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.