Application of Weibull reliability model for functional safety of electro-hydraulic system 1
When the safety of the machinery users relies on a reliable function of the control system, than a safety function is required. Source: 10 steps to Performance Level Performance Level (PL) as integrity measure of a safety function. 2
PL determined based on probabilistic quantity PFH D, estimated through Category, Diagnostic Coverage (DC) and MTTF D Source: 10 steps to Performance Level 3
λ t λ PFH D : State modelling and reliability models MTTF = 1 λ Time (C OK ) OK R R Haz. situation (C H. ) S (C SAFE ) D (C D. ) safe failure dem dang. failure t D,U PFH D D,D (C DD ) D.U. failure (C DU. ) D.D. failure Exponential failure distribution λ(t) = λ Weibull failure distribution λ t = β 1 η β t β 1 4
Approach of this work ISO 13849-1:2015 Category MTTF D DC avg Figure 5 or Table K.1 PFH D Category Proposed method MTTF D DC avg Internal parameters PFH D 5
Approach of this work ISO 13849-1:2015 Figure 5 or Table K.1 PFH D Category MTTF D DC avg Proposed method Comparison of results Postprocessing Preprocessing Internal parameters...... Reliability Model Solver Simulation parameters E N t σ PFH D 6
Approach of this work ISO 13849-1:2015 Figure 5 or Table K.1 PFH D Category MTTF D DC avg Proposed method Category η and β W DC T M Postprocessing Preprocessing Internal parameters...... Reliability Model Solver Simulation parameters E N t σ PFH D 7
Estimation of PFH D through discrete event simulation For a cylinder safe stop, a valve which fails open is a dangerous fault PFH D = Average no. of transitions to unsafe state Service time Channel 1 DD DD DU DU Channel 2 CCF Ch 1 Ch 2 Safety function availability DU DD OK DU DD OK Hazard Safe state Operating DU: Dangerous fault, Undetected by control system DD: Dangerous fault, Detected by control system CCF: Common Cause Failures time 8
Results obtained by proposed method PL a PL b PL c PL d PL e 9
Application example: safety-related stop Directional valve Source: LASHIP - UFSC Redundant hydraulic circuit for safety-function Equivalent to ISO 13849-1:2015 category 3 Three sets of reliability data for these hydraulic valves Shut-off valve 10
Failure probability Application of Weibull reliability model for Failure probability Application example: considered reliability data for hydraulic valves Weibull Weibull Exponential Exponential Time Time Cumulative distribution function of directional valve 1V1 Cumulative distribution function of shut-off valve 0V1 11
Application example: Results and analysis PL d PL e No PL defined Service time 12
Functional safety for machinery is dependent on reliability; Mechanical, pneumatic and hydraulic systems have high reliability at beginning of service life; This is however not accurately considered by functional safety standards; Method allows estimation of PFH D considering more accurate reliability models for hydraulic systems, such as: Lifetime testing, or Field failure analysis; Better Performance Level (PL) could be claimed, if other conditions are fulfilled; Method opens possibility for further usage of hydraulic components in safety critical applications, as: Complex machinery with many interrelated actuators; Challenging applications as subsea oil and gas industry. 13
Thank you for your attention! Contact: Igor Kuhlhoff Bosch Engineering GmbH Igor.Kuhlhoff@de.bosch.com +49 7062 911-7797 www.boschrexroth.com/safety 10 Steps to Performance Level Available at Amazon 14