Abstract: Aiming at the problem that the current magnetic float level gauge in the cryogenic liquid environment application of the level gauge has condensed ice in some parts, which reduces the measurement accuracy of the instrument and affects its service life, according to the principle of vacuum insulation, the level gauge is Improve the structure of the measuring cylinder and the upper and lower balance pipes to add a vacuum insulation layer to prevent the appearance of condensed ice, ensure the normal use and display of the magnetic float level gauge in a deep cold environment, and improve the performance of the instrument. At present, cryogenic liquids There are many ways to measure liquid level, each with its own advantages and disadvantages. Among them, due to the reliable sealing of the magnetic float level gauge and the clear observation, it has been applied in some markets. But under the working conditions required by cryogenic liquids, these instruments need to be further improved.
1 Structural characteristics of ordinary magnetic float level gauge
1.1 working principle and purpose
The magnetic float color level gauge is a measuring instrument that uses a magnetic float as the sensing element, and reflects the measured liquid level or interface through the coupling of the magnetic float and the magnetic body in the display color bar. The magnetic float level gauge and the container to be measured form a communicating device to ensure that the liquid level between the container to be measured and the measuring tube is equal. When the float in the measuring tube of the level gauge changes with the measured liquid level, the magnetic body in the float interacts with the magnetic body in the color code displayed on the display bar to turn it over. Red (or green) means there is liquid, and white ( (Or red) means no liquid, in order to achieve the purpose of accurately displaying the liquid level. The on-site display magnetic float level gauge has the advantages of intuitive and eye-catching display, no power supply, convenient and reliable installation, small maintenance, and low maintenance cost. It is an upgraded product of glass tube and glass plate level gauges. It can be widely used in petroleum, Observation and monitoring of the liquid level of tanks, tanks, tanks and other containers in chemical, power station, pharmaceutical, metallurgical, shipbuilding, water/sewage treatment and other industries. According to the actual needs of the project, in conjunction with remote transmission facilities, the magnetic float level gauge can not only display the liquid level on-site, but also realize remote observation in the main control room through a computer or digital display instrument. According to the needs of industrial process control, alarm or control the liquid level.
It is a schematic diagram of the structure of an ordinary magnetic float level gauge. A measuring cylinder connected with the liquid container, a magnetic float placed in the measuring cylinder, and a liquid level indicator composed of multiple flaps or columns fixed outside the measuring cylinder. Among them, the flap or column is equipped with magnetic material, the magnetic float floats up and down with the level of the liquid in the measuring cylinder, and the flap or column at the same height as the magnetic float is turned over due to the action of the magnetic material on the two elements. . There are two visually distinguishable colors on the flap or column. You can see the liquid level in the container by looking at the color dividing line of the flap or column in the liquid level indicator. Valves are installed at the upper and lower ends of the measuring cylinder in the vertical direction. The upper valve is used to balance the air pressure, and the lower valve is used to release residual liquid.
1.3 Problems in using the magnetic float level gauge in a cryogenic liquid environment When this level gauge is used for some special purposes, the above structure will have problems. For example, when compressed and liquefied gas or other cryogenic liquids are stored in the container, the temperature of the liquid in the container is very low, resulting in the temperature of the upper and lower balance connecting pipes connecting the measuring cylinder of the level gauge and the cryogenic pressure vessel. It’s low. In this case, the cryogenic liquid in the measuring cylinder will condense the outside air, making it easy to condense a large amount of ice on the outside of the measuring cylinder. What is more serious is that the display on the level gauge will also freeze. The flip of the display is broken, making it unable to display its proper color code normally, and losing the proper function of the level gauge.
2 Improved the structure of the magnetic float level gauge
2.1 The principle of vacuum insulation
According to the three ways of temperature propagation: conduction, convection, and radiation. Based on the Dewar principle, using a vacuum layer to isolate between two environments with unequal temperatures will block the above-mentioned temperature propagation, so that their respective temperatures will not affect the temperature of the other environment. Both conduction and convection require heat transfer media. Since there is no medium in the vacuum (it is impossible to have an absolute vacuum in engineering), it can prevent conduction and convection, so it can be kept warm or cold (but there is still radiation heat transfer, such as the inner wall of a vacuum thermos bottle is coated with mercury to prevent radiation heat transfer). Based on this physical concept, technicians improved the structure of the level gauge.
