In industrial production sites, it is necessary to measure the liquid level in a large number of containers, which requires the use of various liquid level measuring instruments. The accuracy of the liquid level data provided by the level gauge will have a direct impact on production. Incorrect measurement results may even damage equipment and pose a hidden danger to personal safety. Therefore, liquid level detection, as a measurement method for material storage in containers, provides important basis for the normal operation of the production process. A good liquid level measurement scheme should be accurate, easy to install, simple to maintain, and cost-effective. This article introduces the application and common troubleshooting of differential pressure level gauges (transmitters) in closed containers.

Overview of Differential Pressure Liquid Level Gauge
1.1 Measurement principle of differential pressure level gauge
For a closed container, assuming the bottom pressure is P, the pressure on the liquid surface is P1, and the liquid level height is H, then:
P=P1+pgH
In the formula: p is the density of the medium, and g is the acceleration due to gravity.
From the above equation, it can be concluded that:
△ P=P-P1=pgH
The density of the measured medium is usually known, and the pressure difference △ P is directly proportional to the liquid level height H. By measuring the pressure difference, the height of the measured liquid level can be determined. When the tested container is open, the gas phase pressure is atmospheric pressure. The negative pressure chamber of the differential pressure gauge can be vented to the atmosphere, and a pressure gauge can also be used to measure the liquid level; If the container is sealed, the negative pressure chamber of the differential pressure gauge needs to be connected to the gas phase of the container.
1.2 Characteristics of Differential Pressure Liquid Level Gauge
① The detection element takes up almost no space in the container, only one or two holes need to be opened on the container wall; ② The detection element only has one or two pressure pipes, with a simple structure, easy installation, convenient operation and maintenance, and reliable operation; ③ The use of a flange type differential pressure transmitter can solve the problem of liquid level measurement in media with high viscosity, easy solidification, easy crystallization, corrosiveness, and suspended solids; ④ Differential pressure level gauges have strong universality and can be used to measure liquid levels, as well as parameters such as pressure and flow rate.
1.3 Precautions for measuring liquid level using differential pressure level gauge
When encountering a measured medium containing impurities, crystals, aggregates, or prone to self aggregation, using a regular differential pressure transmitter may cause blockages in the connecting pipeline. In this case, a flange type differential pressure transmitter is required; When an isolation tank is installed between the differential pressure transmitter and the container, zero point migration is required.
Introduction to Steam Jet Pump 2
2.1 Working principle of steam jet pump. The principle of a steam jet pump is as follows: the steam jet pump works at a certain pressure, and the steam passes through a Laval nozzle, where it is reduced in pressure and increased in speed (the potential energy of the steam is converted into kinetic energy) and sprayed into the mixing chamber at supersonic speed. It is mixed with the pumped medium for energy exchange, and the mixed gas enters the diffuser for deceleration and pressure increase (kinetic energy is converted into pressure energy). In order to reduce the pumping load of the subsequent pump, a condenser is configured, and heat exchange is carried out through convection of two media with a certain temperature difference to achieve the purpose of condensing high-temperature media.
2.2 Characteristics of Steam Jet Pump
① This pump has no mechanical moving parts and is not limited by friction, lubrication, vibration and other conditions, so it can be made into a pump with high pumping capacity. Reliable work and long service life. As long as the structural material of the pump is selected appropriately, it is extremely advantageous for eliminating gases with corrosive properties, gases containing mechanical impurities, and water vapor. ② Simple structure, light weight, and small footprint. ③ The working steam pressure is 4-9 × 105Pa, which is a common steam source in general chemical enterprises. Due to the above characteristics, steam jet pumps are widely used in various tanks of the spinning workshop bath station.

