Orifice Plate Meter Error Stuart Gibbons, Extraordinary Offtake Arrangements Workstream Meeting: 16...
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Transcript of Orifice Plate Meter Error Stuart Gibbons, Extraordinary Offtake Arrangements Workstream Meeting: 16...
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
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 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.
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
10
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
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.