2.2 Improved CSY double-layer cold insulation magnetic float level gauge  On the basis of the original level gauge structure, a vacuum insulation layer is added outside the measuring cylinder. Since the liquid in the measuring cylinder is the same as the liquid in the container, the vacuum insulation layer outside the measuring cylinder blocks the temperature influence of the cryogenic liquid in the measuring cylinder and the outside air. Practice has proved that this improvement does have a good temperature insulation effect. However, with this structure in the actual operation of the device, a large amount of ice will condense around the balance connecting pipe. Through analysis, since the measuring cylinder and the container are connected by two upper and lower balance pipes, when the level gauge can indicate normally, the lower balance pipe is always full of cryogenic liquid. Because the vacuum insulation layer added outside the measuring cylinder cannot block the direct contact between the balance tube and the outside world, a large amount of ice will still appear around the balance tube. Based on the above situation, the structure is improved again, that is, a vacuum insulation layer is also added outside the balance connecting pipe. The specific implementation plan is shown in the figure
2. In the original liquid level gauge tube structure, an outer tube is added outside the measuring cylinder, and an outer tube is made outside the balance tube. The space between the outer tube 1 and the outer tube 6 and the measuring cylinder 2 and the balance tube 7 is evacuated into a vacuum as the vacuum insulation layer 8. The upper drain valve is used to discharge residual gas and balance the air pressure, and the temperature is relatively high, so the handle of the valve used is shorter and smaller in size. A metal pipe is used to directly connect the measuring cylinder and the upper drain valve. Here, the measuring cylinder and the outside can conduct heat to the outside through the pipe and valve. However, due to the small contact area and the pipe passing through the middle vacuum insulation layer, low temperature transmission It is not obvious, and there will be no condensation and icing at the upper drain valve; the lower drain valve is used to discharge residual liquid, and the temperature of the liquid in contact is lower, so the handle of the valve is longer and larger. The flange 3 is used to connect the lower drain valve 4 and the measuring cylinder. Since the low-temperature liquid contact area between the flange and the measuring cylinder is relatively large, the heat conduction will be a lot, and the flange is connected to the metal cylinder, and the sealing reliability of the connection cannot be guaranteed. Long-term use of liquid may leak from the flange connection, so a T-shaped PTFE gasket 5 is installed at the flange connection and the bottom of the measuring cylinder to avoid the low temperature liquid and the flange. Direct contact, and prevent the seepage of liquid.
3 Use effect Figure 3 and Figure 4 show the use effect of the incomplete and comprehensive vacuum cold insulation level gauge. It can be seen from Figure 3 and Figure 4 that the level gauge with no vacuum insulation layer in the balance connecting pipe has condensed ice. After adding, all the condensed ice cubes disappeared.
4 Actual usage The ethylene plant of Shanghai SECCO Petrochemical Co., Ltd. (hereinafter referred to as SECCO) has about 30 low-temperature magnetic float level gauges, with a low design temperature of -171 ℃. Originally used a floating color standard level gauge with a common structure. During the commissioning process, it was found that although there is a cold insulation layer at the balance connecting pipe and measuring cylinder of the level gauge, the external display, magnetic color standard plate, and ruler are still With thick ice, it is impossible to identify the true position of the float in the measuring cylinder.
Beginning in February 2006, SECCO has gradually replaced the ordinary magnetic float level gauge with a vacuum double-layer cold insulation CSY magnetic float level gauge. There is no sign of icing in the viewed part, and no cold insulation is required, and the actual level indication is accurate. , Clear, responsive, long service life, no maintenance.
In the reconstruction and expansion project of SECCO’s ethylene plant in May 2009, 18 new low-temperature CSY magnetic float level gauges were added, which are in good actual use.
5 Conclusion The use of vacuum insulation layer to keep the low temperature liquid level gauge measuring cylinder and the upper and lower balance pipes insulates, which can effectively eliminate the icing phenomenon of the liquid level gauge. There are many methods for cryogenic liquid level measurement, but in actual applications, the liquid level measurement method or combination should be reasonably selected according to the medium, working pressure and temperature in the container, combined with the actual situation, in order to achieve both safe and economical purposes.
Post time: 26-11-21