Process safety

From processdesign
Revision as of 10:30, 26 January 2015 by Jian (Talk | contribs)

Jump to: navigation, search

Authors: Anne Disabato, [2014] Tim Hanrahan, [2014] Brian Merkle, [2014] and Spencer Saldana [2015]

Stewards: David Chen, Jian Gong, and Fengqi You

Date Presented: January 19, 2014



The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals, and pharmaceuticals. Process safety focuses on the prevention of dangerous situations, such as fires, explosions, and the release of chemicals.

The American Institute of Chemical Engineers emphasizes a culture of process safety through four pillars[1]:

1. Commitment to Process Safety: a workforce that is actively involved and an organization that fully supports process safety as a core value will tend to do the right things in the right way at the right time – even when no one else is looking

2. Understanding Hazard and Risk: the foundation of a risk-based approach which will allow an organization to use this information to allocate limited resources in the most effective manner

3. Manage Risk: the ongoing execution of risk based process safety tasks. Risk management can help a company to better deal with the resultant risks and sustain long-term accident free and profitable operations

4. Learn from Experience: Metrics provide direct feedback on the workings of RBPS systems, and leading indicators provide early warning signals of ineffective process safety results. Organizations must use their mistakes and those of others as motivation for action and view as opportunities for improvement.

For the prevention and management of specific safety hazards, such as fires, explosions, or the release of toxic chemicals, please see Process Hazards.

Safety Organizations and Terminology



The Occupational Safety and Health Administration (OSHA) is a federal agency that focuses on the enforcement of safety and health legislation.


The Environmental Protection Agency (EPA) is a U.S. agency whose purpose is the protection of the health of both humans and the environment through the writing and enforcement of regulatory laws.


The Department of Transportation (DOT) oversees federal highway, air, and maritime transportation, and can be involved in the safe transport of chemicals.


The Department of Energy (DOE) is a governmental department tasked with the advancement of energy technology in the United States.



Health, Safety, and Environmental - This term refers to all health, safety, and environmental concerns that arise at each stage of the design process. Companies are required to analyze each part of the process from an HS&E perspective to create a safe and healthy work environment.


Material Safety and Data Sheet - Every chemical has an MSDS which contains all the information regarding safe handling and how to deal with spills or other accidents involving the substance. Relevant information includes how to identify the substance, hazard information, and how to handle spills, fires, and exposure, among other things.


Failure Mode and Effects Analysis - FMEA is an early stage approach to identifying critical technical risks using a semi-quantitative procedure. The analysis encompasses safety, environmental, and operational feasibility. When performing FMEA, engineers look to see places in a potential design that could fail, and then quantify how likely that failure is, how severe the results would be, and then offer potential solutions to minimize the risk. A step-by-step guide to performing FMEA is shown below[2]:

  1. Brainstorm for failure modes
  2. For each FM, rate severity of impact (SEV, 1 - 10).
  3. For each FM, brainstorm for possible causes (there may be multiple).
  4. For each cause, rate likelihood of occurring (OCC, 1 - 10).
  5. Rate the probability that the systems currently in place will detect and prevent the problem before it has an impact (DET, 10 - 1). Do not assume that something that will be added to the design later will take care of the problem.
  6. Overall Risk Probability Number RPN = SEV x OCC x DET. Most practitioners use the 1,4,7,10 scale below to increase granularity. Note that the DET scale is inverse to SEV and OCC.


Figure 1: Suggested scale to be used for quantifying risk and detection of failures in FMEA. Taken from ChE 351 Slides.

The sum of all information collected is implemented into a spreadsheet like the one shown below:


Figure 2: Example spreadsheet used to organize FMEA data.


Hazard and Operability Study - For more information regarding HAZOP, please refer to Process Hazards.


Safety Integrity Levels - The SIL is the relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. A SIL is determined based on a number of qualitative factors such as development process and safety life cycle management. Several methods are used such as risk matrices, risk graphs, layers of protective analysis (LOPA). Three levels of safety integrity are assigned depending on the “availability” of the safety instrumented system (SIS), as shown below[4]:


Figure 3: Table of safety integrity levels based on availability of system. Taken from [4].

Redundant system means instrumentation is duplicated; higher level of redundancy of trip systems give higher SIL. The required SIL should be determined during a process hazard analysis and depends on risk of operator exposure and injury.

