Power Plant · Flow
Duct / Flue-Gas Flow — Pitot Traverse (EPA Method 1–3)
Flue-gas or air flow in a duct from a multi-point pitot traverse: equal-area points, gas density from molecular weight (Orsat), moisture and absolute pressure, point-by-point velocities, and flow at actual, standard and normal conditions with an independent EPA-formula cross-check.
Verified against published standard
Duct
Gas
0.84 for an S-type pitot, 0.99 for a standard (Prandtl) pitot — use the calibration certificate.
Velocity-head readings
Use equal-area points: for a circle with N points on a diameter, distances from the wall (% of D) are 50·[1 − √((2i−1)/N)] and their mirror images (N = 12: 2.1, 6.7, 11.8, 17.7, 25.0, 35.6, 64.4, 75.0, 82.3, 88.2, 93.3, 97.9 %). Take readings on two perpendicular diameters.
Enter the inputs and press Calculate.
About This Calculation
Flow in large ducts is measured by traversing a pitot tube across equal-area points (EPA Method 1), computing each point velocity from the velocity head and gas density, and averaging the velocities. Gas density comes from absolute pressure, temperature, dry molecular weight (Orsat) and moisture (EPA Methods 2 and 3).
Frequently Asked Questions
Why average velocities and not velocity heads?
Velocity goes as the square root of velocity head, so the mean of square roots is smaller than the root of the mean; averaging heads would overstate flow.
What pitot coefficient should I use?
Use the calibration certificate; typical values are 0.84 for an S-type and about 0.99 for a standard Prandtl pitot.
How many traverse points are needed?
EPA Method 1 specifies the number from the duct diameter and the distance to the nearest flow disturbance; typically 12 to 24 points for a circular duct. More points reduce the error from an uneven velocity profile.
Why is molecular weight needed?
Gas density sets the velocity for a given velocity head. Flue gas is heavier than air because of CO₂ and water vapour; the Orsat analysis and moisture content give the molecular weight and so the density.