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
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,
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.
the
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, www.chemsafety.gov
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.
PC
N2 Supply
Vent Gas
• Assume that the storage tank is equipped with
a “pad/de-pad” vent system to control pressure.
Cyclohexane
Storage
Tank
- What hazard scenarios might occur from
this system?
- What are the consequences of these
scenarios?
- What Safeguards might we choose to
mitigate the risk?
What If…?
Initiating Cause
Consequence
Safeguards
1. There is High
Pressure in the
Cyclohexane
Storage Tank?
1.1 Failure of
the pressure
regulator on
nitrogen
supply line.
1.1 Potential for pressure in tank to rise due
to influx of nitrogen through failed
regulator. Potential to exceed design
pressure of storage tank. Potential tank
leak or rupture leading to spill of a
flammable liquid. Potential fire should an
ignition source be present. Potential
personnel injury should exposure occur.
1. Pressure relief vent (PRV)
sized to relieve
overpressure due to this
scenario.
2. Pressure transmitter with
high alarm set to indicate
high pressure in
Cyclohexane Storage Tank.
Mitigating Process Risk
The operating risk is determined by the PHA using an
appropriate Risk Assessment Methodology.
Process Risk
–
Level of
Acceptable
Operating Risk
Inherent
Risk
–
Operating
Safeguards
This risk is mitigated through the application of
safeguards that reduce the risk to an acceptable level.
Layer of Protection Analysis
Core Process
• LOPA is a quantitative
technique for reducing the
RISK of a process.
1st Layer of
Protection
• The theory of LOPA is based
on not “putting all your eggs
in one basket”.
2nd Layer of
Protection
• The layers mitigate the
process RISK as determined
by the PHA.
3rd Layer of
Protection
• Each layer reduces the RISK
of operating the process.
Each layer must be: Independent;
Effective; Reliable; Auditable.
LOPA Example
• Failure of Transfer Pump
leading to overfill of Process
Vessel.
Liquid In
• Potential release of material to
the environment requiring
reporting or remediation.
LAH
• Potential personnel injury due
to exposure to material.
• Severity would be based on
properties of the material
released.
LT
Liquid Out
Process Vessel
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
Category
Example
1 Intensification
Continuous reactor vs. batch reactor
2 Substitution
Change of feedstock
3 Attenuation
Alternate technology
4 Limitation of effects
Minimization of storage volume
5 Simplification
Gravity flow vs. pumping
Each of these inherently safer design choices is applied
in the conceptual phase of development.
Inherently Safe Process Design
•
Azeotropic Distillation vs. Pervaporation
Entrainer
Vessel
Azeotrope
Column
Solvent
Column
2
3
7
1
Stream s:
1 Solvent Feed
2 Hexane Feed
3 Entrained Azeotrope
4 Waste Water
5 Aqueous Phase
6 Organic Phase
7 Hexane Recycle
8 Recovered Solvent
5
6
8
4
Inherently Safe Process Design
•
Traditional Process
–
Sample Risk Assessment using What If? Methodology
What If…?
1. There is higher
pressure in the
Entrainment
Vessel?
2. There is higher
level in the
Entrainer
Vessel?
–
Initiating Cause
Consequence
1.1 External fire in the
process area.
1.1 Potential increased temperature and pressure leading to
possible vessel leak or rupture. Potential release of
flammable material to the atmosphere. Potential personnel
injury due to exposure.
1.2 Pressure regulator for
inert gas pad fails open.
1.2 Potential for vessel pressure to increase up to the inert gas
supply pressure. Potential vessel leak or rupture leading to
release of flammable material to the atmosphere. Potential
personnel injury due to exposure.
2.1 Potential to overfill vessel with cyclohexane. Potential to
flood vent line with liquid leading to flammable liquid
reaching the vent gas incinerator. Potential to overwhelm
incinerator leading to possible explosion. Potential
personnel injury due to exposure.
2.1 Vessel level transmitter
fails and indicates lower
than actual volume.
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
Pervaporation
Unit
Azeotrope
Column
Solvent
Column
4
2
1
5
Stream s:
1 Solvent Feed
2 Azeotrope
3 Waste Water
4 Solvent Rich Phase
5 Water Rich Phase
6 Recovered Solvent
6
3
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.
Upset Scenario
External Fire
Overfill
Overpressure
Traditional Process
Large volume of flammable liquid
circulating in process.
Cyclohexane entrainer more
volatile than 1-propanol.
Larger liquid hold-up leads to
higher severity in the event of a
release.
Inherently Safer Process
Flammable volume limited to
recovered solvent only.
Minimal liquid hold up in
Pervaporation Unit.
Volume limited to solvent
distillation hold-up.
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
1.
2.
3.
4.
5.
6.
7.
8.
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
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Next Lecture – March 21
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Role of design engineer in technology development
– Bob Kline, Eastman Chemical
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Control strategy development
– Jennifer Kline, Eastman Chemical
Next Lecture – March 26
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Property prediction and CAMD