Why Measure Trace Oxygen?

A common misconception is that oxygen will not find its way into a pressurized gas pipeline even if there are leak points in the system. The intuitive conclusion is that any leak would only result in the pipeline gas blowing out under pressure to the atmosphere. Unfortunately, this is only partially true.

To understand how oxygen enters a pressurized pipeline, the gases must be thought of on a molecular level, instead of the gross level that operators are used to dealing with. Gases are not homogeneous, non-compressible fluids but instead are a collection of molecules with large spaces between them. In fact, the relative size of any leak path is massive when compared to an oxygen molecule. Because of Dalton’s Law of Partial Pressures, gas molecules will act independently of each other and expand to fill the container without affecting the pressure of another gas. Therefore, when a leak forms, a large volume of molecules will move from the high pressure to the low-pressure area, while a meaningful amount of gas (ambient air) will transfer from the low pressure to the high-pressure side.

Some common sources for oxygen entry include:

• Pipe, flange or fitting leakage in both vacuum and pressurized systems
• Compressors packing leakages
• Improper operation of vapor recovery units (VRUs)
• Failure or lack of a tank blanketing system
• Flaring without proper systems to prevent air infiltration
• Using only purging instead of a “Block & Bleed” technique on a pig trap during pigging operations