Mechanical Seals Versus Packing

Sealing with a packed stuffing box is impractical for many conditions of service. In an ordinary packed stuffing box, the sealing between the rotating shaft or shaft sleeve and the stationary portion of the stuffing box (or seal chamber) is accomplished by means of rings of packing forced between the two surfaces and held tightly in place by a packing gland. The leakage around the shaft is controlled by merely tightening or loosening the packing gland nuts (or bolts). The actual sealing surfaces consist of the axial rotating surface of the shaft or shaft sleeve and the stationary packing. Attempts to reduce or eliminate all leakage from a conventional packed stuffing box increase the compressive load of the packing gland on the packing. The packing, being semiplastic, forms more closely to the shaft or shaft sleeve and tends to reduce the leakage. After a certain point, however, the leakage between the packing and the rotating shaft or shaft sleeve becomes inadequate to carry away the heat generated by the packing rubbing on the rotating surface, and the packing fails to function. This failure can result in burned packing, packing "blow out," and severely damaged shaft or shaft sleeve surfaces. If the sealing surface is coated, this coating may be destroyed. Even before this condition is reached, the shaft or shaft sleeve may be severely worn and scored by the packing, so that it becomes impossible to pack the stuffing box satisfactorily.

These undesirable characteristics prohibit the use of packing as a sealing method if some leakage of the pumpage to the atmosphere is not acceptable. Packing is limited in its application pressure and temperature range (see Section 2.2.2), and it is usually not acceptable for any flammable or hazardous pumping services. To address these limitations, the mechanical seal was developed (see Section 2.2.3). The mechanical seal has found general acceptance in nearly all pumping applications. Packing is still used in certain low-pressure, low-temperature applications where leakage of the pumpage is not a problem and a history of satisfactory, economical service exists.

Mechanical seals are not always the solution to every sealing situation. Seals are still subject to failure, and their failure may be more rapid and abrupt than that of packing. If packing fails, the pump can many times be kept running by temporary adjustments until it is convenient to shut it down. If a mechanical seal fails, most often the pump must be shut down immediately. As both packed stuffing boxes and conventional mechanical face seals are subject to wear, both are subject to failure. Whether one or the other should be used depends on the specific application and the experience of the user. In some cases, both give good service and the choice becomes a matter of personal preference or cost. Table 2

TABLE 2 Comparison of packing and mechanical seals Advantages Disadvantages

Packing

1. Lower initial cost

2. Easily installed as rings and glands are split

3. Good reliability to medium pressures and shaft speeds

4 Can handle large axial movements (thermal expansion of stuffing box versus shaft)

5. Can be used in rotating or reciprocating applications

6. Leakage tends to increase gradually, giving warning of impending breakdown

Mechanical seals

1. Very low leakage/no leakage 1. Higher initial cost

2. Require no maintenance 2. Easily installed but may require some disassembly of pump (couplings and so on)

3. Eliminate sleeve wear/shaft wear

4. Very good reliability

5. Can handle higher pressures and speeds

6. Easily applied to carcinogenic, toxic, flammable, or radioactive liquids

7. Low power loss

1. Relatively high leakage

2. Requires regular maintenance

3. Wear of shaft of shaft sleeve can be relatively high

4. Power losses may be high

Source: John Crane Inc.

adds other comments and summarizes some advantages and disadvantages of packing and mechanical seals.

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Renewable Energy 101

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