Natural Gas Pipeline Oxygen Limits: Published Tariff Values from 10 ppm to 0.2 %

What oxygen limits do natural gas pipelines publish?

The published limits below come from pipeline tariffs, utility gas rules and an industry compilation. They show how wide the range is, and they are examples only. Tariffs change, so confirm the current text with the pipeline before you design to any of these numbers.

Pipeline or utility	Published oxygen limit	In ppm by volume	Source type
Southern Star Central Gas Pipeline, Line Segments 315, 340 and 385	10 ppm	10	FERC tariff text accepted in 2018
Colorado Interstate Gas (2009 compilation)	0.001 %	10	AGA Report 4A, 2009 compilation
Northwest Pipeline, receipt points connected to the La Plata Facilities	0.1 %	1,000	FERC tariff
Questar Pipeline (2009 compilation)	0.10 %	1,000	AGA Report 4A, 2009 compilation
PG&E (California utility), Gas Rule 21	0.1 %	1,000	Utility tariff rule
Northwest Pipeline, other receipt points	0.2 %	2,000	FERC tariff
Southern Star Central Gas Pipeline, other receipts	0.2 %	2,000	FERC tariff text accepted in 2018
SoCalGas (California utility), Rule 30	0.2 %	2,000	Utility fact sheet
Florida Gas Transmission (2009 compilation)	0.25 %	2,500	AGA Report 4A, 2009 compilation
Gas Transmission Northwest (2009 compilation)	0.40 %	4,000	AGA Report 4A, 2009 compilation

The rows marked 2009 compilation are taken from a table in a SoCalGas fact sheet that cites the American Gas Association's 2009 Report 4A. Those values are from 2009, and current tariffs may differ. The Southern Star rows reflect tariff text FERC accepted in 2018, and a later Southern Star filing proposed changes, so check the current tariff. Each source is linked at the end of this article.

Why do pipeline oxygen limits differ so much?

Pipeline oxygen limits differ because each operator weighs its own corrosion risk, its downstream delivery obligations and the gas it already receives. A system that delivers into pipelines with tighter limits has to hold its own gas to a tighter number, or it cannot hand the gas on.

A 2021 Southern Star proposal to its customers shows that reasoning. That proposal, which was not adopted and which Southern Star later replaced, would have lowered the general oxygen limit from 0.2 % to 10 ppm to reduce internal pitting and crevice corrosion, noted that oxygen makes CO₂ and H₂S corrosion more severe, and cited consistent deliveries to pipelines with more stringent limits. Southern Star's later customer update replaced the systemwide 10 ppm idea with a plan to keep the 0.2 % limit and add a 10 ppm oxygen option for gas carrying more CO₂. A gas processing paper from Trimeric Corporation also reports that oxygen limits in new contracts have trended downward over recent years.

Some tariffs also cap oxygen inside a combined limit. Northwest Pipeline, for example, caps total inerts, including oxygen, at 3 % by volume, and the SoCalGas fact sheet lists a 4 % total inerts limit.

How do I find the oxygen limit for my interconnect?

The limit for your interconnect is in the quality section of the receiving pipeline's tariff, often in the General Terms and Conditions, or in your gas purchase or gathering agreement if that agreement sets its own number. Check the following points before you rely on a number:

1. Find the quality section of the tariff and read the oxygen clause in full, including any segment-specific or receipt-point-specific limits.
2. Check whether a separate contract or interconnect agreement sets a tighter limit than the tariff.
3. Check for combined limits on inerts or on nitrogen plus oxygen, because those can bind before the oxygen limit does.
4. Ask the pipeline whether the limit is applied as an instantaneous value or over an averaging period, because the answer changes how you set alarms.
5. Record whether the pipeline may accept off-spec gas at its discretion, because that acceptance is not something to plan around.

Some tariffs leave room for operator discretion. Southern Star's tariff, for example, states that the pipeline may accept gas above 10 ppm on its restricted segments based on operational and engineering considerations, including its ability to blend that gas and still meet downstream specifications.

How much air does each limit allow?

Air is about 21 % oxygen, so each oxygen limit corresponds to a small amount of air leaking into gas that held no oxygen before the leak. The arithmetic shows why a 10 ppm system leaves almost no room for air ingress.

