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Use Differential Pressure Transducer Measure Liquid Level

It is difficult for a differential pressure transducer to measure the level of a cryogenic liquid below -20 ℃. If we need to measure the liquid level of low temperature medium such as liquid ammonia and liquid nitrogen, we can use indirect measurement by gas isolation. As shown in the figure, the purpose for measuring the liquid level of the low temperature medium is achieved by measuring the gas pressure in the pressure guiding tube. Since the liquid in the pressure guiding tube on the high pressure side is at a normal temperature state above the boiling point, the liquid in the pressure guiding tube evaporates into a gas until a saturated gas is formed in the pressure guiding tube. When selecting a differential pressure transducer for measurement, the method uses the principle of liquid column static pressure. At this time, the pressures on the high and low pressure sides in the differential pressure transducer are as follows respectively:
PH=Hx × ρ + Px ; PL=Px
P=PH + PL=(Hx × ρ + Px ) - Px=Hx × ρ, its range: ∆P=(Hx × ρ + Px), then the correspondence between the differential pressure and the current is as follows.
Differential pressure sensor measuring liquid level
The differential pressure transducer should be installed at a position higher than the highest liquid level, and the high pressure side is connected with the liquid phase pressure guiding tube, so that the transducer can be in a working environment at normal temperature, and the low temperature liquid can be fully vaporized. In order to adjust the instrument zero position and replace the instrument, a balance valve should be installed between the high and low pressure sides of the differential pressure transducer. After the differential pressure transducer is installed, the zero position should be adjusted, then open the gas pressure pilot valve PL, and adjust the zero position again under the working pressure. Then open the liquid phase pilot valve PH, wait a moment to fully vaporize the cryogenic liquid, and then close the balancing valve to put the pressure transducer into operation. When checking or zeroing during use, first open the balancing valve to balance the pressure on the high and low pressure sides of the differential pressure transducer, then close the liquid phase pressure regulating valve to check or adjust the zero position. If the measured vaporized gas of the liquid is in danger of coming into contact with air, certain safety measures should be taken during maintenance.
  • How to measure the liquid level of cryogenic liquid when the differential pressure transducer installation position is limited
When using the differential pressure transducer to measure the liquid level of the low-boiling liquid, in the case of the normal temperature and the dry gas phase, the pressure guiding tube drawn from the container that one is a liquid phase tube and the other one is a gas phase tube. However, since the measured liquid with low boiling point will have a vaporization phenomenon in the pressure guiding tube, whose influence of vaporization will cause distortion of the measurement value. For this reason, there are certain requirements for the preparation of the pressure guiding tube, such as the installation of a gas phase container, and certain requirements to the installation position for the differential pressure transducer, which increases the workload and construction difficulty. Some sites cannot meet the installation requirements for the differential pressure transducer due to environmental constraints. How to deal with it?
Schematic diagram of differential pressure sensor installation in liquid level measurement
Now we introduce a simple installation method, which is using a tube to connect the air tube to the liquid tube as shown in figure. It can be seen from the figure that the gas generated by the measured liquid vaporization rises along the tube to the top of the container. The liquid flows down the tube. This tube can be selected as a Ф 14 × 2. For corrosive medium, select a stainless steel tube. This method is also feasible for liquid level measurement of low boiling liquids using the differential pressure method. The most important thing is that it does not have strict requirements on the installation position of the differential pressure sensor, which is convenient for on-site construction.

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