Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Mechanical Seals 377
less than 100, the seal is referred to as balanced. Figure 17-17 illustrates
common balanced and unbalanced seals.
The selection of a rotating or stationary seal is determined by the speed
of the pump shaft. A seal that rotates with a pump with the shaft is a
rotating seal assembly. Examples are shown in Figures 17-15 and 17-16.
When the mating ring rotates with the shaft the seal is stationary (Figure
17-18). Rotating seal heads are common in industry for normal pump
shaft speeds and where stuffing box space is limited. As a rule of thumb,
when the shaft speed exceeds 5,000 ft/min, stationary seals are required,
Higher speed applications require a rotating mating ring to keep unbalanced
forces that may result in seal vibration to a minimum. Also, for
certain applications like vacuum tower bottom pumps, a stationary seal
allows a steam quench to be applied to the entire inside diameter of the
seal to prevent hangup. Stationary seals require a large cross section
stuffing box.
Figure 17-17. Common unbalanced and balanced seats.