Rule of thumb for the required power in normal cases:

Required power 10 W/m pipe:

– if the pipe diameter is max. 50 mm
– if the insulation is min 50 mm mineral wool
– the lowest ambient temperature is -30°C

In other cases the required power is calculated as per below:

P/m = 2 x π x dt x k x S / In (dy / di)

P/m = required power in W/m
dt = The difference in temperature in °C.
(the lowest permitted temperature of the medium minus the lowest frequent ambient temperature).
k = The permeability of the insulation material in W/m°C (J/m°C x 1.16)
For mineral wool 0.046 W/m°C
S = Safety factor, is put to 1.3 -1.7 depending how it is exposed to weather and wind.
dy = The interior diameter of the insulation in mm.
di = The outer diameter of the pipe in mm.