Ⅳ. OPERATION RECORDS

RIKEN Accel. Prog. Rep. 53

Operation of the BigRIPS cryogenic plant
K. Kusaka,∗1 M. Ohtake,∗1 K. Yoshida,∗1 M. Ohshima,∗2 A. Mikami,∗2 H. Hazama,∗2 H. Miura,∗2 H. Shiraki,∗2H. Hirai,∗2 M. Haneda,∗2 K. Kimura,∗2 and M. Nagano∗2
We performed two continuous operations of the BigRIPS cryogenic plant in 2019. The first operation period was from March 1 to June 24, and the second wasfrom October 23 to December 11. The total operation
time of the compressor unit was 72,222 h.
During these continuous operations, we encountered
two significant incidents. One incident was the failureof the compressor unit in June. We first noticed an unusual noise that was produced by the main motor of thecompressor unit. Although the noise level gradually increased daily, we maintained the continuous operation
while monitoring the vibrations of the main compressor
and the motor unit. However, the compressor unit suddenly stopped on June 13. Though no interlock alarm
event was recorded, we consider that the reason was theover current of the main motor unit because a sudden increase of the main motor current from 29.9 A to 32.6 Awas recorded in the control system. We could restart thecompressor unit and maintain the continuous operation
till the end of the beam time.
The main motor unit was disassembled on site to investigate the origin of the unusual vibrations. In a bearing
unit on the coupling side, damage was found on a surface
of its inner ring and the bearing balls (Fig. 1). We also
found that the size of the housing for the damaged bearing unit was 5 µm larger than its dimension standard(+0–25 µm. The motor unit was reassembled with new
bearing units, and the housing was re-manufactured witha tolerance of +0.0 µm. After the regular maintenanceof the whole compressor unit, a test operation was successfully performed, and no unusual noise was producedin the second operation period.
Figure 2 shows the vibration acceleration in the vertical and horizontal directions as a function of the total
operation time. Two rapid increases of the vibration acceleration at operation times of 59,000 h and 71,000 h
indicate the damage of the bearing unit occurred in Dec.
20161) and in June 2019. The vibration acceleration re-Fig. 1. Damaged bearing unit, disassembled from the coupling side of the motor unit.
∗1∗2

RIKEN Nishina Center
Nippon Air Conditioning Services Co., Ltd.

Fig. 2. Vibration acceleration of the compressor unit. Twooperation periods are indicated by arrows.

mained less than 8 m/s2 in the second operation period.
The other incident consisted of the failures of the first
expansion turbine T1 in the refrigerator. The T1 turbine
did not start rotation in the starting up of the refrigerator. The starting sequence of the refrigerator unit usually proceeds automatically as follows. The purge linevalve opens for 30 s so that room-temperature heliumgas flows into the gas-bearing cartridge of the expansion turbines T1 and T2. Subsequently the control valve
CV3130 in the turbine inlet line gradually opens, andthe inflow of the cold helium gas starts the rotation ofT1 and T2 simultaneously. The opening of CV3130 increases up to 50% and the rotation speeds of T1 andT2 turbine increase beyond 3000 rps depending on thetemperature of the inlet helium gas.
The first failure of T1 turbine occurred on June 13,
when a sudden stop of the compressor caused refrigerator trip. After having the compressor unit restarted, werestarted the refrigerator operation. After the turbine
trip caused by the low-temperature interlock of the inlet
helium gas during the start up, T1 did not start rotationin the automatic starting sequence. We somehow started
the refrigerator without the purging process and could
proceed with the beam time till June 19. We stopped therefrigerator in the usual manner and visually checked T1and T2 in the summer maintenance. Although no visualabnormality was found in the maintenance, the same failure occurred in the test operation on September 25. T1
did not work during start up.
Both turbine cartridges were sent to its manufacturer
Linde and were disassembled. While T2 was not damaged, the gas bearing of T1 had problems. Scratches
and particles were found on the lower axial bearing. After replacing the lower axial bearing with a new one andpolishing the shaft, T1 turbine cartridge was reassembled. The repaired turbines were remounted on October
24, and the refrigerator successfully started on October
25. The origin of the scratches is under investigation.
Reference1) K. Kusaka et al., RIKEN Accel. Prog. Rep. 50, 285 (2017).

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