Process hazards

Title: Process Hazards

Authors: Anne Disabato, Tim Hanrahan, Brian Merkle

Date Presented: February 9, 2014

Introduction

The design and production of chemical processes is inherently hazardous, which is why process safety is of paramount importance to every company working in the chemical, fuels, and pharmaceuticals industry. While “process safety” focuses on the prevention of dangerous situation throughout the design, “process hazards” focuses on how to manage the unavoidable hazards in the final design. In the case of fires, explosions, or the release of toxic chemicals, proper safety hazard analysis will help minimize injuries and damage to the facility and environment.

In addition to moral and ethical obligations to safety, law requires it and the costs (human, social, economic) of non-compliance can be catastrophic. Listed below are three major pieces of safety legislation:

1. The Occupational Safety and Health Act (OSHA); 29 U.S.C. 651 et seq. (1970)

• Employers must provide a place of employment free from recognized hazards to safety and health, such as exposure to toxic chemicals, excessive noise levels, mechanical dangers, heat or cold stress, or unsanitary conditions.

2. The Emergency Planning & Community Right-To-Know Act (EPCRA); 42 U.S.C. 11011 et seq. (1986)

• To help local communities protect public health, safety, and the environment from chemical hazards.

3. The Toxic Substances Control Act (TSCA); 15 U.S.C. s/s 2601 et seq. (1976)

• Allows EPA to track industrial chemicals and ban their manufacture or import

For safety organization and terminology, safe design tactics, and the economic cost of safety, please see Process Safety

Chemical Plant Hazards

The complex nature of chemical plants increases the number of hazards associated with operation and facility maintenance. Understanding the scope of (1) material and (2) process hazards is essential to safe design and operation. Below are examples of chemical plant hazards.

Material Hazards

Toxicity

Nearly every chemical plant is holding large quantities of various chemicals, which can be of serious concern for workers and local residents. Even chemicals with low toxicity can be deadly in the quantities used in manufacturing. Most exposure to high toxicity chemicals occurs from inhalation. Process design needs to consider the elimination or substitution of the most hazardous compounds, prevention of releases, containment, disposal, ventilation, and emergency procedures.

The following are important toxicity definitions

• Acute Effects- Symptoms that develop rapidly, usually as a result of short-term exposure. These effects can be a result of oral, dermal, gas, vapor, dust, or mist inhalation.
• Chronic Effects: Symptoms that develop over a long period of time, often as a result of long-term exposure. Example: Cancer
• LD50- Lethal dose at which 50% of test animals are killed. Indicates acute effects only, expressed in mg/kg body mass
• Threshold Limit Value (TLV) or Permissible Exposure Limit (PEL)- Concentration the average worker can safely be exposed to for 40 hr/week
  • PEL published by OSHA : http://www.osha.gov/SLTC/healthguidelines/
  • TLV published by American Conference of Government Industrial Hygienists. http://www.acgih.org/home.htm

Toxic Substance Control Act or TSCA (15 U.S.C. s/s 2601 et seq., 1976) is USEPA’s version of the Food and Drug Act. The TSCA allows EPA to regulate the 75,000 chemical substances used in industry (including confidential materials). Additionally, it requires extensive review before approval is given by USEPA to manufacture, import and sell a new chemical in the USA. Under TSCA, USEPA can ban or restrict the import, manufacture and use of any chemical, and anyone has the right and obligation to report information about new or alleged health/environmental effects caused by a chemical.

Flammability

Flammability is the measurement of how easily a material will burn or ignite, resulting in a fire or combustion. A fire requires three things: fuel, oxidant, and source of ignition (or auto-ignition). Possible sources of ignition at a chemical facility should be assessed and eliminated; this include electrical equipment such as motors or actuators, open flames from furnaces, incinerators or flare stacks, and undefined sources such as matches, lighter or mobile phones.

Important flammability related properties must be measured:

• Flash Point – function of vapor pressure; lowest temperature at which the material will ignite from an open flame
• Auto-ignition temperature- temperature at which the substance ignites in air spontaneously
• MSDS information
• Flammability limits- highest and lowest concentrations in air (NTP) at which a flame will propagate through the mixture
  • LFL (lower flammable limit): mixture of fuel and air below this is too lean
  • UFL (Upper flammable limit): mixture of fuel and air below this will not burn

The LFL and UFL of mixtures can be calculated using Le Chatelier’s Equation:

$FL_{m}={\frac {1}{\sum _{i=1}^{C}(y_{i}/FL_{i})}}$

where FL_i is the flammability limit of a specific component, and y_i is the concentration. While the LFL is relatively independent of pressure, the UFL changes at different pressures according to the following equation:

$UFL_{P}=UFL+20.6[log(P)+1]$

where P is in MPa and UFL is the upper flammability limit at 1 atmosphere.

