Introduction

With the development of instrument technology, the improvement of people's understanding, and the strengthening of enterprise management, in order to ensure safe production, meet the needs of monitoring and measuring the operation of oil tanks, and reduce the labor intensity of inspectors, in recent years, oil production plants often use some advanced instruments in the tank farm. Develop the tank farm monitoring system and LAN connection to realize information resource sharing, so that the tank metering can reach a new level.

At present, radar level transmitters, servo level transmitters, electronic level transmitters, steel strip level transmitters, and optical fiber level transmitters are widely used in the oil tank. Because radar liquid level has the characteristics of high accuracy, no movable parts, stable and reliable operation, small maintenance, ease to be arranged into a LAN, low energy consumption, strong anti-interference, and wide range of dielectric constant applicable to the measured medium, it is used more and more in the liquid level measurement of large oil tanks.

Radar level transmitter mainly includes:

Frequency modulated continuous wave (FMCW) radar level transmitter and microwave pulse time travel (PTOF) radar level transmitter for non-contact continuous measurement: time domain reflection (TDR) radar level transmitter for contact continuous measurement.

Measurement principle

FM continuous wave radar level transmitter 

This type of level transmitter emits microwave signals of about 10GHz bandwidth that continuously change frequency to the liquid surface. When the signal propagates down to the liquid surface and returns to the antenna, it will be mixed with the signal being transmitted at the moment, and the result will produce a low-frequency signal proportional to the distance from the radar level transmitter to the surface. This signal can be processed and calculated to obtain high-precision space height. The tank height minus the empty height is the high liquid level.

Microwave pulse time travel radar level transmitter

This level transmitter is used to measure the distance from the reference point (i.e. the flange of the process connection point) to the material surface. The antenna sends out microwave pulses, which reflect on the surface of the measured material and are received by the radar system. The round-trip time of the microwave pulse is measured, that is, the time travel. The distance from the reference point to the medium surface is proportional to the time travel of the microwave pulse. The liquid level=empty tank high - the distance from the sensor to the liquid level.

Time domain reflection radar level transmitter

This liquid level transmitter is also called guided wave radar liquid level transmitter. Its measuring principle is similar to that of microwave pulse time travel radar liquid level transmitter. The difference is that the microwave pulse does not propagate through space, but through one (or two) guided wave bodies that extend from the tank top to the tank bottom. The guide wave body can be a metal hard rod or a metal cable. The transmitting pulse is reflected when it reaches the surface of the material to be measured, and then transmitted back along the guided wave body and received by the antenna. The propagation distance and liquid level height can be calculated according to the time difference between the transmitting pulse and the echo pulse.

Structure and composition of radar level transmitter

Taking the RTC radar level transmitter produced by an enterprise as an example, the equipment consists of a generator head (TH) and an antenna.

Inside the generator, the head is an electronic unit (THE), which is composed of microwave unit signal processing, data communication, power supply, and transient protection circuit board.

Technical specifications of transmitter head:

• Accuracy: 1mm;
• Ambient temperature: - 40~70C;
• Field bus (standard): TRI/2 bus (FSK, half duplex, two-wire system, current isolation, 4800bit, based on Modbus);
• Six Pt100 thermal resistors (RTDs) with common wires.

Some enterprises produce radar-level transmitters for FMR as integrated instruments. It is directly composed of an antenna, microwave unit, signal processing, data communication, field operation panel, power supply, etc.

Applications

At present, radar level transmitter is widely used in enterprises, including almost all medium oil tanks. The radar level transmitter is connected to the tank body through a flange or thread.

For the products of some enterprises, the corresponding radar level transmitter shall be selected according to the characteristics, temperature, pressure, and tank shape of different media. For example, the horn antenna radar level transmitter can be installed on the dome roof tank without a waveguide tube and is applicable to measuring the liquid level of various petroleum and chemical products, except asphalt and similar products. 

The maximum temperature in the tank is 230C, and the measuring range is 0.85~20m below the flange; Parabolic antenna radar level transmitter can be installed on the dome roof tank without a waveguide tube or on the floating roof tank to measure the liquid level of various petroleum and chemical products, including asphalt, tar, and condensate products; The guided wave tube radar level transmitter is applicable to all types of storage tanks. In addition, there are radar level transmitters for liquefied petroleum gas and natural gas.

Structure of radar level transmitter measuring system

The structure of radar level transmitter measuring systems produced by various enterprises is quite different. As an example, a measurement system is composed of an RTG level transmitter, FCU field communication unit, RTL/2 field bus, DAU field data acquisition unit, RTD temperature measuring element, etc. The liquid level transmitter is powered by 220VAC, and the signal is an RTL/2 field bus (ESK, half double T.. Two-wire system, current isolation, 4800bit. based on Modbus) communication digital signal. You can also customize other home communication protocols, such as Profibus, as required.

One field communication unit FCU2160 can be equipped with four RTL/2 field buses and two RS-232C communication ports. One bus can be equipped with 8 radar level transmitters, that is, one FCU can be equipped with 32 radar level transmitters. The signal lines of 8 liquid level meters can be connected in parallel at the site, and then lead a bus into the control room. FCU is connected to PLC, DCS, or PC through an RS-232C communication port. The FCU continuously collects the field data through the RTL/2 field bus and stores the data in the buffer memory. When the upper computer needs to extract the field data, the FCU can directly extract and send the data from the buffer memory. The RTL/2 bus uses shielded twisted pair cable, with a minimum cable sectional area of 0.5mm 'and a maximum bus length of 4km.

The DAU2100 field data acquisition unit can access up to 14 thermal resistance (RTD) signals. The unit is powered by and communicates with the radar level transmitter connected to it. The collected value and RTG liquid level value can be displayed on-site, and the collected signal is transmitted to the room through RTC. The slave DAU is intrinsically safe and powered by Zener barriers in the radar level transmitter. There are many types of thermoelectric anode, commonly using Pt100 resistance.

Installation, commissioning, calibration, and maintenance of radar level transmitter

The radar level transmitter shall be installed and debugged by professional technicians according to the instrument manual. After installation and commissioning, it is generally necessary to conduct at least one calibration to make the measurement accuracy meet the requirements. The commissioning and calibration shall be carried out on the measuring scale and computer. Calibration is to compare the manually measured liquid level values of a group of different liquid level heights (including low liquid level, high liquid level, and intermediate liquid level) with the measured values of the corresponding radar level transmitter, and correct the relevant parameters of the instrument according to their errors so that the instrument measurement can reach the required accuracy.

 After calibration, it can be put into formal use. Some high-quality radar level transmitters are considered maintenance-free. After long-term use, the antenna can be cleaned. As it is an intelligent instrument with a self-diagnosis function, if there is a fault, it can be diagnosed and handled by checking its fault code.

Supmea is a manufacturer of radar level transmitters. We provide more than 10 radar level transmitters for liquid level measurement under various working conditions. 50% ultrasonic level transmitter, 15% guided wave radar level transmitter, 20% low-frequency (26 GHz) radar level transmitter, and 15% high-frequency (80 GHz) radar level transmitter.

Supmea radar liquid level transmitter is mainly used for liquid level measurement of various liquids (water, seawater, oil, mud, etc.). If you need to know your product requirements, whether it is a radar level transmitter, ultrasonic level sensor, flow meter, or other equipment, please call us or leave a message on the website.