In the petrochemical industry tank farm, there are many types of storage tanks, including: spherical tanks, internal floating roof tanks and fixed roof tanks. The capacity of the storage tank varies, and the storage medium includes crude oil and residual oil with large viscosity and poor fluidity, liquefied olefin with small dielectric constant, and corrosive acidic substances. For the liquid level measurement of these storage tanks, flange type liquid level gauge, radar liquid level gauge, servo liquid level gauge and optical guide liquid level gauge are commonly used. The author introduces the principle, characteristics and applicability of various types of liquid level gauges in different storage tanks and different media in the petrochemical tank farm, so as to obtain more accurate measurement results and ensure the normal and safe production of the device.
1, Flange type liquid level gauge
The single-flange liquid level gauge is based on the principle that the pressure generated by the liquid level is proportional to the height of the liquid level, and the height of the liquid level is reflected by the pressure H, that is, H=p/pg, where p is the density of the medium and p is the pressure generated by the liquid. It can be seen that the height of the liquid level is related to the density of the medium, but the density often changes with the temperature, so that the liquid level also changes.
For pressurized tanks, single-flange level gauges are no longer suitable. The pressure at the bottom of the tank is the sum of the pressure in the tank and the pressure generated by the liquid, and the single-flange liquid level gauge cannot distinguish whether the liquid level changes or the liquid level height changes caused by the pressure changes in the tank. In this case, a double-flanged liquid level gauge can be used to set a pressure port at the position of the gas phase space at the top of the tank, and the pressure difference between the upper and lower pressure ports is used to measure the liquid level. Single and double flanged level gauges are much cheaper than radar level gauges, servo level gauges and photoconductive level gauges, and are recommended for storage tanks with low level measurement requirements.
In addition, different models of single and double flanged level gauges are suitable for viscous, easy crystallization, strong corrosion or highly toxic media. For 2000m3 residuum atmospheric pressure storage tank of Jinzhou Kaiyuan Petrochemical Co., LTD., due to the low requirement of liquid level measurement accuracy in the process, insert cylinder type single flange liquid level gauge is used, which is in good field use condition.
2, Radar level gauge
Radar level meter emits electromagnetic waves during measurement, which are reflected by the surface of the measured object and received by the antenna.
At present, the radar level meter generally adopts frequeny-modulated continuous wave (FMCW) technology, that is, the frequency of linear change is transmitted at a certain time. Since the transmission speed of electromagnetic wave is constant, the liquid level height distance D of the tank can be calculated by measuring the time of transmitting a certain frequency and receiving the frequency. After setting the height of the tank H, the internal software of the radar level meter can calculate the height of the liquid level L=H-D. When selecting a radar level meter, one of the parameters to be considered is the dielectric constant. The measurement effect of radar level meter will be affected by the dielectric constant value of the medium. In order to facilitate the understanding of the dielectric constant, it can be associated with "dielectric", "dielectric" is a substance with low electrical conductivity. Low electrical conductivity, good dielectric insulation, or low dielectric constant; The higher the dielectric constant, the higher the dielectric constant, the stronger the radar reflection signal, the better the measurement effect of the level meter. In petrochemical tank farm, for the storage tank with small dielectric constant (such as olefin), the effect of using radar level gauge is poor, and false level phenomenon often occurs.
The measurement method of radar level meter is non-contact, which is not affected by the viscosity, density and corrosion of the medium. In order to achieve accurate measurement of radar level meter, good installation is essential. When installing the radar wave, it should avoid the obstacles in the tank (such as: pipes, reinforcement rods, agitators and the entrance of materials, etc.). If the radar wave irradiates to these strongly reflected metal objects, the echo generated will be far stronger than the echo of the medium, and the measurement of the medium will be inaccurate. In order to avoid liquid level fluctuations or other interference, the radar waveguide tube should be installed in the storage tank so that the radar wave is transmitted in the waveguide tube. In addition, for the internal floating roof tank, the radar can measure the liquid level below the floating disk through the waveguide tube, and the waveguide tube is required to be smooth without burrs on the inner wall, because even a small metal burr is stronger than the reflection of the medium on the wave, which will interfere with the radar and lead to misjudgment of the liquid level.
The characteristics of the medium also affect the normal use of the radar level gauge. Jinzhou Kaiyuan Petrochemical Co., Ltd. asphalt intermediate tank, with radar liquid level
The meter does not measure well. When the asphalt enters the tank, about 200℃ of the asphalt generated smoke, such smoke will reflect the electromagnetic wave of the radar, so that the radar can not judge the liquid level correctly, resulting in poor measurement effect of the radar level meter. In addition, the asphalt smoke will condense at the radar flare, and over time it will condense into an oily substance, which is difficult to remove clean, affecting the accurate measurement of the radar level meter.
3, Magnetic flap level meter
3.1 Working Principle
The magnetic turnover level meter works according to the principle of buoyancy and the principle of magnetic coupling. When the liquid level in the measured container rises and falls, the float in the main tube of the liquid level gauge also rises and falls, and the permanent magnetic steel in the float is transferred to the field indicator through magnetic coupling, driving the red and white tip column to turn over 180°. When the liquid level rises, the tip column turns from white to red, and when the liquid level drops, the tip column turns from red to white. The red and white boundaries of the indicator are the actual height of the medium liquid level in the container, so as to realize the indication of the liquid level.
SCUHZ series side-mounted magnetic flap liquid level meter products can achieve high sealing, anti-leakage and safe and reliable liquid level measurement under high temperature, high pressure, high viscosity and strong corrosion. There is no blind area in the whole measurement process, the display is eye-catching, the reading is intuitive, and the measurement range is large. Equipped with liquid level alarm and control switch, the upper and lower limit alarm and control of liquid level or boundary can be realized. Equipped with a liquid level transmitter, the liquid level boundary signal can be converted into a two-wire 4-20MADC standard signal to achieve remote detection, indication, recording and control.
Closing remarks
The three kinds of liquid level meter described by the author have their own characteristics, advantages, application scope and limitations. For different applications, it is necessary to select the type according to the specific process conditions and the characteristics and applicability of the liquid level meter, so that the liquid level meter can play its advantages, carry out accurate measurement of the relevant liquid level, and ensure the normal and safe production of the device.
