SIL and Exida
-
Upload
thugpandit -
Category
Documents
-
view
178 -
download
14
description
Transcript of SIL and Exida
IEC 61508 – IEC 61511Presentation
G.M. International s.r.l Via San Fiorano, 70
20058 Villasanta (Milano)
ITALY
www.gmintsrl.com
Document last revised 20 May 2005
Standard Definitions
Title: Standard for Functional Safety of
Electrical / Electronic / Programmable Electronic
Safety-Related System
IEC 61511 has been developed as a
Process Sector of IEC 61508
Title: Safety Instrumented Systems for the
Process Industry
Standard History
The IEC 61508 was conceived to define and harmonize a method to reduce risks of human and/or valuable harms in
all environments.
The IEC 61508 integrates and extends American Standard ISA-S84.01 (1996)
and German DIN 19250 (1994).
Standard Requirements
Other related standards
• DIN 19250 (1994)
Title: “Fundamental Safety aspects to be considered for measuring and control equipment”
Deals with Quantitative Risk Analysis used for Part 5 of IEC 61508, classification in AK classes 1-8 similar to SIL levels
• ISA-S84.01 (1996)
Title: “Application of Safety Instrumented Systems (SIS) for the process industry”
Defines Safety Lifecycles assuming Risk analysis and SIL been carried out.
Fundamental Concepts• Risk Reduction and Risk Reduction Factor (RRF)• Safety Integrity Level (SIL)• Independence Levels and consequences• Probability of Failure on Demand (PFD)• Reliability• Availability• Failure Rate (λ)• Proof Test Interval between two proof tests (T[Proof])• Failure In Time (FIT)• Mean Time To Failure (MTTF)• Mean Time Between Failure (MTBF)• Mean Time To Repair (MTTR)• Safe Failure Fraction (SFF)• Safety Lifecycle• Safety Instrumented System (SIS)
Risk Reduction
As Low As Reasonably Practicable or Tollerable Risk
(ALARP ZONE)
Fundamental Concepts
Risk ReductionFundamental Concepts
Safety Integrity Level (SIL)• SIL levels (Safety Integrity Level) • RRF (Risk Reduction Factor) • PFD avg (Average Probability of Failure on Demand)
SIL Table for Demand and Continuous mode of Operation
Fundamental Concepts
Independence LevelsAssessement Independence Level
as a function of consequences
Fundamental Concepts
PFDavg / RRF
Correlation between Probability of Failure on Demand
and Risk Reduction Factor
Fundamental Concepts
Reliability• Reliability is a function of operating time.• All reliability functions start from reliability one and decrease to reliability
zero. The device must be successful for an entire time interval.• The statement: “Reliability = 0.76 for a time of 100.000 hs” makes perfect
sense.
R(t) = P(T>t)
Fundamental Concepts
Reliability
Reliability is the probability that a device will perform its intended function when required to do so, if operated within its specified design limits.
– The device “intended function” must be known.– “When the device is required to function” must be judged.– “Satisfactory performance” must be determined.– The “specified design limits” must be known.
Mathematically reliability is the probability that a device will be successful in the time interval from zero to t
in term of a random variable T.
Fundamental Concepts
Availability• Availability is the probability that a device is successful at
time t. • No time interval is involved. • A device is available if it’s operating.• The measure of success is MTTF (Mean Time To Failure)
Fundamental Concepts
MTTF
MTTF is an indication of the average successful operating time of a device (system) before a
failure in any mode.
• MTBF (Mean Time Between Failures) • MTBF = MTTF + MTTR• MTTF = MTBF - MTTR• MTTR (Mean Time To Repair)• Since (MTBF >> MTTR) MTBF is very near to MTTF in value.
Fundamental Concepts
MTBF and Failure Rate
Relation between MTBF and Failure Rate λ
Failure per unit time 1
λ = ----------------------------- = ------------
Quantity Exposed MTBF
1 Quantity Exposed
MTBF = ------ = ----------------------------
λ Failure per unit time
Fundamental Concepts
MTBF - Example• Instantaneous failure rate is commonly used as measure of
reliability.
• Eg. 300 Isolators have been operating for 10 years. 3 failures have occurred. The average failure rate of the isolators is:
Failure per unit time 3 λ = ------------------------------- = ----------------- =
Quantity Exposed 300*10*8760
= 0.000000038 per hour = = 38 FIT (Failure per billion hours) == 38 probabilities of failure in one billion hours.
