TUGAS K3 DALAM INDUSTRI KIMIA CHAPTER 11 RISK ASSESSMENT CHEMICAL PROCESS SAFETY – Fundamentals with Applications, 2nd Edition Daniel A. Crowl/Joseph F. Louvar SITI SITAWATI (NPM : 1006735574) Rev. 1 - 22 April 2011 DEPARTEMEN TEKNIK KIMIA - PROGRAM STUDI MANAGEMEN GAS PROGRAM PASCA SARJANA - UNIVERSITAS INDONESIA

CONTENTS 11-1 Review of Probability Theory 11-2 Event Trees 11-3 Fault Trees 11-4 Quantitative Risk Analysis (QRA) & Layers of Protection Analysis (LOPA)

11-1 REVIEW OF PROBABILITY THEORY EQUIPMENT FAILURES Occur as a result of interaction of individual components POISSON DISTRIBUTION Probability that the component will not fail during the time interval (0,t): R(t) = e-mt (11-1) Where: R = reliability m = faults/time t = time

11-1 REVIEW OF PROBABILITY THEORY Plot Failures: Failure Rate, m Failure Density, f(t) (c) Failure Probability, P(t) (d) Reliability, R(t)

FAILURE PROBABILITY (UNREALIBILITY) 11-1 REVIEW OF PROBABILITY THEORY FAILURE PROBABILITY (UNREALIBILITY) P(t) = 1 – R(t) = 1 - e-mt (11-2) MEAN TIME BETWEEN FAILURES Time interval between two failures of the component E(t) = MTBF = 1 / m (11-3)

Typical Bathtub Failure Rate Curve for Process Hardware 11-1 REVIEW OF PROBABILITY THEORY Typical Bathtub Failure Rate Curve for Process Hardware

Failure probabilities for individual components: 11-1 REVIEW OF PROBABILITY THEORY Failure probabilities for individual components: P = S Pi (11-4) Where: n = total number of components Pi = failure probability of each component Reliability probabilities for individual components: R = 1 - S (1 - Ri) (11-5) Where: Ri = reliability of an individual process component R = S (Ri)

Failure Rate Data for Selected Process Components 11-1 REVIEW OF PROBABILITY THEORY Failure Rate Data for Selected Process Components

11-1 REVIEW OF PROBABILITY THEORY Computation of Component Linkage : Simultaneous failure in parallel: logical AND function. Simultaneous failure in series: logical OR function

11-1 REVIEW OF PROBABILITY THEORY Revealed Failures Immediately obvious to operator and can be fixed in a negligible amount of time Component Cycles for Revealed Failures

11-1 REVIEW OF PROBABILITY THEORY Without operator being aware of the situation until it affects Unrevealed Failures Component Cycles for Unrevealed Failures

11-1 REVIEW OF PROBABILITY THEORY Mean time between failures (MTBF) for revealed and unrevealed: MTBF = 1 / m = tr + t0 (11-12) Where: t0 = time that the component is operational, period of operation tr = period of inactivity/downtime ti = inspection interval

11-1 REVIEW OF PROBABILITY THEORY Probability of Coincidence: Is required when there are dangerous due to process upset occurs and unavailability of emergency system Average frequency of dangerous episode: Where: ld = dangerous frequency l = frequency pd = dangerous process episode U = unavailability of emergency system Ti = time interval

11-1 REVIEW OF PROBABILITY THEORY Mean Time Between Coincidence (MTBC): Reciprocal average frequency of dangerous coincidences Where: ld = dangerous frequency l = frequency m = failure rate (failure/year) ti = inspection period (year)

11-2 EVENT TREES EVENT TREES Inductive approach that provides information on how a failure can occur and the probability of occurrence Used quantitatively if data are available on the failure rates of the safety function and the occurrence rate of the initiation event. Useful for providing scenarios of possible failure modes. Difficulty is that for most real processes the method can be extremely detailed, resulting in huge event tree.

11-2 EVENT TREES Event trees begin with an initiating event and work towards a final result with typical steps: Identify an initiating event of interest Identify the safety functions designed to deal with the initiating event Construct the event tree Describe the resulting accident event sequences

11-2 EVENT TREES EVENT TREE for loss of coolant accident for reactor:

11-2 EVENT TREES Computational Sequence in an Event Tree

11-2 EVENT TREES Typical Event Tree of a Reactor

11-3 FAULT TREES FAULT TREE Is a deductive method for identifying ways in which hazards can lead to accidents: Well-defined accident  top event  works backward toward the various scenarios that can cause the accident Preliminary steps before actual fault tree is drawn: Define precisely the top event Define existing event Define unallowed events Define the physical bounds of the process Define the equipment configuration Define the level of resolution

11-3 FAULT TREES Typical Fault Tree Contributing to a Flat tire

11-3 FAULT TREES Logic Transfer Component of a Fault Tree

11-3 FAULT TREES Typical Fault Tree of Reactor Overpressure

11-3 FAULT TREES Minimal Cut Set Is various sets of events that leads to top event. Determined using Fussel & Vesely Procedure Some of the minimal cut set have higher probability than others Ordered with respect to failure probability Quantitative Calculation Using Fault Tree Computation by Fault Tree Diagram, using AND gate & OR gate until top event Computation by Minimal Cut Set Procedure

