Pre-Odorization Or “Pickling” Of New Natural Gas Pipe

Natural gas, odorless on its own, is odorized in order for people to be able to detect gas leaks by sense of smell.

When a new gas pipeline in commissioned, the line itself will absorb and react with the odorant, leaving the gas odorless at the outlet, which constitutes a safety hazard. The line will continue to consume the odorant until it becomes saturated with odorant; only then will the odorized gas flowing through remain odorized until it reaches the end-users. In order to accelerate this process and be able to safely put the new line in service, it is recommended that installers pre-odorize or “pickle” the line.

This article presents the case study of the pre-odorization of a new pipeline installed to service a residential subdivision in the city of Kitchener, Ontario, Canada. After initial static “slugging” of the line for three weeks, over-odorized natural gas was flowed through the line while odorant levels were monitored at the tail end of the pipe. The additional odorant was injected using a temporary odorant injection station. The pickling appeared complete and the line saturated when the odorant levels at the tail-end were maintained and comparable to those at the inlet, without additional injection being necessary to compensate for odorant loss. However, after a few months, odor fade again occurred. A bypass odorizer was installed to compensate for odor fade and to continue the process of saturating the pipe with odorant.

Following a natural gas explosion in a Texas school in 1937 where 298 children and teachers lost their lives, the U.S. and Canada legislated the use of odorant in natural gas to ensure detectability in case of gas leaks. The regulations state that gas distributed to end-consumers must be detectable at one-fifth of its lower explosive limit (LEL). The LEL of natural gas is 5% in air; therefore, the average person must be able to detect odorized gas at a concentration of 1% in air.

When a new natural gas steel pipe is installed, the porous inner wall of the pipe contains metal oxides (rust and mill scale) which will react with odorant, e.g. tert-butyl mercaptan (TBM), to produce disulfides which are less odorous than TBM. Therefore, perfectly odorized gas entering the pipe will be stripped of odorant as it flows down the pipe and will be left odorless at the outlet, hence a potential hazard. For the safety of all end-users of the pipeline, a pre-odorization step is required to saturate the new line with odorant before commissioning the line for service, so that odorized natural gas entering the new line remains adequately odorized upon reaching end-users. The process of pre-odorizing or saturating the line with odorant is commonly called “pickling”.

The City of Kitchener owns and operates the natural gas distribution system within the confines of the city limits in the municipal division that also operates its sister water distribution system. The natural gas distribution system supplies 60,000 residential, commercial and light-industrial customers. The majority of the distribution system operates at pressures below 60 psig, fed from a high-pressure distribution system generally operating in excess of 220 psig. This southwest Ontario utility receives its natural gas from the Union Gas transportation system at two locations within the southwest and central-west regions of the city.