Overview of Control System Design

Transcription

1 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. 2. Environmental lregulations. Id Industrial ilplants 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. 1

2 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. 2

3 Operator s View of Process Control A Day in the Life of a Plant Operator V.Venkatasubramanian

4 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 10:0000 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

5 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 ,000 people killed 100,000 injured $ Billion in losses V.Venkatasubramanian

6 The BIG Ones: More recently. Mina Al-Ahmedhi Refinery,KPCL,Kuwait, June 2000 Leak led to flammable vapor release and explosion 7 people killed, 50 injured $400 Million in losses Petrobras, Brazil, March 2001 Off-shore oil platform explosion 10 people killed, $5 Billion in losses Platform sank into the Atlantic Ocean V.Venkatasubramanian

7 The BIG Ones: More recently. Ammonium Nitrate Explosion in Toulouse - France 21 September People Killed 2442 Injured Losses in Hundreds of millions dollars V.Venkatasubramanian

8 Relatively Minor Incidents happen more often Mobil, Torrance, CA explosion & fire, 10/94 Conoco Lake Charles, LA, cat cracker fire, 10/94 Miles chemical plant, Baytown, TX, acid leak, 11/94 Koch, Corpus Christi, TX, separator explosion, 11/94 Mobil, Paulsboro, NJ, chemical releases, 11/94 Terra Industries, Sioux City, IA, explosion, 12/94 Chevron, El Segundo, CA, furnace fire, 1/95 Mobil, Torrance, CA, gasoline spill, 2/95 Unocal, San Francisco, acid overflow/leak, 3/95 Amoco, Cartere, NJ, depot leak/fire, 3/95 Clark, Blue Island, IL, refinery fire/extended closure, 3/95 Ultramar, Wilmington, CA, tank leak/fire, 3/95 Conoco, Ponca City, OK, crude topping unit fire, 3/95 Sun Oil, Philadelphia, l gas leak, 4/95 Napp Technologies, Lodi, NJ, explosion & fire, 4/95 Rhone-Poulenc, Philadelphia, granulator explosion and fire, 5/95 Reichhold Chemicals, Grundy Co, IL, rupture/fire/spill, 5/95 BP, Lima and Toledo, OH refinery fires, 5/95 Ultramar, Wilmington, CA, crude unit fire, 6/95 Unocal, San Francisco, naptha tank fire, 6/95 Tosco, San Francisco, crude unit fire, 6/95 Murphy Oil, New Orleans, solvent extraction unit fire, 7/95 Amoco Oil, Texas City, cat cracker explosion & fire, 7/95 Conoco, Ponca City, OK, refinery fire, 7/95 24 incidents: 12 deaths, hundreds hurt, $1B+ losses, $10B+ impact V.Venkatasubramanian Source: Honeywell ASM Consortium

9 Process Safety and Process Control Primary concern of the process industries. Increased public awareness of potential risks, stricter legal requirements, and the increased complexity of modern industrial plants. Overview of Process Safety Process safety is considered at various stages in the lifetime of a process: 1. Preliminary process design. Chapter 10 9

10 2. At the final stage of the design stage hazard and operability (HAZOP) studies, failure mode, and fault tree analysis are used. 3. After plant operation begins, HAZOP studies are conducted on a periodic basis in order to identify and eliminate potential hazards. 4. Proposed plant or operating conditions changes require formal approval. This considers the potential impact of the change on the safety, environment, and health of the workers and the nearby communities (may require governmental approval, e.g., pharmaceutical industry). 5. After a serious accident or plant incident, a thorough review is conducted to determine its cause and to assess responsibility. Chapter 10 10

11 Multiple Protection Layers In modern plants, process safety relies on the principle of multiple protection layers; see Figure Each layer of protection consists of a grouping of equipment and/or human actions, shown in the order of activation. Chapter 10 11

12 Figure Typical layers off protection i in a modern p chemical plant (CCPS 1993). Chapter 10 12

13 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). Chapter 10 13

14 Types of Alarms Type 1 Alarm: Equipment status alarm. Pump is on or off, or motor is running or stopped. 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. Chapter 10 14

15 Fig A general block diagram for an alarm system. Chapter 10 15

16 Fig Two flow alarm configurations. 16

17 Fig Two interlock configurations. Chapter 10 17

18 Safety Interlock System (SIS) The SIS in Figure 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. Chapter 10 18

19 A Final Thought 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. Chapter 10 19