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How Does a Gas Flow Meter Work on Saturated Vapor Lines
Quick Answer: A gas flow meter on a saturated vapor line must handle wet gas, density changes, and condensate slugs. The standard choice is a vortex flow meter with pressure and temperature compensation for saturated steam and wet gas. Thermal mass flow meters only work on dry gas lines. This article covers saturated vapor line principles that affect selection.
Why Saturated Vapor Lines Create Measurement Problems
Saturated vapor sits at the edge of condensation. A small pressure drop or heat loss across the pipe wall turns part of the vapor into liquid droplets. This two phase flow condition changes the density and the velocity profile inside the pipe. A gas flow meter calibrated for dry gas starts to read high or low when liquid droplets pass the sensor.
In practice, many steam lines in Southeast Asia run at 6 bar to 12 bar. At 6 bar saturated steam temperature is about 159 °C. At 12 bar it is about 191 °C. A steam line after a boiler often carries 2 percent to 5 percent moisture. We have seen this on customer sites many times. A flow meter that cannot handle that wetness will drift after startup.
Vortex Flow Meter Working Principle
A vortex flow meter has a bluff body inside the pipe. When vapor passes the bluff body, vortices form downstream. The shedding frequency is proportional to flow velocity. This gives a volumetric flow reading. A pressure transmitter and a PT100 RTD convert that volumetric signal to mass flow in kg/h.
Silver Instruments supplies vortex flow meters from DN15 to DN300. The meter body can be 316L stainless steel. Outputs include 4 to 20 mA HART, pulse, and RS485 Modbus. For a DN100 line carrying 8 bar saturated steam at 175 °C, the meter with pressure compensation gives a mass flow reading that matches boiler feedwater loss within 1 percent.
Wet steam is acceptable up to about 20 percent liquid fraction for most vortex designs. Above that, install a separator upstream. A palm oil mill in Indonesia had slugs on a DN150 steam line. The vortex meter survived, but the reading was noisy. After they added a separator and steam trap, the fluctuation dropped from plus or minus 12 percent to plus or minus 2 percent.
Thermal Mass Flow Meter Limits in Wet Gas
Thermal mass flow meters measure heat removal from a heated sensor to the moving gas. They are good for dry compressed air, biogas, and natural gas. Saturated vapor lines are a different case. Droplets hitting the sensor cause instant cooling spikes. Most engineers skip this part and learn the hard way.
A beverage plant in Vietnam asked us about a thermal mass meter for a compressed air line with water carryover. We told them to fix the condensate issue first. They installed a refrigerated dryer and a coalescing filter. After that, a thermal mass flow meter with 4 to 20 mA output worked fine. But before the dryer, a vortex meter was the safer choice.
Pressure and Temperature Compensation
Gas density depends on pressure and temperature. A vortex meter without compensation will only report actual cubic meters per hour or normal cubic meters per hour if you assume a fixed density. That assumption fails on a saturated vapor line because pressure changes throughout the day.
For saturated steam, pressure alone defines temperature. You can use a pressure transmitter and a steam table inside the flow computer. For gas mixtures or superheated steam, use both pressure and temperature. Silver Instruments can supply the vortex flow meter with an integrated pressure sensor and a PT100 input. The output can be 4 to 20 mA HART with a totalizer on the local display.
Installation Rules That Matter
A vortex flow meter needs a straight run. Standard practice is 15D upstream and 5D downstream. That means for a DN80 meter, you need 1200 mm of straight pipe upstream. Install the meter in a location where condensate cannot collect. For horizontal steam lines, avoid low points. A steam trap upstream helps remove liquid before the mete
We have seen a water treatment plant in Saudi Arabia with a DN50 vortex meter installed after an elbow with only 3D straight run. The reading was 7 percent high. The customer moved the meter to meet the 15D requirement. The error dropped below 1 percent. This is a common fix.
A sugar mill in Zambia had a DN100 steam line with frequent startup slugs. The flow meter was installed in a vertical run with upward flow. That kept condensate from pooling at the meter.
Selecting a Gas Flow Meter for Saturated Vapor Lines
For saturated vapor, wet gas, and saturated steam, start with a vortex flow meter. It handles moderate wetness and has no moving parts. For dry gas and low flow, a thermal mass flow meter is often better. For dry gas batching or high accuracy mass flow, a Coriolis mass flow meter works well, but the cost is higher and slugging is a risk.
A chemical plant in Thailand needed a meter on a DN80 line with 6 bar saturated vapor. They sent us the pressure in bar, temperature in °C, pipe size in DN, and flow range in kg/h. We selected a vortex flow meter with integrated pressure and temperature compensation. The meter has run for two years without calibration drift.
Common Mistakes We See on Customer Sites
One common mistake is using a dry gas calibration for a wet gas line. The meter will read high because liquid droplets increase the effective density. Another mistake is ignoring straight run. A third mistake is installing a thermal mass meter on a steam trace line. A small DN20 steam line can carry slugs during warm up. The thermal sensor fails quickly.
We saw a food factory in Argentina try a thermal mass insertion meter on a saturated vapor line. The sensor failed in three months. They replaced it with a DN40 vortex flow meter and a separator. The total cost was lower than the first repair job.
Recommended Silver Instruments Models
Silver Automation Instruments offers several gas flow meter options for saturated vapor and dry gas service. The vortex flow meter range covers DN15 to DN300 with 4 to 20 mA HART, pulse, and Modbus output. ATEX Zone 1 approval is available. A PT100 input and pressure transmitter input allow direct mass flow calculation. The thermal mass flow meter works well on dry gas from DN15 to DN200, with insertion and inline styles. The Coriolis mass flow meter covers DN3 to DN150 for dry gas mass flow and liquid batching. For liquid lines, we also supply electromagnetic flow meters for conductive liquids from 5 µS/cm, ultrasonic flow meters, oval gear flow meters for fuels and oils from 1 cP to 1000 cP, and pressure transmitters.
Need a specific quote for a saturated vapor line? Send us the pressure in bar, temperature in °C, pipe size in DN, and flow range in kg/h. Contact Silver Instruments at Tel: +86-25-68650347, WhatsApp: +86-25-52155837, WeChat: +86 15365082610.
FAQ: Gas Flow Meter on Saturated Vapor Lines
Question 1: Can a vortex flow meter measure saturated steam mass flow?
Answer: Yes. Pair the vortex flow meter with a pressure transmitter or a pressure and temperature input. The flow computer calculates density and reports mass flow in kg/h.
Question 2: What is the maximum wetness a vortex flow meter can handle?
Answer: Most industrial vortex flow meters handle up to 20 percent liquid fraction. Above that, install a separator or use a different technology.
Question 3: Can a thermal mass flow meter work on a saturated vapor line?
Answer: No. Thermal mass meters are for dry gas. Droplets cause spikes and sensor failure.
Question 4: What pipe sizes do you supply for gas flow meters?
Answer: We supply vortex meters from DN15 to DN300 and thermal mass meters from DN15 to DN200. Larger sizes are available on request.
Question 5: What information do you need for a quote?
Answer: Send the pressure in bar, temperature in °C, pipe size in DN, flow range in kg/h, and gas type. We will propose a model and price.
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