Problem encountered during measurement of differential pressure level gauge
3.1 Installation of differential pressure level gauge in closed containers. To measure the true liquid level, the pressure of the container should be subtracted from the measured pressure at the bottom of the container. To do this, open a pressure tap at the top of the container and connect it to the negative pressure side of the transmitter. In this way, the pressure in the container acts on both the positive and negative pressure sides of the transmitter, and the resulting differential pressure is proportional to the liquid level height. ① Dry pressure connection. If the gas on top of the liquid does not condense, the connecting pipe on the low-pressure side of the transmitter remains dry. This situation is called dry pressure connection. The method for determining the measurement range of the transmitter is the same as the method for measuring the liquid level of an open container. ② Wet conductive pressure connection. If the gas on top of the liquid condenses, the liquid will gradually accumulate in the pressure tube on the low-pressure side of the transmitter, causing measurement errors. To eliminate this error, a liquid tank is pre filled into the low-pressure side pressure conduit of the transmitter, which is called a wet pressure connection.
3.2 Common Fault Handling
3.2.1 Common troubleshooting of liquid level gauges during dry pressure connection. The mature barrel uses a dry pressure connection, opens the compressed air valve, and relies on the pressure of the compressed air to release the adhesive from the can. During measurement, a common phenomenon is that when the compressed air valve is opened, the liquid level suddenly rises and then immediately returns to normal. After analysis and on-site inspection, it was found that the cause of this malfunction is a leakage at a certain node in the pressure pipe. Usually, we use soap bubbles to quickly detect leaks when dispensing glue, and after tightening, it returns to normal. Processing is very convenient and effective. Another fault is that after the liquid level exceeds the range, the liquid level is measured again and the display is low. This is because after exceeding the range, adhesive enters the pressure tube, and cleaning the pressure tube can restore normal operation. We have summarized several situations where the liquid level of the matured barrel is not accurate, and corresponding solutions: ① When the liquid level in the container remains constant and the sealed container is pressurized, the liquid level rises. After stopping the pressurization, the liquid level returns to its original value, indicating that there is a leak in the negative phase pressure tube and the various connection points of the negative phase pressure tube need to be checked. ② When the liquid level rises by more than 35% when the sealed container is pressurized, it may be due to blockage of the negative phase pressure pipe, and the negative phase pressure pipe and collection tank need to be cleaned. ③ When the liquid level in the sealed container remains low under normal conditions, it indicates that there may be too much liquid accumulation in the negative phase pressure tube, and it is necessary to drain the negative phase pressure tube. ④ When the liquid level drops when the sealed container is pressurized, and rises again when the pressurization is stopped, it may be the conjunctiva at the positive phase pressure film, and the positive phase pressure film needs to be cleaned.
3.2.2 Malfunctions and improvement measures for liquid level measurement of closed containers with steam jet pumps. During the measurement, it was found that the liquid level measurement was accurate without adding steam, and the phenomenon of virtual high liquid level appeared as soon as the steam valve was opened. After analysis, the virtual high liquid level is due to the sudden influx of a large amount of steam into the sealed tank after the steam valve is opened. There are two pressures inside the sealed container, one is the liquid level pressure and the other is the internal air pressure. After the steam enters the tank, the internal air pressure suddenly rises, and the pressure on the positive pressure chamber of the liquid level transmitter increases accordingly. At this time, the pressure in the negative pressure chamber does not change, resulting in the phenomenon of virtual high liquid level. Previously, the desulfurization tank was not a closed tank, but had holes on the top of the tank, and the gas phase pressure was equivalent to atmospheric pressure, so there was no situation of virtual high liquid level. Recently, in order to prevent leakage and dripping, the opening on the top of the tank has been sealed, forming a sealed container.

There are two improvement plans for this issue; One is to use a dual flange level gauge for measurement; The second is to add a pressure tube on the negative pressure side for measurement. If we choose the * scheme, we need to prepare a double flange level gauge for direct installation. For the convenience of renovation and cost considerations, we have chosen the second scheme, which adds a pressure pipe on the negative pressure side for measurement.
Using the flange opening on the top of the tank to add a pressure pipe and a collection tank, but due to the tendency of steam to condense, the collection pipe needs to be cleaned frequently, which is very inconvenient for daily maintenance. In production, many containers are in a high temperature state, and the medium inside the container is often in a gas-liquid equilibrium state. At this time, due to the lower temperature at the upper end of the pressure pipe than the temperature inside the container, steam will enter the pressure pipe through small holes and condense into liquid. By utilizing this feature, it is possible to pre inject the medium to be contained in the future container into the negative phase pressure tube, and then perform negative transfer (zero adjustment) on the differential pressure level gauge. Therefore, during the renovation, place the collection tank at the top of the tank and open the top plug of the collection pipe to fill the negative pressure tube and collection pipe with buffer solution. After completing this series of preparations, reset the position of the liquid level gauge to zero. In this way, even if the steam condenses, it will flow back into the desulfurization tank, and the height of the buffer liquid in the collection pipe will not change. The pressure on the negative pressure side of the liquid level gauge will not change, so it will not affect the liquid level measurement.
After this series of modifications, the liquid level measurement of the sealed tank with steam jet pump is very accurate, and the original liquid level transmitter is still used without increasing the cost, which is of great significance for the liquid level measurement of such containers.