Sliding velocity (v_g) is the circumferential speed of the rotating sliding face relative to the (normally) stationary sliding face of a mechanical seal, calculated with the mean sliding face diameter:

v_g =

dm = mean diameter of sliding face

n = (differential) speed [rpm]

In the low and medium range up to approximately 25 m/s it is usual to employ mechanical seals with rotating spring backing, in the high speed range above v_g = 25 m/s mechanical seals with stationary springs are customary.