As many continue the debate around energy isolation methods associated with entry into a Permit-Required Confined Space, I offer another example of why getting the energy isolation plan RIGHT is SO IMPORTANT. I know there are safety professionals out there who feel strongly that single valve isolation or the use of an “automated valve” being controlled by a PLC is perfectly safe and compliant; but this incident can at least provide a first-hand account of how an improper energy isolation plan, which utilized a single UNLOCKED block valve and an AUTOMATED valve on a liquid Anhydrous Ammonia (NH3) line feeding a “dryer” injection system. Here is the incident summary of this tragic accident…
In a fertilizer production plant, an ammonia leak killed one worker and injured two more. On December 1st, 1994, the weekly cleaning of the dryer was scheduled.
- The tubular reactor of this dryer was shut off at 6:25 am then cooled and purged.
- At 9:30 am, the dryer’s ventilation was turned off
- At 9:45 am, the electrical unit undertook its semi-annual replacement of the inverter protecting the automated production system. This operation consisted of stopping all programmes, cutting the electrical power supply of the in-service inverter, connecting the backup inverter and then reinitializing the programmes.
- At 10 am, three (3) maintenance workers entered the dryer. All production chains were shut down; two of the workers were cleaning the reactor supply impellers in front of the ammonia injection nozzle, while the third was inspecting the hot gas intake upstream of the injection nozzle.
- At 10:30, the automatic valve controlling ammonia injection into the dryer OPENED. The gas contained inside the pipe running between the mechanical valve and a manual valve, which was closed at the time and located 32′ upstream, was released.
- One worker was able to escape via the dryer supply chute after passing underneath the injection nozzle, while the two others facing the nozzle tried to reach the dryer access hatch located over 80′ away.
- Only one (1) of these two (2) was able to exit the dryer. After scrambling less than 30′, the second man tripped.
- Wearing an SCBA, the maintenance foreman evacuated the crew member unable to advance (within 3 min).
- All three (3) men complained of facial burns and breathing difficulties and were taken to the hospital.
- One (1) was slightly affected by the exposure, the second more seriously, while the worker who fell died six (6) days later subsequent to the pulmonary burns he had sustained.
The accident was due to poor coordination of these maintenance works, since the replacement of inverters and the cleaning, performed by two different teams, should never have been carried out simultaneously. Upon completion of the inverter replacement operation, the step of reinitializing the automation programmes caused the two automatic valves controlling ammonia and phosphoric acid injection inside the dryer to open.
The workers were verifying that pipes had successfully drained using a manometer, which was only able to indicate the pressure drop but could not confirm the pipe had been fully drained.
When the valve opened, the ammonia left in the pipe (1.3 gallons) was released.
Following this accident, an additional valve was installed at the entrance to the reactor upstream of the automatic valve. In addition, specific guidelines were introduced that mandated:
- prohibition of all simultaneous operations;
- a 1-hour purge of the pipe between the two valves;
- modification of computer programmes to ensure that no valve could remain open during the re-initialization sequence;
- placement of a full seal after the manual valve during onsite work.
- A study of the plant’s SMS was planned
Source: Ministère de la Transition écologique et solidaire (Ministry of ecology and solidarity)
