Abstract : The principle of measuring the oily medium in water by radio frequency admittance and differential pressure level transmitters is used to monitor whether there is a large amount of oily medium in the cooling water tank, so as to judge whether the heat exchange tube in the styrene cooler has leakage or not, according to water and benzene. The dielectric constant and density of ethylene solve this problem.
1. Introduction In the design of styrene cooling system, the process professional requires monitoring whether there is a large amount of oily medium styrene in the cooling water tank to determine whether the heat exchange tube in the styrene cooler has leaked. Since styrene is insoluble in water, it cannot be judged by using a water analyzer to measure chemical changes in water, so a special method needs to be considered. By comparing the physical properties of water and styrene, it was found that the dielectric constant of water is 8O, which is much larger than the dielectric constant of styrene 2.45; the density of water in the standard state is 1000kg/, and the density of styrene is 909kg/. The problem can be solved by starting from the dielectric constant and density of water and styrene.
2. Measurement principle The application of capacitance principle to measure the material level is a mature technology. Capacitance level measurement method is to directly change the level change into the capacitance change of the detection element, and then convert the capacitance into a unified electrical signal for transmission, processing, and finally display the level. The capacitance detection element works according to the principle of a cylindrical capacitor. Since the measured medium water is a conductive liquid, the structure is shown in FIG. 1 . The electrode is covered with an insulator (such as polytetrafluoroethylene) as an intermediate medium, and the liquid together with the metal outer cylinder serves as an external electrode.
The capacitance C of the detection element can be expressed by equation (1):
Where ε1 - permittivity
l - the length of the electrode immersed in the conductive liquid;
R, r—insulation cover outer radius and inner electrode outer radius.
Since ε1 is a constant, C is proportional to l. The value of l can be known from the size of C.
The use of the principle of static pressure to measure the material level is to measure the liquid level through the static pressure of the liquid column. It is a widely used method. The principle is shown in Figure 2.
According to the principle of fluid mechanics, the pressure difference between A and B points: â–³p=PB-PA=HÏg
In the formula
PA—static pressure (phase pressure) at point A in a closed container;
PB - static pressure at point B;
H - liquid column height;
Ï - the density of the liquid;
g - Gravity acceleration at the location of the closed container.
If Fig. 2 is an open container, PA is atmospheric pressure, so formula (2) can be changed as: P=PB-PA=H?g
The P-B gauge pressure in the formula.
In the measurement, if Ïg is a constant, the pressure difference Δp between the two points A and B in the closed container is proportional to the height H of the liquid level, while in the open container, the pressure P at the point B is proportional to H. In other words, if Δp or P is measured, the height of the liquid in the closed container or the open container can be known.
3, the specific application of radio frequency admittance level transmitter is the application of capacitance principle to measure the level of the instrument, and the differential pressure level transmitter is to measure the level using the principle of static pressure.
Now to discuss whether the tank water contains styrene. Install the radio frequency admittance level transmitter and the differential pressure level transmitter on the same tank to make the lower limit of the measurement range consistent. If the tank water is not mixed with other media, the two transmitters will have the same liquid level measurement result within the error range of the transmitter. If there is styrene in the tank water, let H be the height of the water column in the tank.
It is the height of the styrene liquid column in the tank, ε1 is the dielectric constant of water, ε2 is the dielectric constant of styrene, Ï1 is the density of water, and Ï2 is the density of styrene. According to formula (1)
The capacitance of the measuring element of the RF admittance level transmitter
Look at the differential pressure level transmitter. The water tank is an open container. According to the formula (3), the transmitter can measure the pressure P2=H1Ï1g+H2Ï2g. The preset value of the differential pressure level transmitter is the density of water, so the transmitter measures the height of the liquid in the tank H2 = P2 / Ï1g = (H1Ï1 + H2Ï2) / Ïl = H1 + (Ï2 / Ï1) H2. Because the values ​​of Ï1 and Ï2 are not much different, the value of the liquid height H2 in the tank measured by the differential pressure level transmitter and the actual height H1+H2 of the liquid in the tank are also not much different. It can be seen that, in the case of styrene mixed in the tank, the differential pressure liquid level transmitter measured the height of the liquid in the tank H measured 2 is actually greater than the radio frequency admittance level transmitter measured tank liquid height H And, with the increase of the height H of the styrene liquid column in the tank, the difference between the two transmitters' measured values ​​becomes larger and larger.
In summary, the styrene cooling system works normally. When there is no leakage in the heat exchange tube in the cooler, only water is in the cooling water tank. The measured values ​​of the RF admittance level transmitter and the differential pressure level transmitter are basically the same. Consistent; When the two transmitters have measured deviations and are getting larger and larger, it can be judged that there is a leak in the heat exchanger tube inside the cooler.
4. Conclusion In the same chemical plant or storage tank, selecting two or three transmitters to measure the same parameter is a commonly used practice in petrochemical design. This design is also based on common practice, and the transmitter used is also a common instrument. The price is not high. However, taking into account the error factors between two sets of transmitters using different measuring principles, it needs to accumulate a certain time, the two transmitters measured values ​​only appear obvious deviations, it can not be used for important equipment or emergency stop system. In this design, the radio frequency admittance level transmitter needs to use a probe that measures the conductive medium such as water, and it is not suitable to use a probe that measures non-conductive medium.
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