An averaging pitot tube measures flow by sensing differential pressure across multiple ports spanning the full pipe diameter.
Unlike a single-point pitot tube, it averages the velocity profile, giving a more accurate and representative flow measurement across the whole cross-section.
We will understand how multiple sensing ports across the pipe diameter produce a pressure signal that represents the average flow velocity, and how that signal relates to volumetric and mass flow.
We will also cover the flow formula, a step-by-step worked calculation example, the key installation requirements, and a comparison between the averaging pitot tube and other DP flow elements.
The averaging pitot tube produces very low permanent pressure loss.
Its insertion design allows installation in large pipes and ducts where orifice plates and venturi tubes would be impractical or too costly.
Averaging Pitot Tube Working Principle: Bernoulli and Multiport Averaging
The averaging pitot tube works on Bernoulli's equation. When fluid flows against the front face, it decelerates to zero velocity at the impact port.
The kinetic energy converts to pressure at this stagnation point, which is higher than the static pressure of the undisturbed flow.
The difference between total pressure at the front and static pressure at the rear gives the differential pressure (DP). This DP is proportional to the square of the fluid velocity.
Taking the square root of the DP gives the local velocity at that point.
Multiple impact ports face upstream across the pipe diameter. Each port senses stagnation pressure at its radial position.
These pressures combine and average inside the HP tube chamber. The HP port connects to the high port of the DP transmitter.
One or more ports on the downstream face of the tube sense the static pressure. This pressure is lower than total pressure by an amount proportional to the velocity head.
The LP port connects to the low port of the DP transmitter.
The word averaging refers to hydraulic averaging of total pressure from multiple ports across the pipe.
The pressures from all HP ports combine in the internal chamber to produce a single averaged total pressure signal. This compensates for the non-uniform velocity profile across the pipe cross-section.
Averaging Pitot Tube Flow Calculation Formula
The velocity of the fluid is derived from Bernoulli's equation applied to the stagnation point. The flow formula for an averaging pitot tube is:
The velocity flow coefficient Kv accounts for the tube geometry and port averaging effect.
The manufacturer determines Kv during flow calibration. A typical Kv value is between 0.58 and 0.75 depending on the tube design and pipe Reynolds number.
Once average velocity is known, volumetric flow rate is calculated by multiplying by the pipe cross-sectional area:
For mass flow, multiply volumetric flow by fluid density:
Worked Calculation Example
Pipe internal diameter D = 200 mm = 0.200 m
Fluid: natural gas at process conditions
Gas density rho = 0.82 kg/m3
Differential pressure DP = 500 Pa
Velocity flow coefficient Kv = 0.65
Step 1: Calculate pipe cross-sectional area
A = pi / 4 x D2 = 0.7854 x 0.04
A = 0.03142 m2
Step 2: Calculate average velocity
V = 0.65 x sqrt ( 2 x 500 / 0.82 )
V = 0.65 x sqrt ( 1219.5 ) = 0.65 x 34.92
V = 22.70 m/s
Step 3: Calculate volumetric flow rate
Q = 22.70 x 0.03142 = 0.713 m3/s = 2566 m3/h
Step 4: Calculate mass flow rate
qm = 0.713 x 0.82 = 0.585 kg/s = 2105 kg/h
Port Positioning and Velocity Profile Compensation
Ports on an averaging pitot tube are not spaced at equal distances. They are positioned by the log-linear method, placing more ports near the wall where the velocity gradient is steepest.
This ensures each port represents an equal cross-sectional area, giving a true area-weighted average of the velocity profile.
Installation Requirements and Straight Run
A minimum upstream straight pipe run is required to allow the velocity profile to develop before the sensing element.
The required straight run is shorter than for an orifice plate but longer than for a vortex or ultrasonic meter.
| Upstream Disturbance | Minimum Straight Run Required | Note |
|---|---|---|
| Single elbow in same plane | 10 to 15 x pipe diameter | Most common installation scenario in plant piping. |
| Two elbows out of plane | 20 to 25 x pipe diameter | Two elbows out of plane create swirl that distorts the profile. |
| Fully open gate or ball valve | 10 x pipe diameter | Fully open valves have minimal distortion effect. |
| Partially open control valve | 25 to 30 x pipe diameter | A throttled valve creates a severely asymmetric profile. Avoid if possible. |
| Concentric reducer | 5 x pipe diameter | Concentric reducers have a smaller effect on the profile than elbows. |
| Downstream straight run | 3 to 5 x pipe diameter | Required between the sensing element and any downstream fitting. |
For gas service, avoid horizontal installation with ports at the bottom, as condensate can collect in the HP lines and block the ports. For liquid service, ensure the impulse lines slope continuously upward from the tap to the DP transmitter to prevent air pockets. For steam service, use condensate pots to equalise the HP and LP leg temperatures before connecting to the DP transmitter.
Averaging Pitot Tube vs Other DP Flow Elements
| Parameter | Averaging Pitot Tube | Orifice Plate | Venturi Tube |
|---|---|---|---|
| Permanent pressure loss | Very low (1 to 3% of line pressure) | High (30 to 60% of measured DP is lost) | Low (10 to 15% of measured DP is lost) |
| Pipe size range | 50 mm to 3000 mm and above | 25 mm to 1000 mm | 50 mm to 1200 mm |
| Insertion method | Hot tap insertion possible | Full pipe break required | Full pipe break required |
| Accuracy | 0.5 to 2% of reading with calibration | 0.5 to 1% with calibrated bore | 0.5 to 1% with machined throat |
| Turndown ratio | 4:1 to 6:1 (DP based) | 3:1 to 5:1 | 4:1 to 6:1 |
| Suitable for dirty fluids | No (ports can block) | Yes (with drain/vent holes) | Yes |
| Cost for large pipes | Very low | High (large flanged orifice plate) | Very high (large venturi body) |
Watch: Averaging Pitot Tube Flow Meter Explained
Averaging Pitot Tube Questions Engineers Ask
Related Articles on This Site
- DP Flow Transmitter 5-Point Calibration Procedure
- Differential Pressure Transmitter Working Principle
- Flow Nozzle vs Orifice Plate vs Venturi: Comparison
- Vortex Flow Meter Working Principle
- Basics of Differential Pressure Transmitter
External References
- ISO 3966: Measurement of Fluid Flow in Closed Conduits | ISO
- Rosemount 485 Annubar Averaging Pitot Tube | Emerson
What We Learn Today
- An averaging pitot tube measures flow using multiple upstream ports that sense stagnation pressure at several radial positions across the pipe. These pressures are hydraulically averaged in the HP chamber. The DP between the averaged HP and the downstream LP gives the velocity signal via Bernoulli's equation: V = Kv x sqrt (2 x DP / rho). Volumetric flow is then Q = V x A.
- Ports are positioned using the equal area or log-linear method so each port represents an equal cross-sectional zone of the pipe. A typical 4-port tube has ports at approximately 13%, 37%, 63% and 87% of the radius. The velocity flow coefficient Kv (typically 0.58 to 0.75) is determined by the manufacturer during calibration and must be taken from the device datasheet.
- The averaging pitot tube produces very low permanent pressure loss and can be hot-tap inserted into large pipes without shutting down the process. Its main limitations are a 4:1 to 6:1 turndown ratio and port blockage in dirty or particle-laden fluids. Minimum upstream straight run is 10 to 25 pipe diameters depending on the upstream disturbance type.