As a special leak detection and early warning system for most natural gas distribution systems, gas odorization warrants further discussion. An important component of the potential leak impact from natural gas distribution systems is the use of odorization. Methane has very little odor detectable to humans. Natural gas that is mostly methane will therefore be odorless unless an artificial odorant is introduced. It is common practice to inject an odorant at such levels that gas will be detected at levels far below the lower flammable limit of the gas in air—often one-fifth of the flammable limit, meaning that accumulations of 5 times the detection level are required before fire or explosion is possible. This allows early warning of a gas pipe leak anywhere in the system or in a customer’s building and reduces the threat of human injury. This is a consequence mitigation measure.
Gas odorization can be a more powerful leak detection mechanism than many other techniques discussed. While it can be argued that many leak survey methods detect gas leaks at very low levels, proper gas odorization has the undeniable benefits of alerting the right people (those in most danger) at the right time.
Aspects of optimum system design include selection of the proper odorant chemical, the proper dosage to ensure early detection, the proper equipment to inject the chemical, the proper injection location(s), and the ability to vary injection rates to compensate for varied gas flows. Ideally, the odorant will be persistent enough to maintain required concentrations in the gas even after leakage through soil, water, and other anticipated leak paths. The optimum design will consider gas flow rates and odorant absorption in some pipe materials (new steels) to ensure that gas at any point in the distribution piping is properly odorized.
Odorant injection equipment is best inspected and maintained according to well-defined, thorough procedures. Trained personnel should oversee system operation and maintenance. Inspections should be designed to ensure that proper detection levels are seen at all points on the piping network. Provisions are needed to quickly detect and correct any odorization equipment malfunctions.
Evidence should confirm that odorant concentration is effective (provides early warning to potentially hazardous concentrations) at all points on the system. Odorant levels are often confirmed by tests using human subjects who have not been desensitized to the odor. When new piping is placed in service, attention should be given to possible odorant absorption by the pipe wall. “Over-odorizing” for a period of time is sometimes used to ensure adequate odorization. When gas flows change, odorant injection levels must be changed appropriately. Testing should verify odorization at the new flow rates. Odorant removal (de-odorization) possibilities should be minimized, even as gas permeates through soil or water.
The role that a given gas odorization effort plays as a consequence reducer can then be assessed.
Reference PLRMM 3rd Ed