Chapter 10
Overview of Control System Design
1. Safety. It is imperative that industrial plants operate safely
so as to promote the well-being of people and equipment
within the plant and in the nearby communities. Thus, plant
safety is always the most important control objective and is
the subject of Chapter 10.
2. Environmental Regulations. Industrial plants must comply
with environmental regulations concerning the discharge of
gases, liquids, and solids beyond the plant boundaries.
3. Product Specifications and Production Rate. In order to be
profitable, a plant must make products that meet
specifications concerning product quality and production
rate.
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Chapter 10
4. Economic Plant Operation. It is an economic reality that the
plant operation over long periods of time must be profitable.
Thus, the control objectives must be consistent with the
economic objectives.
5. Stable Plant Operation. The control system should facilitate
smooth, stable plant operation without excessive oscillation in
key process variables. Thus, it is desirable to have smooth,
rapid set-point changes and rapid recovery from plant
disturbances such as changes in feed composition.
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Operator’s View of Process Control
A Day in the Life of a
Plant Operator © V.Venkatasubramanian
Operator’s View of Process Control
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Pump A pumping oil has tripped - Cause Unknown
You switch to Pump B. That also trips - Cause Unknown
Soon hundreds of alarms are going off – Cause(s) Unknown
With in minutes you have an explosion and a fire. Two people
are killed and a few hurt at this point.
It is 10:00 in the night.
The plant manager is in Aberdeen, Scotland, and not available.
You are on top of an off-shore oil platform in the middle of the
North Sea.
You are the Shift Supervisor:
What do you do?
© V.Venkatasubramanian
Process Safety is a Major Concern:
The BIG Ones
Piper Alpha Disaster, Occidental
Petroleum Scotland, 1988
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Off-shore oil platform explosion
164 people killed
$2 Billion in losses
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Union Carbide, Bhopal, India,
1984
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MIC release into atmosphere
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3000-10,000 people killed
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100,000 injured
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$0.5-1.0 Billion in losses
© V.Venkatasubramanian
AEM: Abnormal Event Management
$20B+ impact on U.S. economy; $10B impact
on petrochemical companies
 Petrochemical companies have rated AEM their
#1 problem
 Modern plants are more difficult to control,
diagnose and manage
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Complex configurations, very large scale
 Running process at its limit reduces margin for error
 Plant-wide integration makes reasoning difficult
 Advanced control puts process in states which operators
have difficulty managing in the event of an upset
 Fewer experienced operating personnel due to downsizing
 Lack of adequate training of operators
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© V.Venkatasubramanian
Texas City BP Accident
http://www.csb.gov/videoroom/detail.
aspx?VID=16
T2 Laboratories Accident
Before
After
At 1:33pm, 19 December 2007 a powerful explosion at T2
Laboratories in Jacksonville, Florida killed 4 employees, injured
32 (4 employees and 28 members of the public) and destroyed
the facility.
• A runaway exothermic reaction in the production of
methylcyclopentadienyl manganese tricarbonyl (MCMT) (fuel
octane booster) due to cooling loss led to the explosion
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equivalent to 1400 pounds of TNT.
•
T2 Laboratories Accident
http://www.csb.gov/investigations/d
etail.aspx?SID=8
Schematic of Reactor
CAUSES OF ACCIDENT
1. T2 did not recognize runaway
reaction hazard with the MCMT
it was producing despite earlier
indications.
2 . Cooling system was susceptible
to single-point failures due to
lack of design redundancy.
3. MCMT reactor relief system was
incapable of relieving the pressure
from the runaway reaction. 10
Runaway Reactions
Metalation Reaction
Reaction of Sodium and Diglyme Solvent
+ Na
New Test Cell
Burst Test Cell
?
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Operating regimes for exothermic chemical reactors.
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Modeling Needs
Why Simulate the Reactor?
1. Determine cooling requirements
2 . Determine conditions that lead
to runaway conditions, such as
increasing batch size, change in
cooling water temperature, etc.
