『High Temperature Air Flow Meter Systems: Sizing Pitot Tubes and Venturis for Exhaust Gas Stacks』Related information(flow rotameter|digital flow meter|balo meter|hydraulic flow meter|volumetric meter|positive displacement meter|pitot tube flow meter|displacement meter|cfm meter|variable area flow meter|inline flow meter|differential pressure flow meter|calibrated flow meter|velocity meter|paddle wheel flow meter|oval gear flow meter|anemometer hvac|multiphase flow meter|doppler flow meter|thorpe tube flowmeter|heat flow meter)
High Temperature Air Flow Meter Systems: Sizing Pitot Tubes and Venturis for Exhaust Gas Stacks
Quick Answer: For exhaust gas stacks above 250 degree C, use an averaging pitot tube or a Venturi flow meter paired with a DP transmitter that has a low span such as 0 to 250 Pa or 0 to 1 kPa. Size the primary element for a gas velocity between 5 and 30 m/s and check the straight run before mounting. Send your stack diameter, gas temperature, pressure, and flow range to Silver Instruments for a free sizing review.
Sizing an air flow meter for a hot exhaust stack is not the same as sizing a water line or a compressed air pipe. The gas is usually dirty, the temperature swings with load, and the static pressure is often close to atmospheric or slightly negative. In practice, most engineers skip this part and then find the DP transmitter range does not match the actual flow.
This article focuses on two proven primary elements for high temperature exhaust gas stacks: averaging pitot tubes and Venturi meters. Both work with a differential pressure transmitter and both survive continuous operation above 250 degree C when the right materials and mounting hardware are selected.
Why Exhaust Gas Stack Flow Measurement Is Different
Exhaust gas stacks routinely operate between 180 degree C and 600 degree C. The gas density is low at these temperatures. A gas at 20 degree C and 1.2 kg/m3 can drop to 0.45 kg/m3 at 500 degree C. Low density means low differential pressure for the same velocity. For example, a 20 m/s flow at 350 degree C with a gas density of 0.56 kg/m3 gives a theoretical DP of only 112 Pa. A standard DP transmitter with a 0 to 25 kPa span will not resolve this properly.
Dust, moisture, and corrosive compounds like SO2 or HCl often appear in stack gas. This rules out thermal mass flow meters in many heavy oil or biomass exhaust applications. Pitot tubes and Venturi meters tolerate particles better and can be purged with instrument air or nitrogen.
Key Sizing Inputs You Need Before Selecting a Pitot Tube or Venturi
Start with the actual stack inside diameter in mm or DN. Do not use the outside diameter. Then get the normal, minimum, and maximum gas flow in kg/h or Nm3/h. Add the operating temperature in degree C and the static pressure in bar or mbar. Finally, note the gas composition or moisture content because density and corrosion behavior depend on it.
Use these five values for the sizing calculation: stack inside diameter, flow range, gas density at operating conditions, gas viscosity, and allowed permanent pressure loss. For pitot tubes, permanent pressure loss is very low, often below 5 percent of the DP signal. For Venturi meters, pressure loss depends on the beta ratio and is usually 10 to 30 percent of the generated DP.
Sizing Averaging Pitot Tubes for Hot Flue Gas
An averaging pitot tube measures the velocity pressure across the stack cross section. Multiple sensing holes along the probe average the flow profile. This is better than a single point pitot because stack velocity profiles are rarely uniform after bends, dampers, or fans.
The basic sizing formula is DP = 0.5 times density times velocity squared. Multiply the result by the pitot coefficient, usually 0.82 to 0.86 for an averaging probe. Example: a stack at 320 degree C with a gas density of 0.58 kg/m3 and a normal velocity of 18 m/s gives a DP of about 94 Pa. Multiply by 0.84 and the transmitter sees about 79 Pa. So a DP transmitter with a span of 0 to 250 Pa or 0 to 500 Pa is a good match. Do not try to force a 0 to 10 kPa transmitter here; the turndown will be poor.
For stack diameters up to DN 1200, a single insertion probe often works. For larger stacks, use two or three probes connected in parallel to the same DP transmitter. Mount the probes at least 7.5 stack diameters downstream of any bend or damper. If the straight run is shorter, add a flow conditioner or accept a higher measurement uncertainty.
Sizing Venturi Flow Meters for Exhaust Gas Stacks
A Venturi meter accelerates the gas through a reduced throat and produces a higher differential pressure than a pitot tube. This is often the better choice when the gas is very dusty or when you need accuracy better than 1 percent of reading. The beta ratio, which is the throat diameter divided by the pipe diameter, is normally between 0.4 and 0.7 for stack gas service.
For the same example as above, a Venturi with a beta ratio of 0.6 produces a DP that is several times higher than a pitot tube, typically 0 to 5 kPa depending on flow and gas density. The permanent pressure loss is around 20 percent of the DP, which is acceptable in forced draft or induced draft systems where the fan has margin.
