Tip: Mechanical Seals

The start-up process for pumps can be a critical time when problems arise with the mechanical seals they contain. The pump manufacturers do not necessarily give instructions on the right procedures to follow to protect the mechanical seals when the pumps are turned on and the seals can be ruined if the proper steps are not followed. For example, most pumps are self-venting but that does not mean that the stuffing box area is vented. To make sure the mechanical seals do not run dry, the stuffing box must be relieved of air.

Sometimes a pump in a critical application will be hooked up to a stand-by pump in case there is a problem. However, if the mechanical seals in the stand-by pump are not made from the right elastomer, they could have a tendency to harden, distort their shape or stick to the shaft if the pump is not used for a long period of time. Obviously, then when the stand-by pump is needed and turned on, the mechanical seals in it will fail.

If you have mechanical seals that have failed, it is important to look at them to determine what caused the failure. There are many factors that cause failure:

Chemical Degradation: The mechanical seals may exhibit many signs of degradation including blisters, cracks, voids or discoloration. It may be honey combed, flaky or starting to crumble. In some cases, the degradation is observable only by measurement of physical properties.

Extrusion or Nibbling: The mechanical seals develop ragged edges (generally on the low-pressure side) which appear tattered.

Thermal Degradation: The seals may exhibit radial cracks located on the highest temperature surfaces. In addition, certain elastomers may exhibit signs of softening - a shiny surface as a result of excessive temperatures.

Fretting Corrosion: This causes leakage at the secondary seals and will cause problems with the sleeve directly beneath the secondary seal area. This area will be pitted and shiny.

Vaporization: You will see places on the seal faces where vapor was blown out. There will be chipping on the inside and outside diameters and the whole area will be pitted.

Oxidation & Coking: The mechanical seals will have a film or sludge on the side that is exposed to the atmosphere. This not only can lead to the seal faces wearing quicker than usual, but it can cause the seals to want to hang up.