DESCRIPTION
Failure of the main shaft seal ( mechanical seal ) of compressor #2 released ammonia vapor from the refrigeration system into the machinery room.
INCIDENT OVERVIEW
There was an ammonia leak from the main shaft seal, isolated to compressor #2 in the machinery room.
INVESTIGATION CONCLUSIONS
The machinery room is attached to the facility on the southwest of the building. Two compressors in the machinery are installed to operate the refrigeration system.
Ammonia is contained within the machinery room and chilled brine (calcium chloride) is transferred to 4 ice rinks. The compressors require refrigeration oil to lubricate their components.
To prevent refrigerant (ammonia) gas leakage, the compressor’s driveshaft is installed with the main shaft seal which is one of the components of the compressor.
The main shaft seal is composed of ‘rotating ring’ on the driveshaft and ‘stationary ring’ in the seal cover. The rotating ring turns with the shaft and slides against a stationary ring while maintaining a very small clearance or gap, the ‘sliding surface’.
In order to form a proper sealing and reduce any possibility for the seal to have premature failure or wear-out under normal operation, oil film lubrication is used to reduce friction and heat generation in the main shaft seal and the sliding surface.
Failure of main shaft seals are classified into two types:
- premature failure and
- normal wear out
Insufficient oil flow can result in premature failure of the main shaft seal. Oil lubrication system requires daily inspections indicated in the manufacturer operating manual
Normal wear out indicates that the main shaft seal has served the life span according to the operating hours recommended by the manufacturer
The manufacturer’s operating manual indicates the preventive maintenance for the main shaft seal to be checked and replaced if abnormal for every 10,000 hours.
Facility operators bi-hourly record the start and stop time of compressor #2 during daily normal operation. The facility operation manager assumed that the compressor is running on an average of 6 hours per day. No annual operating hours of the compressor were recorded.
FAILURE SCENARIO(S)
The sequence of events:
- Compressor #2 had several ‘Oil Sump’ alarms throughout the week of November 24th. Operators reset the alarms and restarted compressor #2.
- At 10:00 am on November 30, 2019, an operator received a phone call from the alarm monitoring company reporting an ammonia alarm. He entered the machinery room and approached compressor #2 which had the oil sump alarms issues. He smelled strong ammonia odor from the compressor and quickly left the room. Ammonia level was recorded 50 ppm.
- At 12:40 pm the ammonia gas sensor above compressor #2 detected the ammonia level high enough to trigger the ammonia alarm, 99 ppm.
- At 13:17 responding refrigeration mechanic arrived and checked the ammonia level at 101 ppm.
FACTS AND EVIDENCE
Witness statements:
- Logbook stated that on Saturday, November 30, 2019 ammonia alarms as high as 99 ppm.
- Service report on November 30, 2019, stated that an ammonia leak on compressor #2 main shaft seal had to be replaced as soon as possible.
- The incident report submitted by the facility stated that compressor #2 was repaired on Monday, December 02, 2019.
- Service report dated December 02, 2019, stated compressor #2 had the main shaft seal replaced.
- Service report dated December 09, 2019, stated that compressor #2 oil filter changed.
- The facility operations manager on January 03, 2020, verbally stated that daily operating hours of compressor #2 were estimated 6 hours per day and the compressor did not have a preventive maintenance program.
CAUSES AND CONTRIBUTING FACTORS
- No service report indicated when the main shaft seal of compressor #2 was ever replaced prior to the ammonia leak incident.
- It was likely that the failure of main shaft seal of compressor #2 was due to normal wear out.
- It is possible that with proper maintenance as per the manufacture operating manual, the leak from main shaft seal could have been prevented.
Photos:
Photo 1: Compressor #2
Photo 2: Main shaft seal casing
Photo 3: Ammonia Emergency Procedures
Source: https://www.technicalsafetybc.ca/alerts/main-shaft-seal-compressor-failure-releases-ammonia