During an ongoing freeze-drying cycle, a freeze dryer power failure occurred, which resulted in multiple alarms in the system after power restoration. The alarms observed included a freeze dryer pressure deviation alarm during drying and a hydraulic oil excess temperature alarm. The system was unable to reset the alarms automatically, and therefore a detailed investigation was required to identify the exact cause of the issue and restore the equipment to normal operating condition.
During the root cause analysis, it was identified that the slot door feedback sensor signal became unhealthy due to the power dip. Because the feedback signal was not available, the nitrogen purging valve did not open properly, which resulted in a drop in chamber pressure. This condition triggered the freeze dryer pressure deviation alarm during the drying phase. Additionally, the hydraulic pump continued operating due to the missing feedback signal, which caused the hydraulic oil temperature to increase beyond the alarm limit.
As part of the fix, the power supply for the safety switch for the slot door feedback sensor was turned back on. This brought the sensor signal back to normal and made the system work normally again. After the sensor signal was recovered, the nitrogen purging valve started functioning correctly, and all alarms were successfully reset. The freeze dryer was monitored after restoration and was found operating normally, confirming that the issue was resolved with minimal impact on the freeze-drying process.
Incident Description
During the ongoing freeze-drying cycle, a sudden power failure occurred, which caused the freeze dryer system to trip. After restoration of the power supply, multiple alarms were generated in the system. The following alarms were observed:
- Pressure deviation too low during drying (Non-Critical Alarm)
- Pressure deviation too low during drying (Non-Critical Alarm)
- Hydraulic Oil Excess Temperature Critical Alarm
After power restoration, the system alarms were not resetting automatically, and the machine required investigation to identify the root cause of the issue.
Root Cause Identification
The investigation revealed a compromised slot door feedback sensor signal. Due to the loss of the feedback signal from the slot door sensor, the nitrogen purging valve did not open properly.
Because the nitrogen valve did not operate correctly, the chamber pressure dropped below the set limit, which triggered the pressure deviation alarms during the drying cycle.
At the same time, due to the missing feedback signal, the hydraulic pump continued running continuously, which caused the hydraulic oil temperature to increase above the alarm limit, resulting in the hydraulic oil excess temperature alarm.
To resolve the issue, the safety switch power supply for the slot door feedback sensor was restarted. After restoring the sensor power supply, all system signals became healthy, and the alarms were reset successfully. The nitrogen purging valve resumed normal operation, and the freeze dryer was observed to be functioning properly.
Therefore, the root cause of the incident was identified as the loss of the feedback signal from the slot door sensor due to a power dip during the freeze-drying cycle.
Impact Evaluation
The incident occurred due to a power dip during the freeze-drying cycle, which temporarily affected the feedback signal of the slot door sensor. This resulted in improper operation of the nitrogen purging valve and caused pressure deviation alarms.
The vacuum pump and compressor, on the other hand, kept working, and the temperature in the chamber stayed within the acceptable range. After restoring the sensor power supply and resetting the alarms, the system returned to normal operating condition.
No mechanical damage to the equipment was observed during the incident. The freeze dryer was monitored after corrective action and found to be operating normally. Therefore, the impact on equipment performance and product quality was considered minimal.
Corrective Action
- The system parameters and alarms were reviewed after power restoration.
- The slot door feedback sensor signal was checked and found unhealthy.
- The safety switch power supply for the slot door feedback sensor was restarted.
- After restoring the power supply, all sensor feedback signals were verified and found healthy.
- The alarms were reset from the system.
- The operation of the nitrogen purging valve was verified and found to be working normally.
- The freeze dryer system was monitored for stable operation, and it was found running satisfactorily.
Preventive Action (CAPA)
- Ensure a stable power supply to the freeze dryer system to avoid signal interruptions during power dips.
- Perform regular inspections of the slot door sensors during preventive maintenance.
- Include sensor signal verification in routine equipment checks.
- Verify the operation of the nitrogen purging valve periodically to ensure proper functioning.
- Monitor the hydraulic pump and hydraulic oil temperature during operation.
- Train operators to check sensor feedback signals immediately after power restoration.
- Maintain proper documentation of alarms, power failures, and corrective actions in equipment logbooks.
Conclusion
A power dip affected the slot door feedback sensor signal, generating alarms during the freeze-drying cycle. This resulted in improper operation of the nitrogen purging valve and continuous running of the hydraulic pump, which triggered pressure deviation and hydraulic oil temperature alarms.
After restarting the safety switch power supply of the slot door feedback sensor, all signals were restored, and the freeze dryer returned to normal operation. The equipment was monitored, and no abnormality was observed. Therefore, the issue was resolved successfully with minimal impact on equipment performance.
Summary
A power dip during an active freeze-drying cycle caused multiple alarms in the system. These included pressure deviation alarms and a hydraulic oil temperature alarm. The investigation revealed that the feedback sensor for the slot door lost signal due to the power failure. The failure prevented the nitrogen purging valve from opening and caused chamber pressure deviation. Meanwhile, the hydraulic pump continued running, increasing the oil temperature. Restarting the safety switch restored the sensor signal, allowing the alarms to reset and the system to return to normal operation.
Frequently Asked Questions freeze dryer power failure
What caused the alarms in the freeze dryer system?
The alarms were triggered due to a power failure during the freeze-drying cycle, which disrupted system feedback signals.
Which alarms appeared during the incident?
The system generated pressure deviation alarms and a hydraulic oil excess temperature alarm.
Why did the pressure deviation alarm occur?
The pressure deviation occurred because the nitrogen purging valve did not open due to missing feedback from the slot door sensor.
What caused the hydraulic oil temperature alarm?
The hydraulic pump continued running due to missing feedback signals, which increased the oil temperature beyond the safe limit.
What is the role of the slot door sensor?
The slot door sensor ensures that the system receives correct operational signals for safe machine operation.
How was the issue resolved?
The safety switch power for the slot door sensor was restarted to restore proper signal communication.
Did the machine return to normal operation?
Yes, after restoring the sensor feedback signal, all alarms were reset, and the machine operated normally.
Why is nitrogen purging important in freeze drying?
Nitrogen purging helps maintain proper chamber pressure and prevents contamination during the drying process.
How can such issues be prevented in the future?
Regular sensor checks, stable power supply, and preventive maintenance can reduce the risk of similar incidents.
Why is root cause analysis important in freeze dryer alarms?
Root cause analysis helps identify the exact reason for system failure and ensures corrective actions prevent recurrence.