Leakage rate (leakage flow). A mechanical seal's total leakage is conditional on both theoretically calculable and incalculable factors affecting leakage. With externally pressurized mechanical seals under boundary friction conditions (friction types) it is possible to determine the proportion of leakage governed by the theoretically calculable factors for differential pressure to be sealed of up to 50 bar:

Q = 7,5 · 10^-15 · · (1 = 0,0013 · D)³ · n^1,9 · h^-0,1 · (k + )^-0,9 · Dp^0,1

Q = leakage rate (ml/h)

D = outside diameter of the sliding face (mm)

d = inside diameter of the sliding face (mm)

n = speed (min^-1)

h = dynamic viscosity (Pa·s)

k = balance factor

p_f = spring pressure (spring force on seal face, usually approx. 3 bar) (bar)

Dp = pressure difference to be sealed (bar)

The leakage rate of internally pressurized mechanical seals is higher by roughly the power of 10 than the leakage of an externally pressurized mechanical seal under the same conditions.

Charts are used to obtain a rough value for the leakage rate.

The theoretically incalculable factors affecting leakage can lead to positive as well as negative deviations from the calculated value.