What Is a Pitot Tube?
Pitot tubes measure fluid velocity at a specific point in a pipe or duct by sensing the difference between the total (stagnation) pressure and the static pressure. Named after Henri Pitot, who first used the principle to measure water velocity in the Seine River in 1732, the device remains one of the simplest and most widely used velocity-sensing instruments in industrial piping, HVAC, and aerospace applications.
In oil and gas and process piping, Pitot tubes are classified as differential pressure (DP) flow elements and are standardized under ISO 3966 and ASME PTC 19.2. They are particularly valuable for large-diameter pipes and ducts where full-bore flow meters would be prohibitively expensive.
Operating Principle
A Pitot tube has a small opening (impact port) pointed directly into the flow, which senses the total pressure (P_total = static pressure + dynamic pressure). A second port, oriented perpendicular to the flow or located on the outer tube surface, senses the static pressure (P_static). The differential pressure between these two ports equals the dynamic pressure:
dP = P_total - P_static = 0.5 * rho * V^2
Rearranging for velocity:
V = C * sqrt(2 * dP / rho)
Where C is the Pitot tube coefficient (typically 0.98 to 1.00 for standard designs) and rho is the fluid density at flowing conditions.
Types of Pitot Tubes
| Type | Description | Typical Application |
|---|---|---|
| Standard (single-point) Pitot tube | L-shaped tube with impact and static ports at one location | Laboratory, HVAC ducts, point velocity surveys |
| Pitot-static tube (Prandtl type) | Concentric tube design with hemispherical nose; static ports on outer tube | Precise single-point measurement; aircraft airspeed indicators |
| Averaging Pitot tube (Annubar, Torbar) | Multiple impact ports across the pipe diameter, averaging the velocity profile | Large process pipes (6” to 96”+), stacks, ducts |
| S-type (Stausscheibe) Pitot | S-shaped tube for particulate-laden flows | Stack emissions testing, dirty gas measurement |
Averaging Pitot Tubes
Standard single-point Pitot tubes measure velocity at one location, requiring traversal across the pipe to determine the average velocity. Averaging Pitot tubes incorporate multiple sensing ports distributed across the pipe diameter at equal-area points, so the averaged signal represents the true mean velocity.
| Parameter | Single-Point Pitot | Averaging Pitot Tube |
|---|---|---|
| Accuracy | +/- 1-2% (with traverse) | +/- 0.75-1.5% of rate |
| Turndown | 3:1 | 8:1 to 10:1 |
| Permanent pressure loss | Negligible | Very low (<1% of line pressure) |
| Pipe size range | Any (insertion type) | 2” to 96”+ |
| Installation | Simple insertion | Welded or flanged nozzle; hot-tappable |
| Cost | Very low | Low to moderate |
Installation Considerations
Pitot tubes are insertion devices entering the pipe through a single penetration (threaded coupling, flanged nozzle, or compression fitting). Key installation factors:
- Orientation: The impact port must face directly into the flow. Even a 5-degree misalignment introduces 1-2% velocity error.
- Straight run: Minimum 15-25 pipe diameters upstream and 5 downstream.
- Blockage ratio: The tube cross-section should be less than 3% of pipe area to minimize flow disturbance.
Materials
| Service | Pitot Tube Material |
|---|---|
| Air, clean gas, water | 316 stainless steel |
| High-temperature steam, flue gas | 310 or 316 stainless steel |
| Corrosive chemicals | Hastelloy C-276 |
| Sour gas (H2S) | Duplex stainless steel, Inconel 625 |
Advantages and Limitations
Advantages: Very low pressure loss, low cost, applicable to very large pipe sizes, simple design, hot-tappable (averaging type), minimal maintenance.
Limitations: Lower accuracy compared to full-bore meters (orifice plates, Venturi meters), sensitive to alignment errors, impact ports can plug in dirty or particulate-laden fluids, limited low-velocity sensitivity.
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