Pickling or pre-odorization is a process used to prep new gas pipe before commissioning for use. In new gas lines, it is very common to experience odor fade or have the natural gas odorant scrubbed or removed from the gas stream, therefore making it odorless, a significant safety hazard.
In new pipe odorant molecules attach to the pipe wall. Therefore, the odorant no longer remains in the gas stream, and it becomes odorless. The pipeline continues to adsorb the odorant until the line becomes saturated, then the odorant begins to stay in the gas. It is unclear, however, how long this process can take which is why it is common to “pickle” the line to speed up the progression.
In addition to the adsorption issue, the inner porous wall of new steel pipe contains mill scale and rust which includes metal oxides. The metal oxides react with some odorant (tert-butyl mercaptan) and create disulfides which are less odorous than mercaptan. In this case, odor fade occurs.
What is Pickling?
Pickling is the process used on new gas lines to pre-odorize or saturate the line with odorant so that the pipe stops adsorbing the odorant. In this case, the line may be initially dosed with the extra odorant, perhaps by using a temporary odorizer. The odorant is monitored on the back end of the pipeline for detection. The pickling usually continues until the odorant level at the tail end of the line is comparable to the level at the odorizer.
There are three methods of pickling gas pipes. 1) Injection of highly odorized gas (>40 PPM). 2) Slugging and 3) Continuous injection of a controlled dosage of liquid odorant through the pipeline. When slugging, a liquid odorant is poured directly into the pipe and permitted to pickle for an extended period without flow.
Many parameters will impact the speed of pre-odorizing gas lines. Among them are pipe material, the length and diameter of the pipeline, flow rate and the odorant used. The target, of course, is to have the gas on both ends of the line be detectable by a person with a “normal” sense of smell at 1/5th the lower explosive limit, and the LEL of natural gas is 5%. Therefore, the gas must be detectable when present at a 1% concentration in air.
If you are interested in a case study on pickling, visit “Pre-Odorization or “Pickling” of New Natural Gas Pipe”
Pre-odorization or “pickling” of new natural gas pipe .., http://www.mrrus.com/wp-content/uploads/2011/07/Pickling_Article_in_PGJ-Nov_2009_issue-reprint.pdf (accessed June 17, 2016).