Inherently safer design is now a recognized
and generally accepted good chemical engineering practice. It should now be standard
practice to apply this approach in design,
construction, operation, and maintenance
of process plants. How?
When planning to produce a new
fuel or chemical product, examine
alternative products and alternative
process technologies to get the best
inherently safer results.
Review process design early in the
creation or modification of a process
plant to ensure that opportunities for
inherent safety are incorporated. This
can be done by a separate inherently
safer review of the process or by
incorporating the inherently safer
concepts in the process hazards
reviews during the design process.
For existing process plants, look for
inherently safer applications when
doing the periodic reauthorization
process hazards analyses.
It is also important to remember that
although it is easier to design for inherent
safety than to retrofit existing plants, one
can still improve older plants by modifications to incorporate inherent safety principles and practices.
As professional engineers, we are obligated to be ever mindful of safety, and we
have the opportunity to continue to incorporate inherently safer design, construction,
and operation. One need look no further
than the new generation of process plants
that will be built in response to abundant
new sources of natural gas and oil from
shale (“Oil & Gas Rush,” PE, April 2013).
When designing these new facilities,
comprehensive process safety management programs will be necessary, and
inherently safer design will play a very
important role. PE
NSPE member Victor H. Edwards, Ph.D.,
P.E., retired in July after 30 years at IHI
E&C International Corp., where he served
in process engineering positions, including
director of process safety. He chaired the
American Institute of Chemical Engineers’
Global Congress on Process Safety in 2013
and is an AIChE Fellow.
Creating an inherently safer plant can also
be accomplished by using less hazardous
conditions, a less hazardous form of a material, or facilities that minimize the impact of
a release of hazardous material or energy
(also called attenuation).
In the case of ammonia, which has
numerous industrial uses, diluting it with
water can create a safer environment.
Anhydrous (100%) ammonia at typical
ambient temperatures must be stored in
pressure vessels (at 21 degrees C, the
vapor pressure of anhydrous ammonia is
8. 8 atmospheres). If containment is lost, the
ammonia will quickly vaporize, forming a
toxic and potentially flammable vapor cloud.
If ammonia is diluted with water to a
concentration of 19%, however, the partial
pressure of ammonia at typical ambient
conditions is much less than one atmosphere. Therefore, the aqueous ammonia
will not boil and form a large vapor cloud if
containment is lost.
Similarly, the use of a lower temperature for storage of chlorine can create a
safer environment. The lower temperature
reduces the vapor pressure and can greatly
decrease the size of any vapor clouds
formed on accidental release.
Kletz’s final rule is to design facilities that
Other Inherently Safer Methods
eliminate unnecessary complexity, make
operating errors less likely, and are forgiving
of errors when they happen (also called
error tolerance). For engineers, this could
mean using welded, not flanged, piping
for highly toxic chemicals and designing
vessels to withstand full vacuum to prevent
collapse during vacuum conditions.
In addition to Kletz’s four steps to inherently
safer processes, three additional approaches
are proposed here that deserve attention.
Hybridization: This method involves
maintaining the original chemistry of the
reaction but adding an additional chemical
that transforms a potentially hazardous
reaction process into a much safer one.
This concept is based on work by Jenq-
Renn Chen presented in a 2004 article in
Process Safety Progress. Chen reported that
adding water to cyclohexane decreased the
flammability of oxygen/cyclohexane vapors
without adversely affecting the basic cyclo-
hexane oxidation process. This innovation
prevents combustion from occurring in the
gas phase in a gas-liquid reaction.
Stabilize or ensure dynamic stability:
Not all process designs are inherently stable,
but stable operation is achieved using
instrumentation and controls. Modify the
process design so that it has wide operating
limits and is less sensitive to variations in
operating parameters. One example of this
approach is to increase the rate of heat
removal from a strongly exothermic reactor
in a way that prevents runaway reactions.
Limit hazardous effects during concep-
tual and detailed engineering: Increase the
spacing of process equipment and of poten-
tially hazardous units to reduce the likelihood,
severity, and consequences of vapor cloud
explosions and other overpressure incidents.
Methods of Reducing Process Risks
To prevent process plant incidents, there are four basic risk management strategies:
Inherent safety: Eliminate the hazard by using materials and process conditions
that are nonhazardous.
Passive safety layers: Minimize the frequency or the consequence of any hazard
by a design feature that does not require the active functioning of any device, for
example, dikes and blast walls.
Active layers of protection: Use controls, alarms, safety instrumented systems,
and mitigation systems to detect and respond to process deviations, for
example, a control loop that shuts off feed to a reactor when an abnormally high
temperature is detected in the reactor.
Procedural safety layers: Use policies, procedures, training, administrative
checks, and emergency response to prevent incidents or to minimize the effects
of an incident.