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Thread: fluid mechanics in liquid nitrogen vessel

  1. #1 fluid mechanics in liquid nitrogen vessel 
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    I am attempting to use differential pressure to measure the level within a small cryogenic container which automatically fills and maintains it's own preset pressure. There is an inlet for liquid nitrogen to enter the vessel, as well as a vent gas exit orifice. When the system is static, the measurement works well, even though the differential measurement is quite small. I am only dealing with approximately 10 inches of LN2. The problem is that when the system vents, or fills, the differential readings become unreliable. Since my automation system is dependent upon this measurement system, I have a situation which can result in an overfill condition. I have tried pressure snubbers in the differential sensor location, with no significant improvement. I believe that the velocity of the fluid moving through either path is creating a small siphon pressure at the sensor which, due to the fact that the total pressure being compared is so slight, is causing major fluctuation in my level measurement. Could someone please offer advice on dynamic systems and differential pressure. I fully understand that in this case, diff. pressure is not the ideal method of level measurement, but I am not in a position to totally rework the system at this time. I need a "patch" method for temporary fix. Any suggestions would be appreciated.

    Thank you


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  3. #2  
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    Could you more fully describe your level indicating system? Is it a sight glass or is there an electronic d/p cell type of arrangement? How is the level standpipe configured? Where is the vent in relation to the standpipe?I don't have experience with liquid nitogen but could probably help you reason it out.


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    I'm guessing you have a sensing tube connecting the vapor space to the low pressure side of the DP cell, and another sensor tube connecting the bottom of the vessel to the high pressure side, so the differential measures the liquid level.

    As liquid flows past the open end of the lower sensing tube it causes eddying that might be producing the fluctuations in your readings. It might even be boiling locally at a low pressure spot.

    You might consider installing a stiff diaphragm seal between the liquid N2 and the DP cell. You will need to fill the tubing between the diaphragm seal and the cell with a fluid that stays in the gas phase at liquid nitrogen temperatures. Helium?
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    The upper part of the system could be a problem too. If the vent is located near the upper tap of the sensing line it could create a local low pressure area that would mess up your d/p reading.
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  6. #5 Thanks for the advice. 
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    I will say that my belief has been that there is a slight "siphon" effect at the point of the pressure tap, which is causing errors to the sensor reading. To clarify slightly, the diff. pressure is being measured with an electronic sensor. The low pressure side is connected to a small capillary tube in communication with the vapor space of the container, and the high pressure side is connected through a similar tube to the bottom of the container. Everything works very smoothly when it is allowed to vent to atmosphere and control it's own level. However, when I create a pressurized container with a controlled release of excess pressure, sometimes the reading falls to a point lower than the level actually is, which can result in the inlet valve staying open longer than necessary and actually overfilling the vessel. Any suggestion on preventing the area of low pressure, "siphon", or "eddy" situation from affecting my system? Thank you again for all of your help.
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    When you throttle down the vent valve you increase the pressure on top ot the container. This should be communicated to both the high and low side of the dp cell, which would compensate the level reading. If it is not communicatd fast enough to the low side it will appear as an increased level. Perhaps the upper capillary line is too small or restricted. Perhaps you could increase the size of that tubing or throttle the vent valve much more slowly and carefully to give the system time to adjust.

    Another thing I am wondering is if you have liquid or gas in the lower sensing line and dp cell. If it's not insulated, it could have gas in it, then when you pressurize the container that condenses, which could cause some sort of transient. Maybe try putting more insulation on the connection to the dp cell.
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  8. #7  
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    when I create a pressurized container with a controlled release of excess pressure
    Do you have both inflow and outflow at this stage? Is the inlet opening opposite the pressure tap? If so it's possible you have a jet impinging on your upper pressure tap. This would add the stagnation pressure of the jet onto the static pressure in the vapor space.

    You could overcome this with a deflection baffle inside the vessel.
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