Emerson mass flow measurement density issue
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Dear experts, I’m using Emerson mass flow meters to measure the flow rate of anhydrous ethanol, with feed under negative pressure (around -0.08 MPa). The fluid flows from one main pipe to two branch pipes. One of the flow meters shows a density of 850 kg/m3, while the other indicates a density of 810 kg/m3; the normal density for anhydrous ethanol is 790 kg/m3. Is this considered normal? Some expert please help answer.
The density displayed by the mass flow meter is the measured value. The medium density changes with temperature, pressure, and composition. The key is to check whether the mass flow reading of the flow meter is reasonable. .
Emerson’s quality accuracy is 0.1% and its density is 0.5%; if the manufacturer selects the appropriate model based on the data you provide, such large errors generally do not occur. Also, was this instrument calibrated before installation, and was it found to be compliant? If qualified. Whether the pipes installed on-site meet the requirements. And other factors like that.
What is the difference between the temperature during measurement and the standard temperature? The density under vacuum also differs significantly from the standard density!
The density of the mass flow meter can be calibrated, and water can be used for this purpose. The ProLink software is very user-friendly; I have carried out calibrations using it
This post was last edited by qugd on 2022-5-16 09:11. The data clearly shows excessive deviations, and the reason for this is likely the significant disruption to the flow pattern of the medium caused by negative pressure feeding. The installation location of the flow meter was chosen without giving sufficient consideration to the effect of negative pressure on the flow behavior of the medium. With such high negative pressure, the medium may enter a two-phase flow state, which exceeds the range that the mass flow meter can handle; it is therefore necessary to increase the resistance to the flow of the medium in order to prevent phase separation. This situation has nothing to do with whether correction is performed or not. Constantly calibrating instruments is a sign of a lack of field experience; do you really think it’s that easy to calibrate an instrument on site? It involves switching to a bypass medium, introducing the calibration medium, then replacing it again – operating the device in this way is nothing but a formality. The boiling point of ethanol drops significantly under negative pressure; during transfer, phase separation may occur, and even continuous gas bubbles might form. Can such disturbances be resolved through calibration? It can only eliminate the case of two-phase flow, by appropriately increasing the static pressure in the pipeline or by selecting a different location for the flow meter.
Another possibility is that the water content in the medium is too high, meaning that the water content in anhydrous ethanol is already quite high.
It’s fine as long as it contains water; each batch needs to be tested. In the past two days, high temperatures caused large fluctuations in flow rates, and the flow meter displayed error code 102; the manufacturer attributed this to two-phase flow. These days, as the temperature has dropped, operation is normal with no alarms, but the density remains high. I wonder if problems will arise again when the temperature is higher in a while.
A mass flow meter measures mass flow rate, while density and temperature are auxiliary measurements. It is generally not used as a W-process quantity. The measurement result of the mass flow meter is independent of density. Density conversion is only required for flow rates that measure volume, such as orifice plates
It makes perfect sense – ethanol is inherently a volatile medium with a low boiling point, and under negative pressure, it tends to form a two-phase flow.