49 CFR §192.355

1. (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.
2. (b)Service regulator vents and relief vents. Service regulator vents and relief vents must terminate outdoors, and the outdoor terminal must—
   1. (1)Be rain and insect resistant;
   2. (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. (3)Be protected from damage caused by submergence in areas where flooding may occur.
3. (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.