Operations to chemically change the nature of materials have often been the source of major catastrophes, causing severe plant damage and death and injury to workers and surrounding communities. Risks to life and property from incidents in chemical process plants may come from fires, explosions or toxic chemical releases. The energy of destruction often comes from uncontrolled chemical reaction of process materials, combustion of fuels leading to pressure waves or high levels of radiation and flying missiles that can cause damage at large distances.
Plant operations and equipment
The first stage of design is to understand the chemical processes involved and their potential for energy release. Lees (1980) in his Loss Prevention in the Process Industries sets out in detail the steps required to be undertaken, which include:
· proper process design
· study of failure mechanisms and reliability
· hazard identification and safety audits
· hazard assessment—cause/consequences.
The assessment of the degrees of hazard must examine:
· potential emission and dispersal of chemicals, particularly toxic and contaminating substances
· effects of fire radiation and dispersal of combustion products
· results of explosions, particularly pressure shock waves that can destroy other plants and buildings.
More details of process hazards and their control are given in Plant guidelines for technical management of chemical process safety (AIChE 1993); Sax’s Dangerous Properties of Industrial Materials (Lewis 1979); and the NFPA’s Industrial Fire Hazards Handbook (Linville 1990).
Siting and exposure protection
Once the hazards and consequences of fire, explosion and toxic releases have been identified, siting of chemical process plants can be undertaken. Again, Lees (1980) and Bradford (1991) provided guidelines on plant siting. Plants must be separated from surrounding communities sufficiently to ensure that those communities cannot be affected by an industrial accident. The technique of quantitative risk assessment (QRA) to determine separation distances is widely used and legislated for in the design of chemical process plants. The disaster in Bhopal, India, in 1984 demonstrated the consequences of locating a chemical plant too close to a community: over 1,000 people were killed by toxic chemicals in an industrial accident.
Provision of separating space around chemical plants also allows ready access for fire-fighting from all sides, regardless of wind direction. Chemical plants must provide exposure protection in the form of explosion-resistant control rooms, worker refuges and fire-fighting equipment to ensure that workers are protected and that effective fire-fighting can be undertaken after an incident.
Spill control
Spills of flammable or hazardous materials should be kept small by appropriate process design, fail-safe valves and appropriate detection/control equipment. However, if large spills occur, they should be confined to areas surrounded by walls, sometimes of earth, where they can burn harmlessly if ignited. Fires in drainage systems are common, and special attention must be paid to drains and sewerage systems.
Heat transfer hazards
Equipment that transfers heat from a hot fluid to a cooler one can be a source of fire in chemical plants. Excessive localized temperatures can cause decomposition and burn out of many materials. This may sometimes cause rupture of the heat-transfer equipment and transfer of one fluid into another, causing an unwanted violent reaction. High levels of inspection and maintenance, including cleaning of heat transfer equipment, is essential to safe operation.
Reactors
Reactors are the vessels in which the desired chemical processes are undertaken. They can be of a continuous or batch type but require special design attention. Vessels must be designed to withstand pressures that might result from explosions or uncontrolled reactions or alternatively must be provided with appropriate pressure-relief devices and sometimes emergency venting.
Safety measures for chemical reactors include:
· appropriate instrumentation and controls to detect potential incidents, including redundant circuitry
· high quality cleaning, inspection and maintenance of the equipment and the safety controls
· adequate training of operators in control and emergency response
· appropriate fire suppression equipment and fire-fighting personnel.