The double-flange level gauge measures the liquid level; it shows a reading even when the tank is empty
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Help: I have a double-flange transmitter for measuring liquid level. The distance between the upper and lower flanges is 6.6 m. There is no measuring medium inside the tank, but there is nitrogen at a pressure of about 6 MPa. A zero-point adjustment is carried out before use; the procedure involves closing the shut-off valves on both sides and opening the flush valve to connect it to the atmosphere. A handheld communicator is used to perform the one-click adjustment, and after opening the shut-off valves, a pressure reading of 5 kPa is observed on the double-flange transmitter. Even with the valve closed, the reading of the double-flange transmitter remains 0. This indicates that there is no fault with the transmitter; this phenomenon occurs with both flange transmitters installed on the same device. Question: Under normal conditions, the operating pressure inside the tank is 6 MPa. For positive and negative pressure measurements to be accurate, the gauge should show 0; why is there a pressure of 5 kPa? Could nitrogen itself have gravity? Is the pressure of the gas itself not taken into account?
The accuracy of your table is already quite good; I calculated that under those conditions, the pressure drop caused by the weight of nitrogen itself is approximately 4.85 Kpa. Hope this can help you
Thank you very much. Could you please explain the method of calculation?
The high static pressure test on empty tanks does indeed affect level measurement, but I can’t match your data. Flange spacing of 6.6 meters, gravity acceleration of 9.8 – this results in a differential pressure of around 5 kPa. Is it low-temperature nitrogen?
To match your data, the following conditions must be met: nitrogen at a pressure of 6 MPa and a temperature of 0 degrees; under these conditions, the density of nitrogen is 76 kg/m3. Only then can it roughly match your 5kPa data. 76x9.8x6.6=4915pa. Is the nitrogen temperature at the site 0 degrees?
Since it’s an empty tank, why cut out a valve for zeroing? Can’t it be zeroed directly at a pressure of 6 MPa? The nitrogen pressure is 6 MPa, and the pressure under operation is roughly at this level as well. It is best to calibrate the instruments under actual operating conditions; therefore, there is no need to disconnect them.
We also just discovered this issue not long ago. Level measurement in a closed tank under high static pressure: the high static pressure at the upper layer can affect level measurement. Therefore, when performing range transfer, it is necessary to wait until the hydrostatic pressure stabilizes before carrying out the transfer. Additionally, the gas layer above must be taken into account; this upper gas layer is regarded as the medium at the interface level, and interface-level measurement methods are used to conduct the transfer calculations.