overview of control system design chapter 10 1.safety. it is imperative that industrial plants...

Overview of Control System DesignC

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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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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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© V.Venkatasubramanian

Operator’s View of Process Control

A Day in the Life of a Plant Operator

© V.Venkatasubramanian

Operator’s View of Process Control Operator’s View of Process Control

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 Off-shore oil platform explosion 164 people killed $2 Billion in losses

Union Carbide, Bhopal, India, 1984

MIC release into atmosphere 3000-10,000 people killed 100,000 injured $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

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

Texas City BP Accident

http://www.csb.gov/videoroom/detail.aspx?VID=16

T2 Laboratories Accident

• 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 equivalent to 1400 pounds of TNT.

Before After

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T2 Laboratories Accident

http://www.csb.gov/investigations/detail.aspx?SID=8

Schematic of Reactor

CAUSES OF ACCIDENT

1. T2 did not recognize runawayreaction hazard with the MCMTit was producing despite earlierindications.

2 . Cooling system was susceptibleto single-point failures due tolack of design redundancy.

3. MCMT reactor relief system wasincapable of relieving the pressurefrom the runaway reaction.

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Runaway ReactionsMetalation Reaction

Reaction of Sodium and Diglyme Solvent

+ Na ?

New Test Cell Burst Test Cell 11

Operating regimes for exothermic chemical reactors.

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Modeling Needs

Why Simulate the Reactor?

1. Determine cooling requirements

2 . Determine conditions that leadto runaway conditions, such as increasing batch size, change incooling water temperature, etc.(so-called parametric sensitivity)

3. Size the pressure relief valve and bursting disk pressure

4. Develop a training tool13

Elements for Model

1. Unsteady Material Balance

2 . Unsteady Energy Balance

3. Reaction Rates includingtemperature dependence (must come from the lab)

4. Simulation of the model equations

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• 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.

Multiple Protection Layers

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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.

• 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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Liquid Chlorine

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• Inadequate precision of temporal information (e.g., lack of true alarm order).

• Excessive nuisance alarms

• Inadequate anticipation of process disturbances.

• lack of real-time, root-cause analysis (symptom-based 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.

Typical Complaints from Operators

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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.

Types of AlarmsType 1 Alarm: Equipment status alarm. Pump is on or off, or motor is running or stopped.

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Fig. 10.4 Two interlock configurations.

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Safety Interlock (Instrumented) System (SIS)

• 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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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.”

A Final Thought…

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