Chemical and process hazards: an overview

Many plants use chemicals that can burn, explode, poison or corrode: refrigerants such as ammonia, cleaning and sanitizing chemicals, acids and caustics, solvents, compressed gases, fuels, resins and the materials being processed. Some plants run processes with large inventories of highly hazardous chemicals at high temperatures and pressures. When those processes fail, the result can be a major accident that kills workers and harms people far outside the fence. This lesson gives an overview of chemical and process hazards so that everyone in a plant understands what is at stake. HSE-311 covers process safety management, HSE-312 covers chemical handling and storage, HSE-313 covers combustible dust, and HSE-102 covers hazard communication.

Personal safety and process safety

Occupational or personal safety deals with hazards to individual workers: slips, cuts, strains, falls and contact with machinery. Process safety deals with the prevention of fires, explosions and toxic releases from the loss of containment of hazardous materials or energy. The two need different measures. A plant can have an excellent personal injury record and still be heading for a major accident, because the indicators it watches, such as lost-time injuries, say little about the condition of its relief valves, alarms, inspections and operating procedures. The 2005 explosion at the BP Texas City refinery, which killed 15 workers, happened at a site that had been proud of its low injury rate.

Lessons from major accidents

Major accidents are rare in any one plant, so most people never experience one. That is why industries study them closely. The same causes appear again and again: hazards that were not understood, safeguards that had been bypassed or allowed to deteriorate, changes made without review, alarms ignored or overwhelmed, and warning signs that were missed or normalized. HSE-311 examines these cases in depth; the short summaries below give the flavor.

Common chemical hazards in ordinary plants

You do not need to work in a refinery to face serious chemical hazards. Ordinary plants have plenty of them, and some of the most common are easy to underestimate because they are familiar.

Ammonia refrigeration

Anhydrous ammonia is the refrigerant in many food plants, cold storage warehouses and arenas, because it is efficient and has little effect on climate. It is also toxic and corrosive. A large leak can produce a cloud that injures people inside and outside the plant. Most people can smell ammonia at concentrations well below dangerous levels, which gives some warning, but the smell also causes panic. Ammonia systems need trained operators, leak detection, ventilation, emergency shut-offs, relief systems discharging to safe locations, and an emergency response plan coordinated with the local fire department.

In the United States, ammonia refrigeration systems with more than 10,000 lb of ammonia are covered by OSHA's process safety management standard (1910.119) and EPA's risk management program rule. Smaller systems are covered by the General Duty Clause and industry standards from the International Institute of All-Natural Refrigeration (IIAR), and OSHA has run regional emphasis programs on ammonia refrigeration.

How hazardous a release could be

Process safety specialists estimate how far a release could spread and at what concentration, to plan emergency response and site buildings. The full methods are complex, but a simple calculation shows why even a modest leak inside a closed room can quickly reach dangerous levels.

# How quickly could an ammonia leak reach a dangerous level in a closed room? (simplified, illustrative)
# Assumes perfect mixing and no ventilation, at about 20 C and 1 atm.
room_m3 = 15 * 10 * 5           # a 750 m3 machinery room
leak_g_per_min = 300            # a small leak at a valve packing
mol_mass = 17.03                # g/mol for ammonia
molar_volume = 24.1             # L per mol at 20 C
def ppm_after(minutes):
    gas_l = leak_g_per_min * minutes / mol_mass * molar_volume
    return gas_l / (room_m3 * 1000) * 1e6
for t in (5, 15, 30, 60):
    print(f"After {t:>2} min: about {ppm_after(t):6.0f} ppm")
print("For comparison: smell threshold about 5 ppm; IDLH 300 ppm (NIOSH)")

After  5 min: about   2830 ppm
After 15 min: about   8491 ppm
After 30 min: about  16982 ppm
After 60 min: about  33964 ppm
For comparison: smell threshold about 5 ppm; IDLH 300 ppm (NIOSH)

Layers of protection

Process safety relies on several independent layers of protection, so that if one fails another stops the accident. The layers start with inherently safer design, such as using less of a hazardous material or a less hazardous one, and continue through the basic control system, alarms and operator response, safety instrumented systems that shut the process down automatically, relief devices, physical containment such as dikes, and finally emergency response. The image of several slices of Swiss cheese, each with holes, is often used: an accident happens only when the holes in every layer line up. Each layer must be maintained and tested, because a layer that has quietly failed gives no protection.

# Independent layers multiply: how often might an initiating event become a release? (illustrative)
initiating_per_year = 0.1          # e.g. a control valve failure once in 10 years
layers = {"alarm and operator response": 0.1,
          "safety instrumented shutdown": 0.01,
          "relief valve": 0.01}
f = initiating_per_year
for name, pfd in layers.items():
    f *= pfd
    print(f"after {name:<30} {f:.1e} per year")
print(f"About once in {1 / f:,.0f} years, if every layer is independent and maintained")
print("If the relief valve is out of service (probability 1), that becomes once in", f"{1 / (f / 0.01):,.0f} years")

after alarm and operator response    1.0e-02 per year
after safety instrumented shutdown   1.0e-04 per year
after relief valve                   1.0e-06 per year
About once in 1,000,000 years, if every layer is independent and maintained
If the relief valve is out of service (probability 1), that becomes once in 10,000 years

Management of change

Many major accidents followed changes that seemed harmless: a substituted chemical, a new bypass line, a modified procedure, a change in staffing, a temporary repair that became permanent. Management of change (MOC) is the process of reviewing changes for their safety effects before they are made, updating drawings, procedures and training, and checking the change before start-up. In ordinary plants, MOC applies to changes in equipment, chemicals, processes, procedures, software and organization, not just to large projects. HSE-103 introduced MOC; HSE-311 covers it for process safety.

Carbon monoxide from fuel-burning equipment

Carbon monoxide is one of the most common toxic gas hazards in ordinary plants and one of the easiest to miss, because it has no color or smell. Propane forklifts, gas-fired heaters, furnaces and generators produce it, and it can build up in closed buildings in winter or in trailers being loaded by forklifts. Controls include maintenance and emission testing of engines, ventilation, carbon monoxide detectors, electric forklifts where possible, and training to recognize headaches, dizziness and nausea as warning signs.