Here are some possibilities: The product is viscous and some products become more viscous with agitation (e.g., cream becomes butter with agitation). These products are called dilatants.
A recirculation line from the discharge of the pump is aimed at the seal and interfering with its movement.
A foreign object is in the stuffing box.
A protruding gasket is touching the movable part of the seal.
(A) Something is restricting the free movement of the seal.
(B) The shaft is being displaced, causing the seal to hit something as it rotates, or causing the rotating face to run off the stationary face.
The pump is operating off of its best efficiency point (B.E.P.), causing the shaft to bend.
The rotating assembly is out of dynamic balance.
The shaft is bent.
There is misalignment between the motor and the pump.
Pipe Strain is twisting the pump stuffing box.
Heat causes expansion, leading to the possibility for rubbing or wear.
Cavitation, slip stick, harmonic vibration, bad bearings or some other form of vibration is causing excessive movement of the shaft.
The shaft sleeve is not concentric with the shaft, causing it to run “off center.”
The pump designed with sleeve or babbitted bearings and shaft movement is excessive.
(C) The seal face is being distorted by either temperature or pressure.
(D) The product is vaporizing between the seal faces, causing the faces to blow apart.
If boiler feed water vaporizes, it leaves behind all of the chemicals that were added to the water to prevent hardness, adjust PH, soften boiler scale, etc.
In cryogenic (cold) applications, the vaporizing fluid can freeze any lubricant that might have been placed on the seal faces. This frozen lubricant can damage the carbon/graphite seal face.