Failures in Process Industries

Transcription

1 Fault Tree Analysis

2 Failures in Process Industries Single Component Failure Data for failure rates are compiled by industry Single component or single action Multiple Component Failure Failures resulting from several failures and/or actions Failure rates determined using FTA

3 Failure Rates Data Instrument Faults/year Controller 0.29 Control valve 0.60 Flow measurements (fluids) 1.14 Flow measurements (solids) 3.75 Flow switch 1.12 Gas liquid chromatograph 30.6 Hand valve 0.13 Indicator lamp Level measurements (liquids) 1.70 Level measurements (solids) 6.86

4 Failure Rates Data Instrument Faults/year Oxygen analyser 5.65 ph meter 5.88 Pressure measurement 1.41 Pressure relief valve Pressure switch 0.14 Solenoid valve 0.42 Stepper motor Strip chart recorder 0.22 Thermocouple temperature meas Thermometer temperature meas Valve positioner 0.44

5 Failure Rates Data Some data are per hour Failure Frequency Failure Frequency Component (hr -1 ) Component (hr -1 ) Gasket Failure (leak) 1.00 x Pump Seal Failure 8.00 x Gasket Failure (total) 1.00 x Alarm Failure 1.00 x Pipe Rupture (> 3 in) 1.00 x Operator Error 2.00 x Pipe Rupture (< 3 in) 1.00 x Hose Rupture 2.00 x Valve Rupture 1.00 x 10-08

6 Frequency, Reliability and Probability Component p = 1- e -mt where p is the annual probability of occurrence, m is the annual frequency and t is time period (i.e., 1 year). Failure Rate m (faults/year) Reliability R=e (-mt) Failure Probability P=1-R Control Valve Controller DP Cell Conversion is important in OR gate (dimensional homogeneity)

7 Frequency and Probability - Example taking the case of gasket failure and assuming that we have 10 gaskets, the annual probability of occurrence is: p 1 exp 1 x 10 hr hr year x 10 3 year 1

8 What is Fault Tree Analysis Fault Tree is a method by which a particular undesired system failure mode can be expressed in terms of component failure modes and operator actions. The system failure mode to be considered is termed the top event and fault tree is developed in branches below this event showing it causes.

9 Fault Tree Analysis Fault tree analysis is typically carried out by a group or people or an individual. These individuals must have knowledge on the process so that causes of undesirable events can be understood The following information is important process and equipment description and specification process flow diagram, process instrumentation diagram and design information plant operation, human factors and environmental factors

10 Two basic Element The two mostly used gate symbol are and & or gates. And gate is used to indicate that output event occurs if all input event occurs simultaneously. Or gate is used when output event occurs if any one of the input event occurs. Event symbol mostly used is Rectangle to show any event. Signify the TOP EVENT by a double box.

11 FTA Procedure 1. Define top event 2. Choose events identified by hazard identification method (i.e HAZOP) which can lead to this top event. 3. Decide on the hierarchical construction of fault tree 4. Construct fault tree. All inputs to a particular gate should be completely defined before further analysis of one of them is undertaken. 5. Quantify the base events 6. Quantify the top event

12 FTA Procedure 7. Analyze results to determine the significance of particular base events or combination events 8. Carry out sensitivity analysis to test the following factors: uncertainty of basic data effect of improving reliability of plant and control systems effect of varying method of operation on the plant effect of plant modernization effect of improved training of operators

13 Underlying Principles Causes of undesirable events can only be understood with knowledge on how the system functions through: chemical/physical processes in the plant specific information on the whole process data on hazardous properties of materials process flow diagram and process instrumentation diagram equipment specification plant operation human factors and environmental factors

14 Example: Pump A system to pump acetic acid from the supply tank to the process is illustrated in figure. The system function automatically. When the regulator is energized, one of the pumps is started and acid passes through the feed pipes; if no acid is detected in the feed pipe the second pump is started. Construct a fault tree with the top event no flow to the process. To make your life easier, consider failure modes listed here. Is there any other notable failures not listed should be considered?

15 Example: Pump S E C1 C2 F1 F2 P1 M R P2 E : ELECTRICITY F1,F2 : FEED PIPES M : MANIFOLD C1, C2 : CABLES P1,P2 : PUMPS R : REGULATOR S : SUPPLY TANK

16 Failure Modes to Consider Component Symbol Failure Mode Cables C1 + C2 short-circuit Electricity supply E power cut Feed pipes F1 + F2 rupture of pipe Manifold M rupture Pumps P1 + P2 fail to start Regulator R fail to open on Supply tank S level too low

17 Fault Tree NO FLOW TO PROCESS PROBLEMS WITH PUMPS GENERAL PROBLEMS PUMP P1 PROBLEMS PUMP P2 PROBLEMS Tanks level too low Regulator fails Manifold M fails Power cut Pipe P1 ruptures Pump P1 fails to start PROBLEM 1 - SIMPLIFIED SYSTEM Cable C1 short circuits Pipe P2 ruptures Pumps P2 fails to start Cable C2 short circuits

18 Unit on Fault Tree and Rules Frequency (failure/year) = probability of failure per operation number of operation per year AND GATE rules : OR GATE rules : can multiply P and P = unit of probability can multiply P and F = unit of F cannot multiply F and F = unit F 2 (for example failure/yr 2 ) can add P and P = unit of P can add F and F = unit F cannot add F and P =different unit RULES for AND GATES P(A.B) = PA.PB F(AB) = FA.PB

19 Boolean Algebra and Minimal Cut Set Boolean Rules Differences to numerical manipulation Indempotent A+A=A A.A=A Absorption A+A.B=A A.(A+B)=A For example : (M+W). (M+Z) = M.M + M.Z +W.M +W.Z = M + M.Z +W.M +W.Z = (M + M.Z +M.W) + W.Z = M+ W.Z A CUT SET = combination of basic events which will produce TOP EVENT In the example : M, M.Z, W.M, W.Z are all cut set But Minimal CUT SET is a CUT SET if any basic event is removed the TOP EVENT will not occur Therefore MINIMAL CUT SET is M and W.Z can redraw the FAULT TREE..

20 Example Minimal Cut Set PUMP FAIL PUMP A FAILS PUMP B FAILS Failure of Power Supply Pump A Mechanic al Failure PROBLEM 1 - SIMPLIFIED SYSTEM Failure of Power Supply Pump B Mechanic al Failure M W M Z

21 Unit on FTA Quantify Fault Tree Electrical supply failure, P = 0.1 Single pump failure, P = 0.25 Referring to Fault Tree : Before minimal cut set, Probability of pump fail = After minimal cut set, Probability of pump fail =

22 Example -Minimum Cut Set PUMP FAIL FAILURE OF POWER SUPPLY MECHANICAL FAILURE OF PUMPS M PROBLEM 1 - SIMPLIFIED SYSTEM Pump A Mechanical Failure W Pump B Mechanical Failure Z

23 Boolean Algebra-Minimum Cut Set TOP EVENT A B D E C D E C

24 Boolean Algebra-Minimum Cut Set (A + B). [ (C + D). (E + C) + (D.E) ] = (A + B). (C.E + D.E + C.C + D.C + D.E ) = (A + B). (C.E + D.E + C + D.C + D.E ) = (A + B). (C + C.E + D.E + D.C + D.E ) = (A + B). (C + C.D + C.E + D.E + D.E ) INDEMPOTENT LAW = (A + B). (C + C.D + C.E + D.E) ABSORPTION LAW = (A + B). (C + D.E )

25 Boolean Algebra-Minimum Cut Set TOP EVENT A B C D E