The utilization and development of natural gas are crucial to optimizing our country's energy structure and advancing ecological civilization. Representatives from the National Development and Reform Commission, the Ministry of Environmental Protection, the Ministry of Finance, and other ministries, as well as the State Grid Corporation and the five major power groups, three major oil companies, gas enterprises, the Development Research Center of the State Council, and relevant research institutions and professional industry associations, among others, have engaged in exchanges and discussions to contribute their wisdom to promoting the use and development of natural gas in our country.
One of the advantages of natural gas - safety
The gas density of natural gas is half that of air. Compressed natural gas is lighter than air, highly volatile, and not easily explosive. Methane, ethane, propane, and other components in natural gas are inherently non-toxic. In terms of safety, pipeline gas is superior to bottled gas. This is because pipeline gas employs a centralized storage and supply management system at gasification stations, avoiding the safety risks associated with the scattered storage of bottled gas, liquefied natural gas tanks, and self-use in households. The indoor pressure of pipeline gas is only 1/130 of that of tap water and 1/170 of bottled gas pressure. The storage capacity of the gas pipeline itself is very small, approximately 1/3000 of bottled gas when calculated based on a 12-meter indoor pipeline per household. In case of a leak or fire due to other reasons, simply closing the main valve or the unit riser valve indoors can cut off the gas supply and eliminate the danger. In contrast, when faced with the same situation, it is difficult to effectively close or remove the petroleum liquefied gas cylinder, which can easily lead to explosions.
Second advantage of natural gas - Economic
Economic affordability is one of natural gas's advantages and a key factor behind its rapid nationwide adoption. Compared to other fuel prices at current market rates, using piped natural gas can save at least 25% to 35% on monthly costs under the same usage volume (calorific value) compared to bottled LPG. This not only eliminates the expenses of purchasing and regularly inspecting steel cylinders but also reduces the costs and inconvenience associated with transporting and replacing LPG in daily life. The cost of using natural gas is more than 35% cheaper than electricity, and approximately 25% to 35% cheaper than diesel or heavy oil. The third advantage of natural gas – environmental friendly Natural gas is composed of 98% methane, which is a simple molecule (CH4) consisting of one carbon atom and four hydrogen atoms.
Maintain and repair
Routine Maintenance
The housing must not be subjected to any impact, and operations should be conducted as per regulations. The externally fitted valves and piping components should remain clean and intact, with valves capable of smooth opening and closing, and regular calibration as required. Under normal use, an annual inspection and maintenance of all valves, piping components, and instruments should be conducted, and timely replacement of consumables (such as valve seals) should be ensured.
2. Insulation performance maintenance
One of the key requirements for low-temperature liquid storage tanks is excellent insulation performance, with the quality of vacuum being crucial to the insulation.
Once the vacuum of the storage tank is compromised, it cannot store low-temperature liquids. Therefore, special attention should be paid to protecting the vacuum integrity of the tank. The explosion-proof device on the tank shell and the exhaust valve are directly connected to the vacuum interlayer. The device must not be tampered with unless the vacuum integrity is not compromised or there is no need to replenish and re-evacuate with pearly sand. The exhaust valve is sealed with lead at the factory and must not be moved arbitrarily; otherwise, it will damage the vacuum integrity of the tank.
The tank shell is an external pressure vessel, bearing atmospheric pressure. It is strictly prohibited to strike or bump it to prevent damage to the shell and affect the vacuum degree.
3. Inspection
(1) Routine inspection
Is the valve in the correct open/close position?
2) Accuracy and reliability of pressure gauge and differential pressure (level) meter measurements.
3) Pipeline and valve for leakage and blockage issues.
4) If the container pressure reaches the set pressure of the safety valve and the valve does not activate, the safety valve's set pressure should be calibrated immediately to ensure the safety of the storage tank.
5) Are the turbocharger flange bolts loose, and is there any pipe deformation?
(2) Regular maintenance
1) The pressure gauge is calibrated annually.
2) The safety valve is to be inspected annually.
3) Measure vacuum annually.
4) Thermocouple vacuum gauges (purchased by the user, not within the supply scope) should be calibrated according to the manufacturer's instructions before each measurement.
5) Inspect the tank grounding resistance annually; less than 10Ω is considered合格. If greater than 10Ω, check the connection status promptly.
