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 GMR  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 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. Orifice bore dimension (d / mm)

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 dimensional (CMM) machine undertaken on an annual basis by a UKAS accredited calibration facility.  Bias introduced following a calibration undertaken on the  Bias removed following a calibration undertaken on the

8 Impact.  All orifice plates measured during this period will have been in error.  Results in a nominal dimensional error of ~ 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.