The chart may be used to give quick estimates of flow through boiler blow down lines if the system resistance K and the drum operating pressure is known. Critical flow conditions generally prevail in the line due to the low back pressure. see my book Steam Plant Calculations Manual for more information on the calculation procedure . K=fL/d,where f is the friction factor and L is the total equivalent length of the blow down line,including valves,fittings,bends etc. Table below shows typical equivalent lengths in ft for a few common resistances.
Example: A boiler operating at 590 psia or 40 atm has a 2 in blow down line whose length from the drum to the flash tank is 50 ft.The total equivalent length is 100 ft.The system resistance is K=0.023x100x12/2=13.8.From the chart, the mass flow rate=8000 kg/m2s. Hence flow rate=3.14x.05x.05x8000x3600/4=56500 kg/h.[pipe id=.05 m]