Process safety is absolutely critical in the chemical process industry. At an industrial scale, processes yield more product, but also have inherent risk. There is a risk that the process can become unsafe and become uncontainable. Depending on the process, there may be different safety measures in place to minimize the risk and danger associated with that process. Layers and regulations are put in place to implement safety measures in all processes so that any process can be safe. Provided that these layers and regulations are followed by using good safety practices, the industrial process should be able to be contained and produced safely. This module covers several basic topics relating to process safety, including methods for occupational hazard control, layers of protection, and regulatory symbols and documentation.
The other sections in the safety section of the encyclopedia include a general introduction to process control, and more specifically reaction control, pressure control, temperature control, and the prevention of fires and explosions.
Occupational Hazard Control
The goal of occupational hazard controls is to provide protection to workers at every level against hazards and injury. The standard hierarchy of hazard controls established by the National Institute for Occupational Safety and Health, NIOSH, is shown below and is applicable to every industry. This chart is designed to be read from the top down, in which higher-level controls are more effective and recommended over lower-level ones. A brief overview of controls at each level follows.
Elimination
Elimination is the process of removing a hazard from a system. For example, leaving flammable substances in the open is a safety risk. Instead, they should be properly stored in the appropriate flammable container. Inherently safer design is a popular strategy based on the premise that something cannot pose a hazard if it’s not present in a system. For more information about inherently safer design, see the Reaction Control module.
Substitution
Risky behavior or process can be replaced by one that results in less risk. A caustic or flammable chemical in a process could present a safety risk. A less hazardous chemical that is capable of accomplishing the desired task should be considered over one that is more hazardous.
Engineering Controls
Measures should be taken in the design of a building or process to eliminate risk. Engineering controls are typically some of the most varied as they are the first line of defense that does not involve removing the hazard. Thus, they should only be considered if the hazard cannot be removed altogether.
To enclose a hazard, an area may be kept at slightly negative pressure, for example. Equipment designed for handling flammable materials should also be designed to eliminate the risk of static buildup that can lead to ignition. A vessel to store a volatile liquid should contain vents to prevent a boiling liquid from expanding vapor explosion (BLEVE). Process controls can be used in conjunction with a computer system to slow or halt a chemical process if the temperature of a reactant is nearing its flashpoint. Other examples of engineering and administrative controls are included in the layers of protection section below.
Administrative Controls
Administrators have the power to change workplace procedures if they pose a risk. Implementing an extensive maintenance and safety training program can help increase employee awareness of risky behavior and ensure that all equipment is handled with care. It is also important to follow and promote governmental regulations and safety.
Personal Protective Equipment (PPE)
Personal protective equipment is required whenever a hazard is present and otherwise unavoidable. It is the last line of defense and shouldn’t be considered as a replacement for removing the hazard by the methods above Laboratory coats, safety glasses, and gloves are common PPE in a laboratory environment. Hearing protection, such as earplugs or earmuffs, is required in plants where noise levels are typically over 85 decibels, the threshold at which hearing damage can occur with chronic exposure. The exact PPE required depends on the nature of the hazard present; examples of PPE configurations can be found later in this article under the Hazardous Materials Identification System subsection.
Layers of Protection
Alongside the hierarchy of controls, there are layers of protection. While the hierarchy of controls represents actions that can be taken to protect workers, the layers of protection refer to the way that an established process can be safely maintained and disasters can be mitigated. The layers of protection exist within the engineering controls and administrative controls sections of the hierarchy of controls. The layers of protection are comprised of prevention layers and mitigation layers, as described below.
Prevention Layer: Basic Process Control System
The basic process control system for a process typically contains a feedback control loop that will monitor and automatically shift system parameters to maintain safe operating conditions. This is done through the use of a sensor transmitter, a controller, and a final control element. See the process control fundamentals module, as well as the pressure control and temperature control modules for more information on process control and feedback control loops.
Prevention Layer: Operator Intervention
The next layer of prevention is operator intervention. This layer refers to the opportunity of the operator to manually change system parameters before an emergency situation arises.
Prevention Layer: Safety Instrumented Systems (SIS)
Safety instrumented systems (SIS) provide additional protection if a control loop were to fail. SIS contain essentially the same components as a feedback control loop; however, they do not continuously adjust system parameters. An SIS only alters its control element when the controller determines that the process is beyond safe operating conditions. The SIS will then activate its control element to provide relief to the system. It operates independently from the feedback control loop to avoid potential interference. At this point, the emergency shutdown of the process will occur.
