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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.
To keep the seal faces together, the springs are constantly adjusting the seal in relation to the moving shaft.
When an elastomer is used between the rotating face and the shaft, the elastomer moves back and forth on the shaft. This creates a polishing action that repeatedly removes the protective oxide coating from the corrosion-resistant material of the shaft and eventually forms a groove at that point on the shaft. The groove causes leakage and necessitates repetitive repair or replacement of the shaft. To combat this problem, a sacrificial shaft sleeve is usually installed in the way of the stuffing box.
However, the only lasting solution to the problem of fretting corrosion lies in the elimination of the dynamic seal. Most major seal manufacturers now produce non-fretting seals that protect the pump parts from fretting corrosion.
Balanced and unbalanced seals
The balance of a mechanical seal determines the magnitude of the closing force on the faces. This force depends upon the effective cross-sectional areas of the seal, as well as the pressure in the stuffing box.
An unbalanced seal exposes the full cross-sectional area of the reverse side of the rotating face to the stuffing box pressure and creates a high closing force between the seal faces, which can increase the operating temperature and accelerate wear rate. These conditions can dramatically reduce seal life in high temperature services or where the liquid is aggressively abrasive.
Balancing a mechanical seal reduces the closing force and extends the life of the seal. It is usually achieved by reducing the effective cross-sectional area of the rotating face by using a stepped shaft or sleeve. However, this is never taken to a point approaching a net closing force of zero because it is possible that the condition between the seal faces can become unstable and might be blown open by any sudden change.
While the balanced seal might appear to be the answer to all sealing problems, certain services might be better served with the unbalanced seal. For example, some applications might require more emphasis on security from leaks than seal longevity; this might translate into a greater desire for a high closing force in the selected seal. Also, when sealing a cold liquid, an increase in operating temperature might be of little concern.
Regardless of any other consideration, a balanced seal is usually recommended when the stuffing box pressure exceeds 50 psi.
Inside and outside seals
Positioning the seal inside the stuffing box is the most popular arrangement. While this requires disassembly of the pump wet-end to carry out any maintenance on the seal, it offers a major advantage in the ease with which the seal environment can be controlled.
An outside seal reverses the orientation of the stationary face and locates the rotating unit on the shaft outside the stuffing box gland. It offers five key benefits:
1. ease of installation;
2. relatively low cost;
3. the ability for continual monitoring and cleaning;
4. suitability for stuffing boxes too small for an internal seal; and
5. less susceptibility to shaft deflection difficulties as it is located closer to the bearings.
The major drawback is that centrifugal force will throw any solid particles into the seal faces from the underside of the seal. Consequently, this seal is primarily used with clean, nonabrasive liquids.
The split seal is an important addition to the outside seal in recent years. The split seal is a complete assembly that is placed between the stuffing box and the bearing housing and is designed to eliminate the need to dismantle the pump every time the seal needs to be changed. These seals are gradually being developed to incorporate all the other design criteria discussed. Because of the simplicity of changing the seal with this design, it is important to resist the temptation to merely change the seal and not investigate the root cause of a failure.
The cartridge seal is a completely self-contained assembly that includes all the components of the seal, gland and sleeve in one unit. Because it does not require any critical installation measurements, this type of seal simplifies installation procedures while simultaneously protecting the faces and elastomers from accidental damage. These benefits also translate into reduced maintenance time for changeouts.
Cartridge arrangements are available for almost every type of seal on the market, and therefore can eliminate the risk factors and extra maintenance hours inherent in the use of conventional component seals.
Double seals and barrier fluids
Using a seal with two sets of faces instead of a single seal gives a higher degree of leakage protection. Such double seals most frequently handle volatile, toxic, carcinogenic, hazardous and poorly lubricating liquids.
There are three distinctive arrangements of double seals, all of which require the use of a barrier fluid system to maintain a liquid or gas barrier between the two sets of seal faces.
A commonly used low-cost double seal arrangement is referred to as the back-to-back seal. It positions the rotating faces in opposite directions. It always should have a barrier fluid pressurized to about 20 psi above the stuffing box pressure; this ensures that the inner seal is lubricated at all times by the barrier fluid and also contributes to the closing force on the seal faces.
In the more sophisticated face-to-face seal, the rotating faces point toward each other (Figure 6); they often act on opposite sides of the same stationary face. This seal can use either a high- or low-pressure barrier fluid system.
The third arrangement, the tandem seal, has both rotating faces pointing in the same direction, away from the impeller. Here, the barrier fluid pressure is normally lower than the pump pressure, and the two seals combine to operate as a two-step pressure breakdown device.
All types of double seals require barrier fluid systems. They usually are external closed-loop systems containing a fluid that is normally different, but compatible, with the process liquid. The system contains a reservoir that should be as close as possible to the seal.
The design of these systems varies widely. Some use a pumping ring in the seal, whereas others rely upon a thermosyphon effect where the difference in fluid temperature between two legs in a loop initiates a continuous flow around the system. Auxiliary heating or cooling frequently is provided for fluid in the reservoir. In addition, an alarm might be included to alert staff to changed conditions.
Depending upon the nature of the fluid being sealed, the barrier fluid system might be operated at a higher or lower pressure than the pressure in the stuffing box.
To further the drive toward zero emissions, the seal industry has developed gas barrier sealing, which uses an inert gas such as nitrogen in place of a liquid barrier system. With any failure of the inner set of faces on a gas-barrier double seal, only the inert gas will leak; no liquid will escape to cause possible contamination of the environment.
Regardless of whether a liquid or gas is used, the barrier system must be dedicated to the specific seal and alarmed in such a way that any failure of the inner set of faces can be immediately recognized so appropriate action can be taken.
In many applications, reliable operation of a single seal requires control of the environment in which the seal is located. Keep the following pointers in mind:
1. Mount the seal on a strong shaft with minimal deflection. Although the industry standard defines a maximum deflection of 0.002 in. at the seal faces, an even stiffer shaft often is desirable.
2. Locate the seal in a large-bore seal chamber. These enhance seal reliability and are available from almost all pump manufacturers.
3. Control the pressure in the stuffing box to avoid flashing.
4. Keep the temperature in the stuffing box within the operating parameters of the seal materials.
5. Maintain the cleanliness of the fluid in the stuffing box.
Some final thoughts
In the selection of a mechanical seal, it always is important to recognize the benefit of onsite experience. If a particular seal has a history of operating well in the same service under similar conditions, then this should take precedence over any other consideration.
When the pump supplier is left to select the mechanical seal for its pump, the user needs an effective method for evaluating the seals chosen by various vendors. (A copy of one such evaluation form is available by contacting the author via e-mail.) It, however, is far better if the user becomes sufficiently well informed about mechanical seals and their ongoing development to be able to specify the particular seal needed in any application. Requiring all pump suppliers to include that model in their pricing can simplify the evaluation process.
A practical resource
The material for this article is excerpted from The Practical Pumping Handbook. It offers real-world advice for solving problems that cause frequent and repetitive downtime, and also incorporates reference data, including material recommendations, viscosity correction charts, friction loss tables, conversion data and essential formulae. This resource can be ordered online at www.practicalpumping.com.
Ross Mackay has more than 40 years of experience in the pump industry and is president of Ross Mackay Associates Ltd., Aurora, Ontario, a firm he started in 1990 to provide training and consulting on pumps. He can be reached at 1 (800) 465-6260 or via e-mail at firstname.lastname@example.org.