(3) Vacuum Level Inspection and Re-evacuation
The tank's vacuum level is measured once a year. To measure it, simply unscrew the protective cap of the metal thermocouple tube, insert the plug of the thermocouple vacuum gauge, and the interlayer vacuum level can be known.
After several years of use, the vacuum level in the tank may drop to 65Pa, necessitating a re-evacuation to improve insulation performance. Prior to re-evacuation, the liquid in the tank should be drained first, and then heated with dry, oil-free air or nitrogen at 80-100℃ to blow out until it returns to room temperature. Connect the vacuum pipeline, start the pump to remove the moist air from the pipe, and then open the vacuum valve to evacuate.
To shorten the vacuuming time, the container can be heated with dry, oil-free air below 100℃. The vacuum degree of the jacket seal should be ≤3Pa. If necessary, the user may contact the manufacturer regarding vacuuming matters, and both parties can agree on the specific process for re-vacuuming.
4. Fault Handling
(1) Safety Valve Fault Handling
1) Safety valve leakage
The valve disc and seat sealing surface may experience leakage beyond the allowable limit under operating pressure, the possible reasons for which could be:
There is debris on the sealed surface.
If the pre-installation cleaning with compressed air is not thorough, or if solid impurities enter during the trial run, or if it causes necessary movements, debris can become trapped between the valve seat and the sealing surface. If it's easy to remove the debris, do so manually. However, if there's still leakage after removal, consider the following reasons: the valve seat and sealing surface may be damaged; or debris may be embedded between them. If this is difficult for the user to handle, return it to the manufacturer for repair.
② Internal Pipeline Force
a) Human Factors
During the process of installing the safety valve on the pipeline, if it is forcibly rotated, the valve seat may rotate, and at this point, the position of the safety valve adjustment ring may change or the sealing surface may be forcibly worn. A decrease in operational performance may occur. In such cases, depending on the situation, it can either be handled on-site by the user or returned to the factory for processing, but it is necessary to fully grasp the situation and make a quick judgment.
b) Internal Force Factors
The adverse effect of welding residue's force within the pipeline on the safety valve is almost identical to the issue mentioned above. Therefore, it is crucial to fully consider the absorption of the force within the pipeline during installation. This is a very important point.
c) The setting pressure of the safety valve is too close to the normal operating pressure of the equipment, resulting in an excessively low sealing ratio pressure on the sealing surface. This makes leakage more likely when the safety valve is subjected to vibration or fluctuations in the medium pressure. Under conditions of meeting strength requirements, the setting pressure of the safety valve should be appropriately increased during design.
③ Spring relaxation reduces the set pressure, causing the safety valve to leak.
The primary cause of spring relaxation may be that after the safety valve has been adjusted and the check valve is completed, the adjustment screw is not properly tightened, causing it to loosen during equipment operation. This results in spring relaxation, a decrease in preload, and the premature opening of the safety valve. It is necessary to re-inspect the safety valve.
2) Inflexible safety valve operation
The reason may be:
Improper adjustment of the safety valve's regulating ring can cause an extended opening process or slow return to seat. It should be re-adjusted. By adjusting the lower regulating ring, increase the return seat pressure.
②The exhaust pipe of the safety valve has excessive resistance, resulting in significant back pressure during discharge, which causes the safety valve to open at an insufficient height. By changing the exhaust pipe of the safety valve to a straight-through design and removing the middle bend, the safety valve will function normally.
3) Safety valve frequent chatter or vibration
The reason may be:
The relief valve's discharge is excessive. The rated discharge of the selected relief valve should be as close as possible to the equipment's required discharge.
② The imported pipeline diameter is too small or the resistance is too high.
③ Excessive resistance in the exhaust pipe causes high back pressure during emissions. Reduce the resistance in the exhaust pipe.
④Improper adjustment of the adjusting ring results in excessive return seat pressure. The position of the adjusting ring should be re-adjusted.
4) Operation of Three-way Valve A-5 during the replacement of the safety valve
During normal operation, both safety valves operate simultaneously, meaning both channels are open at the same time, and the handle position is as shown in Figure 1.
②When replacing the safety valve, rotate the handle 90 degrees (as shown in Figures 2 and 3), close the side channel to be replaced, and empty the remaining medium inside the channel, then replace the safety valve.
