Orifice Plate Meter Error

Stuart Gibbons, Extraordinary Offtake Arrangements Workstream Meeting: 16 October 2009

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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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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.

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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.

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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)

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Route cause of the error.

Error traced back to calibration of the KEMCO 700 coordinate measuring

machine (CMM) used to determine the orifice bore.

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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.

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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.

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Timeline – general.

Calibration of Kemco CMM

Meter AuditWhitwell

Auditor issued draft report

Measurement & process

investigateCalibration of Kemco CMM

Orifice Plate Meter Error Timeline

Jan feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan feb Mar Apr May Jun Jul Aug Sep2006 2007 2008 2009

Orifice plates installed

Orifice plates removed

Bias introduced Bias removed

Joint Office notification

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Timeline – site example.

Site: Gt Wilbraham

Calibration of Kemco CMM

Orifice plate recorded in laboratory

Calibration of Kemco CMM

Orifice Plate Meter Error Timeline

Jan feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan feb Mar Apr May Jun Jul Aug Sep2006 2007 2008 2009

Orifice plates installed

Orifice plates removed

Bias introduced Bias removed

Orifice plate calibrated

Orifice plate installed

Orifice plate removed

Orifice plate dispatched to

network

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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.

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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.