(so-called parametric sensitivity)
3. Size the pressure relief valve
and bursting disk pressure
4. Develop a training tool
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Elements for Model
1. Unsteady Material Balance
2 . Unsteady Energy Balance
3. Reaction Rates including
temperature dependence
(must come from the lab)
4. Simulation of the model
equations
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Multiple Protection Layers
Chapter 10
• In modern plants, process safety relies on the principle of
multiple protection layers; see Figure 10.1.
• Each layer of protection consists of a grouping of equipment
and/or human actions, shown in the order of activation.
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Chapter 10
Figure 10.1.
Typical layers
of protection
in a modern
chemical plant
(CCPS 1993).
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• Basic process control system (BPCS) is augmented with two
levels of alarms and operator supervision or intervention.
Chapter 10
• An alarm indicates that a measurement has exceeded its
specified limits and may require operator action.
• Safety interlock system (SIS) is also referred to as a safety
instrumented system or as an emergency shutdown (ESD)
system.
• The SIS automatically takes corrective action when the process
and BPCS layers are unable to handle an emergency, e.g., the
SIS could automatically turn off the reactant pumps after a high
temperature alarm occurs for a chemical reactor.
• Rupture discs and relief valves provide physical protection by
venting a gas or vapor if over-pressurization occurs (also flares
for combustibles).
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pressure relief valve
(“What Pressure Relief Really Means,” Chem. Engr. Progress, Sept. 2010)
Chlorine Vaporizer
• Provides chlorine vapor to a reactor that converts
alkane (C12H26) to C12H25Cl, which in turn is alkylated
with benzene ring.
• When reactor is shut down, the vaporizer undergoes a
pressure surge that trips a relief valve/rupture disk
(undesirable behavior). Why does it occur(modeling
application)?
• The chlorine gas passes through the relief system and is
transferred to beds of clamshells in water, which
neutralizes the Cl2 to CaCl2.
• Analyze the P & ID and the valve failure conditions for
shutdown.
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4
Liquid Chlorine
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Typical Complaints from Operators
• Inadequate precision of
temporal information (e.g.,
lack of true alarm order).
• Excessive nuisance alarms
• Inadequate anticipation of
process disturbances.
• lack of real-time, rootcause analysis (symptombased alarming).
• Lack of distinctions
between instrument failures
and true process deviations.
• Lack of adequate tools to
measure, track, and access
past records of abnormal
situations.
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Types of Alarms
Type 1 Alarm: Equipment status alarm. Pump is on or off, or
motor is running or stopped.
Chapter 10
Type 2 Alarm: Abnormal measurement alarm. Measurement is
outside of specified limits.
Type 3 Alarm: An alarm switch without its own sensor. When it is
not necessary to know the actual value of the process variable,
only whether it is above (or below) a specified limit.
Type 4 Alarm: An alarm switch with its own sensor. This serves as
a backup in case the regular sensor fails.
Type 5 Alarm: Automatic Shutdown or Startup System.
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Chapter 10
Fig. 10.4 Two
interlock
configurations.
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Safety Interlock (Instrumented) System (SIS)
Chapter 10
• The SIS in Figure 10.1 serves as an emergency back-up system
for the BPCS.
• The SIS automatically starts when a critical process variable
exceeds specified alarm limits that define the allowable
operating region (starting or stopping a pump or shutting down a
process unit).
• Only used as a last resort to prevent injury to people or
equipment.
• SIS must function independently of the BPCS; (e.g., due to a
malfunction or power failure in BPCS). Thus, the SIS should be
physically separated from the BPCS and have its own sensors
and actuators.
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Safety Instrumented Systems Video
http://www.youtube.com/watch?v=4AbmZ7vjUZk
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A Final Thought…
Chapter 10
As Rinard (1990) has poignantly noted, “The regulatory control
system affects the size of your paycheck; the safety control system
affects whether or not you will be around to collect it.”
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Chapter 10
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