49 CFR 192.355
Customer meters and regulators: Protection from damage.
(a)Protection from vacuum or back pressure. If the customer's equipment might create either a vacuum or a back pressure, a device must be installed to protect the system.#
(b)Service regulator vents and relief vents. Service regulator vents and relief vents must terminate outdoors, and the outdoor terminal must—#
(1)Be rain and insect resistant;#
(2)Be located at a place where gas from the vent can escape freely into the atmosphere and away from any opening into the building; and#
(3)Be protected from damage caused by submergence in areas where flooding may occur.#
(c)Pits and vaults. Each pit or vault that houses a customer meter or regulator at a place where vehicular traffic is anticipated, must be able to support that traffic.#