Pneumatic testing is carried out in certain cases where water should not be permitted in the system and for other design considerations. However pneumatic testing can be deadly if certain precautions are not followed. The Alberta Boiler Safety Association has provided the following tips:
"Due to the large amount of energy stored in compressed gas and the potential hazard of a sudden release of this energy, pneumatic testing should be avoided if at all possible. The data in Table 1 illustrate the comparative risk of a pneumatic test versus a hydrostatic test:
A pneumatic pressure test should only be considered if a hydrostatic test has been carefully reviewed and determined not to be feasible. When pneumatic testing is necessary, there are many critical safety precautions that must be considered. Some important considerations are:
a) Code of construction requirements for pneumatic test (e.g., UW-50 for ASME Section VIII Div. 1).
b) A determination of the energy stored in the test fluid. Calculations may be made based upon the isentropic expansion of a confined gas.
c) Test site preparations and related precautions including removal of unauthorized personnel, isolation of test site and a determination of the restricted distance for the pneumatic pressure test. The restricted distance is the distance from the item(s) under test at which barriers are placed to prohibit access, and the distance at which the test is monitored.
For the example cited above, with approximately 2,000,000 ft-lbs of stored energy (equivalent to one pound of nitroglycerine) the restricted distance using the NASA Glenn Research Center methodology would be 80 feet.
d) Test medium, pressure source and pressure and temperature ranges during testing.
e) Provision of pressure relief valves, which must be sized to handle the maximum output of the pressure source, to avoid excessive testing pressure.
f) Material specifications of the vessel or system involved in the test. For materials whose resistance to brittle fracture at low temperature has not been enhanced, a test temperature above 60 ºF (16 ºC) should be used to reduce the risk of brittle fracture during the pneumatic test.
g) Precautions taken to prevent gas expansion temperature drop and thermal stresses due to temperature gradients".
Read the article in this link.
In February 2009, a LNG pipeline undergoing pneumatic testing killed a worker when it exploded.Here is the link for that accident along with pictures.