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Instrument Performance Evaluation according to ISO 107231

Introduction

One of the most frequently asked questions EffecTech receives is what is the ISO 10723 performance evaluation and why is it necessary? This short technical paper has been prepared to answer these questions and allow potential customers to make an informed decision.

Background

ISO 10723 Natural gas - Performance evaluation for analytical systems is an international standard which defines a method for evaluating the performance analytical instruments used for analysis of natural gas. The standard does not stipulate a pass or fail criteria but allows a quantitative assessment to be made of the instrument under evaluation. Based upon these quantitative benchmarks, the user / operator of the instrument can then decide if the instrument is fit for its intended purpose. In addition, ISO 69742 Natural Gas - Determination of composition and associated uncertainty by gas chromatography requires a performance evaluation according to ISO 10723:2012 for analysers operating in both type 1 and type 2 modes.

Natural gas is traded on the total energy delivered, which is the product of volume flow and gas quality (energy per unit volume or mass). It follows that errors and uncertainties in the measurement of either volume flow or gas quality will affect the total energy calculation. Due to the large volumes of natural gas flowing across fiscal custody transfer points even a very small error in either flow or gas quality will lead to a large financial discrepancy. Many of our clients have responded to this by saying sometimes with a particular instrument, you win and sometimes with another instrument, you lose. However, whilst this is true, current technologies exist such that errors in gas quality measurement can be almost eliminated. Until an assessment has been made of the instrument performance used for gas quality measurements, the client does not know if any errors exist and in which direction they may lie. Simply buying industry standard equipment will not insure you against the potential of large financial discrepancies through mis-measurement.

Evaluation, validation and calibration

The ISO 10723 performance evaluation is used for a number of different applications

Factory Acceptance Test (FAT) A factory acceptance test allows the purchaser of an instrument to assess the instrument’s fitness-for-purpose before it leaves the factory. Gas chromatographs are complex instruments and in some cases are sent to site in a condition which will not fulfil the client’s requirements. Identifying problems at this stage often means that they can be remedied quickly preventing a protracted commissioning period or, in some cases, shipment of the instrument back to the vendor’s factory. In most cases, problems identified with the instrument will only become evident following an ISO 10723 performance evaluation. Visual inspections or simple measurements of the proposed site calibration gas are insufficient.

Site Acceptance Test (SAT) A site acceptance test allows the client to be confident that the instrument has been transported, installed and commissioned satisfactorily. Often instruments remain packaged for freight for many months after assembly and before shipment. Correct operation and instrument performance may have deteriorated since manufacture dependent upon transport and storage conditions. During commissioning, instruments can be operated in error, for example, without correct carrier gas supply leading to detector failure. This and other commissioning faults can cause further deterioration of instrument performance observed in the factory.

1 ISO 10723 : 2012 Natural gas - Performance evaluation for analytical systems

2 ISO 6974 : 2012 Natural gas - Determination of composition and associated uncertainty by gas chromatography

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Emissions Trading (EU/ETS3)

Any industrial installation in the EU emitting significant quantities of carbon dioxide (categories B & C) are subject to monitoring and reporting requirements as outlined in the Commission’s “Monitoring and Reporting Regulations” of the EU Emission Trading Scheme (EU/ETS). The MRR gives information on the use of process gas chromatographs and the ISO 170254 requirements of the calibration gas and instrument performance testing according to ISO 10723. The MRR document sets out the performance requirements for the instruments used on these sites and frequency of calibration. This is European law. This work has to be carried out by an accredited organisation with the appropriate calibration scope.

Annual validation and inspection Regular validations and inspections following the installation and commissioning of the analyser show objectively that the analyser and pressure reduction system is maintaining (or otherwise) its performance. This ensures the ongoing accuracy of your gas quality measurements.

Fiscal / custody transfer measurements: When measurement errors are discovered, the re-apportionment is often back to the last known validated measurement. Obviously, the financial risk is greater the longer the interval between evaluations. The cost of the performance evaluation is very small compared to the potential financial risk caused through mis-measurement of the gas passing through the fiscal metering point.

Safety requirements: Safety legislators always place the onus on the operator to adopt industry best standards and to have proof that good practice is being maintained.

EU/ETS: Annual validations are mandatory under EU law for the carbon dioxide emission trading scheme.

What added value is there from having an ISO 10723 performance evaluation?

The ISO 10723 performance evaluation will not only give you confidence in your analytical instrument but can give you:

Analysis function parameters to reduce analytical bias (errors). Analysis function parameters are calculated during the data processing following the ISO 10723 evaluation. ISO 6974:2012 refers to these instruments using analysis function parameters operating in a type 1 mode. Assuming the instrument has the capabilities to operate in a type 1 mode, these parameters can be programmed into the instrument to reduce analytical bias. Type 2 instruments as defined by ISO 6974 operate with a single point calibration and are subject to ISO 10723 evaluation to establish any inherent non-linearity in the detector response for each of the components. Figure 1 below shows a typical performance evaluation dataset containing 30,000 simulated gas compositions. The red markers indicate the spread of results assuming the instrument is operated in a type 2 mode and has a linear response function for all components. The blue markers show the same instrument operated in a type 1 mode, demonstrating significantly less spread of results and almost zero error.

3 COMMISSION DECISION of 18/VII/2007 establishing guidelines for the monitoring and reporting of greenhouse gas

emissions pursuant to Directive 2003/87/EC of the European Parliament and of the Council of Brussels, 18/VII/2007 C (2007) 3416 final. 4 ISO/IEC 17025:2005 General requirements for the competence of testing and calibration laboratories

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Calibration gas advantages Once operating the GC in a type 1 mode; i.e the analysis function parameters have been included in the instrument firmware, a calibration gas composition may be used which may not be close to the stream gas composition. This has a large impact where the stream gas is hydrocarbon wet and a low fill pressure calibration gas is necessary to prevent retrograde hydrocarbon condensation. A good example of this was where a client mirrored the calibration gas to the stream gas and could only achieve a 4 bar fill pressure in the calibration gas cylinder. After the changing from type 2 to type 1 mode of operation, by implementing the analysis functions, a new calibration gas composition was designed which could be filled to 45 bar without fear of condensation. This increased the longevity of the calibration gas by a factor of more than 10 times. The new calibration gas composition was used in the ISO 10723 data processing and showed nominally the same bias as when used with the hydrocarbon wet composition.

Typically, gas analysers have dedicated calibration gases for each particular stream. Once the GC has been changed from type 2 to type 1 mode of operation, the instrument is able to achieve a much wider measurement range. An example of this is a client operating several off-shore platforms each receiving two or three different streams. The gas then was then sent to the mainland and again analysed at the on-shore terminal. The client had each instrument evaluated according to ISO10723 and the analysis function parameters updated in the instrument firmware. Following this, a single gas composition was suggested for all offshore and terminal chromatographs. This has reduced the logistics of having many different compositions. E.g. Stocks of calibration gases with identical composition could now be held which since have significantly reduced calibration gas procurement time.

Variable stream gas compositions from blending, or gases from different sources can be measured by a single analyser using a single appropriate calibration gas if the GC is operated in a type 1 mode.

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