SECTION 9: CONCLUSION
The investigation team concluded that the explosion that occurred in the ammonium nitrate plant at the Terra Port Neal Complex initiated in the neutralizer, and the explosion in the neutralizer initiated an explosion in the rundown tank virtually instantaneously.
The investigation team developed three scenarios that could have initiated the sensitized contents of the neutralizer. Based on the evidence available, the investigation team concluded that the explosion initiated in the ammonium nitrate solution in the neutralizer as a thermal decomposition reaction that accelerated into a deflagration.
The investigation team concluded that the explosion occurred as a direct result of a series of unsafe operating procedures and conditions at the Terra Port Neal Complex. The team concluded that the explosion could not have occurred without the presence of the following conditions. The conditions were a direct result of unsafe operating procedures. They are as follows:
1. Strongly Acidic Conditions in the Neutralizer.
The acidic conditions in the neutralizer resulted primarily from purging nitric acid from the acid line beginning in the acid plant and terminating at the neutralizer.
The investigation team concluded that had the pH of the neutralizer and rundown tank been adjusted to a value of 6.0 or greater, the explosion probably would not have occurred. The presence of free ammonia inhibits acid catalysis in thermal decomposition reactions in the ammonium nitrate.
2. Application of 200 psig Steam to the Nitric Acid Spargers.
200 psig steam was applied directly to the nitric acid line in the AN plant, and subsequently to the nitric acid spargers. The steam valve was completely open. The only limit to the amount of steam applied to the spargers was the ability of the steam system to deliver the steam through the one-inch diameter steam line. The temperature of the steam, with the addition of any superheat, created temperatures in and around the spargers higher than the maximum temperatures of concern provided by Terra personnel. Application of the steam in this manner would also allow any additional superheat generated by other demands on the steam system to reach the neutralizer. This probably occurred when the nitric acid plant compressor was turning in the "slow roll" phase of startup.
The investigation team determined that the explosion would not have occurred without the unrestricted application of 200 psig steam to the spargers.
3. Introduction of Low Density Zones (Bubbles).
Low density zones were created by the application of compressed air, then 200 psig steam through the nitric acid spargers. Low density zones in the AN solution:
a) Reduce the activation energy necessary to initiate thermal decomposition of the AN;
b) Lower thermal inertia in these zones allowing local heating; and
c) Increase propagation of resulting pressure waves through adiabatic compression of the bubbles.
Low density zones were created in the neutralizer for a period of 15 hours before the explosion. Terra did not have written operating procedures for introduction of air or steam into the neutralizer.
4. Lack of Flow in Vessels.
The lack of flow in the neutralizer and from the neutralizer into the rundown tank isolated sensors critical for monitoring the AN solution. The lack of flow also contributed to thermal and contaminant stratification within the vessels and the significance of local events and conditions, such as heating and diffusion. This allowed for local conditions to develop without warning to the operator and without being diffused throughout the AN mass. This condition existed in the neutralizer for a period of 15 hours before the explosion.
Flow in the neutralizer and rundown tank was possible using one of the product pumps and a recirculation line from the pump to the neutralizer. Terra did not have procedures or guidelines for limiting the amount of time that the "no flow" condition could exist before additional monitoring or procedures were initiated.
5. Contamination.
Analytical data proved that chlorides were present in the nitric acid and ammonium nitrate. The presence of chlorides in ammonium nitrate also decrease the activation energy necessary to initiate thermal decomposition, and react synergistically with many metals to further decrease this activation energy. Terra did not monitor the nitric acid for chlorides.
6. Process Monitoring.
The lack of periodic monitoring of the AN plant when it was shut down with AN in the neutralizer and rundown tank allowed otherwise detectable conditions to exist from the time the plant was shut down until the explosion. Terra did not have operating procedures that required periodic parameter monitoring when the plant was shut down with vessels charged.