• MTBF = 1 / λ = 303 years (for constant failure rate)
Fundamental Concepts
Failure Rate Categories
λ tot = λ safe + λ dangerous λ s = λ sd + λ su λ d = λ dd + λ du λ tot = λ sd + λ su + λ dd + λ du
Where:
sd = Safe detected su = Safe undetected dd = Dangerous detected du = Dangerous undetected
Fundamental Concepts
FIT
Failure In Time is the number of failures per one billion devices hours.
1 FIT = 1 Failure in 109 hours =
= 10-9 Failures per hour
Fundamental Concepts
SFF (Safe Failure Fraction)
Fundamental Concepts
SFF summarizes the fraction of failures, which lead to a safe state and the fraction of failure
which will be detected by diagnostic measure and lead to a defined safety action
Type A SFF Chart
Type A components are described as simple devices with well-known failure modes and a solid history of operation
Fundamental Concepts
Type B SFF Chart
Type B: “Complex” component (using micro controllers or programmable logic); according 7.4.3.1.3 of IEC 61508-2
Fundamental Concepts
HSE Study
Results of system failure cause study done by English “Health and Safety Executive” (HSE)
Fundamental Concepts
Safety Lifecycle OriginFundamental Concepts
Safety Lifecycle 1/5
Fundamental Concepts
Safety Lifecycle 2/5
First portion of the overall safety lifecycle
ANALYSIS (End user / Consultant)
Fundamental Concepts
Safety Lifecycle 3/5
Realisation activities in the overall safety lifecycle
Fundamental Concepts
Safety Lifecycle 4/5
Safety lifecycle for the E/E/PES
(Electrical / Electronic / Programmable Electronic)
Safety - Related System (IEC 61508, Part 2)
Fundamental Concepts
Safety Lifecycle 5/5
Last portion of the overall safety lifecycle
OPERATION (End User / Contractor)
Fundamental Concepts
SIS
SIS (Safety Instrumented System)
according to IEC 61508 and IEC 61511
Fundamental Concepts
IEC 61511
Safety Instrumented Systems
for Process Industry
• IEC 61511 has been developed as a Process Sector implementation of the IEC 61508.
• The Safety Lifecycle forms the central framework which links together most of the concepts in this standard, and evaluates process risks and SIS performance requirements (availability and risk reduction).
• Layers of protection are designed and analysed.• A SIS, if needed, is optimally designed to meet particular process
risk.
Process sector system standard
IEC 61511
IEC 61511 Parts
The Standard is divided into three Parts
• Part 1: Framework, Definitions, Systems, Hardware and Software Requirements
• Part 2: Guidelines in the application of IEC 61511-1• Part 3: Guidelines in the application of hazard and risk
analysis
IEC 61511
IEC 61511 Part 3
Guidelines in the application of hazard and risk analysis
IEC 61511
FMEDA
Failure Modes and Effects Diagnostic Analysis (FMEDA)
Is one of the steps taken to achieve functional safety assessement of a device per IEC 61508 and is considered to be a systematic way to:
• identify and evaluate the effects of each potential component failure mode;
• classify failure severity;• determine what could eliminate or reduce the chance of failure;• document the system (or sub-system) under analysis.
FMEDA
The following assumptions are usually made during the FMEDA
• Constant Failure Rates (wear out mechanisms not included)• Propagation of failures is not relevant• Repair Time = 8 hours• Stress levels according IEC 60654-1, Class C (sheltered location),
with temperature limits within the manufacturer’s rating and an average temperature over a long period of time of 40°C
FMEDA
1oo1 Architecture
PFDPFDavg avg (T1) = (T1) = λλdddd * RT + λ * RT + λdudu * T1/2 * T1/2
because RT (avg. repair time) is << T1
PFDPFDavgavg = = λλdudu * T1/2 * T1/2
λdu = λdu (sensor) + λdu(isolator) + λdu(controller) + λdu(final element)
SIL level is the lowest in the loop.
1oo2 Architecture
PFDavg = λduc * (T1/2) + λddc * RT+(λddn* RT)2 + (λddn* RT * λdun* T1)2/2 + (λdun* T1)2 /3
PFDavg = (λdun* T1)2/2 + (λdun* T1)2 /3
2oo3 Architecture
PFDavg = λduc * (T1/2) + 3[λddc * RT+(λddn* RT)2 + (λddn* RT * λdun* T1)2/2 + (λdun* T1)2 /3]
SIL3 using SIL2 subsystem
SIL3 Control Loop or Safety Function using SIL2 SubSystems in 1oo2 Architecture
Safety Manual
A Safety Manual is a document provided to users of a product that specifies their responsabilities for installation and operation in order to maintain the design safety level.
The following information shall be available for each safety-related sub-system ..
