The liquid level of the liquid ammonia storage tank is accurately measured, which can eliminate potential safety hazards.
Choosing a suitable monitoring instrument for liquid ammonia storage is a prerequisite for ensuring safe production. The main instruments in the liquid ammonia tank area include liquid level, temperature, pressure, toxic (flammable) gas alarms, etc. For tank farms where the system is rebuilt, and environments where wiring is difficult, wireless instrumentation solutions can be used.
This article analyzes the application problems of several common liquid ammonia storage tank level measuring instruments. So as to provide a basis for the selection of measuring instruments.
About Liquid Ammonia Storage Tank
Liquid ammonia is a common chemical raw material. To facilitate transportation and storage, gaseous ammonia is usually pressurized or cooled to obtain liquid ammonia. When liquid ammonia is dissolved in water, ammonium ion NH4+ and hydroxide ion OH- are formed, and the solution is alkaline. Liquid ammonia is mostly stored in pressure cylinders or steel tanks. Liquid ammonia is widely used in industry, is corrosive and easily volatile, so its chemical accident rate is very high.
Application Analysis of Common liquid Ammonia Storage Tank Level Measuring Instruments
External liquid level sensor
The external level gauge is an instrument that uses the sonar ranging principle and “micro-vibration analysis” technology to measure the level from the outside of the container. No need to make holes on the tank wall, no flanges, no fire, no tank cleaning. Do not touch the liquid and gas in the tank. It can realize online installation and maintenance. It is a completely non-contact isolated instrument.
HS-2000 External Mounted Ultrasonic Tank Level Sensor- (referred to as “external level gauge”). It adopts advanced signal processing technology and high-speed signal processing chip. It breaks through the influence of container wall thickness. Realizes the height of the liquid level in the closed container True non-contact measurement.
The ultrasonic liquid level sensor (probe) is installed directly under the outer wall (bottom) of the container under test. Calibrate the outer wall of the probe. There is no need to open holes for the container to be tested, easy to install, and does not affect production. It can accurately measure the level of various toxic substances, strong acids, strong alkalis and various pure liquids in high-pressure airtight containers. The HS-2000 External Mounted Ultrasonic Tank Level Sensor has no special requirements for the material of the liquid medium and the container. The explosion-proof design is adopted to meet the explosion-proof requirements.
The external liquid level sensor continuously and accurately measures the liquid level in the tank from outside the tank. No contact with the liquid and gas in the tank at all, realizing a true isolation measurement. Therefore, the characteristics are as follows:
- Isolated non-contact liquid level measurement technology is very safe.
- Installation and debugging, thereby reducing construction and maintenance costs.
- It is more reliable and durable. There are no mechanical moving parts in the measuring probe and host, and it is strictly sealed, isolated from the outside world, and will not be worn or corroded.
- Pass the “functional safety integrity” level certification.
Differential pressure transmitter
One end of the differential pressure transmitter is connected to the liquid phase, and the other end is connected to the gas phase. The liquid level of the liquid ammonia storage tank can be measured using the principle of hydrostatics.
In the process of applying this instrument and equipment, it is necessary to strictly control the installation height of the equipment.
Generally, it needs to be installed below the lower interface of the liquid level pressure interface of the container under test. And choose easy to maintain and observe. At the same time, it is also close to the pressure taking part.
Taking into account the low boiling point and easy vaporization of liquid ammonia, it is necessary to use a double flange differential pressure transmitter to ensure the stability of the measurement signal.
If this device is used, the installation of the device will only be limited by the length of the capillary tube.
Specifically, it is necessary to take corresponding protection and insulation measures, and ensure that the bending radius of the capillary is greater than 50mm. When using this measuring instrument, if the installation elevation changes, the output value of the device will be affected.
Differential pressure(DP) level transmitter is often used as a tank level transmitter. The DP transmitter is connected with a stainless steel flange by capillary. Flange installation. The pressure is sensed by a remote transmission device installed on container. The pressure is transmitted to the body of the transmitter through the capillary. The transmitter convert the pressure or differential pressure to 4 ～ 20mA. The DC signal is output. It can communicate for the setting and monitoring by cooperating with the HART communicator. Explosion proof available.
