In the internal compression process air separation plant, we generally select the the multistage centrifugal pump as the liquid nitrogen pump. In order to ensure the stable operation of the liquid nitrogen pump, the pump seal mostly adopts the structure of gas-filled labyrinth seal.
The seal gas needs enough flow to warm the cryogenic liquid leaking from labyrinth seal. When the seal gas flow decreases to a critical point, the exhaust temperature of the seal gas will drop greatly and cause the interlock shutdown of the pump. With the pump running in the unstable state for a long time, the labyrinth seal will be worn and the gap will be increased, resulting in the increase of seal gas consumption, seal mixture outlet with liquid phenomenon, and causes the exhaust temperature of seal gas to decrease. In addition, the blockage of inlet pipeline filter by mechanical impurities and impurities in the filter screen of self regulating valve will also cause the seal gas flow to be too low.
The use principle of seal gas is not only to prevent liquid leakage along the outside of the pump, but also not to allow too much seal gas into the pump, resulting in the liquid nitrogen pump with gas phenomenon. It is suggested that the seal gas pressure can be adjusted by adding self regulating valve. When adjusting the seal gas pressure, it is necessary to ensure that the seal gas inlet pressure is higher than the labyrinth inlet pressure, which is 0.1-0.4Mpa, and the labyrinth inlet pressure is greater than the mixed gas pressure at the labyrinth outlet, which is 0.1-0.2Mpa. In order to prevent the equipment from being damaged by low temperature, it is necessary to ensure that there is no leakage of cryogenic liquid at the labyrinth seal. It is suggested to set a temperature sensor on the side near the pump body in the coupling guard to measure whether there is leakage at the sealing point, so as to give an alarm in time.