Fire protection is best accomplished by containing flammable materials. Other tactics include:

• Inerting- an inert gas is added to reduce the oxygen concentration below the minimum oxygen concentration, MOC, at which explosions can occur
• Reducing static electricity- by installing ground devices or using antistatic additive to increase conductivity
• Explosion-proof equipment- designed to absorb shock after explosion and prevent the combustion from spreading.
• Flame arrestors- specified on vent lines of equipment that contains flammable materials to prevent a flame from propagating back from the vent
• Sprinkler systems

Incompatibility

When certain hazardous chemicals are stored or mixed together, violent reactions may occur because the chemicals are incompatible. Combination of interest include:

1. Acids and Bases
2. Acids and Metals
3. Fuels and Oxidants
4. Free radical initiators and Epoxides, Peroxides, or Unsaturates.

Chemical incompatibility can lead to runaway reactions, and material incompatibility can lead to corrosion of vessels, internals, and instruments as well as the softening of gaskets, seals, and linings.

Material Hazards Conclusions

Material hazards account for a wide variety of incidents. Six factors should be considered in design for material hazards: (1) substitution, (2) containment, (3) prevention of releases, (4) ventilation, (5) disposal, and (6) provision of emergency equipment. Consulting the Material Safety Data Sheets (MSDS) is also essential in accounting for hazards. Please see the MSDS section under “Process Hazard Analysis Tools.”

Process Hazards

Overpressure

Overpressure occurs when mass, moles or energy accumulates in a contained volume (or space with restricted outflow), and can be extremely dangerous. The rise in pressure is determined by the rate of accumulation. Process controls are one tool used to control process pressures, but in the case of overpressure, they may not be able to response quickly enough. If pressure is not released by a pressure safety value, a vessel could rupture or explode resulting in the loss of containment. Please see Process hydraulics and Pressure Vessels for help with creating an appropriate design. Pressure relief values and rupture disks should be installed on all pressure vessels.

Fires and Explosions

Chemical plant fires can quickly damage control systems and equipment, causing overpressure, loss of containment, and explosions. In addition to protecting expensive equipment, the safety and lives of workers, local residents, and the environment are put at risk if a fire starts. It is important to follow fire protection guidelines (NFPA 30, API RP 2001, API Publ 2218) and legal requirements set by OSHA (29 CFR 1910 L).

As mentioned in the “Flammability” section, the use of electrical equipment in chemical plants can ignite a fire. Electrical equipment use is regulated by law through OSHA standard 29 CFR 1910.307 and industry design codes, National Electrical Code NFPA 70 and NFPA standards 496,497, API RP 500, 505. These codes define equipment and installation regulations, and specific precautions that must be taken in risky areas.

An explosion is a worst-case result of a fire, defined as the sudden, catastrophic release of energy causing a pressure wave. The following are explosion related definitions:

• Deflagration- an exposition where combustion zone propagates at subsonic flame speed, usually < 30 m/s, and pressure wave < 10 bar. Deflagration can turn into detonation when propagating along a pipe.
• Detonation- an exposition where combustion zone propagates at supersonic velocity, 2000 – 3000 m/s, and pressure wave 20 bar. The principal heating mechanism is shock compression, and requires confinement of a high-intensity source
• Expansion factor- measure of the increase in volume resulting from combustion:

$E={\frac {molardensityofreagents}{molardensityofproducts}}$

where the maximum value of E is for adiabatic combustion.

• Flame speed- the rate of propagation of a flame front through a flammable mixture, with respect to a fixed observer

Explosivity properties can be found in textbooks such as An Introduction to Fire Dynamics by D. Dugdale, as shown below.

Figure 1: Explosivity Properties. Dugdale, D. An Introduction to Fire Dynamics. Wiley, New York: 1985.

Loss of Containment

Loss of containment can occur due to pressure relief, operator error, poor maintenance procedures, such as failure to drain and purge properly, or leaks from degraded equipment.

Containment is one of the six principles of “Inherently Safe Design,” laid out in the wiki page for Process safety. If hazardous materials cannot be eliminated, they should at least be stored in vessels with mechanical integrity beyond any reasonably expected temperature or pressure excursion. This is an old but effective strategy to avoid leaks. However, it is not as inherently safe as substitution, intensification, or attenuation.

Noise

Noise may not seem like a process hazard when compared to explosions, but can cause permanent damage to hearing. Compressors, turbines, motors, and solids handling can be very noisy, both within the plant and in the surrounding neighborhood.

Sound is measured in decibels, defined by the following equation:

$SoundLevel=20\log _{10}{\frac {RMSsoundpressure(inPa)}{0.00002}}$

The result is a sound level in dB, and it is advised to wear ear protection in areas over 80 dB, as permanent damage can be caused by noise over 85 dB.

Process Hazard Analysis Tools

Exposure Evaluation

At each stage of the process, every chemical, intermediate, and catalyst must be inventoried and analyzed. Risk can then be prioritized through a combination of chemical toxicity and exposure source (valve, pump, etc.). Exposure is highly variable, and should be measure over relatively large time period, longer than chemicals with long half-life. Finally, compliance with OSHA requires designer to make calculations of concentrations and exposure time of plant personnel during normal operation.