In the production process of melamine powder by the low-pressure method, the system pressure and the liquid level of the melting kettle are closely related. Due to the physical and chemical properties of solid urea, ammonia gas is released during melting, and it is easy to crystallize. In addition, the structure of the melting pot is complex, with both insulation jacket layers and agitation installed, which requires strict conditions for measuring the urea liquid level.
Commonly used flange level gauges, radar level gauges, gamma ray level gauges, etc., cannot be used normally due to the structure of the melting pot equipment and the physicochemical properties of the liquid level urea. After testing, it has been found that the blown gas level gauge is very suitable for melamine production and can reliably, with low maintenance and at an economic cost, measure the liquid level of molten urea.
The working principle of the blowing liquid level gauge is based on the relationship between gas pressure and liquid height, using non-contact measurement (except for gas pipelines) to protect key components:
Insert the catheter into the melting kettle. Compressed nitrogen gas (preferred gas source) is filtered, depressurized to constant pressure (PA), and regulated to a stable flow rate through a flow meter.
A constant flow of gas escapes from the bottom of the conduit, forming uniform bubbles that pass through the liquid and are discharged into the atmosphere.
The pressure measured by the differential pressure transmitter (PB) equals the liquid’s hydrostatic pressure.
In order to adapt to the special conditions of urea melting pots in melamine factories, targeted modifications and material selection must be carried out:
Communication pipe section: Use Φ 14 × 2 304 stainless steel to achieve basic corrosion resistance.
CD pipe section: Made of DN20 316L stainless steel – its superior corrosion resistance and high temperature stability solve the strong corrosiveness and crystallization tendency of molten urea.
Valve: Choose a 304 stainless steel Φ 10 straight ball valve to ensure durability and leak prevention.
Steam-heating pipeline: Add a steam-heating system to the CD pipe section to maintain the fluidity of liquid urea and prevent crystallization blockages.
Flushing flange mouth: Install a flushing flange on the CD pipe section for easy maintenance and unblocking during crystallization.
Flow stability: Equip pipelines with constant flow valves and rotor meters to ensure stable gas flow and significantly improve measurement accuracy.
Compared with traditional liquid level gauge technology, the blown gas liquid level gauge has significant advantages in melamine production:
Very suitable for measuring media with high temperature (molten urea ≈ 139 ° C), high viscosity, strong corrosiveness, and easy crystallization – perfectly matching the conditions of urea melting tanks.
Only the blowing tube comes into contact with the measured medium; Other components (transmitters, flow meters) should be installed outside the pot to avoid corrosion, crystallization, and mechanical damage. This reduces maintenance frequency and extends service life.
The investment cost is lower than that of gamma-ray level gauges (commonly used in melamine plant molten-salt tanks).
No radiation hazards (unlike gamma ray meters), ensuring safer operation and compliance with environmental regulations.
Simple structure, easy debugging, and reduced operation and maintenance costs.
Suitable for open and closed containers, laying the foundation for improving high-temperature equipment (such as ≈ 400 °C molten-salt tanks) in future melamine plants.
Gas source selection: Use dry, clean nitrogen gas to avoid contamination of molten urea and to prevent blowpipe blockages.
Flow control: Maintain a stable flow rate (≈ 100 liters/hour) to ensure accurate pressure measurement; fluctuations may cause errors in water-level readings.
Temperature monitoring: Monitor the medium temperature (approximately 139 ° C for molten urea) and adjust the gas pressure and flow rate as needed to prevent crystallization.
Blockage: If the bubbles stop or the flow rate decreases, rinse the flange mouth with steam or solvent to remove crystalline deposits from the pipeline.
Inaccurate readings: Check for gas leaks, flow meter malfunctions, or pressure fluctuations – calibrate the transmission and adjust the constant flow valve to restore stability.
Corrosion issues: Regularly inspect pipeline sections; if corrosion is detected, replace 304/316L components to avoid leakage.
Q1: Why is nitrogen used as the blowing medium?
A1: Nitrogen is inert, dry, and insensitive to the production medium of molten urea or melamine, avoiding pollution and ensuring process safety.
Q2: Can the blown air level gauge be used for high-temperature media (such as a 400 ° C molten salt tank)?
A2: Yes. The successful transformation of the 139 ° C urea melting kettle laid the foundation for adapting to a 400 ° C molten salt tank, by adjusting materials (such as higher-grade stainless steel) and heating systems as needed.
Q3: How to prevent urea crystallization from causing pipeline blockage?
A3: Use steam to heat and cool the pipe section, maintain stable gas flow, and regularly flush the pipeline through the flange to eliminate potential crystallization.
Q4: How does the blown air level gauge compare to the radar level gauge in the melamine factory?
A4: Radar instruments are susceptible to interference from the movement of ammonia and urea melting kettle agitators, resulting in inaccurate readings. The blowing-type liquid level gauge avoids direct contact with steam and mechanical interference, providing higher reliability.
The blowing liquid level gauge solves the key challenge of measuring liquid level in the urea melting kettle for melamine production, thanks to its adaptability to harsh media, low maintenance requirements, high cost-effectiveness, and high safety. Through targeted material selection (316L stainless steel), structural optimization (steam heating, flushing port), and flow stability control, they outperform traditional meters in reliability and practicality.
With the development of melamine production technology towards greater efficiency and safety, the blowing liquid level gauge will continue to play a key role, not only in urea melting kettles but also in the modification of future high-temperature equipment, such as molten salt tanks.
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