11-3 FAULT TREES Drawing Fault Tree: Draw the top event at the top of the page Determine major events that contribute to the top event Parallel  connected by AND gate ; Series  connected by OR gate Determine intermediate events that contribute to the top event Expand intermediate events that contribute to the top event

11-3 FAULT TREES Disadavantages of Fault Trees For complicated process becomes enormous Not certain if all failure modes have been considered A particular item of hardware does not fail partially Failure of one component does not stress the other components Subjective dependence of individuals Requires failure probabilities of all events in the fault tree

11-3 FAULT TREES Advantages of Fault Trees: It begins with a top event, which is selected by user to be specific to the failure of interest Used to determine the minimal cut sets, which provides enormous insight into various ways for top events to occur Enables application of computers, which is available for construct fault trees, determining minimal cut set, calculating failure probabilities

11-4 QRA & LOPA Quantitative Risk Analysis Identify where operations, engineering, or management systems can be modified to reduce risk. Design to provide managers with a tool to help them evaluate the overall risk of a process. Evaluate potential risks when qualitative methods cannot provide an adequate understanding of risks Relatively complex procedure that requires expertise and a substantial commitment of resources and time.

11-4 QRA & LOPA Major steps of QRA study include: Define potential event sequences and potential incidents Evaluate incident consequences (typical tools for this step include dispersion modeling and fire explosion modeling) Estimate potential incident frequency using event trees and fault trees Estimate incident impacts on people, environment, and property, and Estimate the risk by combining the impacts and frequencies, and recording the risk using a graph

11-4 QRA & LOPA Layer of Protection Analysis Semi-quantitative too for analyzing and assessing risk Simplified methods to characterize the consequences and estimate the frequencies, Various layers of protection are added to a process to lower frequency of the undesired consequences Consequences and affects are approximated by categories, the frequencies are estimated, and the effectiveness of the protection layers is also approximated. Individual companies use different criteria to establish the boundary between acceptable and unacceptable risk.

11-4 QRA & LOPA Typical Layer of Protection Analysis of a Specific Accident Scenario

11-4 QRA & LOPA Major steps of QRA study include: Identify a single consequence Identify an accident scenario and cause associated with the consequence Identify the initiating event for the scenario and estimating the initiating event frequency Identify protection layers available for consequence and estimating the probability of failure on demand (PFD) for each protection layer Combining the initiating event frequency with the PFD for the independent protection layers to estimate a mitigated consequence frequency Plotting the consequences versus the consequence frequency to estimate the risk Evaluating the risk for acceptability

11-4 QRA & LOPA Consequence Most common scenario of interest for LOPA is loss of containment of hazardous material occurred through variety of incidents such as leak from a vessel, ruptured pipeline, gasket failure, release from a relief valve   Consequences are estimated using the following methods: Semi-quantitative approach without the direct reference to human harm Qualitative estimates with human harm Quantitative estimates with human harm

11-4 QRA & LOPA Semi-Quantitative Consequences Categorization

11-4 QRA & LOPA Frequency Methods to determine frequency includes the following steps: Determine failure frequency of initiating event Adjust the frequency to include the demand Adjust the failure frequency to include probabilities of failure on demand (PFDs) for each independent layer of protection Probabilities of failure on demand (PFD) for each independent protection layer (IPL) varies from: 10-1 for a weak IPL 10-2 for a common practice IPL 10-5 for a strong IPL

11-4 QRA & LOPA Three rules for classifying a specific system or action of an IPL: IPL is effective in preventing the consequence when it function as designed IPL functions independently of the initiating event and the components of all other IPLs that are used for the same scenario IPL is auditable, that is, the PFD of the IPL must be capable of validation including review, testing, and documentation

11-4 QRA & LOPA Frequency Values Assigned to Initiating Events

11-4 QRA & LOPA PFD concept is used when designing emergency shutdown system called safety instrumented functions (SIFs). A SIF achieves low PFD figures by: Using redundant sensors and final redundant control elements Using multiple sensors with voting systems and redundant final control elements Testing the system components at s specific intervals to reduce the PFD by detecting hidden failures Using deenergized trip system (i.e., a relayed shutdown system)

11-4 QRA & LOPA PFDs for Passive IPLs

11-4 QRA & LOPA PFDs for Active IPLs and Human Actions

11-4 QRA & LOPA Consequence Frequency of Specific Scenario Endpoint Consequence Frequency of Multiple Scenario Endpoint Where:

11-4 QRA & LOPA Safety Integrated Levels (SILs) for emergency shutdown system: SIL1 (PFD = 10-1 to 10-2): implemented with a single sensor, a single logic solver, a single final control element, and requires periodic proof testing SIL2 (PFD = 10-2 to 10-3): typical fully redundant, including the sensor, a single logic solver, a single final control element, and requires periodic proof testing SIL3 (PFD = 10-3 to 10-4): typical fully redundant, including the sensor, a single logic solver, a single final control element, and requires careful design and frequent validation test to achieve low PFD figures.  

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