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『How to Calculate Flow Rate in Venturi Meter: Apply Bernoulli’s equation using the measured differential pressure, throat area, and discharge coefficient.』Related information(v cone flow meter|rotary gas meter|gallon meter|gear meter|metal tube rotameter|glass tube rotameter|nutating disk meter|transit time ultrasonic flow meter|wedge flow meter|pd meters|hydraulic flow meter|ultrasonic water flow meter|water flow meter|air flow meter|magnetic water flow meter|oxygen flow meter|gas flow meter|argon flow meter|air velocity meter|gasoline flow meter|fuel flow meter|ultrasonic water meter|digital water meter|nitrogen flow meter|solid flow meter|Oxygen tank flow meter|digital water flow meter|diesel fuel flow meter|steam flow meter|02 flow meter|compressed air flow meter|oil flow meter|liquid flow meter)
How to Calculate Flow Rate in Venturi Meter: Apply Bernoulli Equation Using Measured Differential Pressure, Throat Area, and Discharge Coefficient
Quick Answer: Flow through a venturi meter is not measured directly. You measure the differential pressure between the pipe inlet and the venturi throat, then calculate flow using the Bernoulli equation, throat area, fluid density, and a discharge coefficient. For a DN50 line with a 25 mm throat, water at 20 °C, a differential pressure of 20 kPa, and a discharge coefficient of 0.98, the flow rate is roughly 11.3 m3/h.
Venturi Meter Flow Formula
First confirm where the pressure taps are located. The differential pressure is measured between the upstream pressure tap and the throat tap. The working formula for liquid flow is:
Q = C A2 sqrt ( 2 Delta P / rho / ( 1 - beta^4 ) )
Here Q is volumetric flow rate in m3/s. C is the discharge coefficient. A2 is the throat area in m2. Delta P is differential pressure in Pa. Rho is fluid density in kg/m3. Beta equals the throat diameter divided by the pipe diameter, so beta = d/D.
Do not use the upstream pipe area in this formula. The throat area controls the velocity change. Most engineers skip this part when they convert a DP signal to flow in a spreadsheet.
Step by Step Calculation for a Venturi Meter
Step 1. Measure the pipe internal diameter D and the throat diameter d. For a DN50 venturi tube, d is often 25 mm.
Step 2. Convert all dimensions to metres. A 25 mm throat is 0.025 m.
Step 3. Calculate beta and the throat area. Beta = d / D. A2 = pi d^2 / 4. If you are not sure about the actual internal diameter, use the certified dimensions from the meter datasheet.
Step 4. Convert the measured differential pressure to Pa. A 20 kPa reading is 20000 Pa.
Step 5. Apply the formula. For water at 20 °C, rho is about 998 kg/m3. For a DN50 line with D = 0.05 m, d = 0.025 m, beta = 0.5, A2 = 0.0004909 m2, and C = 0.98, you get 0.003147 m3/s. Multiply by 3600 to get 11.33 m3/h.
Discharge Coefficient and Installation Effects
The discharge coefficient C is not a fixed number for every venturi tube. A machined venturi tube can have a C value from 0.95 to 0.99 depending on beta ratio and Reynolds number. Real installation effects matter. The meter needs enough straight pipe upstream and downstream. Flow conditioners, elbows, valves, and pumps change the velocity profile and reduce accuracy.
Use the calibrated C value from the meter certificate. If you use a generic 0.98 for every meter, the flow error can be 1 percent to 3 percent at high flow. For gas and steam, add the expansion factor Y. Because density changes through the throat, Y is below 1. The gas formula is Q = C A2 Y sqrt ( 2 Delta P / rho / ( 1 - beta^4 ) ).
Real Field Example from Southeast Asia
Last year a customer in Vietnam asked us about a DN200 cooling water line. He used a venturi tube with a 100 mm throat and a DP transmitter ranged 0 to 30 kPa. Water temperature was 25 °C. We calculated the flow at 30 kPa because the PLC only received the raw 4-20 mA signal. For water at 998 kg/m3, the result was about 222 m3/h.
Here is the thing. Differential pressure flow is nonlinear. At 25 percent of differential pressure, the actual fl
Pairing a Venturi Meter with a Differential Pressure Transmitter
At the transmitter side, a 4-20 mA HART differential pressure transmitter is the standard choice. Silver Automation Instruments supplies DP transmitters for venturi flow measurement in water, steam, gas, and chemical lines. You can order a DP transmitter with square root output or send linear DP to a PLC and calculate the square root there. For gas and steam, pressure and temperature compensation is required for mass flow measurement in kg/h or t/h.
Common pressure ranges are 0 to 10 kPa, 0 to 30 kPa, 0 to 100 kPa, and 0 to 500 kPa. Pipe sizes can run from DN15 up to DN600. The fluid can be water, diesel, air, natural gas, steam, or chemicals. In hazardous areas, use an Ex d or Ex ia DP transmitter rated for ATEX Zone 1. We have seen many customer sites where long impulse lines are not purged or sloped. Long impulse lines that are not purged or sloped create offset errors and unstable readings.
Common Mistakes in Venturi Flow Rate Calculation
One common mistake is using the inlet pipe area instead of the throat area. Another issue is forgetting to convert kPa to Pa or using m3/h inside the formula. Some engineers also treat differential pressure as linear. Flow is proportional to the square root of differential pressure. A DP of 25 percent means 50 percent flow.
Most engineers skip the discharge coefficient and installation checks. That can cause errors above 2 percent even with a good transmitter. If the fluid is heavy oil or has viscosity above 200 cP, a venturi meter is often not the right choice. An oval gear flow meter may work better.
How to Get a Quote for a Venturi Flow Metering Kit
Send us your pipe size DN, operating pressure in bar, operating temperature in °C, flow range in m3/h or kg/h, fluid name, and required output signal. We will select a venturi tube or DP transmitter and return a quotation. You can reach Silver Automation Instruments at Tel +86-25-68650347, WhatsApp +86-25-52155837, or WeChat +86 15365082610. You can also check the product pages on flow-meter.com.au for differential pressure flow meters and pressure transmitters.
FAQ: Venturi Meter Flow Rate Calculation
What is the main venturi meter flow formula? Use Q = C A2 sqrt ( 2 Delta P / rho / ( 1 - beta^4 ) ) for liquids. Add the gas expansion factor Y for gas and steam.
Why is the discharge coefficient needed? Real flow is lower than ideal Bernoulli flow because of friction and velocity profile effects. The discharge coefficient corrects the calculated flow to match calibration data.
How do I convert differential pressure to flow in m3/h? Calculate Q in m3/s first, then multiply by 3600. Never enter m3/h directly into the formula because the units will not balance.
Can a venturi meter measure gas flow? Yes. Use the expansion factor Y and upstream or downstream density. For mass flow in kg/h, add pressure and temperature compensation.
What information does Silver Automation Instruments need for a venturi flow meter quote? Pipe size DN, fluid, flow range, operating pressure, operating temperature, and signal type are enough for the first quotation.