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Maintenance and management of cryogenic liquid pumps in large air separation equipment

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Release date:2020-05-06 Author: Clicks:

With the rapid development of domestic coal chemical projects, the design, installation, operation and maintenance of large internal compression process air separation equipment are being continuously optimized. Among them, as an important part of the high pressure liquid oxygen pump and liquid storage system backup liquid pump, its operating state directly determines whether the whole air separation equipment can operate stably and whether the gas can be normally supplied when the fault state.

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1, make the liquid oxygen pump in the performance curve of a better working point can avoid cavitation phenomenon

The high-pressure liquid oxygen pump in a factory is a multi-stage centrifugal liquid oxygen pump. During the commissioning, after the operator changes the output of high-pressure oxygen, the outlet pressure of the high-pressure liquid oxygen pump fluctuates periodically, and the motor current display value also fluctuates periodically, with a fluctuation range of about 10A. The periodic fluctuation of motor current was improved to some extent, and then the working state was relatively stable, and the large fluctuation range was also reduced to less than 2A. The outlet pressure and high-pressure oxygen flow of the high-pressure liquid oxygen pump gradually tended to be stable. After inspection found that the operator in the process of high pressure liquid oxygen pump load without strictly according to the characteristic curve of pump, high pressure liquid oxygen pump flow and outlet pressure fluctuate after also did not cause enough attention, leading to a lower outlet pressure lower than the corresponding flow operation flow, and the operation of the high pressure liquid oxygen pump at a higher speed, which under the condition of constant power output, the impeller make part of liquid on less fluid power, and this is an important cause of the cryogenic liquid pump cavitation.

In order to avoid the recurrence of this running state, in addition to strictly following the characteristics of the cryogenic liquid pump curve to make the pump running speed, outlet pressure and flow to achieve a better match, it is essential to strengthen the monitoring of the running current of the pump motor, because the current is a more sensitive parameter reflecting the cryogenic liquid pump process state.

It is recommended that the running current signal of cryogenic liquid pump motor be introduced into DCS control system of air separation equipment for monitoring. If the current fluctuates, even if the fluctuation range is only about 2A, it also indicates that the pump appears cavitation during operation. After the current of cryogenic liquid pump motor fluctuates, whether it is caused by external conditions (such as the process system before and after), or by electrical reasons such as frequency converter, the cryogenic liquid pump process state fluctuations need to be comprehensively considered and the fault should be eliminated as soon as possible, otherwise the pump body will be damaged if it is in a long period of failure.

2. Control the cryogenic liquid pump to operate above the lower allowable pressure at the inlet

The control of low inlet pressure of cryogenic liquid pump is also an important factor that should be paid attention to. The low pressure at the inlet of cryogenic liquid pump is above the low allowable pressure at the inlet, which is a factor to prevent cavitation and ensure the stable operation of cryogenic liquid pump.

Inevitably in the process of air separation equipment installation for section aluminum pipe and produce some crumbs, especially, under the tower separation on arrangement of the air separation equipment, after throttling the tower into the tower, and then through the structured packing tower distillation of the cryogenic liquid oxygen pump into the circulation again, will drive in the original stage of scour of aluminum to the circulating liquid oxygen pump entrance, caused by circulating liquid oxygen pump under certain operation speed, allowing its flow under low flow rate; And because the inlet filter resistance increases, once the circulating liquid oxygen pump speed is too high, according to the corresponding allowed suction vacuum, the pump inlet pressure is lower than the corresponding liquid saturation pressure, the pump inlet liquid began to vaporize, the pump body will appear larger vibration, long time vibration will cause pump damage due to cavitation.

Large internal compression process air separation equipment using upper and lower tower split type is decorated, in the process of the original testing, once found cycle cannot effectively conveying liquid oxygen pump, its performance for the current in a relatively stable level at first, but can't meet the requirements of liquid transportation, as if by improving circulation liquid oxygen pump running speed way to increase the transmission capacity, will cause the pump inlet section of liquid, long run will result in circulating liquid oxygen pump damage.

3, liquid storage system cryogenic liquid pump maintenance matters needing attention

Liquid nitrogen pump in liquid storage system is often used as an important component of the standby security nitrogen system and its protection should be paid great attention to. Due to liquid nitrogen pump inlet pressure for storage tank liquid column static pressure, so its running state is complex, and once choose the multistage centrifugal pump with large flow, high rotating speed and, if the holding tank liquid level drops quickly, very easy to appear because of the suction pressure saturated pressure and make the liquid nitrogen pump suction part of liquid, in turn, cavitation failure occurs, long running will cause the sealing of the pump and the pump body seriously damaged. In addition, in general, due to the short operation time of the equipment of the standby security system, the pump inlet valve USES manual valve, so it is impossible to feedback the actual valve opening on the DCS control system, once the wrong operation will cause a large equipment failure.

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