Which testing medium should not be used to pressure test a fuel gas piping system?

Using oxygen as a testing medium for pressure testing a fuel gas piping system is inappropriate and potentially dangerous. The primary reason is that oxygen can greatly enhance combustion. In a scenario where there is a leak or a failure in the piping system, the presence of oxygen can lead to an explosive environment, especially when mixed with flammable gases.

In contrast, air and nitrogen are commonly accepted media for pressure testing. Air is often utilized for preliminary pressure tests, while nitrogen is effective for testing because it is inert and does not react with the components of the fuel gas system. Similarly, carbon dioxide is also inert and can be used in specific circumstances, but it carries its own set of handling considerations.

Employing oxygen for pressure tests compromises safety by introducing a significant risk of fire or explosion, illustrating why it is completely unsuitable as a testing medium in this context.