Oil storage tank is an important equipment for oil refinery to store crude oil and refined oil. By measuring the liquid level, temperature, pressure, density and other parameters of the liquid medium in the storage tank, the volume and quality reserves of the liquid in the storage tank can be calculated, so as to realize the demand for asset management in modern production.  The level of storage tank is one of the important parameters in the process of oil gathering and transportation. In order to prevent the occurrence of accidents such as tank roof falling and vacuuming, the reliability and stability of the level gauge measurement are required more strictly.  Due to the different sizes and types of storage tanks, the characteristics of the measured medium and process conditions are also different, so only from the actual situation of the field process, reasonable selection of liquid level gauge and design of liquid level measurement, in order to better meet the measurement needs of the process, to better ensure the safety of production process.  The radar level meter is the most widely used level measurement instrument in oil storage tank area in recent years. It adopts the measurement method without contact with the medium, and has the advantages of convenient installation, low failure rate, easy maintenance, high accuracy and small influence by the medium.  

1、Working principle, characteristics and classification of radar level gauge

1.1 Working principle of radar level gauge

    Radar wave is a high-speed electromagnetic wave propagating at the speed of light C. When the transmitted electromagnetic wave hits the liquid surface of the medium, it will be reflected back. Radar determines the distance between the antenna and the reflecting surface (altitude H) by measuring the delay t between the transmitted wave and the reflected wave.  T = 2 h/c.  (c=300000km/s) The speed of light C is not affected by the medium environment, and the propagation speed is stable. When the delay time t is measured, the space height H can be obtained.  

    The radar system continuously transmits the linear frequency modulation signal (that is, the signal with a linear relationship between frequency and time), and the difference frequency between the transmitted signal frequency and the reflected signal frequency can be obtained, which is proportional to the delay time T, that is, proportional to the space height H.  After data processing of the difference frequency signal, the empty height value H can be obtained. The difference between the tank height value and the empty height value is the liquid level value.  

1.2 Characteristics of radar level gauge

（1）High reliability.  Non-contact measurement, no moving parts, low failure rate, therefore high reliability.  

（2）High measurement accuracy.  The measurement accuracy is not affected by the temperature, pressure and dielectric constant of the medium.  

（3）Wide range of applications.  It can be used in various oil storage tanks, especially in high precision, easy crystallization and high temperature and high pressure media.  

（4）Digital output technology.  Rs-232, RS-485 standard interface, digital transmission between the field and the control room, can overcome the error caused by analog signal transmission.  

（5）Small size, light weight, convenient installation, simple maintenance.

（6）Have fault self - diagnosis and prompt function.

1.3 Classification of radar level gauges

● Radar level meter according to the working frequency, usually divided into: 6.8GHz, low frequency, 26GHz, high frequency, 80GHz, high frequency three.  Different frequency radar level gauges can be adapted to different occasions:  

a. Low frequency has better resistance to antenna hanging material; 

b. Low frequency has better performance on fluctuating liquid surface; 

c. Low frequency has better water vapor foam penetration; 

d. High frequency has a narrow beam width, which can avoid obstacles;

e. High frequencies have better reflection on tilted solid surfaces because they have shorter wavelengths;  

f. Smaller antennas can be used for higher frequencies.

● According to the type of antenna, radar level meter is divided into rod type, horn type, rod type, ball type, hemisphere type, paraboloid, planar antenna and so on.  

a. Rod type and rod type antennas are common in 6GHz, low-frequency radar, with the characteristics of small antenna diameter, suitable for the installation of riser small diameter, strong corrosive, condensed water application conditions.  

b. Horn antenna is the most common radar antenna, suitable for all kinds of frequency radar level meter.  Different sizes of bell mouth performance difference, the larger the bell mouth, the better the performance.  

c. Short-rod antenna radar is often used in 26GHz, high frequency radar level gauge. It has the characteristics of small antenna size, strong signal and strong corrosion resistance, and is widely used in strong corrosive conditions.  

d. Spherical or parabolic antenna is used to generate a very small beam Angle and greatly enhance the microwave energy by focusing on the principle of echo transmission. It is often used in high-precision radar or solid radar.  

e. Planar antenna is based on the principle of microwave transmission focusing, which can produce smaller beam Angle.  At present, it is mainly used for measurement of solid material level, but not widely used in petrochemical liquid level measurement.  