Safety Manual Requirements1. Functional specification and safety function2. Estimated rate of failure in any mode which would cause both
undetected and detected safety function dangerous failures3. Environment and lifetime limits for the sub-system4. Periodic Proof Tests and/or maintainance requirements5. T proof test time interval
6. Information necessary for PFDavg, MTTR, MTBF, SFF, λdu, λtotal
7. Hardware fault tolerance and failure categories8. Highest SIL that can be claimed (not required for proven in use
sub-systems)9. Documentary evidence for sub-system’s validation (EXIDA)10. Proof Test ProceduresProcedures to reveal dangerous faults which are
undetected by diagnostic tests.
Standard referencesRemembering that:
• SIL (Safety Integrity Level) • RRF (Risk Reduction Factor) • PFD avg (Average Probability of Failure on Demand)
SIL Table for operative modes “high” and “low” demand
Using the Safety Manual
Standard references
Remembring definitions given for type “A” and “B” components,
sub-systems, and related SFF values
Using the Safety Manual
Loop PFDavg calculation
1oo1 typical control loop
PFDavg(sys) = PFDavg(tx) + PFDavg(i) + PFDavg(c) + PFDavg(fe)
Using the Safety Manual
Loop PFDavg calculation
For calculating the entire loop’s reliability (Loop PFDavg), PFDavg values for each sub-systems must first be found and be given a proportional
value (“weight”) compared to the total 100%.
This duty is usually assigned to personnel in charge of plant’s safety, process and maintainance.
Using the Safety Manual
Loop PFDavg calculation
Equation for 1oo1 loop
Where:
RT = repair time in hours (conventionally 8 hours)
T1 = T proof test, time between circuit functional tests (1-5-10 years)
λdd = failure rate for detected dangerous failures
λdu = failure rate for undetected dangerous failures
Using the Safety Manual
Loop PFDavg calculation
If T1 = 1 year then
but being λdd * 8 far smaller than λdu * 4380
Using the Safety Manual
Example 1
PFDavg = λdu * T1/2
For D1014 λdu is equal to 34 FIT (see manual)
Therefore
PFDavg = 34 * 10-9 * 4380 =
= 0,000148920 = 148920 FIT
Using the Safety Manual
Example 2
“Weights” of each sub-system in the loop must be verified in relation with expected SIL level PFDavg and data from the device’s safety manual.
For example, supposing SIL 2 level to be
achieved by the loop on the right in a low
demand mode:
• PFDavg(sys) is between 10-3 and 10-2 per year• “Weight” of D1014 Isolator is 10%
Therefore PFDavg(i) should be between 10-4 and 10-3 per year.
Using the Safety Manual
Example 2
Given the table above (in the safety manual) conclusions are:
1. Being D1014 a type A component with SFF = 90%, it can be used both in SIL 2 and SIL 3 applications.
2. PFDavg with T proof = 1yr allows SIL3 applications
3. PFDavg with T proof = 5yr allows SIL2 applications
4. PFDavg with T proof = 10yr allows SIL1 applications
Using the Safety Manual
1oo2 architecture
What happens if the total PFDavg does not reach the wanted SIL 2 level, or the end user requires to reach a
higher SIL 3 level?
The solution is to use a 1oo2 architecture which offers very low PFDavg, thus increasing fail-safe failure probabilites.
Using the Safety Manual
1oo2 architecture
For D1014S (1oo1):
PFDavg = λdu* T1/2
PFDavg = 148920 FIT
For D1014D (1oo2):
PFDavg = (λdun* T1)2/2 + (λdun* T1)2 /3
PFDavg = 75 FIT
In this case a 1oo2 architecture gives a 2000 times smaller PFDavg for the sub-system
Using the Safety Manual
Final considerations
• Always check that the Safety Manual contains information necessary for the calculation of SFF and PFDavg values.
• Between alternative suppliers, choose the one that offers:
• highest SIL level,
• highest SFF value,
• longest T[proof] time interval for the same SIL level,
• lowest value of PFDavg for the same T[proof].
• When in presence of units with more than one channel and only one power supply circuit, the safety function allows the use of only one channel. Using both of the channels is allowed only when supply is given by two independent power circuits (like D1014D).
• Check that the Safety Manual provides all proof tests procedures to detect dangerous undetected faults.
Using the Safety Manual
Credits and Contacts
G.M. International s.r.l
Via San Fiorano, 70
20058 Villasanta (Milan)
ITALY
www.gmintsrl.com
Document last revised 20 May 2005
TR Automatyka Sp. z o.o.
ul. Lechicka 14
02-156 Warszawa
POLAND
www.trautomatyka.pl