The expander used in the cryogenic device belongs to a velocity turboexpander, which transfers energy in response to changes in the velocity of natural gas. The high-pressure natural gas is close to isentropic expansion in the expander, causing a drop. At the same time, the output work, consume energy, increase the enthalpy drop, reduce the natural gas outlet temperature, and achieve the purpose of refrigeration. The expansion impeller belongs to the radial axial flow reverse radial impeller, the impeller of the coaxial brake centrifugal turbocharger is semi-open, and the expander and supercharger are single. The expander is the main equipment of the cryogenic station refrigeration system, which plays an important role in cooling and recovering energy. The design flow rates of the high and low pressure expanders are 18623/17388 Nm3/h, the rotational speed is 39000/47000r/min, the inlet temperature of the high and low pressure expanders is -69.15/21.55 °C, and the outlet temperature is -105.32/-39.55 °C. Through analysis, the main factors affecting the bearing temperature of the expander are lubricating oil, water cooler, sealing gas, accessory quality, maintenance quality, etc. The oil film thickness of the lubricating oil is the main reason for restricting the bearing temperature, and the bearing temperature is the influence. The key to long cycle and efficient operation of the expander.

1 Problems with the expansion unit

In May 2005, the expansion unit showed high bearing temperature and chain shutdown phenomenon for a week (data shown in Table 1), which caused the unit to be in a lower load operation state. The refrigeration negative temperature was around -50 °C, which was far from the designed cooling temperature.

 

2 Expander bearing lubrication mechanism

 

The lubrication and cooling of the shaft and bearing of the expander are mainly based on lubricating oil. The main function of the lubricating oil is to provide lubrication and cooling for the shaft, bearing and seal. A dynamic pressure oil film of a certain thickness is formed between the journal and the bearing to form an oil film contact, the oil film holds the shaft, the bearing and the shaft are not in direct contact, and the shaft and the bearing are in fluid friction, and the friction factor is small. At the same time, the metal chips and impurities falling off between the friction pairs are taken away by the lubricating oil, which reduces the abrasive wear, removes the heat generated by the friction, and prevents the shaft and the bearing from being too hot to burn the tile.

 

3 Low-pressure expander structure and lubricant flow

 

The structure of the low-pressure expander is mainly composed of an expander casing, a rotor shaft, an impeller, a bearing, a key, a back bolt, a heat insulation plate, a back pressure seal, a sealing rubber ring, a temperature probe, a vibration probe and the like. The auxiliary system includes a supercharger, a sealed gas system, a cold blowing system, an accumulator, a balancing valve, and the like.

 

Lubrication of bearings and shafts of low pressure expanders

 

Lubricating oil of L-TSA68 is lubricated. The viscosity index of lubricating oil is 55.08~82.28mm2/s, and the flash point (opening) index is ≥185°C. The lubricating oil process is lubrication of the oil tank-rough filter-oil pump-lubricating water cooler-double filter-line valve-pre-machine filter-shaft and bearing of the expander.

 

4 Troubleshooting and analysis

 

4.1 Checking the instrumentation system

 

Look at the low expansion machine bearing return oil temperature field data and the main control room computer display bearing temperature data comparison, and then with the infrared thermometer to measure the data comparison, the measured data is basically the same, the error is about 2 ° C; check the low expansion machine Temperature probe installation, if the probe is loosely installed or the probe is not linear, it will also affect the data change, re-calibrate the temperature of the field, re-confirm the sensitivity of the instrument probe, and check that no abnormalities are found.