49 CFR § 192.706 - Transmission lines: Leakage surveys.
Leakage surveys of a transmission line must be conducted at intervals not exceeding 15 months, but at least once each calendar year. However, in the case of a transmission line which transports gas in conformity with § 192.625 without an odor or odorant, leakage surveys using leak detector equipment must be conducted -
(a) In Class 3 locations, at intervals not exceeding 7 1/2 months, but at least twice each calendar year; and
(b) In Class 4 locations, at intervals not exceeding 4 1/2 months, but at least four times each calendar year.