Why Water Contained Gas is Hard to Measure?

Gas flow measurement is often being asked by our customers, and we always provide perfect gas flow meter solution to different kinds of gas measurement.
We choose vortex flow meter, gas turbine flow meter, thermal mass flow meter, variable area flow meter, or coriolis mass flow meters for different gas measuring with different technical specifications.
No mater clean gas, dirty gas, or mix gas, we all have solutions, but wet gas is a tough job for us, let’s see a case to analysis why moist gas flow measurement is a problem.

Case:
One of the customers provide below applications and questions:
1) Gas flow meter for exhaust gas from diesel engine: 0- 1500m3/h, pipe size DN150. ; Medium temperature: 70- 80oC
2) Gas flow meter for exhaust gas from diesel engine: 0- 4000m3/h, pipe size DN250. ; Medium temperature: 70- 80oC
One important point, the gas is wet, contained a lot water, and we want to use gas turbine flow meter, vortex flow meter or thermal mass flow meter for the exhaust gas, can you give us some advice?

After see their request, I told them none of the mentioned flow meters can work, because it contained too much water!
Reasons as below:

Gas Turbine Flow meter
Catalog reference: http://www.silverinstruments.com/product/flow-measurement/gas-turbine-flow-meter.html
Gas turbine flow meters, if moist gas passes through the gas meter measuring tube, water will take away the grease on the rotor bearing, and the turbine rotor will damage fast without grease on the rotor bearing. You should know the rotor is a vital instrument part for the turbine flow meter, if rotor has problem, the gas flow meter will break down soon. We recommend you dry the gas before flow measurement.

Thermal Mass Flow meter
Catalog reference: http://www.silverinstruments.com/product/flow-measurement/srk-100-thermal-mass-flow-meter.html
Thermal mass flow meter also cannot handle wet exhaust gas flow measurement, because water specific heat capacity is relatively high, and if the water passes through the thermal mass flow probe, it will take away large amount heat on the mass flow meter probe.

We all know that thermal mass flow meter working on below principle:
“Thermal mass flow meters employ the thermal dispersion principle whereby the flow rate of heat absorbed by a gas pass in a pipe is directly proportional to its mass flow”.
From above description, we can see that heat has extremely close relationship with Thermal dispersion flow meter, and thermal mass flow meter should not measure water contained medium.
We often recommend that water content should be less than 20% of the total medium if you want to use thermal Gas meters. If exceed 20%, the accuracy cannot be ensured.
Anyway we recommend you remove the water in the exhaust gas, and then you can have lot choices for the gas flow measurement according to your limit budget.

Vortex Flow meter
Catalog reference: http://www.silverinstruments.com/product/flow-measurement/vortex-flow-meter.html
For Vortex Flow meters, the water not harms the vortex flow sensor, but the water can also pass through the probe of the vortex flow meter, the vortex flow sensor can detect water and also gas which pass the probe, then the measuring result is both the gas also the water flow, we can see that the measuring result is not accurate at all. Also your flow range is small; vortex flow meters may be not sensitive to your provided small flow.

More information can be found here (Silver Automation Instruments).