Which type of tank should not be air tested?

The rationale behind why tanks that have previously contained flammable or combustible liquids should not be air tested lies in safety and risk management. Air testing can create a hazardous situation because, depending on the condition of the tank and the remaining residues of flammable materials, there is a risk of ignition when the tank is pressurized.

When air is introduced into such tanks, there may be residual vapors of flammable or combustible substances that could be ignited by potential sparks or friction generated during the testing process. Therefore, it is essential to avoid air testing for these tanks to mitigate the risk of fire or explosion. Instead, other testing methods, such as hydrostatic testing, which uses water and poses less risk, can be employed to ascertain the integrity of the tank.

For other types of tanks mentioned in the choices, such as new tanks, metal tanks, or those with protective coatings, air testing may be appropriate under the right conditions, as these do not inherently involve the same level of risk associated with residual flammable materials. Thus, the focus on safety is critical in understanding why the correct answer is centered around tanks previously used for volatile substances.