Carbon Blistering

[sinbj1]

All,

I have a theoretical question regarding carbon blistering and use of a relatively viscous barrier fluid such as mineral oil. As I understand it blister patches are created on the carbon face from high shear forces due to higher viscosity fluids. In my experience, the majority of the time (not all the time) the carbon face is used as the rotary face. Is it a valid thought that by changing the stationary face to carbon, blistering could be reduced?

The basis for this thought is that the velocity profile for the stationary face is zero where as it will be however many ft/sec for the rotary face. And that based on the velocity profile of the fluid between the faces the shear forces would be reduced on the stationary face. I completely understand that the fluid film thickness between the faces is extremely small. This is just a wild thought. Feedback is appreciated.

 

[flexibox]

I don't agree because I have seen it in both arrangements. What happens is the carbon absorbs the oil and the heat generated by the seal heats the oil and it expands. The blisters, is the oil expanding inside the carbon face causing it to lift into small craters.
Is this a refrigeration application? As it is common in this industry.

 

[bk19702]

Moving the carbon to the stationary would not alleviate the issue - the blistering is a result of viscous shear at the interface; it's independant of the actual velocity profile of the face itself. There still would be the shearing effect even if the face was stationary. This problem can be remedied sometimes by switching from a resin-impregnated carbon to one that is metal-filled, typically antimony being the metal. However, generally speaking, carbon faces in lube oils above ISO Grade 32 will be prone to blistering. In these cases, the only way to address the problem from a material standpoint is to go to hard faces on both the rotating and stationary face.

 

[v7042800]

My two cents: it takes multiple start/stops to blister carbon faces..