important design elements such as the cartridge seal, drive collar, thrust bolster, flushing system, etc., are found in many different seal types built on the modular assembly principle.

Cartridge seal
The outstanding feature of cartridge seals is that all parts affecting the sealing function can be preassembled and tested outside the machine. The connections to the shaft and flange are usually made up with a shaft sleeve and housing. Secondary sealing elements are employed to isolate the seal rings themselves from these components.

The advantages of this seal design are the ability to restore machine operability quickly during servicing and easier, more reliable assembly of the seal outside the machine.

Modular system
With the modular assembly system, it is a relatively simple matter to combine the few standardized components of a particular seal type to obtain a mechanical seal for a given set of sealing requirements. In operation, standardization of the seal ring geometries and materials facilitates immediate replacement of these wear parts, thus reducing the seal's downtime to an absolute minimum. Besides, as Fig. 6 illustrates, it is possible to design entire seal ring pairs of different seal types for interchangeability. The housing parts are designed to allow direct attachment of other parts, e.g. an integrated bearing or water cooling parts. If a given seal has to stand up to various products, resistance to attack is achieved merely by replacing a few standardized minor components.

In other words, modularly designed mechanical seals meet the need for rapid availability, effectively reduce spare parts stocks, and permit economical solutions for special applications.

Drive collars and thrust bolster
The frictional moment of the mechanical seal has to be transmitted from the machine shaft to the rotating seal rings and from the stationary seats to the abutting housing parts. In addition, the forces applied to the shaft sleeve by the pressure difference have to be absorbed.

The most frequently used components are the split clamping ring that acts directly on the shaft and fits against the rotating parts of the seal, and a shrink disc on the shaft sleeve. Shrink disks necessitate higher machining precision in the adjacent parts and extra space to allow for relatively complicated assembly. Tight-fitting parts on the shaft often cause undesirable scoring.

If the axial forces can no longer be effectively transmitted by friction, e.g. in the case of high pressures and large diameters, tight-fitting bolsters have to be provided at the next shaft shoulder or at a hub or coupling.

Flushing
In the conditions prevailing in the vessel might damage the seal, such as very high temperatures, excessive dust, or a corrosively or chemically aggressive product, the mechanical seal can be protected with a flushing system on the product side. Flushing medium, flow rates, and flushing intervals are adjusted to suit the specific requirements. In cases where the seal is pressurized, it is advisable to tap the reference pressure within the flushed zone.

Materials
Because the seal rings have to withstand high temperatures along with corrosive and abrasive attack, they are usually made of high-grade carbons and ceramics. These materials have the added advantage of not deforming plastically.

The substantial amounts of graphite contained by high-grade carbons gibe them valuable emergency-running properties and high thermal conductivity. Their porosity necessitates impregnation with synthetic resins or metals. Carbon seal rings are regarded as soft because of their relatively low E-modulus. The high-grade carbon composition is selected to suit the duty. For example, special dry-running carbons are used for unlubricated applications.

Seal rings of silicon carbide, tungsten carbide or aluminum oxide are hard and therefore nondeforming. Silicon carbide is used most widely because of its versatility and reasonable price.

Where low pressures are involved, seal ring pairs consisting of a hard/soft combination can perform just as well as a result of higher pressures, the hard ring helps to maintain the seal face's shape while its soft partner provides the necessary emergency-running properties.

High-grade carbons are not suitable for abrasive substances. Pair comprising two hard seal rings are limited with regard to the pressure difference they can withstand and thermal loading, especially in the mixed friction range.