The Five Key Features of Good Mechanical Seal Design
At FITT Resources, we stock a wide variety of Chesterton mechanical seals, and so selecting the right seal for your pump or application needs to take a number of factors into consideration. This guide highlights the key features of the best mechanical seal design, and which seal will best meet your needs in terms of capability, cost, fit and ease of application.
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Protected Springs
As most process fluids are not especially clean, when the spring of a mechanical seal is immersed in process fluid, dirt and debris is likely to collect between the springs. This will ultimately affect how the spring responds to vibration and movement, as well as its ability to keep the seal faces closed. In time, this clogging can lead to premature seal failure.
The most effective mechanical seal design has springs on the atmospheric side, protecting them from the process fluid and so ensuring their ongoing efficacy.
Balanced Design
Seal faces are kept closed through the compression created by the pressure of the seal springs (Ps) and the hydraulic pressure of the liquid in the pump (Pp). When seals are balanced, the seal ring area (Ah) on which the hydraulic pressure of the liquid acts is reduced.
This results in a reduction of the net closing force, enabling better lubrication and therefore lower heat generation. A balanced seal will generally have a higher pressure rating than unbalanced seals.
Mechanical seals have a balance ratio (B), determined by the ratio of the hydraulically loaded area (Ah) and the sliding surface are (As).
In a balanced mechanical seal, the balance ratio will be B<1, while in an unbalanced seal it will be B>1. The balance ratio is often between 0.6 and 0.8. Seals with a lower balance ratio have thicker lubricating films and higher leak rates.
When selecting the balance ratio of a seal, heat generation, leak rate and the stability of the sealing gap all need to be factored in.
Monolithic Seal Faces
A mechanical seal can have either a monolithic seal face or an inserted seal face. Carbon/graphite is used to produce the sacrificial seal face as it offers good running properties, although it can be weaker mechanically than other materials.
A seal with an inserted face will utilises a metal rotary holder in order to transmit the shaft torque to the seal face. The fact that the face and holder material have different thermal expansion coefficients is a disadvantage of inserted face design. This is because it results in a change in the net interference force between both parts when exposed to heat from the process fluid or face friction. Leakage and accelerated water can result from deformity in the seal face.
Monolithic seal faces, on the other hand, are made from the same material. This means that the torque transmission is applied directly to the seal face, whose geometry is designed in such a way that it has the strength to handle the torque.
As a result, there is a more stable fluid film between the faces, and monolithic seal faces become significantly less deformed than inserted faces. This decreases emissions and improves reliability of the seal.
Non-Fretting Design
Mechanical seals will have at least one secondary seal designed to interact with the dynamic movement of the flexible mounted face. This dynamic secondary seal moves with the springs in order to keep the seal faces closed, adjusting with each rotation for any misalignment and parts tolerance; as the springs compensate, the secondary seal moves back and forth, twice per revolution.
This movement prevents a chrome oxide layer, which protects the metal, from forming. This leads to erosion of the unprotected area under the dynamic secondary seal, resulting in a groove being formed. In time, this can become so deep that the O-Ring compression is lost and the seal will begin to leak. When this occurs, the fretted shaft needs to be replaced.
In contrast, in non-fretting seals the dynamic secondary seal rides on a non-metallic surface, generally the sacrificial face.
Stationary Design
A rotary mechanical seal will feature a spring mechanism in the rotating section. For the faces to stay closed, it is important that the stuffing box face is perpendicular to the shaft. As there is always some misalignment as a result of installation and parts tolerances, the springs must adjust with each rotation in order to keep the seal faces closed. At higher speeds, this adjustment is more difficult.
In contrast, the springs in a stationary seal do not rotate with the pump shaft. This means that the springs are not affected by rotational speed, and they do not need to correct or adjust with each rotation. Adjustment is only necessary for misalignment once installed.
Rotary seals tend to be simpler in design (so they are less expensive), but are suitable for lower speeds only. Stationary seals, on the other hand, are suitable for all speed ranges, and their design means they are more often configured as cartridge seals rather than component seals.
Find out more about our range of Chesterton mechanical seals
If you would like a printed pocket guide to mechanical seals, please contact Mathew Howell in our Sealing Division on 1300 653 229 or via email [email protected].