addressing uncertainty in oil and natural gas industry greenhouse

All these factors should be assessed when estimating the overall range of uncertainty for flow
measurements using thin plate, concentric, square-edged metering systems.
In the reapproved 2006 version of the standard, several changes were incorporated to reduce the
uncertainty attributable to installation effects and to improve the rigor of the flow calculation routines.
The revised standard recognizes the lead-time necessary for upgrading existing installations, and leaves
this lead-time to the discretion of facility operators and their data quality targets for flow measurement
data.
However, it should be recognized that if orifice meter installations are not upgraded to conform to the
new recommendations, measurement bias error may occur. This bias might be due to improper upper and
lower distances from bends and points of flow turbulence that might lead to inadequate flow conditioning
prior to measurement. Additionally, even without changing equipment installations, the standard
recommends adopting new calculation procedures and techniques (explained in Part 1 and 3 of the
standard) that represent significant improvements over the previously adopted approach. It is important
to note that the expected uncertainty ranges for flow measurements quoted in Part 1 of the reaffirmed
standard may differ from those obtained in practice when the equipment installation differs.
3.1.2 Measurement of Flow to Flares
The measurement of flow to flares is distinctly different than other flow measurements. Flares are
designed as safety relief systems and typically are capable of handling highly variable flow rates of
widely varying gas compositions. Therefore, some of the practices that are generally applicable to
custody transfer or process control flows have to be modified when addressing flows to flares. API
published a measurement standard addressing gas or vapor flare flow measurements, which also includes
cautionary details about the effects of fouling (due to entrained liquid droplets, aerosol mists, or other
contaminations) on the measurement (API MPMS, July 2007).
Most flare headers are designed to operate during both non-upset conditions at near atmospheric pressure
and ambient temperature, and during flare episodes, at a wide range of pressure, temperature, and flow
velocities. During such episodes, flare gas compositions are also highly variable and could range from
molecular weights approaching that of hydrogen to molecular weights of C 5+ .
As with other flow measurements, the accurate determination of flow to flares is dependent on many
parameters such as the ability to predict – or measure – mixture composition, pressure, temperature,
and/or density. The accuracy of measurements associated with highly variable flare gas mixtures will
depend largely on the meter technology type and the ability of the flare flow measurement system
(FFMS) to achieve the targeted response time and analytical accuracy levels. Exhibit 3-2 below lists the
Pilot Version, September 2009 3-5