What is the effect of instrument quality on ATMOS?

The quality of field instruments for flow, pressure measurements is very important for pipeline integrity monitoring. To eliminate any misunderstanding, the following definitions of terms are given (reference: Scientific Apparatus Makers Association Standard).

Hysteresis: The maximum difference for the same input between the upscale and downscale output values during a full range traverse in each direction.

Accuracy: A number or quantity which defines the limit that errors will not exceed when the device is used under reference operating conditions. Accuracy includes the combined conformity, hysteresis and repeatability errors.

Repeatability: The closeness of agreement among a number of consecutive measurements of the output for the same value of the input under the same operating conditions, approaching from the same direction, for full range traverses. It does not include hysteresis.

Resolution: The degree to which equal values of a quantity can be discriminated by the device.

These definitions indicate that accuracy gives the maximum measurement error, repeatability represents the consistency of measurement results and resolution determines the minimum change an instrument can sense. For pipeline monitoring the above four qualities are of varying importance for ATMOS pipe:

• Instrument resolution determines the minimum leak detectable by any system based on field measurements. If the resolution of flow and pressure meters is 0.1%, for example, it is impossible to use these meters to detect a leak lower than 0.1%.
• Instrument repeatability is critical in determining leak detection reliability. If it is required to detect a leak of a magnitude equal to or lower than the instrument repeatability, then false alarms will be generated. To avoid false alarms, the minimum leak to be detected should be greater than the combined repeatability of the flow and pressure instruments.
• Since ATMOS pipe detects relative changes in the mean value of the corrected mass imbalance term, known instrument errors are accepted as ‘normal’ discrepancy between the instruments assuming no leak is present when the system is first installed. For example, if the system is commissioned after a successful hydrostatic test, an initial mean value of 2% in the mass imbalance term is accepted as the result of instrument errors rather than a leak. Thereafter if the mean increases to, say 3%, then a leak alarm will be generated as the mean has increased by 1%. During leak-free operations ATMOS pipe continuously learns about instrument inaccuracies and tunes itself at a lower rate than leak detection (if all leaks must be detected within 24 hours, the tuning will be carried out over a longer time window, say 72 hours). Therefore it is possible for ATMOS pipe to detect a leak smaller than the instrument errors. Previous field tests have shown that ATMOS pipe detected a leak of 0.5% while the flow accuracy was 1%. To detect such small leaks, one condition is that no leak is present in the pipeline when ATMOS pipe is first commissioned.
• Hysteresis can affect leak detection performance only when different nonlinear characteristics exist between the flow meters at the ingress and egress points. An example of such cases is when different makes of flow transmitters are used at the inlet and outlet of a pipeline. In one application where ATMOS pipe was applied to a pipeline with existing instruments of different makes, the mean difference between the inlet and outlet flow measurements increases from 3 t/d to 11 t/d as the true flow changes from 100 t/d to 450 t/d. Initially such measurement errors can introduce false alarms, once they are recognised as instrument errors it is possible to distinguish them from pipeline leaks. To minimise nonlinear effect of a field instrument, it is good practice to arrange the scale of the meter such that it can be used within its linear range most of the time.

Note that SCADA systems can reduce measurement resolutions if the Analogue to Digital (AD) converters are not selected properly. To maintain a high resolution level, it is recommended to use 16 bit AD converters.