1. Horndon Temperature Error (SMER)
Stuart Gibbons, Offtake Arrangements Workstream Meeting: 25th January 2011

2. Contents What & How the error was identified? What is the error?
Route cause of the error.
Impact.
Timeline of events.
Typical measurement system.
Graphs.
Remediation.
Summary.

3. How was the error identified?
A ME2 validation (and Audit) at Horndon Offtake meter during July 2010 identified an anomaly on the meter temperature measurement system.
ME2 - CP12 Temperature transmitter calibration.
ME2 - CP13 temperature element spot check.
ME2 - CP13 test requires that the temperature element is placed with a reference temperature device to compare the indicated temperature.
Test failed.

4. What was the error?
CP13 test identified a temperature reading error (bias) of +7.3ºC to that of the reference temperature.
The route cause of the error (bias) was identified to be a loose connection on the temperature element (RTD) device.
On tightening the loose connection the indicated temperature came within the CP13 test limits (±0.5ºC).

5. Impact
The resultant positive temperature error (bias) results in a lower density and therefore an under read of volume flow.
Temperature error (bias) not constant, varies (~4 to 6 ºC).
Cannot refer to another temperature at site.
Had to look at upstream / downstream offtakes that were flowing for a valid temperature measurement.
Temperature error (bias) within normal operational temperature range.
Analysis indicates that this error (bias) spanned 20 months.
ME2 validation 08 – ME2 validation 10.
Difficult to analyse impact as only one temperature element at the offtake can be used.

6. Impact 2 ME2 validation done in 2009.
CP13 test passed.
The positive temperature error (bias) results in approximately 125 GWh of under registration of energy.
Error was -0.42

7. Impact 3
Looking further back revealed an intermittent negative temperature error ranging between -1 to -2 ºC.
Error occurred Dec 2005 until July 2008 when the temperature transmitter was replaced.
The replacement of the temperature transmitter is assumed to be the contributing cause of the positive temperature error.
Error was -0.42

8. Timeline
Audit
Temperature reading approx 1 to 2 ºC low (Intermittent )
Temperature reading approx 4 to 6 ºC high

9. Temperature graph 2008 (bias) example

10. Temperature graph 2005 (error) example
Temperature measured low

11. Typical temperature measurement system
RTD Temperature element
Temperature Tx
Line temperature is measured from a 3-wire connected RTD element, located 8.6 diameters
downstream of the orifice plate. The element is connected to a Rosemount 248 temperature
transmitter, calibrated range -10°C to 40°C, located in a junction box approximately 1.5 metres
from the temperature element.

12. Temperature Loop Instrumentation
RTD element
Transmitter
Flow Computer

13. Remediation
Identify in Capex plan upgrade to temperature measurement system.
Looking at duplex RTD sensors.
One used for measurement.
One used for validity check.
Updating to 4 wire system to flow computer.
Improves performance.
Improves fault diagnostics.
Additional checks in HPMIS.

14. Summary Error identified at routine ME2 validation.
Two separate temperature measurement issues.
One results in an under read of energy ~ 125 GWh over 20 months.
One estimated to result in an over read of energy – not quantified.