addressing uncertainty in oil and natural gas industry greenhouse

Part 3 – Natural gas applications; and
Part 4 – Background, development, implementation procedures, and subroutine documentation.
The standard recognizes that many factors contribute to the overall measurement uncertainty associated
with many metering applications, as summarized in Exhibit 3-1.
EXHIBIT 3-1: FACTORS CONTRIBUTING TO MEASUREMENT
UNCERTAINTY FOR ORIFICE METERS
a) Tolerances in prediction of coefficient of discharge
− Derivation of the basic flow equation for an orifice flowmeter is based on physical laws.
− Any derivation is accurate when all assumptions used to develop the equation are valid.
− The empirical equation for the coefficient of discharge that is included in API 14.3 (Reference 16) was developed
from a large database with well-controlled and quantified independent variables.
b) Predictability in defining the physical properties of the flowing fluid
− All empirical equations and standards for concentric, square-edged orifice meters apply to steady state flow
conditions for fluids that are considered to be clean, single phase, and homogeneous, such as all gases – and most
liquids – in the petroleum, petrochemical, and natural gas industries.
− Fluid's flow rates are expressed in volume units at base (standard or reference) conditions, and the volumetric flow
rates that are measured at the operating flowing conditions are then converted to standard volume with respect to the
base conditions.
− Fluid properties are defined as a function of the operating pressure and temperature that are monitored by secondary
devices. Significant temperature variation between the thermal well and the orifice taps will affect the measurement.
c) Fluid flow conditions
− Database is available for the empirical equations for coefficient of discharge for steady-state fully developed pipe
flow profile with negligible or no swirl flows and flow fluctuations.
− Deviations from these conditions are typically due to piping installation upstream of the flowmeter and they
introduce flow measurement uncertainty.
d) Construction tolerances in meter components
− Part 2 of the reapproved API MPMS Chapter 14.3 standard lists the changes recommended in the mechanical
tolerance requirements for the orifice meter components.
− The standard encompasses a wide range of diameter ratios for which experimental results are available and some of
the tolerances are more stringent than the tolerances in the previous standards.
e) Uncertainty of secondary devices/instrumentation
− The secondary devices are the instruments used to monitor the flowing fluid temperature, pressure, and the
differential pressure across the orifice plate.
− Parameters affecting the accuracy of the differential pressure device include: ambient temperature, static pressure,
linearity, repeatability, long-term stability, and drift, as well as the uncertainty of the calibration standard.
− The stated accuracy of most differential pressure-measuring devices is expressed as a percentage of the full-scale
reading, which leads to increased error bands with decreasing differential pressures.
f) Data reduction and computation
− Ultimate errors in flow rate computation depend on the accuracy of defining the physical properties of the flowing
fluid, as computed by the microprocessor-based flow computers.
− Computation of the physical properties, especially for gas flows, is dependent on the constituents of gas in the
flowing fluid.
− All fixed input and critical parameters affecting the flow rate computation should be verified to reduce bias error in
flow measurement.
Pilot Version, September 2009 3-4