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For more than 100 years, the leakage of liquids along the pump shaft from the casing has been minimized by an arrangement of materials collectively referred to as packing. Despite its dubious distinction of being the oldest part of the design of a modern process pump, packed stuffing boxes are still widely used because of their low initial cost and their familiarity to plant personnel.
However, environmental concerns are making packing increasingly unacceptable, particularly for dealing with the more aggressive liquids now common in our industrial processes. Consequently, mechanical seals are replacing packing in a growing number of applications.
A mechanical seal operates by having two flat faces running against each other. The rotating face is secured to the pump shaft, whereas the stationary face is held in the gland. Because one face is moving while the other is kept stationary, this type of seal is referred to as a dynamic seal.
In a basic seal (Figure 1), four possible leak paths must be blocked:
1. between the two seal faces;
2. between the rotating face and the shaft;
3. between the stationary face and the gland; and
4. between the gland and the stuffing box.
The last two leak paths are usually handled by static seals inasmuch as there is no relative motion between the two parts. These seals frequently are referred to jointly as the tertiary seal, and might consist of a flat gasket or an O-ring made of materials that are compatible with the process fluid.
In older seal designs, the secondary seal under the rotating face moves marginally back and forth on the shaft, thereby causing fretting corrosion and premature failure. However, in newer seal designs, the secondary seal remains static, avoiding fretting corrosion problems on the shaft.
In normal pump operation, the rotating and stationary faces are held closed by the liquid pressure in the stuffing box, which acts as the closing force. During startup and shutdown, the stuffing box pressure is augmented (and even possibly replaced) by spring force.
Most mechanical seals are designed with a rotating face made of a softer material that wears on a harder stationary face. For many years, the most popular combination was a carbon rotating face running on a ceramic stationary. These materials are still in popular use, but other face options now include stainless steel or harder materials, such as tungsten carbide or silicon carbide. Detailed discussions with your local expert will normally identify the best material combination available for the particular application.
Regardless of the materials used, a thin film of liquid must exist between the faces to provide some lubrication. However, a combination of spring load and liquid pressure in the stuffing box creates a closing force on the seal faces. Too high a closing force can substantially reduce the amount of liquid between the faces. This will result in increased heat generation and premature wear. If the closing force is too low, the faces can open easily and permit leakage.
Seal manufacturers are constantly trying to improve the flatness of the faces, which undergo lapping with special polishing plates. The finish is then checked on an optical flat using a monochromatic light source. Because of this, careful handling of these faces is essential and installation instructions should be strictly followed to ensure that the seal faces are suitably protected and precisely located.
Seal flexibility options
Any axial or radial movement of the shaft will require some flexibility from the spring(s) to keep the faces closed. However, only a certain degree of flexibility can be provided. The mechanical condition of the pump and its slenderness ratio (a measure of shaft diameter to overhang length, with lower values being better), also play an important role in the reliability of the seal. This seal flexibility is usually supplied by a single large spring, a series of small springs or a bellows arrangement.
The chemical industry traditionally has used seal designs where the force is applied to the rotating face. This is known as a rotary seal because the springs or bellows rotate with the shaft. More recent designs apply the springs or bellows to the stationary face. It is now quite common to find both stationary and rotating faces of a mechanical seal with some kind of flexible mounting arrangement.
Many seals of an earlier design use a single large spring that wraps around the shaft and provides a very strong closing force to the seal faces during the startup of the pump. The action of the seal depends upon the rotation of the shaft to tighten the coil.
Later seal designs (Figure 2) employ a series of smaller springs located around the shaft to provide evener loading to the seal faces. Because the smaller springs can clog more readily, most seals of this type locate the springs entirely out of the pumped fluid.
The most popular design for many aggressive applications is the metal bellows seal. It is made from a series of thin metal discs welded together to form a leaktight bellows (Figure 3). This creates a more uniform closing force between the faces and also eliminates the need for a secondary seal at the seal face, which automatically stops any possible fretting damage.
Although the main closing force normally is provided by the pressure in the stuffing box, the springs and bellows compensate for any shaft movement and keep the seal faces closed during startup and shutdown of the pump.
The problem of fretting
A pump shaft will undergo both radial and axial movement for a variety of reasons, including bearing tolerances, end play, vibration and shaft deflection. In addition, movement within the mechanical seal itself is also quite normal due to the difficulties in maintaining the two faces absolutely parallel. Such movement can be caused by equipment and installation tolerances, thermal growth, pipe strain or shaft misalignment.