Judgment and handling of safety valve leakage during loading

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Reason for occurrence: The safety valve is the main safety device in liquefied gas tankers. The most common sudden failure of safety valves during use is leakage, with several incidents occurring each year on railway lines. There are three main reasons for this: (1) Overfilling during charging causes the safety valve to open and leak. (2) When a liquefied gas tank truck is filled in areas with lower temperatures and transported to areas with higher temperatures, the pressure inside the tank increases as the temperature rises during transportation. When this internal pressure exceeds 1.05 to 1.1 times the design pressure of the tank, which is 2.2 Mpa, the safety valve opens to release pressure. (3) Failure due to fatigue or severe corrosion of the safety valve spring, resulting in the valve remaining open and failing to return to its original position, thereby causing leakage. Method of determination: When a safety valve leaks, it is first necessary to determine the cause. The method for making this determination is to climb onto the top of the tank truck, quickly open the cover plate of the manhole protection cover, and check whether the pressure indicated by the pressure gauge mounted on the manhole cover is within the specified standard range. Since the opening pressure of the safety valve is 1.05 to 1.1 times the design pressure of the tank, which is 2.2 Mpa, when the pressure inside the tank exceeds this opening pressure, the safety valve opens on its own to allow fluid to be released. This phenomenon is not a leak; rather, it is the safety valve performing its protective function due to the excessively high pressure inside the tank. Once such a situation occurs, the true cause of the excessive pressure in the tank should be identified as quickly as possible, and appropriate emergency measures should be taken: (1) If the safety valve is discharging due to excessive filling of the tank with the medium, efforts should be made immediately to unload the material from the tank. (2) If the excessive pressure is caused by abnormally high tank temperature, cooling measures such as spraying cold water should be applied immediately to lower the tank temperature. (3) If both the pressure and temperature inside the tank are normal yet the safety valve still leaks, it is generally caused by a broken spring in the safety valve; in a few cases, it is due to fatigue of the spring material. Once such a situation is detected, it can be determined that there has been a sudden failure in the safety valve itself; immediate emergency measures must be taken to forcibly seal the safety valve in order to prevent the situation from worsening and causing serious consequences. Emergency handling method (1): Lift the rubber cap on the upper part of the safety valve cover, and carefully break the set screw seal. (2) Use an explosion-proof wrench to slowly turn down the set screw, causing it to press the safety valve disc down against the sealing seat of the valve opening, until the liquefied gas leakage is eliminated. (3) If both safety valves are leaking due to failure, emergency measures should be taken, and discharge should be contacted immediately thereafter. Because once both safety valves cease to function, during transportation, an increase in ambient temperature will lead to a corresponding rise in pressure inside the tank. When this internal pressure exceeds 1.05 to 1.1 times the design pressure of the tank, which is 2.2 Mpa, the safety valves will not open to release pressure and provide safety protection, resulting in an overpressure explosion of the tank truck. Therefore, once both safety valves fail, the tank truck is not allowed to continue operating and must be unloaded. Measures to prevent overfilling: (1) Before a liquefied gas railway tank car is brought to the loading platform, it is weighed using an electronic rail scale. The weight of the empty vehicle is compared with the weight indicated on the tank itself; if the weight of the empty vehicle as measured by the scale exceeds the tank’s own weight, it is possible that the liquefied gas inside the tank has not been completely unloaded. Therefore, the excess amount should be deducted when determining the allowable filling volume, in order to prevent overfilling. (2) After the liquefied gas has been filled, it is weighed using an electronic platform scale to determine the weight of the liquefied gas loaded. If the amount filled exceeds the maximum weight that the liquefied gas tank vehicle can carry, the vehicle must be taken back to the loading platform so that the excess liquefied gas can be unloaded until the specified limit is reached. (3) Regularly calibrate and check the zero point of the mass flow meter used for filling measurement, to prevent overloading caused by measurement errors of the flow meter.