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Pitot Tube Flow Meters and Velocity Elements for Large Duct Point Velocity Measurement

Quick Answer: A pitot tube flow meter measures point velocity in large ducts. Use an averaging pitot tube velocity element when you need a low cost insertion flow meter for combustion air, flue gas, or HVAC air. Send us your duct width, duct height, gas pressure, temperature, and flow range for a quote.

Where Pitot Tube Velocity Elements Make Sense

Large duct flow measurement shows up in power plants, cement plants, chemical plants, and water treatment plants. We see these jobs in Southeast Asia, the Middle East, and Australia. Engineers use pitot tube velocity elements for combustion air, flue gas, process gas, HVAC air, and aeration air. The duct can be round or rectangular. Typical round ducts range from DN200 to DN3000. Rectangular ducts often run from 500 mm x 500 mm up to 8 m x 8 m.

Most engineers choose an insertion pitot tube because it costs much less than an inline flow meter in large pipe sizes. An inline electromagnetic flow meter cannot measure gas. A thermal mass flow meter works well in small pipes but becomes expensive for large ducts. A pitot tube velocity element mounts through a nozzle or flange and creates a low permanent pressure loss.

Single Point Pitot Tube vs Averaging Pitot Tube

A single point pitot tube measures velocity at one location. It is simple and low cost. But a large duct has a velocity profile. Air moves faster near the center. Air moves slower near the wall. One point cannot represent the full flow. An averaging pitot tube has multiple sensing holes along its length. It measures the dynamic pressure at several points and outputs one averaged differential pressure. We recommend an averaging pitot tube for ducts above DN400. For rectangular ducts, specify the probe length according to the duct width.

How the Velocity Element Works

The pitot tube velocity element has two chambers. One chamber sees total pressure. The other chamber sees static pressure. The difference is dynamic pressure. The transmitter converts dynamic pressure to velocity. Flow rate equals velocity times duct cross sectional area. You need a differential pressure transmitter with 4-20 mA HART output. For gas flow, you also need a temperature sensor like a PT100 and a pressure transmitter for density compensation. The flow computer or PLC calculates compensated mass flow in kg/h or Nm3/h.

Common Large Duct Applications

We have quoted pitot tube flow meters for combustion air in a cement plant in Vietnam. Another customer in Saudi Arabia needed flue gas flow measurement after an electrostatic precipitator. A water utility in Queensland asked for airflow measurement on aeration blowers. A paint manufacturer in Indonesia used an averaging pitot tube for oven exhaust air. In practice, these point velocity hardware solutions solve a problem without shutting down the process for long.

Accuracy, Materials, and Process Limits

Accuracy depends on installation and velocity profile. A properly installed averaging pitot tube can reach ±1% to ±2% of flow rate in a stable duct with a straight run. If the duct has bends, valves, or dampers nearby, accuracy drops. Standard material is 316L stainless steel. For corrosive gas, we offer Hastelloy C probes. Standard process temperature range is -40°C to 120°C for the transmitter. With a remote diaphragm seal or impulse line cooling, the probe can handle gas up to 400°C. Standard pressure rating is PN16 or PN40.

Installation and Straight Run Requirements

You need a stable velocity profile. The general rule is 10 diameters of straight duct upstream and 5 diameters downstream. For rectangular ducts, use the hydraulic diameter. Mount the probe through a flange or a weld nipple. Do not install the probe right after a damper, elbow, or fan. If space is short, use flow straightene