Mechanical seal failure modes

- Sep 04, 2017-

A mechanical seal is dramatically influenced by numerous factors over which its manufacturer has no direct influence, such as operating temperature, transients from start-stop operations or mechanically induced vibrations resulting from poor pump shaft alignment, for example.

As already indicated, one of the key failure modes of the mechanical shaft seal is dry running. Without the lubricating film, friction can elevate seal temperatures to several hundred degrees Celsius in minutes. This can cause failure of the elastomeric elements of the seal such as the O-rings.

Dry running may be caused by excessive loading or contamination of the fluid film, for example, but low viscosity of the pumped medium or a temperature well above the boiling point at atmospheric pressure can exacerbate potential issues with dry running and a phenomenon in which localized heating and cooling occurs can cause thermal fatigue in the seal faces.

Another significant factor is the presence of suspended and dissolved solids, for example, a coal-fired plant does have some fairly difficult service conditions for a mechanical seal. Services entail water of varying temperatures, pressures and shaft-speeds of the pump, but coal-fired plants in particular have a lot of services that contain abrasives and solids within the fluid which are hard for the mechanical seal because it creates additional abrasion and erosion of the components.

Furthermore, as the lubricating film in the sealing gap is subjected to large gradients in temperature, pressure and velocity this increases the likelihood of precipitation and sedimentation there.

The contamination can also 'hang up' the seal, meaning the seal is no longer flexible. The particles can get into the mechanical seal's O-rings and springs and cause them to go rigid, where the seal is no longer able to move with the shaft movements and pressure deflections. It can be difficult to have a seal operate correctly under these circumstances.

Aside from suspended solids, seal performance can also be dramatically influenced by other aspects of the water quality of the pumped fluid.

For example, two decades ago many thermal plants changed their feedwater treatment system from all volatile treatment (AVT) of boiler feedwater to combined oxygen treatment (COT) to resolve a number of technical issues such as precipitation in the boiler tubes and ammonia extraction. However, this change in combination with these high-speed applications resulted in a significant shortening of the lifespan of mechanical seals.

In these high-speed applications and under particular conditions, when using silicon carbide for example, minute arcs cause electrical corrosion. Under such circumstances the material of the seal itself becomes degraded. This failure mode only affects large, heavy-duty feed pumps, but the result, depending on the sliding velocity, is a very short lifespan.

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