Venturi meters require a long straight run, usually 10 to 30 pipe diameters upstream depending on beta ratio and upstream disturbances. In a stack with limited straight run, a Venturi can still work but you should apply the ISO 5167 installation correction or use a calibrated flow conditioner. For high temperature stacks, specify a welded or flan
Pitot Tube vs Venturi for High Temperature Stacks
Pick an averaging pitot tube when the stack is larger than DN 300, the gas is relatively clean, and you need a low cost insertion meter without cutting the stack. Pick a Venturi meter when you need higher accuracy, the gas carries heavy dust, or the flow range is wide. Venturi meters cost more and require more straight run, but they handle abrasion and high velocity better than most insertion probes.
DP Transmitter and Temperature Compensation
Use a DP transmitter with a high overpressure rating and a low span capability, such as 0 to 250 Pa or 0 to 1 kPa. Silver Instruments supplies the SDP-3051 differential pressure transmitter with 4-20 mA HART for hot stack loops. Many stack gas applications have a static pressure around -5 mbar to +20 mbar, so the transmitter body must tolerate low absolute pressure and condensation. Mount the DP transmitter below the pressure taps so any condensate drains away. Use impulse lines with heat insulation or a heated transmitter enclosure if the gas is above 300 degree C.
For mass flow output, add a temperature sensor and a pressure sensor. A PT100 RTD with a thermowell rated for the stack temperature is a practical choice. The output goes to a flow computer or a PLC with the gas density compensation formula. Silver Instruments supplies the DP transmitter, the RTD, and the paperless recorder in one package for exhaust gas metering loops.
Materials and Mounting for High Temperature Exhaust Gas
For gas up to 400 degree C, 316L stainless steel probes and thermowells are common. For 400 to 600 degree C, use 310 stainless steel or Hastelloy C for acidic gas. Avoid plastic impulse line and standard cable glands near the stack. Use metal braided cable and ceramic insulated wiring for the transmitter and RTD.
Mount the flow sensor on a flanged nozzle or a weld-on compression fitting. The probe length must cover at least 70 percent of the stack inside diameter. If the stack is insulated, calculate the insertion length from the outer wall to the far inner wall, not from the insulation surface.
Application Notes From the Field
Last year a customer in Vietnam asked us for a flow meter on a biomass boiler exhaust stack at 280 degree C. The stack inside diameter was DN 700 and the gas velocity varied from 6 to 22 m/s. A single averaging pitot tube with a 0 to 250 Pa DP transmitter and a PT100 for temperature compensation solved the problem. The customer connected the 4-20 mA HART output to a paperless recorder and now tracks flue gas flow in Nm3/h for environmental reporting.
Another customer in Peru runs a cement plant with dusty kiln exhaust at 340 degree C. We supplied a Venturi flow meter with a beta ratio of 0.55 and a differential pressure range of 0 to 3 kPa. The body was carbon steel with a stainless steel throat and a ceramic coating in the inlet cone. The instrument air purge ports keep the pressure taps clean.
FAQ: High Temperature Air Flow Meter Sizing
Q1: What is the minimum straight run for a pitot tube in an exhaust stack?
A1: For an averaging pitot tube, use at least 7.5 pipe diameters upstream and 3 pipe diameters downstream after a bend or damper. If the straight run is shorter, use a flow conditioner or expect an additional uncertainty of 2 to 5 percent.
Q2: Can a Venturi meter handle flue gas above 500 degree C?
A2: Yes. Use 310 stainless steel or alloy 800 for the throat and inlet cone. Keep the DP transmitter remote with heat resistant impulse lines. With proper materials, Venturi meters work up to 800 degree C in some furnace exhaust systems.
Q3: Why is my pitot tube DP signal very low?
A3: Hot gas has low density. At 350 degree C a velocity of 15 m/s can produce a DP below 100 Pa. Check that the DP transmitter span is not too wide. A 0 to 250 Pa or 0 to 500 Pa transmitter with a square root output gives a usable signal.
Q4: Which flow meter is better for dirty exhaust gas, pitot tube or Venturi?
A4: A Venturi meter is better for heavy dust because the smooth throat and purge ports resist blockage. A pitot tube can work with continuous air purge and a well designed insertion point, but it needs more maintenance in very dirty gas.
Q5: How do you convert the DP signal to Nm3/h?
A5: You need the gas temperature and pressure. The flow computer calculates density at normal conditions and converts the measured volume flow to Nm3/h. Silver Instruments can supply a paperless recorder with the Nm3/h calculation preconfigured.
Get a Sizing Check From Silver Instruments
Send us your gas composition, stack inside diameter in DN or mm, operating temperature in degree C, static pressure in bar or mbar, and the flow range in kg/h or Nm3/h. Silver Instruments will check the pitot tube or Venturi sizing and recommend a DP transmitter with the correct span. Contact Silver Automation Instruments at Tel: +86-
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