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 straighteners or accept lower accuracy. We see this issue often in old plants. A customer in Malaysia had only 3D upstream straight run. We quoted a multi point averaging tube and told them the accuracy would be about ±3%.

Recommended Model Pairing from Silver Instruments

Silver Instruments offers the SI-PT series averaging pitot tube flow meter for large ducts. The velocity element comes in 316L stainless steel or Hastelloy C. The differential pressure transmitter ranges from 0-1 kPa to 0-100 kPa depending on gas density and velocity. Output is 4-20 mA HART. You can add a PT100 temperature sensor and a pressure transmitter. The complete kit includes a valve manifold, impulse tubing, and mounting flange. For hazardous areas, we can supply ATEX Zone 1 transmitters.

What Information to Send for a Quote

Send us your duct dimensions, gas composition, gas temperature, static pressure, flow range, and available straight run. Tell us if the duct is round or rectangular. Tell us the wall thickness and whether you can weld a nozzle or need a flange mounting. A sketch or photo helps. Contact Silver Automation Instruments at +86-25-68650347, WhatsApp +86-25-52155837, or WeChat +86 15365082610. Send your pressure in bar, temperature in °C, duct size in mm, and flow range in kg/h or Nm3/h.

FAQ on Pitot Tube Flow Meters and Velocity Elements

1. Can a pitot tube flow meter measure gas flow in a large rectangular duct?
Yes. Averaging pitot tubes are made for rectangular ducts. You specify the duct width and height. The probe length matches the duct width or height.

2. What is the difference between a single point pitot tube and an averaging pitot tube?
A single point probe measures one point. An averaging probe has multiple holes along its length. The averaging probe gives one differential pressure that represents the average velocity across the duct.

3. What accuracy can I expect on a large duct with an averaging pitot tube?
With 10D upstream and 5D downstream straight run, expect ±1% to ±2% of flow rate. With poor installation, accuracy may drop to ±3% or worse.

4. Can pitot tube velocity elements handle dirty or dusty flue gas?
Yes, but dust can plug the sensing holes. Use a purging system or select a probe with larger holes. For heavy dust, we offer a purge port connection with compressed air or nitrogen.

5. What information does Silver Instruments need for a pitot tube flow meter quote?
Send duct width, duct height or diameter, gas composition, temperature, static pressure, flow range, and available straight run. A simple sketch helps us check probe length and mounting.

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