Process Safety Considerations in Fertiliser Production Plant Engineering
Process safety is an integral element of the overall safety of fertiliser production plants. The goal of process safety management (PSM) is to prevent production or operational major accidents, equipment damage, production interruptions, and harmful emissions that can result from production or operational failures.
Although process safety accidents may be less frequent, their consequences are, at worst, extremely serious and costly, resulting in significant harm to people, the environment, property, and the company’s reputation. This article presents some of the main process safety considerations that must be considered in fertiliser plant engineering.
Hazardous Chemicals and Associated Risks in Fertiliser Production
Fertiliser production involves many hazardous chemicals such as nitric acid, ammonia, sulphuric acid, phosphoric acid, urea, and heavy metals. For example, ammonia is colourless, with a very pungent odour, and a highly irritating gas at room temperature. It is a toxic and flammable gas which can ignite and explode when mixed with air or oxygen.
Fertiliser product ammonium nitrate itself is not a combustible substance, but as an oxidiser, it accelerates the burning of other substances and therefore contact with organic materials must be prevented. Ammonium nitrate is considered safe when it is uncontaminated and properly stored. If it is absorbed into, for example, wood, such as transport pallets, their ignition risk increases substantially. Dangerous reactions may also occur with strong bases and acids, nitrites, galvanised metal and metal powders, and reducing agents. Ammonium nitrate with high concentration can be detonated by shock, friction, or heat.
Exposure to large amounts of fertilisers without adequate Personal Protective Equipment (PPE) can lead to an individual’s acute toxicity. While smaller quantities of fertilisers are irritating or mildly poisonous, larger amounts can be harmful. The toxic chemicals in fertilisers, such as nitrates and nitrites, can lead to symptoms like throat burning, dizziness, diarrhea, nausea, and vomiting.
Furthermore, the production of fertilisers and potential leakages of chemicals poses a risk due to their potential environmental impacts, leading to issues like water pollution, soil acidification, and increased greenhouse gases. It also uses up significant resources, including water, energy, and materials, resulting in solid, liquid, and gaseous wastes.
Prevention and mitigation methods in engineering start from inherent safety and optimal layout design
Inherently safe design principles, such as elimination, minimisation, or separation of hazards, are the basis for the engineering of a fertiliser production plant. The design of simpler units is inherently safer than complex ones as there are fewer opportunities for failures, and units normally contain less equipment or pipelines that can leak. But there are elements that needs to be taken into account regardless of the size of the unit. For example, ammonia synthesis reactors involve high pressure and temperature conditions and the risk of runaway reactions and require prompt design and operation planning.
Effective equipment layout design ensures optimal layout and spacing to enhance safety. Consequence analyses and modelling are an efficient way to demonstrate the impact of potentially hazardous scenarios. Modelled scenarios are chosen based on the worst credible case principle, derived from scenarios identified during risk assessments. Typical examples of the scenarios modelled are:
- pool or jet fires
- gas or dust explosions
- dispersion of toxic chemicals
In engineering projects, various risk assessment methods, such as Hazard and Operability Study (HAZOP) and Layers of Protection Analysis (LOPA), are employed to identify and assess process safety-related hazards and potential loss of containment (LOC) events. These methods help to mitigate risks, identify independent protection layers (IPLs) and ensure the safe and efficient operation of fertiliser production plants. They also provide essential information for layout design and engineering.
Site Selection and Hazard Mitigation
The fertiliser plant must be located so that possible accidents causing heat radiation, overpressure or dispersion of hazardous gases at the site will not cause serious damage to off-site buildings, equipment, structures or harm to people. Possible hazardous events are not allowed to prevent people from being evacuated from the impacted area. Hazardous effects of accidents on adjacent facilities (domino effects), units or functions at the same facility must also be considered.
In the engineering phase, processing areas and storage are located separately from facilities or areas where people are working on tasks other than those directly related to this activity. Incompatible chemicals are stored separately so that they cannot cause additional hazards in the event of an accident or process failure. For example, it is very important to ensure safe storage of ammonium nitrate in ventilated, cool, and dry places separately from the sources of ignition and incompatible materials. Therefore, the results of consequence analyses shall be considered in land use planning, engineering and plant layout.
Material selection should consider choosing corrosion-resistant materials for construction and selection of appropriate containers, piping, and valves that can withstand the pressure and temperature of chemicals, ammonia and nitrate-based fertilisers to limit corrosion. This reduces maintenance requirements and minimises the risk of containment failures. Production and storage facilities shall be designed with non-combustible and chemical-resistant flooring material.
Advanced Monitoring and Control Systems Help to Enhance the Safety
It is also important to engineer and implement remote monitoring and efficient control systems, like basic process control system (BPCS) and safety instrumented system (SIS), to ensure that production processes are within safe limits and tolerable risk level is achieved.
The fertiliser plant processing and storage facilities shall be equipped with separate ventilation systems. The goal is to prevent the gases, vapours and dust released from hazardous chemicals from spreading from one space to another or outside them, such as personnel rooms. Also, other controls, such as fume hoods and scrubbers should be used to reduce the emission and exposure of harmful gases and vapours. The control and monitoring of the quality and moisture content of fertiliser products is essential because they can affect their stability and reactivity.
Key Considerations for Process Safety
In conclusion, ensuring the safe and efficient operation of fertiliser production plants requires careful consideration of several process safety key factors. These include the following 5 process safety considerations:
- Inherently safe design principles, such as elimination, minimisation or separation of hazards, are the basis for the engineering of a fertiliser production plant.
- Comprehensive risk assessments and consequence analysis methods to identify and assess process safety hazards and support land use planning and overall engineering.
- Effective equipment layout design ensures suitable layout and spacing to enhance safety.
- Material selection considering corrosion-resistant materials for construction and selection of appropriate containers, piping, and valves that can withstand the pressure and temperature of chemicals.
Deployment of remote monitoring and efficient control systems to ensure that production processes are within safe limits.
This article was originally published in the World Fertilizer Magazine in October 2024. Access the publication here.
