Mechanical Seals
A mechanical seal is a sealing device which forms a running seal between rotating and stationary parts. They were developed to overcome the disadvantages of compression packing. Leakage can be reduced to a level meeting environmental standards of government regulating agencies and maintenance costs can be lower. Advantages of mechanical seals over conventional packing are as follows:
- Zero or limited leakage of product (meet emission regulations.)
- Reduced friction and power loss.
- Elimination of shaft or sleeve wear.
- Reduced maintenance costs.
- Ability to seal higher pressures and more corrosive environments.
- The wide variety of designs allows use of mechanical seals in almost all pump applications.
The Basic Mechanical Seal
All mechanical seals are constructed of three basic sets of parts as shown in Fig. 9:
- A set of primary seal faces: one rotary and one stationary?shown in Fig. 9 as seal ring and insert.
- A set of secondary seals known as shaft packings and insert mountings such as 0-rings, wedges and V-rings.
- Mechanical seal hardware including gland rings, collars, compression rings, pins, springs and bellows.
Fig. 9 A Simple Mechcanical Seal
How A Mechanical Seal Works
The primary seal is achieved by two very flat, lapped faces which create a difficult leakage path perpendicular to the shaft. Rubbing contact between these two flat mating surfaces minimizes leakage. As in all seals, one face is held stationary in a housing and the other face is fixed to, and rotates with, the shaft. One of the faces is usually a non-galling material such as carbon-graphite. The other is usually a relatively hard material like silicon-carbide. Dissimilar materials are usually used for the stationary insert and the rotating seal ring face in order to prevent adhesion of the two faces. The softer face usually has the smaller mating surface and is commonly called the wear nose.
There are four main sealing points within an end face mechanical seal (Fig. 10). The primary seal is at the seal face, Point A. The leakage path at Point B is blocked by either an 0-ring, a V-ring or a wedge. Leakage paths at Points C and D are blocked by gaskets or 0-rings.
Fig. 10 Sealing Points for Mechanical Seal
The faces in a typical mechanical seal are lubricated with a boundary layer of gas or liquid between the faces. In designing seals for the desired leakage, seal life, and energy consumption, the designer must consider how the faces are to be lubricated and select from a number of modes of seal face lubrication.
To select the best seal design, it's necessary to know as much as possible about the operating conditions and the product to be sealed. Complete information about the product and environment will allow selection of the best seal for the application.
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