The liquid air level in the lower column is one of the core parameters of air separation distillation, and its fluctuation directly affects the product purity and system stability. The causes of liquid level fluctuation can be summarized into four categories: gas source disturbance, valve failure, instrument distortion and abnormal working conditions in the tower.
1、 Inlet gas volume and pressure fluctuation. This is the most common cause. Unstable exhaust pressure of air compressor and sudden reduction of gas volume during switching of molecular sieve adsorber will lead to fluctuation of air volume entering the tower. The lower tower is very sensitive to the change of gas volume, and the liquid level will fluctuate immediately. In addition, the abnormal working condition of the booster or the adjustment of the expander gas volume will also indirectly disturb the liquid level of the lower tower through the cooling capacity balance.
2、 Valve regulating system failure. The liquid air throttle valve (V1 valve) is automatically controlled by the liquid level of the lower tower. If the valve positioner drifts, the valve stem is jammed or there is internal leakage, the actual opening will be inconsistent with the instruction, and the liquid level will oscillate periodically. At the same time, the failure of automatic control valves such as liquid nitrogen reflux valve and waste liquid nitrogen throttle valve will change the reflux ratio of the lower tower, which will lead to the fluctuation of liquid air generation.
3、 The liquid level measuring instrument is distorted. If the sampling pipe of the cryogenic level gauge is partially blocked by molecular sieve powder, carbon dioxide dry ice or hydrocarbon impurities, it will cause gas-liquid entrainment in the negative pressure pipe and form a false level fluctuation. In addition, the insufficient power of the heating block of the liquid level gauge and the unstable liquid seal in the sampling pipe in the cold box caused by the loose fixing will make the liquid level display jump frequently, and the actual working condition has not changed.
4、 The distillation condition in the tower is abnormal. The blockage or flooding of the tray will destroy the normal gas-liquid contact and make the lower tower resistance and liquid level fluctuate abnormally synchronously. During flooding, a large amount of liquid is entrained by air flow to the upper tray, and the liquid level fluctuates periodically; Liquid leakage from the tray will cause the liquid air level to be continuously low and the adjustment is invalid.
Disposal principle: in case of fluctuation, the authenticity of the instrument shall be checked first, then the status of the gas source and valve shall be checked, and finally the working condition in the tower shall be judged by combining the resistance, purity and other parameters, so as to avoid blind adjustment and aggravation of the disturbance.
