You are sealing double-ended pumps whose stuffing boxes are at suction pressure. If the solids have a low specific gravity or density and float on the liquid. Be sure to compensate for the fact that the rotating unit will go out of balance. The seal faces must be vibration dampened. O-Ring seals are equipped with a natural vibration damper because of their dynamic elastomer.
Metal bellows seals re-quire some other dampening method.
Letting the seal face holder rub and vibrate along the shaft is a normal approach used by most metal bellows seal manufacturers. The logic of this is questionable. Give the seal room to move. Unless you have installed an oversized sealing chamber, shaft run-out and vibration can cause the seal's rotating components to contact the inside of the stuffing box. Use motion seals if the run-out or vibration is excessive.