Question 26 TK02 - Tank Vessel Assistant - Liquefied Gas

The Correct Answer is A. **Why option A ("To calibrate the gas leak detectors") is correct:** Span gas (also known as calibration gas or reference gas) is a certified gas mixture of known composition and concentration (typically containing methane or a methane-air mix). Aboard LNG carriers, fixed and portable gas detection equipment (such as those used in compressor rooms, pump rooms, and void spaces) must be regularly calibrated to ensure accuracy. The detectors are calibrated using this span gas as a traceable reference standard to verify that the instrument is reading concentrations correctly (i.e., "spanning" the measurement range accurately). **Why the other options are incorrect:** * **B) To inert the barrier spaces:** Inerting is the process of replacing the air in a space (like the barrier space or secondary insulation spaces surrounding the cargo tanks) with an inert gas, typically nitrogen (N2), to reduce the oxygen concentration below the level required for combustion. Span gas is not used for this large-scale purging or inerting process; it is a specialized calibration mixture. * **C) To odorize the cargo:** Liquefied Natural Gas (LNG) is typically odorless. While natural gas delivered to consumers is odorized (usually with mercaptans) for safety purposes, LNG cargo carried on ships is **not** odorized. Odorizing the vast quantity of LNG cargo would be impractical, unnecessary for marine safety standards, and potentially damaging to cryogenic equipment. * **D) To detect leaks in cargo piping:** While gas detectors *calibrated* by span gas are used to monitor spaces for leaks, the span gas itself is not injected into the cargo piping or used as the medium to physically detect leaks. Leak detection is achieved by the operational gas monitoring systems reacting to the presence of leaked cargo vapor.