Annubar: Definition, Averaging Pitot Tube, and Gas Flow Measurement
An annubar is a multi-port averaging Pitot tube flow meter inserted across the full diameter of a pipe to measure the volumetric or mass flow rate of gases, liquids, or multiphase streams in pipelines and process systems. Unlike a standard single-point Pitot tube, which samples velocity at only one location and introduces significant error in turbulent or asymmetric flow profiles, the annubar contains multiple upstream-facing sensing ports distributed across the pipe cross-section that simultaneously capture the impact (stagnation) pressure at each measurement point. A single downstream-facing port, or in some designs a common downstream chamber, measures the static pressure of the flowing stream. The differential pressure between the averaged impact pressure and the static pressure is proportional to the square of the average fluid velocity, allowing flow rate to be calculated from first principles using the Bernoulli equation. The name "Annubar" is a registered trademark of Emerson Electric Co. through its Rosemount measurement division, though the term has entered general engineering usage as a descriptor for the entire class of averaging Pitot tube instruments produced by multiple manufacturers under various trade names.
Key Takeaways
- The annubar averages velocity pressure readings across the full pipe diameter through multiple sensing ports, overcoming the fundamental limitation of single-point Pitot tubes and producing accurate flow measurements even in turbulent or partially disturbed flow profiles found in typical field installations.
- Permanent pressure loss across an annubar installation is typically 5 to 10 percent of the measured differential pressure signal, compared to 50 to 80 percent for an orifice plate at equivalent flow conditions, delivering significant energy savings and reducing compression costs on long-distance natural gas gathering and transmission systems.
- Annubars can be installed via hot-tap procedure into live, pressurized pipelines without taking the line out of service, making them the flow meter of choice for retrofitting measurement capability onto existing infrastructure at producing wellhead facilities, compressor stations, and custody transfer points.
- The instrument's accuracy of plus or minus 0.5 to 1.0 percent of full scale and rangeability of 10:1 make it suitable for custody transfer measurement of natural gas and liquid hydrocarbons when properly sized, installed, and calibrated in accordance with AGA-3 or ISO 5167 standards.
- Annubar measurement accuracy depends critically on the flow conditioning upstream of the sensing element; the instrument requires a minimum straight pipe run of 20 to 30 pipe diameters upstream and 5 pipe diameters downstream free of elbows, reducers, control valves, and other flow disturbances, or the use of a flow conditioning plate to simulate developed flow profiles in shorter runs.
How an Annubar Works: Operating Principle
The fundamental operating principle of the annubar is derived from Bernoulli's theorem, which states that for an incompressible, inviscid fluid in steady flow, the sum of static pressure, dynamic pressure, and gravitational pressure remains constant along any streamline. In practice, the key relationship for flow measurement is that the stagnation (impact) pressure recorded at the leading face of an obstruction placed normal to the flow direction equals the sum of the static pressure plus the dynamic (velocity) pressure: P_stagnation = P_static + (rho x v^2 / 2). Rearranging, the local velocity at any point in the flow profile is v = sqrt(2 x delta_P / rho), where delta_P is the differential pressure between the stagnation port and the static port, and rho is the fluid density. The challenge in applying this relationship to real pipe flow is that velocity is not uniform across the pipe cross-section: boundary layer effects near the pipe wall, turbulence-induced eddies, and downstream disturbances from fittings all create a complex, non-uniform velocity profile. A single-point Pitot tube measures velocity at only one radial position, and unless that position is precisely at the point of mean velocity in a fully developed turbulent profile, the resulting flow calculation carries a systematic error that can reach 5 to 20 percent in typical field conditions.
The annubar solves this problem by distributing multiple sensing ports across the pipe diameter according to a mathematically determined weighting scheme, typically based on Gauss-Legendre numerical integration or the log-Tchebycheff method described in ISO 5167. The most common annubar designs place 4 to 8 upstream-facing ports spaced at radial positions selected so that each port represents an equal annular area of the pipe cross-section. The stagnation pressures from all upstream ports are hydraulically averaged in a common manifold chamber within the sensing element body, producing a single averaged impact pressure signal that approximates the flow-weighted mean velocity pressure across the profile. The downstream port or ports, positioned in the low-pressure wake behind the sensing element's body, measure the static pressure. The differential pressure transmitter connected between the averaged high-pressure port and the static low-pressure port generates a continuous 4-20 mA or digital signal proportional to delta_P, from which the flow rate is computed by a flow computer applying the relevant flow equation for the specific fluid, temperature, pressure, and pipe diameter conditions.
For compressible fluids such as natural gas, an expansion factor (or gas expansion factor, Y1 or Y2 depending on the reference standard) must be applied to correct for the density change between the upstream pipeline conditions and the lower pressure at the stagnation port. This correction is particularly important for high-pressure gas applications where the differential pressure represents a significant fraction of the line static pressure, or in flare gas measurement applications where the pressure ratio across the meter may be large. Flow computers managing annubar outputs in gas gathering and natural gas processing facilities routinely apply real-time AGA-8 equations of state to compute gas compressibility factors (Z) and densities from measured pressure and temperature, ensuring that calculated mass flow rates reflect actual gas composition rather than assumed ideal gas behavior.
Annubar Designs and Profile Types
Several distinct sensing element geometries are manufactured under the annubar concept, each offering tradeoffs between drag coefficient, pressure drop, vibration resistance, and manufacturing cost. The diamond-profile annubar is the most widely deployed design in the oil and gas industry. Its diamond-shaped cross-section presents a sharp leading edge to the flow, minimizing flow separation and vortex shedding at the leading face while the trailing edges are positioned to create a stable, symmetric low-pressure wake that provides a steady static pressure reference. Diamond-profile annubars manufactured to tight dimensional tolerances carry a discharge coefficient (Cd) that is stable across a Reynolds number range from approximately 8,000 to 10 million, matching or exceeding the Reynolds number stability of standard orifice plates in most gas gathering and transmission service conditions.
The T-profile annubar, also called a T-bar design, consists of a round tube with upstream-facing ports drilled at specified radial intervals. This design is simpler to manufacture and less expensive than the diamond profile, though its blunt leading edge produces more flow separation and greater vortex shedding susceptibility at high velocities. T-profile designs are commonly used in water injection and formation water handling systems where fluid density is high and velocities are moderate, making vortex-induced vibration a lower concern. Round-profile annubars use a cylindrical sensor body and are preferred for low-velocity, high-viscosity fluid applications such as heavy crude oil pipelines where the round profile's superior hydraulic characteristics in laminar-to-transitional flow regimes provide better accuracy than sharp-edged designs.
Multiport averaging Pitot tubes are also available in self-averaging designs that route both high-pressure and low-pressure ports within a single instrument housing, eliminating the external impulse tubing connections between the sensing element and the differential pressure transmitter. These integrated designs, sometimes called "integrated multipoint averaging" instruments, reduce installation time and eliminate a potential source of measurement error from unequal impulse line temperatures or fluid accumulation in condensate-service sensing lines. For wet gas or two-phase flow service, annubar designs with integral drain ports or back-purge capability are specified to prevent liquid condensate accumulation in the high-pressure manifold chamber from introducing a hydrostatic head error in the differential pressure measurement.