14 December 2010

Shaft seal failure: what is damaged? - Results of four years of research

The purpose of a mechanical seal is to prevent refrigerant leakage by sealing with oil. But all seals leak oil to some controlled degree.

Seal failure means that leakage is extreme & uncontrollable. The shaft seal is broken and needs replacement. Over the last 4 years, RM Support has examined 110 broken shaft seals of diverse brands of compressors.

seal-damageIn most cases the seal leakage came from the sealing interface (carbon ring). However, in some situations, leakage emerged from O-rings (the secondary sealing area). In particular, VSD compressor units that operated for a longer time in a low capacity (below 2500 rpm) have more damage from O-rings. In combination with higher oil consumption, this is known as oil carry over.

Here are some interesting observations:

71% of the seals showed damage to the carbon ring. Symptoms indicated overheating. Overheating/thermal attack results in 3 kinds of symptoms: radial cracking, blistering and spalling.

  • 65% within this category of damaged seals showed a deformation of the O-rings. 33% had binder problems due to the heat caused by chemical attack.
  • 15% showed deformation on the O-rings only. In some of the investigated seals we fitted a silicon part instead of the standard carbon ring.
  • 10% showed copper plating on sealing parts. The use of Freon gas and hydrated oil in the seals influenced the creation of copper plating on mechanical seals.
  • 4% of the defects could be related to incorrect lapping, hammered dowel pins, materials, broken or missing springs, or mounting failures.

The conclusion: symptoms indicate the majority of damage is due to overheating (thermal attack). Reducing the heat during the sealing process will reduce the shaft seal damage.

This research was a part of the development of a heat reduced (HR) shaft seal.

Please contact us if you want to know more about this research. Read about our HR shaft seal, which will be available soon, here.

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