Application of Level Gauge in Petrochemical Tank Farm

Application of Level Gauge in Petrochemical Tank Farm？In the tank farm of the petrochemical industry, there are many types of storage tanks, including spherical tanks, internal floating roof tanks and fixed roof tanks. The storage tanks vary in size. The storage medium includes crude oil and residual oil with high viscosity and poor fluidity, liquefied olefin with small dielectric constant, and acidic substance with strong corrosiveness. For the level measurement of these storage tanks, flanged level gauges, radar level gauges, servo level gauges and light guide level gauges are commonly used. The applicability of different storage tanks and different media in the area 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 level gauge measures the liquid level according to the principle that the pressure generated by the liquid level is proportional to the height of the liquid level. The pressure reflects the height of the liquid level H, that is, H = p /ρg, where ρ is the density of the medium, 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 temperature changes, so that the liquid level also changes.
        For pressure tanks, single flange level gauges are no longer applicable. The pressure at the bottom of the storage tank is the sum of the pressure in the tank and the pressure generated by the liquid. The single-flange level gauge cannot distinguish whether it is a change in the liquid level or a change in the liquid level caused by a change in pressure in the storage tank. In this case, a double-flange level gauge can be used, and a pressure tap is set in the gas phase space on the top of the storage tank, and the liquid level is measured by the pressure difference between the upper and lower pressure taps. The price of single and double flange level gauges is much cheaper than radar level gauges, servo level gauges and light guide level gauges. It is recommended to use this type of level gauge for storage tanks that do not require high level measurement. In addition, different types of single and double flange level gauges are suitable for viscous, easy to crystallize, strong corrosive or highly toxic media.
2 Radar level gauge
        The radar emits electromagnetic waves during the measurement process. These waves are reflected by the surface of the object to be measured and then received by the antenna.
        At present, radar level gauges generally use frequency modulated continuous wave (FMCW) technology, which emits a linearly changing frequency for a certain period of time. Since the transmission speed of electromagnetic waves is constant, by measuring the time of transmitting a certain frequency and receiving the frequency, the liquid surface-to-air height distance D of the tank can be calculated. After setting the tank height H, the internal software of the radar level gauge can calculate the height of the liquid level L = H-D. When choosing radar level timing, one parameter to be considered is the dielectric constant. The measurement effect of the radar level gauge will be affected by the dielectric constant value of the medium. In order to facilitate the understanding of dielectric constant, it can be associated with "dielectric". "Dielectric" is a substance with low conductivity. Low conductivity, good insulation of the medium, or low dielectric constant; high conductivity, high dielectric constant, the higher the dielectric constant, the stronger the radar reflection signal, and the better the measurement effect of the level gauge. In petrochemical tank farms, for storage tanks with small dielectric constants (such as olefins), radar level gauges have poor results when measuring liquid levels, and false liquid levels often occur.
         The measurement method of the radar level gauge is non-contact, which is not affected by the viscosity, density and corrosiveness of the medium. To make the radar level gauge achieve accurate measurement, good installation is essential. During installation, the radar wave should be prevented from irradiating obstacles in the tank (such as: pipes, reinforcing rods, agitators, and material inlets, etc.). If the radar wave irradiates these strongly reflective metal objects, the echo generated will be much 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, a radar waveguide should be installed in the storage tank so that the radar wave is transmitted in the waveguide. In addition, for the inner floating roof tank, the radar can measure the liquid level below the floating plate through a still-wave tube. The inner wall is required to be smooth and free of burrs when making stilling wave pipes, because even a small metal burr, the reflection of the radar wave is stronger than the reflection of the medium to the wave, which will interfere with the radar and cause a misjudgment of the liquid level.
The characteristics of the medium also affect the normal use of the radar level gauge. For the asphalt tundish of Jinzhou Kaiyuan Petrochemical Co., Ltd., the effect of radar level gauge measurement is not good. When the bitumen enters the tank, the bitumen at about 200°C produces smoke. This type of smoke reflects the electromagnetic waves of the radar, which makes the radar unable to correctly judge the liquid level, resulting in poor measurement results of the radar level gauge. In addition, the asphalt smoke will condense at the horn of the radar, condensing into an oily substance over time, and this kind of oily substance is difficult to wipe clean, which affects the accurate measurement of the radar level gauge.
3. Magnetic flap level gauge
The magnetic flap level gauge 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 pipe of the liquid level gauge also rises and falls. The permanent magnet in the float is transferred to the on-site indicator through magnetic coupling, driving the red and white turning columns to turn over 180°. When the liquid level rises, the turning column turns from white to red. When the liquid level drops, the turning column turns from red to white. The red and white boundary of the indicator is the actual height of the medium liquid level in the container, so as to realize the liquid level. The side-mounted magnetic flap level gauge product can achieve high sealing, anti-leakage and safe and reliable measurement of liquid level under high temperature, high pressure, high viscosity, and strong corrosive conditions. The whole process of measurement has no blind spots, and the display is eye-catching. , The reading is intuitive, and the measurement range is large. Equipped with a liquid level alarm and control switch, it can realize the upper and lower limit alarm and control of the liquid level or interface level. When equipped with a liquid level transmitter, the liquid level interface signal can be converted into a two-wire system. The standard signal of 4~20mADC realizes remote detection, indication, recording and control.
The three liquid level gauges have their own characteristics, advantages, scope of application and limitations. For different applications, the selection must be made according to the specific process conditions and the characteristics and applicability of the level gauge, so that the level gauge can take advantage of its advantages, carry out accurate measurement of the relevant liquid level, and ensure the normal and safe production of the device.