Mitigation Layer: Active Protection
Once a system has surpassed safe operating conditions and the prevention layers have failed, mitigation steps aim to reduce damage. Active protection includes process equipment used to relieve the system, such as relief valves and rupture discs. This equipment expels the system contents once a set pressure has been reached to lower system pressure.
Mitigation Layer: Passive Protection
Passive protection refers to equipment that does not directly affect system parameters but works to contain the process and potential disasters. An example of passive fire protection is fire-resistant walls that prevent a fire from spreading. Passive protection can also be in the form of secondary containment equipment that treats relieved process fluid for disposal. This includes flare stacks, quench pools, and absorbers.
Mitigation Layer: Emergency Response
If all prevention and mitigation steps fail, the final layer of protection is the emergency response layer. This layer refers to the actions taken when the process can no longer be maintained and steps are taken to minimize damage and environmental harm. Actions may include neutralizing released chemicals, evacuating, or calling the fire department.
Regulatory Symbols and Documentation
The physical and chemical properties of materials used in industry are carefully documented to inform and ensure the safety of workers who handle them. Common forms of documentation include safety data sheets and regulatory labeling.
The OSHA Hazard Communication Standard (HazCom) contains the following requirements: “Employers that ‘use’ hazardous chemicals must have a program to ensure the [safety] information is provided to exposed employees. ‘Use’ means to package, handle, react, or transfer.” The Hazard Communication Standard is designed to inform and protect any worker who will potentially be exposed to harmful chemicals.
Regulatory labels are designed to meet the requirements outlined in the HazCom standard. The information contained in a regulatory label provides quick, at-a-glance overviews of what types of hazards a material could present. Employee safety programs are implemented to meet this standard. Some of the most popular labels used to comply with the hazard communication standard are the NFPA 704, Hazardous Materials Identification System (HMIS), and the Global Harmonized System (GHS).
NFPA 704 standard
The NFPA 704 is a standard designed by the National Fire Protection Association for first responders to understand the severity of hazards presented by chemicals. Regardless, it has become a popular general-purpose safety label. The NFPA 704 standard defines the “Fire Diamond,” used ubiquitously in the U.S. industry to denote the risk factors of a substance. It appears on various containers as a large diamond subdivided into four smaller, colored diamonds, as shown below.
Though the NFPA 704 standard was originally designed for first responders to understand the severity of hazard a material presents, many industries have adopted the symbol as part of employee safety programs. The top three regions each contain a number from 0 to 4, with higher numbers denoting higher levels of risk. The blue region denotes health risks, red indicates a substance’s flashpoint, and yellow denotes how suddenly or violently a substance may react, indicating a potential explosion hazard. Different symbols may appear in the white region to denote a substance’s special hazards. Officially defined in the NFPA 704 standard are the symbols “OX,” which denotes a substance as an oxidizer, “SA” to indicate simple asphyxiants, and “ W ” to denote that the substance is reactive with water. Other symbols may appear in this area to denote other hazards. This information can be critical to firefighters and other first responders.
The following table denotes the meaning of various NFPA hazard levels:
| Hazard Level | Health | Flammability | NFPA hazard |
| 0 | No health hazard | Does not burn | Stable even when exposed to fire |
| 1 | Causes minor irritation, little to no residual effects | Flashpoint greater than 200°F | Unstable at high temperature or pressure |
| 2 | Acute exposure may incapacitate or cause residual injury | Flashpoint 100°F to 200 °F | Reacts violently at high temperature or pressure, or reacts violently with water |
| 3 | Acute exposure may cause serious harm and lasting residual effects | Flashpoint 73 °F to 100°F | Explodes when exposed to a strong initiator |
| 4 | Acute exposure may cause death or long-lasting residual effects | Flashpoint lower than 73°F (below room temperature) | Readily explodes under normal conditions |
Hazardous Materials Identification System (HMIS)
The Hazardous Materials Identification System (HMIS) is another label used to denote the hazards of chemicals. Like the NFPA 704, HMIS ranks each category 0-4, where 4 is the most severe. The health and flammability ratings share nearly identical definitions with NFPA 704. However, the orange physical hazard category is distinct from the analogous instability category in NFPA 704.
