Why pneumatic testing lines instead of hydrotest?

For some supports the worst design condition is the hydrotest when the lines (carrying gas or vapours during operation) are full of water. To take into consideration pneumatic test of lines instead of hydrotest means saving cost in structures.

Moreover complex circuits of small bore lines in category D may be subjected an initial service leak test as stated at para 345.1 ASME B31.3. An example of such kind of lines is the steam tracing system for which the test with water is impractical.

Typical circuit tested with air is the blow down connecting the process unit and the flare. In this case the structure design loads due to the line empty or full of water are very different (anyway a certain part of pipe section, 20 to 30%, to be considered full of condensate).

Pneumatic test is convenient for columns vapour line as well to avoid exceeding allowable stress on the shell and to avoid difficulties in spring support setting (provided that even during the start-up the vapour line is always empty).

Before design a system to be tested with air you should be sure that the pneumatic test is permitted by local regulations, that in some cases require the calculation of the stored energy in the system. The stored energy is the parameter used to measure the dangerousness of the operation. Sometimes the energy calculated is compared with the effects of explosives (i.e. TNT trinitrotoluene).