Production and storage of fertilizers
As a substance, ammonium nitrate has a long history. (It was first produced in 1659). It is a “dual-use” substance from which either fertilizers or explosives can be produced. It is produced at a large scale throughout the world (over 20 million tonneIs in 1998) with over a third of this production based in Europe (over 7 million tonnes in 1998). It is without doubt important for western society. It is an easily absorbed and efficient source of nitrogen for plants and particularly suitable to growth conditions of the European climate. Its efficient absorption rate means that it is relatively friendly to the environment relative to other manufactured fertilizers; the amount of nitrogen lost to the atmosphere is normally low.
History of accidents involving ammonium nitrate fertilizers
Ammonium nitrate caused a few of the most catastrophic events of the 20th century in peaceful times. The two most notorious and disastrous accidents in the western world were the Oppau, Germany, accident from the detonation of 450 tonnes of sulfo-ammonium nitrate fertilizers in storage that killed 561 people. In 1947, in Texas City, Texas (USA) a ship carrying 2600 t of ammonium nitrate exploded and set fire to a nearby
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vessel hold 960 tonnes of ammonium nitrate. 581 people in total were killed. Detailed descriptions and lessons learned from these accidents can be found from many sources. Several books have been written that cover these disasters and there is also substantial information on these and other ammonium nitrate accidents available in open sources online.
Still since then, ammonium nitrate has been involved in numerous accidents causing explosions, fires, and releasing toxic fumes. It has been recognized in many countries that even small storages of ammonium nitrate fertilizers, defined as low as 10 tonnes in some legislation, may place the population at high risk if proper safety measures and procedures are not fully in place (http://ipsc.jrc.ec.europa.eu/fileadmin/repository/sta/mahb/docs /SpecialRegulatoryTopics/Ammonium_nitrate_safety.pdf). The chart below depicts the fatalities and tonnage associated with AN accidents identified by this study from 1916 until present. |
Accident 2
Production and storage of fertilizers
Sequence of events An explosion occurred in an NP buffer in the neutralization process of the production activity. Production in the fertilizer plant had been stopped due to maintenance work in the ammonia storage area, and as a result, there could be no supply of ammonia to the plant. Just prior to the explosion, an automatic fire detector, directly connected to the control room of the local emergency preparedness unit and the plant, went off. In addition, gas was observed by the operators in the factory and the building was evacuated with staff directed to the designated meeting points. Shortly after the evacuation, the explosion took place. The pressure from the explosion caused window damage in the meeting place area and 5 operators were injured due to glass fragments. The explosion caused a fire in the third floor of the building. The fire was extinguished after a little over an hour.
Causes The cause of the accident was identified as decomposition of ammonium nitrate in the NP buffer tank due to high temperature and low pH in the tank. These conditions resulted in the formation of a large amount of gas leading to rupture of the tank from overpressure. The overheating was the result of a leaking steam valve on the 20 bar steam supply to the tank. The NP buffer tank was the last unit before the liquor was pumped to the evaporation and prilling section for making the final product prills. The off gas from the tank is connected to the recovery system for ammonia. In this process, the addition of ammonia neutralizes the acidic liquor from the process immediately prior. The ammonia flow was controlled by an online automatic pH measurement, located at the 25% level of the tank. In addition, ammonium nitrate is added to obtain the correct ratio between N and P in the final product.
Important findings • The NP buffer tank had no instrumented safety functions, but a high temperature alarm on 145 ° C was installed. Also, there was a high and low alarm on the automatic pH measurement and a high alarm on the online chlorine analyser. • No hazards had been associated with the NP buffer tank in the Hazop study or the risk analysis. • Two evenings before the accident a high alarm for temperature went off. It was acknowledged and dismissed without investigation. • The evening of the day before the accident, the temperature continued to register on the high side, but since pH was high and the steam valves were shut, it was assumed that the temperature measurement was wrong.
Lessons Learned • Hazard identification should have drawn attention to the elevated risk associated with the presence of ammonium nitrate in a process tank while the process was idle. Safety procedures and controls for process equipment are usually designed to manage risks when the process is running and cannot be automatically assumed to be capable of also controlling substances safely in abnormal situations. • Hazard identification should pay particular attention to the sensitivity of ammonium nitrate to changes in operating conditions. As such it should also take into account the plant life cycle and unintended events that could adversely affect these conditions in order to establish appropriate safety controls and procedure for these situations. • The installation of appropriate instrumented safety functions are a typical control measure that could assist the operator in limiting consequences from unexpected ammonium nitrate reactions under a wide range of conditions. • Alarm management is a common challenge at many processing plants where there are numerous processes with numerous alarm for each covering a wide range of functions. The failure to respond to the high temperature alarm suggests that the company did not have an adequate system for prioritising alarms to ensure an appropriate and timely response to emergencies. In addition, employee trainee should also instill a heightened awareness in opera tions staff to nonconformities, negative indicators, and pre-emergency alerts during shutdown periods.
[eMARS accident #694]
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Accident 3
General chemicals manufacture
Sequence of events Self-decomposition of NPK fertilizers led to a fire in a storage silo and release of toxic substances, mainly nitrogen oxides. The silo contained approximately 15,000 tonnes of the product, but the fire was detected early enough (probably from the fumes rather than automatic detection) to avoid serious consequences. Five firefighters were treated for minor injuries in hospital and some onsite personnel suffered from eye and throat irritation and burning. Some neighbouring establishments and houses were evacuated and other areas were told to shelter-in-place for some time (duration not specified), In the end, no offsite injuries were reported. The fire was controlled after most of the material was removed by mechanical means.
Causes It was thought that exposure to moisture had caused caking in a portion of the product. In addition, the product may have been in contact with organic material, specifically, pigeon excrement, due to the considerable quantity of pigeons present in the silos. Over a two-month period, some of the product involved in the accident had been exposed to ambient conditions during a period in which the region experienced a lot of rain. Self-sustaining decomposition caused by the presence of a contaminant would have likely been accelerated by the presence of anomalous crystal structures (caking) in the product.
Important findings • A portion of the product had been exposed to ambient temperatures in a period when the region was experiencing a lot of rain. Leaks in the silo roof caused water to fall on the exposed lot, generating a possible recrystallization or caking of the fertilizer. • No documentation available at the installation reflected the possibility that such an accident could take place. • A large amount of the NPK was stored in the same location without proper separation. This practice was actually counter to company practice regarding storage conditions.
Lessons Learned • Storage facilities should strive to eliminate the possibility that impurities are introduced into the ammonium nitrate. Preventive measures should be in place to prohibit birds and animals from contact with the product or, if this is not possible, ammonium nitrate should not be stored in that facility • In storage of ammonium nitrate compounds, exposure to water should be avoided in order to prevent caking. A nonconformity in the fertilizer structure, such as caking, can accelerate oxidisation Facilities should be appropriately constructed and maintained to avoid leaks, flooding, or formation of pockets of moisture in areas where the ammonium nitrate is located. • Employees should be regularly trained and tested on critical safety procedures and periodic monitoring should take place to ensure that procedures have been followed. • The follow-up investigation also recommended that temperature monitors should be installed in each storage silo.
[eMARS accident #263]
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