A rotating bellow is often considered to be “self-cleaning.” If the sealed fluid is on the outside, it will tend to throw out solids rather than plugging up. But, it may be susceptible to erosion. A stationary bellows can handle more angular misalignment since the bellows does not flex with each rotation. But, it may plug up if solids are present. It was common belief in the past that a stationary bellows is more likely to hang up and leak severely. A rotating bellows may be more likely to leak gradually rather than suddenly.
So, for a dirty fluid that poses a high danger of fire, we would prefer a rotating bellows. For a less volatile fluid in a pump that may have high stuffing box face run-out, we would prefer a stationary bellows.
But, every service must be looked at individually.
