December 2014, Vol. 241, No. 12


Analyzing The Analyzer Metrology In Natural Gas Pipelines

John Apgar

Upstream, you have producers and gas processors that deliver natural gas to fill the growing demand for the clean-burning fuel. Downstream, you have hundreds of miles of pipeline infrastructure to deliver the gas to market.

In between, at the points of custody transfer, are the multitudes of analyzers that ensure contaminants such as H2S are at low enough levels to protect the pipelines and keep the public safe. This critical measurement prevents corrosion, which in the worst case can lead to pipeline rupture.

What if the supplier and user each use a different analyzer? Well, there is a saying in the natural gas industry, “If you have one analyzer you have a measurement, if you have two analyzers, you have an argument.”

“Sometimes, you will have a dispute when two different companies are using two different brands,” observed Hunter Brown, a measurement control supervisor for Access Midstream headquartered in Oklahoma. “The readings can be 2 ppm (parts per million) apart and that can mean a lot if the third-party is reading 7 and the supplier is reading 5. In fact, it can result in a shut-in.”

The reason accuracy is so important is simple – money. Neither supplier nor buyer can afford to halt any part of the 70 Bcf of natural gas consumed in the United States on a daily basis. When two analyzers show different readings in the same line, who wins the argument? Whomever can demonstrate the most accurate analyzer.

Final Word
Fortunately, there is a final word in accuracy at the National Institute of Standards and Technology (NIST). In fact, it is the final word in all metrology. If an analyzer is calibrated with highly accurate gas, and that gas can demonstrate NIST traceability, the argument is over.

“Both calibration and validation are important. If either one is wrong, it’s not going to read right,” said David Bromley, a natural gas technical authority for BP. “If I get that test gas in and it is reading 3 instead of 4 ppm, the question becomes, is it the test gas or the analyzer? One of them has to be wrong.”

Increasingly, producers and processors are demanding greater accuracy through NIST traceability, and that has led analyzer manufacturers to respond.

Houston-based SpectraSensors developed the first Tunable Diode Laser (TDL) analyzer to measure impurities, such as hydrogen sulfide in pipelines. On its own, the company soon realized that due to the inherent uncertainties in any measurement, a genuine accuracy claim must trace back to the metrology gatekeeper, NIST.

Originally, analyzer companies simply ordered a calibrated gas bottle from a reliable source and used it to calibrate their analyzers at the factory. Once calibrated, the unit was good to go. However, those companies discovered that not all bottles are created equally, and historically no one traced their standards back to NIST. The problem was they were finding a higher degree of uncertainty in the bottles than the manufacturers claimed, so they went looking for a company that would collaborate with them to solve the problem.

That company turned out to be Air Liquide. “H2S is a notoriously difficult component to blend into a gas mixture and have it remain stable,” said Chief Technical Officer Stephen Miller. “And that problem becomes progressively worse as concentrations become lower.
Neither supplier nor buyer can afford to halt any part of the 70 billion cubic feet of natural gas consumed in the US on a daily basis.
Major Commitment
Both companies realized it would require a major commitment of time and money to build the traceability path back to NIST. But the reward would be worth it.

The H2S concentration needed in the bottles was 4 and 16 ppm, each of which had to be in a matrix gas of methane and nitrogen and everything had to be NIST traceable. The answer was “dual certification.”

A common industry method to produce H2S mixtures is to dilute H2S down to 4 ppm and analyze it in the lab. But that process has accuracy risks, even if done with NIST reference standards, because there is no second check of the actual concentration and no indication of long-term stability. The Air Liquide dual certification process, which employs gravimetric and analytical procedures, both with full NIST traceability, requires these two independent methods come to the same result and agree with each other within close statistical boundaries.

“First, we determine the concentration and the accuracy of the mixture by mass alone, using NIST primary references and stringent statistical methods,” said Miller. “Then, we analyze that same cylinder in the laboratory using instruments calibrated with NIST SRM’s (standard reference materials). When these two independent processes agree with one another, we have high confidence in the certified H2S concentration, and we have the required unbroken chain of comparisons within our measurements all the way back to NIST.”

How does NIST know the values are correct? It’s a three-step process that includes worldwide verification.

“When we create primary standards we design the procedure to develop a suite of standards that define a concentration range we wish to use to support for customers,” said NIST’s Franklin Guenther.

“To verify the primary standards, we analyze the suite on an instrument with a well-defined instrument function, such as gas chromatography, which has a linear function,” he said. “If all the primary standards fall on the linear response function of the instrument, we are confident that the standards are self-consistent and no blunders have occurred in the filling process.”

NIST then analyzes the results against past primary standards. If they agree, the institute is confident the standards are historically consistent and that of the measurement system. A third method of verification is to measure the results against other National Metrology Institutes (NMI) in a bilateral comparison.

The bottom line for everyone in the measurement business is that NIST is the standard; NIST traceability trumps all debates.

Total Measurement System
Miller said he sees it as a total measurement system.

“If you truly care about the H2S content in the pipeline, then all elements of your measurement system must be optimized. If you are not using a total measurement system, then you risk making a less accurate measurement, which leaves you susceptible to economic losses.”

Bromley agreed, saying. “We have to have repeatability and accuracy in our measurement. If you are measuring 4 ppm you must have repeatability and accuracy.”

The best method is to calibrate the analyzer at the factory using the new, dual-certified NIST traceable H2S calibration standards. If a customer wishes he can also validate the unit in the field, again, with a NIST-traceable bottle.

For natural gas suppliers and users, the end of the H2S measurement argument at transfer points in pipelines could finally be in sight.

Author: John Apgar is a freelance writer based out of Chino Hills, CA.

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