The following table denotes the meaning of the HMIS hazard levels:
| Hazard level | Health | Flammability | Physical Hazard |
| 0 | No health hazard | Does not burn | Stable even in a fire |
| 1 | Causes minor irritation, little to no residual effects | Flashpoint greater than 200°F | Stable except at high temperature or pressure. Reacts slowly with water. Polymerizes in the absence of inhibitors |
| 2 | Acute exposure may cause temporary or minor injury | Liquids with flashpoints of 100°F to 200 °F | Unstable at normal temperature and pressure. Reacts violently to water. May form peroxides in air |
| 3 | Causes major injury unless given immediate medical attention | Flashpoint 73 °F to 100°F, low volatility liquids with flash points below 73°F | Capable of explosion with strong initiating source. Reacts explosively with water or undergoes other violent reactions including decomposition or polymerization |
| 4. | Acute exposure may cause death or long-lasting residual effects | Gases or volatile liquids with a flashpoint below 73°F (below room temperature) | Readily capable of explosive reaction with water, explosive reactions including decomposition or polymerization at normal temperature and pressure |
Additionally, the personal protection category describes the PPE requirements when handling the chemical with the letter A-K. Generally, letters later in the alphabet denote more strict requirements for PPE than earlier letters. The following list describes the PPE requirements under each letter:
| Letter | Face PPE Required | Body PPE Required | Hands/Feet PPE Required |
| A | Safety Glasses | ||
| B | Safety Glasses | Gloves | |
| C | Safety Glasses | Apron | Gloves |
| D | Face Shield | Apron | Gloves |
| E | Safety Glasses, Dust Respirator | Gloves | |
| F | Safety Glasses, Dust Respirator | Apron | Gloves |
| G | Safety Goggles, Vapor respirator | Gloves | |
| H | Splash Goggles, Vapor respirator | Apron | Gloves |
| I | Safety Glasses, Dust and vapor respirator | Gloves | |
| J | Splash Goggles, Dust and vapor | Apron | Gloves |
| K | Air Line Mask or hood | Full body Suit | Gloves, Boots |
Global Harmonized System (GHS)
Starting in 2012, OSHA mandated the use of GHS labels on containers used for transportation. In 2015, this standard was further expanded to include applicable labels on all containers which may expose workers. Unlike the systems mentioned previously, GHS does not directly denote the severity of a hazard. The regulatory symbols shown below denote the presence of explosives, flammable material, oxidizers, and compressed gas respectively. A full list of OSHA-required pictograms is available OSHA’s Pictogram Quick Card.
Shown below are the variations used internationally for the transportation of hazardous goods. These are given a color background and a distinct label to enhance visibility from a distance. Additionally, a number is used to indicate the category and subcategory of hazard. The transport pictograms are more descriptive than general pictograms in that each of the above classes in GHS pictograms is broken down into further subclasses, each with its own distinct label.
Other Regulatory Symbols
Other countries and organizations may have additional regulatory labels beyond what is mentioned in this module. In many cases, these will contain the standard GHS pictograms and will be plainly visible on a hazardous substance.
Safety Data Sheets
Safety Data Sheets (SDS) contain the most specific information regarding a substance’s hazardous properties and proper care. Facilities are required to keep SDSs for all hazardous chemicals on site. Further information regarding the current layout of safety data sheets can be found on OSHA’s SDS Quick Card.
Acknowledgments
- National Fire Protection Association (NFPA)
- Occupational Safety and Health Administration (OSHA)
- Center for Disease Control (CDC)
References
- “Frequently Asked Questions on NFPA 704 .” NFPA, NFPA, www.nfpa.org/Assets/files/AboutTheCodes/704/704_FAQs.pdf.
- “The MSDS Hyperglossary: HMIS.” The MSDS HyperGlossary: HMIS, ILPI, http://www.ilpi.com/msds/ref/hmis.html.
- “United States Department of Labor.” Hazard Communication | Occupational Safety and Health
Administration, OSHA, https://www.osha.gov/dsg/hazcom/global.html. - “CDC – Hierarchy of Controls – NIOSH Workplace Safety and Health Topic.” Centers for Disease
- Control and Prevention, Centers for Disease Control and Prevention, 13 Jan. 2015, https://www.cdc.gov/niosh/topics/hierarchy/default.html
Developers
- John Novak
- Alex White
- Austin Potter