Example 2-17 Line Sizing for Vacuum Conditions

Therefore the system is applicable to the basis of the method, since the calculated pressure drop is less than 10% of the final pressure, and w/d = 25.5, which 20. 

For piping with air in streamline flow at absolute pressures in the range between 50 microns and 1 millimeter of mercury, the following is a recommended method. Calculation procedures in pressure regions below atmospheric are very limited and often not generally applicable to broad interpretations. 

For this method to be applicable, the pressure drop is limited to 10% of the final pressure. 

The majority of industrial chemical and petrochemical plants vacuum operations are in the range of 100 microns to 760 torn This is practically speaking the rough vacuum range noted above. For reference  

In general, partially due to the size and cost of maintaining vacuum in a piping system, the lines are not long (certainly not transmissions lines), and there is a minimum of valves, fittings, and bends to keep the resistance to flow low. 

---