Here is a paragraph that will explain the difference between air at pressure and liquids at pressure. PSI is PSI, but the reaction is different. The link to the whole article is at the bottom and comes from the OSHA website.
To Whom It May Concern:
From time to time, I receive inquiries as to the suitability of using PVC pipe land fittings in compressed gas piping systems. While the benefits of use may be enticing, it is a very dangerous and, in some states, illegal thing to do. For example, MIOSHA (Michigan's branch of OSHA) prohibits the use of PVC plastic in compressed gas systems unless properly encased in steel, cement, or some other approved material. Please check your local and state regulations.
The main problem with using PVC pipe and fittings for compressed gas is not that it spontaneously explodes but that PVC is a brittle material that can be broken or shattered with external force unless properly protected. Compressed gasses can be best described as being analogous to a coiled spring. When a PVC pipe or fitting fails when under stress from compressed gas it literally explodes like a bomb, sending shards of plastic flying several feet in all directions. Liquids, on the other hand, being compressed by only 1/10th of 1% contain very little stored energy. When pressurized systems with liquids fail, the energy is dissipated very quickly, thereby creating a much lower potential for hazard.
Colonial Engineering does not recommend the use of PVC plastic pipe fittings in compressed gas service.
If you have further questions regarding this matter please feel free to contact me directly.
ESLON THERMOPLASTICS, INC.
P.O. BOX 15894, CHARLOTTE, NC 28210
DATE: July 11, 1988