From the application effect point of view, the differential pressure transmitter has the characteristics of easy calibration and a wide range of popularization. At the same time, the price of this type of equipment is low, and it has a certain cost-effective advantage.
However, when installing the device, it is necessary to reserve mounting holes. At the same time, due to the close relationship between the equipment and the density change of the medium, the measurement operation will cause the material of the instrument diaphragm to be corroded by the corrosive medium. After a long time of operation, the equipment is prone to deformation of the capsule.
In the application process, if there is no indication, the indication is too large or too small and the indication remains unchanged. It is also necessary to check the power supply, safety barriers, circuit boards, diaphragm capillaries and pressure valves. And appropriately replace the equipment components and open the equipment pressure valve in time.
Magnetic flap level gauge
The application of the magnetic flap level gauge follows the law of float and the principle of magnetism, and needs to be connected to the tank through a flange.
During the measurement, the two-color cylindrical magnetic float inside the device will change under the influence of the magnetic field, and the float junction is the actual liquid level of the storage tank.
In the process of installing the equipment, it is necessary to avoid magnetic materials around the equipment, and determine the heating temperature according to the medium condition. In the case of large gas-liquid-phase opening connections, a flange needs to be installed in the middle of the level gauge.
In the process of equipment debugging, a magnetic steel corrector is needed to adjust the magnetic column to white, and then open the gas phase stop valve and the liquid phase stop valve successively, and observe the liquid level rise and fall.
The magnetic flap level gauge can measure the liquid level and interface of any medium, and the equipment itself has the characteristics of convenient and intuitive installation.
However, if there are too many impurities in the medium, the magnetic column will not flip and the measurement will fail.
At the same time, the application of this device is prone to the problem of magnetic column jamming, which will lead to the failure of remote transmission instructions. However, the price of this equipment is low, and installation holes need to be reserved during installation.
The magnetic flap level gauge is prone to problems such as no signal output from the transmitter, no change in the output signal and the range cannot be adjusted to the corresponding value during the application process. In the application process, the power supply voltage, cables, terminal pins and safety barriers need to be checked.
Cable guided wave radar liquid level sensor
The application of the cable guided wave radar level gauge follows the principle of travel time, and it is necessary to measure the distance from the reference point to the surface of the liquid ammonia by transmitting and receiving high-frequency pulse signals.
When installing the equipment, it is necessary to avoid the liquid ammonia inlet to prevent false reflections from the equipment.
When installing the sensor, it cannot be installed in the center of the tank, and the sensor must be kept away from the tank wall.
When installing the cable probe, the distance between the probe and the smooth metal wall should be greater than 50mm, the distance between the end of the probe and the bottom of the container should be greater than 0mm, and the diameter of the short tube should not be greater than 0mm. In addition, depending on the installation position, the offset of the cable probe may increase.
Guided wave radar is a measuring instrument based on the principle of time travel. The radar wave runs at the speed of light. The operating time can be converted into level signals by electronic components. The probe emits high-frequency pulses and propagates along the cable or rod probe. When the pulse meets the surface of the material, it is reflected back and received by the receiver in the instrument. And convert the distance signal into level signal. SIRD-701 Guided Wave Radar Level Sensor is suitable for liquid and Bulk Solids measurement, and complex process conditions.
The cable-type guided wave radar level gauge has the characteristics of direct contact with the medium and small measurement blind area, and the measurement accuracy is high.
However, this equipment is not suitable for measuring pressure vessels, and the price is relatively high. It is necessary to reserve mounting holes on the top of the equipment during installation.
From the point of view of application failures, the equipment may have failures such as incorrect DCS system configuration, cable probe failure, and unstable DCS display value.
In the application process, it is necessary to carry out on-site measurement before calibration, and it is not necessary to check whether the DCS configuration data is consistent with the actual measurement result in order to obtain the real data.
Before measurement, you need to clean the probe and increase the height of the empty tank appropriately to eliminate the effect of echo.
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