The operating procedures for low-temperature liquid pipeline storage tanks are as follows:
1、 Preparation before use
Equipment inspection: Conduct a comprehensive inspection of the appearance of the storage tank, checking for any dents, deformations, or cracks in the tank body, ensuring that all connection parts are secure, and checking for any signs of damage or leakage in valves and pipelines. Confirm that safety accessories (such as pressure gauges, safety valves, etc.) are complete and functioning properly. The pressure gauge should be regularly calibrated to ensure accurate pressure display; The safety valve should undergo a jump test to ensure timely opening and pressure relief in case of overpressure.
Airtightness test: After installation or restoration of the inner cylinder to room temperature, a system airtightness test should be conducted before filling with low-temperature liquid. The test pressure is the maximum working pressure of the storage tank, and the test gas is oil-free dry air or dry nitrogen. The test time is determined by the size of the storage tank, but shall not be less than 4 hours.
Blow off treatment: After passing the airtightness test, the inner cylinder system of the storage tank needs to be blown off with oil-free dry air or dry nitrogen to remove humid air. After being blown away with dry air or dry nitrogen, the system should also be blown away with product gas. When blowing, gases lighter than air are added from the top of the storage tank (such as a vent valve) and removed from the bottom through a liquid inlet and outlet valve. In order to accelerate the removal of moisture from the tank, the blown gas can be heated to 80-100 ℃. Each pipe valve should be blown off separately, especially the liquid level gauge and pressure gauge, which should be purged of gas from the joint to remove moisture from the pipeline until the dew point of the exhaust gas from the inner cylinder system meets the requirements before filling can begin.
Valve and instrument inspection: Before filling with low-temperature liquid, it is necessary to carefully check whether the valve is in the correct position, whether the instrument is flexible and reliable, and whether the connection of the liquid level gauge is unobstructed.
2、 Liquid filling operation
The filling operation is divided into two situations: the first filling and the replenishment filling.
First filling (referring to the filling of the inner cylinder when it is in a hot state):
Connect the filling pipeline.
Blow off the filling pipeline, which should be done before each filling. Before the upper liquid inlet and outlet valves are opened, a small amount of liquid is discharged into the output liquid pipe through the liquid source discharge valve. At the same time, the residual liquid vent valve of the pipeline is opened to clean the pipeline to remove moisture and dust impurities from the pipeline.
Open the inner pipe vent valve and pressure gauge valve, and start the liquid level gauge at the same time.
Open the upper inlet valve and allow liquid to enter from the upper part. At this time, as the inner cylinder is in a hot state, the opening of the upper inlet valve should be small to allow the pipeline and inner cylinder to slowly and stably cool to the temperature of the low-temperature liquid being filled. When the exhaust valve of the inner cylinder is stable, the upper inlet valve can be opened to increase the filling speed.
When the liquid level gauge indicates a liquid level, open the liquid inlet and outlet valves, close the upper inlet valve, and switch from upper inlet to lower inlet.
When the liquid filling indicator valve (already opened) sprays out liquid, it indicates that the low-temperature storage tank is filled with liquid, and the liquid inlet and outlet valves should be immediately closed to stop filling. Simultaneously open the residual liquid discharge valve of the pipeline to eliminate the residual gas-liquid in the filling pipeline.
After filling, remove the filling pipeline.
Supplemental liquid filling (referring to the fact that the inner cylinder already has low-temperature liquid and no longer needs to be cooled):
The difference is that the filling amount can be increased from the beginning, which is basically the same as the first filling. During the filling process, close attention must be paid to pressure changes. If the liquid is directly imported from the liquid inlet and outlet valve and the pressure rises significantly, the lower (bottom) inlet can be changed to the upper (top) inlet to reduce the pressure.
From the perspective of pressure, it can be divided into atmospheric pressure charging and pressurized charging. During the atmospheric pressure filling process, the inner cylinder vent valve is always open, allowing the inner cylinder to communicate with the atmosphere; During pressurized filling, the vent valve is closed and the pressure inside the tank is higher than atmospheric pressure, but the working pressure of the low-temperature storage tank should be greater than 2Kg/cm ².
3、 Boosting operation
When the liquid in the low-temperature storage tank needs to be filled into other storage tanks or vaporized for use, the inner cylinder pressure needs to be increased. The increase in pressure depends on the actual usage needs of the user, but should not exceed the maximum working pressure of the low-temperature storage tank. The boosting program is:
Check if the pressure gauge and liquid level gauge are in working condition.
Ensure that the gas passage valve is fully open.
Slowly open the valve in front of the booster (booster valve) to allow the liquid to enter the booster and vaporize. If the discharge speed is fast and the inner cylinder pressure drops rapidly, the pressure increasing valve can be opened. When the pressure reaches the required working pressure, turn down or completely close the front valve of the turbocharger, and the liquid inside the turbocharger will continue to evaporate to the required pressure. During the pressurization process, it is necessary to closely monitor the changes in the pressure gauge reading.