1 Instrument

  • if it signals, plant goes down
  • Probability of incident = probability of instrument failure
  • Probability of spurious trip = false positive rate of one instrument

2 Instruments

  • 1 out of 2 voting (1oo2): one instrument signals, plant goes down
    • Probability of incident reduced by duplication
    • Probability of spurious trip doubled
  • 2oo2 voting
    • Probability of incident worse than single instrument (twice likelihood that system is down)
    • Probability of spurious trip reduced

3 Instruments

  • 2oo3 voting
    • Best overall trade-off between reducing incident rate and spurious trip rate
    • One malfunctioning instrument does not cause trip or prevent detection of real incident

Safe Design

Inherently Safe Design

Inherently safe design of a particular process can be achieved by adhering to the following six strategies put forth by Turton et al[3]:

1. Substitution: Avoid using or producing hazardous materials on the plant site. If the hazardous material is an intermediate product, for example, alternate chemical reaction pathways might be used. In other words, the most inherently safe strategy is to avoid the use of hazardous materials.

2. Intensification: Attempt to use less of the hazardous materials. In terms of a hazardous intermediate, the two processes could be more closely coupled, reducing or eliminating the amount of intermediate produced. The inventories of hazardous feeds or products can be reduced by enhanced scheduling techniques such as just-in-time (JIT) manufacturing.

3. Attenuation: Reducing, or attenuating, the hazards of materials can often be affected by lowering the temperature or adding stabilizing additives. By using materials under less hazardous conditions, the potential consequences of a leak can be reduced.

4. Containment: If the hazardous materials cannot be eliminated, they at least should 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.

5. Control: If a leak of hazardous material does occur, there should be safety systems that reduce the effects. For example, chemical facilities often have emergency isolation of the site from the normal storm sewers, and large tanks for flammable liquids are surrounded by dikes that prevent any leaks from spreading to to other areas of the plant. Scrubbing systems and relief systems in general are in this category. They are essential, because they allow controlled, safe release of hazardous materials, rather than an uncontrolled release from a vessel rupture.

6. Survival: If leaks of hazardous materials do occur and they are not contained or controlled, the personnel (and the equipment) must be protected. This lowest level of the hierarchy includes fire fighting, gas masks, and so on. Although essential to the total safety of the plant, the greater the reliance on survival of leaks rather than elimination of leaks, the less inherently safe the facility.

Assessing Preliminary Design

While pilot plants are necessary to design effective plant equipment, there are some dangers associated with the industrial scale of chemical plants. Scaling up without accurate literature and experimental data can be very dangerous.

In every step of process design, the Health, Safety, and Environmental (HS&E) analysis must be carried out with the available technical information[5].



Figure 4: General steps in the design process and analysis to be carried out at each stage. Taken from [5].

Economic Cost of Safety

Price of Safety installations and fire protection systems range from .5 to 1.6 % of fixed-capital investment of a plant; but expenditures are often much higher than this and it is difficult to estimate these expenditures for a given plant

In addition, designers also examine the economic impact of safety and maintenance issues. For instance, they may determine that the plant reactor configuration can be improved, and with improved operator training facilities, it can run with improved safety. These hidden costs must be considered when determining the economic feasibility of operation[5].

Other Process Safety Considerations

The safety and well-being of the consumers using the eventual product should be considered in process safety. The risks involved in using a product should be clearly communicated to the consumer by industrial leaders.

Human Error is another safety risk that is difficult to quantify. The intervention of well-trained operators is a vital layer in process safety[7].

Case Study: Bhopal Disaster

On December 3, 1984 at a chemical manufacturing plant owned by Union Carbide in Bhopal, India, water accidentally flowed into a tank in which the highly reactive intermediate, methyl isocyanate (MIC), was stored. This led to a rapid increase in temperature accompanied by boiling, which caused toxic MIC vapors to escape from the tank. These vapors passed into a scrubber and flare system that unfortunately were not working at the time. As a result of this accident, approximately 25 tons of MIC vapor were released, killing over 2,000 and injuring roughly 20,000 more[6].

As a result of the incident, Union Carbide was forced to pay $470 million, as well as fund a hospital in Bhopal that was used specifically to treat victims of the disaster. Cleanup of the plant site and other legal action are still being determined to this day.


  1. American Institute of Chemical Engineers.
  2. Northwestern University. Chemical Engineering 351 Lecture Slides.
  3. R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz. Analysis, Synthesis, and Design of Chemical Processes. Prentice Hall: Upper Saddle River, 2003.
  4. G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.
  5. L.T. Biegler, I.E. Grossmann, A.W. Westerberg. Systematic Methods of Chemical Process Design. Prentice-Hall: Upper Saddle River, 1997.
  6. W.D. Seider, J.D. Seader, D.R. Lewin. Process Design Principles: Synthesis, Analysis, and Evaluation. Wiley: New York, 2004.
  7. M.S. Peters, K.D. Timmerhaus. Plant Design and Economics for Chemical Engineers, 5th Ed. McGraw-Hill: New York, 2003.