2、 Installation and commissioning

2.1 Install

    The installation of radar level gauge is simple and convenient, but the correct installation will directly affect the measurement of radar level gauge.  Notes are as follows:  

◆  For round or arched tanks, the meter should not be installed in the middle position, multiple reflections will create false signals, so that no valid information will be received;  

◆  For circular tanks, the distance between the level gauge and the tank wall is best kept between 1/6 and 1/4 of the tank diameter, and the closest distance between the installation point of the small diameter tank and the tank wall can not be less than 30cm;  

◆  For flat conical tank, the meter can be installed in the middle of the tank top, so as to ensure the measurement to the conical bottom;  

◆  When the instrument is installed, it can not be installed directly above the liquid inflow port under test;   

◆  Do not install limit switches, human ladders or temperature sensors within the radiation range of the transmitted microwave beam;  

◆  Pay attention to the symmetrical devices in the inner wall of the tank, such as heating coils, space rings and other obstacles, which may directly affect the accuracy of measurement;  

◆  During installation, the central axis of horn antenna should be perpendicular to the liquid level, and the antenna must be extended into the tank at least 10mm to ensure the quality of echo and avoid receiving false echo. 

◆  For the use of guided wave tube installation (guided wave tube or bypass tube), can avoid obstacles in the container, foam, liquid surface fluctuations on the impact of measurement.  Isobaric hole diameter is 5...  10 mm, the minimum distance between the two holes is 100mm.  To avoid impact, the installation position of the wave guide tube should be far away from the inlet and outlet.  

2.2 Debug 

    There are two ways to debug:

a. Through the radar table panel to set the parameters, by viewing the panel data and reading the curve to complete the radar debugging.  

b. Through mobile phone bluetooth to set parameters, read curve to complete the radar debugging.  

3、Common faults in measurement and improvement measures

    In the practical application of radar level meter, the measurement value jump, no output and other related problems, these problems can be detected and avoided.  Common faults and improvement measures are as follows:  

◆  There was no change in the measurements.  When the measured values do not change, most of them cause strong interference because there are fixed reflectors in the beam range.  At this time, we should check whether there are obstacles in the tank wall, extension rod and other parts.  In addition, if the electronic module of the radar level meter fails, it will also cause this problem, so the electronic module should be updated and maintained in time.   

◆  Loss of wave.  The main reason for this problem is that the antenna direction of the radar level gauge is not vertical to the liquid level, and the liquid level has disturbance or interference.  At this time, check whether there is condensation or other adhesive on the radar antenna, clean the antenna in time and adjust the direction of the antenna.   

◆ The measured value jumps within a fixed interval.  There are two reasons for this phenomenon. One is that the measured liquid level is unstable and fluctuates, leading to changes in the measured reflected wave time; the other is that the crude oil hangs on the wall seriously, which will weaken the electromagnetic wave signal and lead to a jump in the measured value.  In the use of radar level gauge, if there is an error code, you can refer to the corresponding fault code marked in the manual for handling.  

◆  When the radar level gauge is transferred from one tank to another tank, it must be reset operation, if only the basic parameter setting will cause the work of the radar level gauge is not stable.  After reset, the radar level meter relearns the new installation environment, and then sets parameters to achieve accurate measurement.  

4、 Economic benefit

◆ The radar level meter has reasonable structure, good performance, high measurement accuracy and good stability, and realizes all-weather measurement and supervision of oil storage tank.  

◆ The radar level gauge is simple in structure design, easy to install and operate, less equipment, free from environmental restrictions, accurate measurement and so on.  

◆ The popularization and application of radar level meter can better solve the profit and loss measurement of crude oil entering the plant and the loss management of finished oil leaving the factory.  

◆ The popularization and use of radar level gauge has improved the ability of industrial automation.   

◆ The application of radar level meter can improve work efficiency and reduce labor intensity of process personnel.