Oxygen limit	In ppm	Equivalent air in the gas
10 ppm (0.001 %)	10	About 48 ppm of air, roughly 1 part in 21,000
0.1 %	1,000	About 0.48 % air
0.2 %	2,000	About 0.95 % air
0.25 %	2,500	About 1.2 % air
0.4 %	4,000	About 1.9 % air

This table is arithmetic only, not a field measurement. The table assumes that air is about 21 % oxygen and that the gas contained no oxygen before air entered.

For the sources of that air, from tank hatches to compressor packing, see oxygen in natural gas: sources, limits and where to measure.

Which analyzer range fits each pipeline limit?

Choose a range that puts your limit well inside the scale, with enough headroom above the limit to see an upset rather than a pegged signal. OxyScout offers five ranges, and the table below matches them to the limit bands in this article.

Your oxygen limit	OxyScout range to consider	Why
10 ppm	Ultra-Trace, 0–50 ppm*	The limit sits low in the scale, and the range leaves headroom to watch a rise above the limit.
Up to about 0.05 % (500 ppm)	Trace, 0–1000 ppm*	The limit sits mid-scale, with room above it for upsets.
0.1 % to 0.4 %	Percent, 0–1 %vol*	The limit sits inside the scale, and the range can follow a larger air leak without saturating.
Upset tracking above 1 %	Percent, 0–10 %vol* or 0–21 %vol*	These ranges follow large air ingress events up to air itself.

On the Ultra-Trace range, the limit of detection is ≤0.5 ppm* and accuracy is ±2 ppm or ±5 % of reading, whichever is greater*. On the Trace range the limit of detection is 50 ppm*, and on the Percent ranges it is 0.01 %vol* (0–1 %) and 0.1 %vol* (0–10 % and 0–21 %). Accuracy is ±0.5 % of full scale* on the 0–1 %vol Percent range, and ±5 % of full scale* on the Trace range and the 0–10 % and 0–21 %vol ranges. Typical response is under 10 seconds*.

On the 0–1 %vol range, ±0.5 % of full scale equals ±0.005 %vol*, which is small relative to a 0.1 % limit and smaller still against limits of 0.2 to 0.4 %.

*Based on controlled lab conditions.

See the full specifications on the optical oxygen analyzer page.

What does an oxygen analyzer do at the custody transfer point?

At the fence line, a continuous analyzer monitors oxygen against your pipeline limit, so operators see a rising trend before gas reaches the receiving pipeline's own measurement. OxyScout sends its reading over 4–20 mA and RS-485 Modbus RTU, so you can set your own alarm point below the limit in your RTU, PLC or SCADA. The receiving pipeline's measurement and its tariff terms still govern acceptance of the gas.

A single fence-line reading shows the cumulative oxygen from every upstream source. To find where the air enters, add readings upstream at gathering lines, compressor stations and vapor recovery units. The custody transfer page shows where the analyzer mounts at the interconnect.

OxyScout is engineered for Class 1 Division 2 (Zone 2) hazardous areas and has no H₂S or CO₂ cross-sensitivity. The analyzer is priced comparably to electrochemical analyzers, and because there are no cells to replace, its total cost of ownership over 2–3 years is much lower. To review your limit, range and sample point with an engineer, book a quote meeting.

Sources and further reading

• Southern Star Central Gas Pipeline, filing to FERC in Docket RP18-276 (2018), GT&C Section 3.2(c) gives the 0.2 % and 10 ppm limits and the discretion clause, and the FERC letter order of May 17, 2018 accepts that text.
• Southern Star, Gas Quality Specifications, proposed tariff changes (June 10, 2021) gives the corrosion and downstream-delivery reasoning of the 2021 proposal.
• Southern Star, Gas Quality Specifications, updated plan (August 16, 2022) shows the later plan that keeps the 0.2 % limit with a 10 ppm option.
• Northwest Pipeline LLC, FERC Gas Tariff, GT&C Section 3.1 gives the 0.2 % and 0.1 % limits and the 3 % inerts cap.
• PG&E Gas Rule 21 gives the 0.1 % limit.
• SoCalGas, Renewable Natural Gas Gas Quality Standards fact sheet gives the Rule 30 limit, the 4 % inerts limit and the 2009 AGA Report 4A table.
• Jones, McIntush and Wallace, Oxygen Removal in Natural Gas Systems, Laurance Reid Gas Conditioning Conference paper (Trimeric Corporation) describes the downward trend in contract limits.