Pitot Tube

A pitot tube is a flow measurement device that determines fluid velocity, and by extension volumetric flow rate, by comparing the stagnation pressure at the tube's open end (where the flowing fluid is brought to rest) against the static pressure of the undisturbed stream. In oil and gas operations, pitot tubes are used to measure gas flow rates in pipelines, flowlines, and flare headers where low-cost, low-maintenance measurement is acceptable and fiscal-grade accuracy is not required. The instrument has no moving parts, installs with a simple penetration fitting, and is applicable across a wide range of pipe diameters and flow velocities.

Historical Background and Operating Principle

The pitot tube takes its name from French engineer Henri Pitot (1695 to 1771), who developed an early form of the device in 1732 to measure flow velocity in the Seine River. The operating principle is based on Bernoulli's equation for steady, incompressible flow along a streamline: the sum of static pressure and dynamic pressure is constant. At the stagnation point, where the fluid velocity is reduced to zero at the tube opening, all kinetic energy converts to pressure. The velocity of the flowing stream is therefore: v equals the square root of (2 times the difference between stagnation pressure and static pressure, divided by fluid density). In practice, a correction factor (coefficient of discharge, typically 0.95 to 0.99 for clean pitot tubes) is applied to account for flow profile and viscous losses. For gas measurement, density is calculated from pressure, temperature, and gas composition using real gas equations of state.

Instrument Types Used in Oil and Gas

Several pitot tube variants are deployed across oilfield applications. The standard single-point pitot tube consists of a small-bore tube (typically 1/8 inch inner diameter) inserted along the pipe centerline, with the open end facing the upstream flow. It measures velocity at a single radial point, requiring a flow profile correction (area-averaged velocity factor) to convert centerline velocity to mean flow velocity across the cross-section. This correction depends on the Reynolds number and pipe roughness and introduces uncertainty if the flow profile is non-uniform. The Annubar (averaging pitot tube) overcomes this limitation by using a probe that spans the full pipe diameter with multiple sensing ports positioned at predefined radial locations corresponding to equal-area annuli. The differential pressures from all ports are averaged internally, producing a signal proportional to the average velocity. Annubar designs are available from manufacturers including Rosemount, Dieterich Standard, and Veris, and are widely used in production facilities for gas flow measurement where orifice meters or ultrasonic meters would be economically or logistically impractical.

Applications in Gas Measurement and Flare Quantification

On producing wells and gathering systems, pitot tubes are used for non-fiscal allocation measurement, separator inlet gas measurement, compressor suction and discharge flow monitoring, and gas lift injection rate verification. They are particularly common in flare header measurement, where regulatory requirements increasingly mandate quantification of volumes sent to flare for greenhouse gas reporting under frameworks such as the Alberta Energy Regulator's Directive 060, the US EPA's Subpart W, and similar regulations. Flare pitot installations typically use Annubar-type averaging probes with a pressure transmitter and temperature correction to account for variable gas composition and flow conditions. The measurement uncertainty of a well-installed averaging pitot on a clean gas service is typically plus or minus 1 to 3 percent of reading, which is adequate for regulatory reporting but not for fiscal custody transfer, where orifice meters, ultrasonic meters, or Coriolis meters are preferred.

Installation Requirements and Limitations

Accurate pitot tube measurement depends on a fully developed, axisymmetric velocity profile at the measurement point. Industry practice following ISO 3966 and API MPMS Chapter 22 recommends a minimum of 10 pipe diameters of straight pipe upstream and 5 diameters downstream of the pitot location, clear of bends, valves, reducers, and other flow disturbances. Where straight-run is unavailable, flow conditioners such as tube bundles or perforated plates can be installed upstream to reduce swirl and profile distortion. Limitations of the pitot tube include sensitivity to flow direction misalignment (small yaw angles reduce accuracy), susceptibility to plugging from liquid carryover, condensate, hydrates, or scale on the sensing ports, and reduced sensitivity at low velocities where the differential pressure signal becomes very small relative to measurement noise. Pitot tubes are not suitable for multiphase or wet gas service without careful engineering consideration, as liquid slugs can block the sensing ports and produce erroneous readings or instrument damage.

Comparison with Other Gas Flow Meters

The pitot tube occupies a specific niche in the hierarchy of gas metering technologies. Compared to orifice meters, pitot tubes have lower permanent pressure loss (which reduces compression energy consumption) and lower installation cost, but higher measurement uncertainty and greater sensitivity to flow profile. Compared to multipath ultrasonic meters, pitot tubes are far less expensive and simpler to install but cannot self-diagnose flow profile issues or provide velocity profile data across multiple chords. Turbine meters have higher accuracy and are used for fiscal metering but have moving parts that wear and require regular calibration. Coriolis meters provide mass flow measurement independent of fluid properties but are limited to smaller pipe sizes and higher cost. The averaging pitot tube sits in a middle ground: it offers better accuracy than a single-point pitot, reasonable installed cost, no moving parts, low maintenance, and is suited for monitoring and regulatory reporting applications where uncertainty requirements of 1 to 3 percent are acceptable.

Key Takeaways

• The pitot tube measures gas velocity by comparing stagnation pressure at the tube opening to static line pressure, using Bernoulli's principle; flow rate is then calculated from velocity, pipe area, and gas density.
• Averaging pitot tubes (Annubar type) sample across multiple radial positions to compensate for non-uniform velocity profiles, offering improved accuracy of plus or minus 1 to 3 percent compared to single-point designs.
• Oilfield applications include gathering system allocation measurement, compressor monitoring, gas lift injection verification, and flare header quantification for greenhouse gas regulatory reporting.
• Minimum installation requirements are 10 pipe diameters upstream and 5 downstream of any flow disturbance; plugging from liquids, hydrates, or scale is the most common maintenance issue in wet gas service.
• Pitot tubes are not suitable for fiscal custody transfer applications, where orifice meters, ultrasonic meters, or Coriolis meters provide the lower uncertainty required by regulatory and commercial agreements.