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Orifice Plate Meter Error

Orifice Plate Meter Error. Stuart Gibbons, Extraordinary Offtake Arrangements Workstream Meeting: 16 October 2009. Contents. What is the Issue? How was the error identified? What is the error? Route cause of the error. Impact. Timeline of events. Remediation. Summary.

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Orifice Plate Meter Error

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  1. Orifice Plate Meter Error Stuart Gibbons, Extraordinary Offtake Arrangements Workstream Meeting: 16 October 2009

  2. Contents. • What is the Issue? • How was the error identified? • What is the error? • Route cause of the error. • Impact. • Timeline of events. • Remediation. • Summary.

  3. What is the issue? • A meter error has been identified potentially affecting 34 of National Grid’s Offtake meters across all 4 National Grid distribution networks. • Meter validations in accordance with ME/2 procedures. • Orifice plates are exchanged every 12 months in accordance with UNC.

  4. How was it identified? • An audit of an Offtake meter during July 2009 identified an anomaly on the physical measurement of the orifice plate. • A number of audits are undertaken on an annual basis at agreed offtakes following consultation with Ofgem. • Audits are conducted in accordance with GMR00022. • Auditor noticed orifice plate dimension of outside of normal tolerance after looking across a number of certificates. • This is not part of the routine audit process.

  5. Orifice bore dimension (d / mm) What is in error? • An error in the measurement of the orifice bore dimension of the orifice plate. • The dimensions of the plate are required in order to calculate the flow of natural gas through the meter.

  6. Route cause of the error. • Error traced back to calibration of the KEMCO 700 coordinate measuring machine (CMM)used to determine the orifice bore.

  7. Calibration. • Calibration of the Kemco 700 3 dimensional (CMM) machine undertaken on an annual basis by a UKAS accredited calibration facility. • Bias introduced following a calibration undertaken on the 13-10-2006. • Bias removed following a calibration undertaken on the 05-10-2007.

  8. Impact. • All orifice plates measured during this period will have been in error. • Results in a nominal dimensional error of ~ - 0.3116 mm in 311mm. • Approximately the same % error regardless of d. • Therefore as dimension to be measured increases then so does the error. • Equates to ~ 0.25% under read of volume gas throughput. • Equates to energy of ~ 600 GWh being underreported from the 34 meters. • Varies between 2-56 GWh per meter.

  9. Timeline – general.

  10. Timeline – site example.

  11. Remediation. • National Grid will ensure that O/P’s that have dimensional changes outside of the uncertainty of measurement are investigated. • National Grid will ensure that the CMM is checked on a routine basis to ascertain suitable repeatability.

  12. Summary. • A potential meter error at 34 orifice plate meter errors across our 4 Networks. • Additional checks now in place. • A potential under registration of ~ 600 GWh of energy at LDZ entry. • The measurement error notifications reported via the Joint Office in accordance with UNC conditions.

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