3. General exemptions.— Nothing in these rules shall apply to vessels which form part of a processing plants. For the purpose of this rule vessels forming part of a processing plant shall mean vessel in which a unit process or unit operation is carried out and vessels which contain, as a process requirement, a compressed gas received from and consumed in the same processing plant, provided that the water capacity of the vessel (s) shall be such that the gas stored therein at the maximum working pressure shall not exceed the requirement for feeding the consuming point (s) for a period not exceeding 16 hours at the designed flow rate. |