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Process Safety and Design. CHEN 4470 – Process Design Practice Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 18 – Process Risk Assessment & Inherently Safe Process Design March 19, 2013

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Process Safety and Design

CHEN 4470 – Process Design Practice

Dr. Mario Richard EdenDepartment of Chemical EngineeringAuburn University

Lecture No. 18 – Process Risk Assessment & Inherently Safe Process Design

March 19, 2013

Material Developed by Dr. Jeffrey R. Seay, University of Kentucky - Paducah


Importance of Process Safety

  • The safety record of the chemical process industry is the responsibility of all of us in the profession.
  • Process safety is important for employees, the environment, the general public, and it’s the law.
  • As process design engineers we are tasked with reducing the risk of operating a chemical manufacturing process to a level acceptable to employees, regulatory authorities, insurance underwriters and the community at large.
  • Recent chemical plant disasters underscore the importance of this point in terms of both human and financial losses.

Recent Incidents

T2 Laboratories Inc – Jacksonville, FL

December 19, 2007

4 Killed and 13 Wounded in reactor explosion in manufacture of gasoline additive.

BP America Refinery – Texas City, TX

March 23, 2005

15 Killed and 180 Wounded in isomerization unit explosion and fire.

West Pharmaceutical Services – Kinston, NC

January 29, 2003

6 Killed and Dozens Wounded in dust cloud explosion and fire from release of fine plastic powder.

Source: U.S. Chemical Safety Board,


Process Safety Terminology

  • Hazard vs. Risk
    • HAZARD is a measure of the severity of the consequences of a catastrophic failure of a given process or system, regardless of the likelihood and without considering safeguards.
    • RISK is the combination of both the severity of the worst case consequence and the likelihood of the initiating cause occurring.
    • In short, for an EXISTING PROCESS, we have little influence on the HAZARD, but through the application of safeguards, we can reduce the RISK of operating the process.

Process Hazard Analysis

  • Process Hazard Analysis (PHA) is a technique for determining the RISK of operating a process or unit operation.
  • PHAs are required by law for process handling threshhold quantities for certain listed Highly Hazardous Chemicals (HHC) or flammables.
  • Approved techniques for conducting PHAs:
    • HAZOP (Hazard and Operability)
    • What If?
    • FMEA (Failure Mode and Effects Analysis)
  • In general, a PHA is conducted as a series of facilitated, team brainstorming sessions to systematically analyze the process.

Risk Assessment Example

  • Consider a low design pressure API storage tank filled with cyclohexane.
  • Assume that the storage tank is equipped with a “pad/de-pad” vent system to control pressure.
  • What hazard scenarios might occur from this system?
  • What are the consequences of these scenarios?
  • What Safeguards might we choose to mitigate the risk?

Mitigating Process Risk

  • The operating risk is determined by the PHA using an appropriate Risk Assessment Methodology.
  • This risk is mitigated through the application of safeguards that reduce the risk to an acceptable level.

Core Process

1st Layer of


2nd Layer of


3rd Layer of


Layer of Protection Analysis

  • LOPA is a quantitative technique for reducing the RISK of a process.
  • The theory of LOPA is based on not “putting all your eggs in one basket”.
  • The layers mitigate the process RISK as determined by the PHA.
  • Each layer reduces the RISK of operating the process.

Each layer must be: Independent; Effective; Reliable; Auditable.


Liquid In

Process Vessel



Liquid Out

LOPA Example

  • Failure of Transfer Pump leading to overfill of Process Vessel.
  • Potential release of material to the environment requiring reporting or remediation.
  • Potential personnel injury due to exposure to material.
  • Severity would be based on properties of the material released.

Inherently Safe Process Design

  • Inherent safety is a concept based on eliminating the causes and/or reducing the consequences of potential process upsets.
  • Inherently Safe Process Design is a technique applied during the conceptual phase of process design.
  • Inherently Safe Process Design targets the HAZARD, rather than reducing the RISK after the fact.
  • This technique is based on making inherently safer design choices at a point in the process development where the engineer has the most influence on the final design.

Inherently Safe Process Design

  • Definitions
    • Inherently safe process design can be grouped into 5 categories
    • Each of these inherently safer design choices is applied in the conceptual phase of development.





Continuous reactor vs. batch reactor



Change of feedstock



Alternate technology


Limitation of effects

Minimization of storage volume



Gravity flow vs. pumping


Inherently Safe Process Design

  • Azeotropic Distillation vs. Pervaporation

Inherently Safe Process Design

  • Traditional Process
    • Sample Risk Assessment using What If? Methodology
    • Consider what types of safeguards would be required to mitigate the Process Risk due to these scenarios.

Inherently Safe Process Design

  • Azeotropic Distillation vs. Pervaporation

Inherently Safe Process Design

  • Inherently Safer Process
    • When considering the potential upset scenarios for the process, the benefits of the inherently safer process become clear.

Inherently Safe Process Design

  • Inherently Safer Process (Cont’d)
    • Based on this risk comparison, it is clear that multiple independent protection layers would be required to mitigate the operating risk of the traditional process.
    • This risk can be reduced by designing an inherently safer, ie, less hazardous process.
    • Although a complete economic analysis would be required, this example has illustrated that the need for independent protection layers is reduced in the inherently safer process design.

Summary 1:2

  • Conclusions
    • Clearly, process safety is a critical component of process design. In industry, no process is put into service without a comprehensive risk assessment.
    • It is important to realize that the management of operating risk is the key focus of process safety. As design engineers, we have responsibility for and the most influence on the overall hazard of a process.

Summary 2:2

  • References
  • R. Sanders, Chemical Process Safety – Learning from Case Histories, 3rd Edition, Elsevier, Inc, 2005.
  • D. Nelson, Managing Chemical Safety, Government Institutes, 2003.
  • Environmental Protection Agency, Process Hazard Analysis, 40 CFR 68.67, 2005.
  • Occupational Safety and Health Administration, Process Safety Management of Highly Hazardous Chemicals, 29 CFR 1910.119, 2005.
  • Center for Chemical Process Safety, Layer of Protection Analysis – Simplified Process Risk Assessment, AIChE, 2001.
  • T. Kletz, Process Plants: A Handbook for Inherently Safety Design, Taylor and Francis, 1998.
  • Center for Chemical Process Safety, Guidelines for Engineering Design for Process Safety, AIChE, 1993.
  • Seay, J. and M. Eden, “Incorporating Risk Assessment and Inherently Safer Design Practices into Chemical Engineering Education”, Journal of Chemical Engineering Education, 42(3), pp. 141-146, 2008.

Other Business

  • Next Lecture – March 21
    • Role of design engineer in technology development
      • Bob Kline, Eastman Chemical
    • Control strategy development
      • Jennifer Kline, Eastman Chemical
  • Next Lecture – March 26
    • Property prediction and CAMD