4、 Liquid discharge operation
There are three forms of liquid discharge:
Vaporizer discharge: When the pressure inside the tank reaches the required level, the discharge valve can be opened to supply liquid to the vaporizer, causing it to heat up and vaporize before being sent to the site of use. This is the main discharge method for low-temperature storage tanks with vaporizers.
Discharge by liquid inlet and outlet valves: This type of discharge uses infusion hoses to supply liquid to tank trucks or other larger low-temperature storage tanks. The operating procedures are basically the same as filling liquid, except that the liquid inlet and outlet valves have changed from inlet to outlet.
Direct discharge through a drain valve (also known as a liquid discharge valve) or through a Dewar tube: This type of discharge supplies liquid to movable small containers such as Dewar tanks. Connect the metal hose of the small container to the joint at the drain valve (or through a Dewar tube), open the liquid outlet valve, and the liquid can be supplied. This type of discharge volume is relatively small, and for fixed low-temperature storage tanks, liquid can be supplied to the outside without pressurization according to specific circumstances.
5、 Storage operation
Liquid storage includes two methods: atmospheric pressure storage and pressurized storage:
Atmospheric storage: During the atmospheric storage process, the vent valve of the inner cylinder is always open, allowing the naturally evaporated gas to be discharged to the atmosphere through this valve, and the pressure inside the tank will not increase. Due to minimal natural evaporation, the vent valve only needs to be slightly opened.
Pressure storage: During the process of pressure storage, the vent valve is closed because the naturally evaporated gas remains in the tank, and the pressure in the inner cylinder gradually increases. At this time, the pressure gauge displays the pressure. When the inner cylinder pressure reaches the working pressure, the vent valve should be opened immediately to reduce the pressure.
6、 Maintenance and upkeep
Regular inspection: Do not dismantle or tamper with the explosion-proof device and vacuum valve of the outer cylinder, otherwise it will damage the vacuum degree of the low-temperature storage tank. The outer shell belongs to an external pressure container that can withstand atmospheric pressure. Collision is strictly prohibited to prevent damage to the outer shell and affect the vacuum degree. The vacuum degree of the low-temperature storage tank should be checked every six months. When measuring, simply unscrew the protective cover on the metal heat gauge tube and plug in the thermocouple vacuum gauge plug to determine the authenticity of the interlayer. After several years of use, if the vacuum degree of the low-temperature storage tank may drop below 66.66Pa, it is necessary to vacuum it again.
Calibration of safety accessories: Regularly calibrate the pressure gauge, liquid level gauge, and safety opening valve according to regulations, such as checking and calibrating every 2 years, and replacing the bursting disc of the inner cylinder blasting device every year.
7、 Safety precautions
Personal protection: Suitable protective equipment should be worn during operation, such as cold clothing, protective gloves, goggles, etc., to prevent frostbite caused by contact between low-temperature liquids and the skin or eyes. Do not wear synthetic clothing with oil stains or electrostatic effects, and do not wear shoes with nails.
Operating standards: Operators should be familiar with the equipment process flow, the functions and operating procedures of various valves, instruments, and safety accessories. Filling and draining should be strictly carried out according to the operating procedures. All valves should be opened and closed slowly to prevent excessive force from damaging the valves. When the valve freezes and cannot be opened or closed, hot air or hot water should be used to thaw it. It is strictly prohibited to use hard objects to knock, fire or electric heating.
Environmental monitoring: During use, it is necessary to regularly check whether the valves are in the correct opening and closing positions, whether the measurements of pressure gauges and level gauges are accurate and reliable, and whether there are any air leaks or blockages in equipment pipelines and valves. When the pressure of the container reaches the tripping pressure of the safety valve and the safety valve does not move, the pressure of the safety valve should be calibrated immediately to ensure the safety of the low-temperature storage tank.
Medium characteristics: When the stored medium is liquid oxygen, the tank must undergo degreasing treatment because oxygen is a strong combustion supporting gas. It is strictly prohibited to use tools with grease, protective equipment, and steel tools that are prone to sparks during operation. Rubber or brass tools without grease should be used instead.
Exception handling: If the pressure inside the tank rises abnormally, the gas vent valve should be immediately opened and the cause should be identified to prevent accidents caused by overpressure. When not in use for a short period of time, do not empty all the liquid in the low-temperature storage tank. Leave 5% -10% of the liquid volume according to the duration of the interruption to reduce evaporation loss during refilling. After the liquid in the low-temperature storage tank is emptied, the pressure inside the tank should be maintained at 0.02 MPa to prevent humid air from entering the pipeline system and causing valve blockage.