What should be noted during the loading and unloading of LPG railway loading arms?
LPG railway loading arms are the main equipment for loading and unloading operations of railway tank cars. There are two types of LPG railway loading arms: one is the universal unloading arm, and the other is the hose flange unloading arm.
The universal loading arm consists of a vertical pipe and a universal joint. The universal joint composed of elbows with "rolling balls" can be conveniently aligned and connected to the gas and liquid phase pipes of the tank car. The hose flange loading arm is composed of a vertical pipe, a threaded joint, a high-pressure armored oil-resistant hose, a flange joint, a threaded globe valve, a loose flange, etc.
The LPG railway loading and unloading trestle is an operation platform set up for loading and unloading operations. It is generally built together with the loading arm. There is a hanging ladder (with an inclination angle not greater than 60°) from the trestle to the tank car, through which operators can go up to the tank car for operation.
The loading and unloading methods of LPG railway tank cars mainly depend on the topographical conditions of the LPG warehouse and the structural form of the tank cars.
(1) Gravity loading and pumping loading
Gravity loading mainly uses the high-level difference of the terrain to carry out loading by gravity. This loading method not only saves investment and reduces costs, but more importantly, it is not affected by the power supply, safe and reliable, but it needs to have the topographical conditions for gravity loading. Pumping loading is a method used when the topographical conditions for gravity loading are not available. At present, many reserve oil depots in China are built in mountainous areas, and most of the storage areas are arranged in the depth of mountains or canyons, with most of the elevations higher than those of the operation areas. Therefore, most oil depots adopt pump unloading and gravity loading.
(2) Top unloading
Top unloading mainly involves inserting the rubber hose or movable aluminum pipe at the end of the loading arm into the tank from the tank car cover on the top of the liquid tank car, and then unloading by pump or siphon gravity. When filling Class A and B liquids into liquid railway tank cars from the top, a loading arm inserted into the bottom of the tank car should be used. The flow rate of the liquid in the loading arm should not exceed 4.5 m/s. When using a pump to unload liquid, it is necessary to ensure that the pump suction system is filled with liquid, and that no gas blockage and flow interruption occur at the top of the loading arm and any part of the suction system. Therefore, a vacuum pump must be equipped to meet the requirements of priming the pump and pumping the bottom liquid. Its characteristics are that the liquid unloaded from the bottom of the tank car can be directly pumped to the liquid storage tank without passing through the zero-position tank, which reduces the evaporation of LPG, but it needs to be equipped with tall loading arms, trestles, vacuum systems, etc. There are many equipment, the operation is complex, the fire risk is not small, and gas resistance is formed, which affects normal oil unloading.
In order to prevent the high-pressure armored oil-resistant hose from breaking or cracking, its allowable working pressure should be at least 4 times the maximum working pressure of the system. Generally, high-pressure armored oil-resistant hoses with steel wires are selected, and the length is usually about 4 m. For the convenience of operation, the hose flange unloading arm is fixed on a special hanger, and the relative position between the flange joint and the railway tank car can be changed by means of the loading arm hanger for alignment. The structural size and installation position of the loading and unloading arm must comply with the relevant provisions of "Code for Construction Gauge for Standard Gauge Railways" (GB 146 - 1983). The horizontal extension distance of the loading arm must not be less than 2.6 m, otherwise it cannot reach the loading and unloading port in the tank car. The height difference between the lowest position of the non-detachable part of the loading arm extending to the end of the tank car and the rail top is not less than 5.5 m. To prevent static electricity or stray current discharge from causing an explosion, the loading arm is equipped with a grounding device. The structural form of the loading arm requires convenient operation, no air leakage, good sealing, safety and reliability.
(1) When the LPG loading and unloading operation line is combined with the Class C B liquid loading and unloading operation line and they are operated at the same time, the distance between the two loading arms should not be less than 24 m; when they are not operated at the same time, the distance between the two loading arms is not limited.
(2) The distance from the center line of the large loading arms arranged on both sides of the two special lines with a walking distance of ±2 m to the center of the railway special line should be 2.6 m, and the distance between adjacent loading arms should be 12 m. The distance from the oil loading and unloading arm to the wall and railway gate should not be less than 20 m.
(3) In the railway loading and unloading area of flammable liquids, the distance from the internal combustion locomotive to the loading arm of another trestle: for Class A and B liquids, it should not be less than 12 m; for Class C liquid loading arms, it should not be less than 8 m. The distance between the loading arms of two adjacent trestles should not be less than 10 m, but the distance between the loading arms of two adjacent trestles for loading and unloading Class C liquids should not be less than 7 m. When flammable liquids are loaded and unloaded in a closed manner, their fire protection distance can be reduced by 25%.
The loading flow rate should be controlled within 4.5 m/s. Since the liquid phase port of the LPG loading arm is only DN50, a loading flow rate of 30 m³/h is more appropriate. If the loading time permits, it is better to keep the flow rate of LPG below 3 m/s.
When the liquid level of the tank car is higher than the zero-position liquid tank and there is a sufficient liquid level difference, siphon gravity unloading can be adopted. However, equipment for vacuum pumping or liquid filling must be available to fill the siphon of the loading arm. The advantage of this oil unloading method is that there are few equipment and the operation is simple; the disadvantage is that a zero-position oil tank is added, which requires one more transfer and increases the evaporation of LPG. The use of submersible oil pumps for upper unloading of oil products is also applied. This submersible oil pump is installed at the end of the hose of the unloading arm. The pump is driven by a motor, and both are installed in a closed shell. The motor is cooled by the transported oil products. This unloading method is flexible and effective, mainly suitable for field operations, but the motor must meet the explosion-proof grade requirements.
