Section-20 Safety Instrumented Systems

Haward Technology Middle East 1Section 20

Process Control, Instrumentation and Safeguarding

Section 20

Safety Instrumented Systems



TOPICS

Introduction

Probability of Failure

System Architecture

Safety PLC

Major Systems

Typical Questions & Answers




Introduction




INTRODUCTION




INTRODUCTION

“The prevention of accidents should not be considered a question of

legislation, but instead, our responsibility to fellow human beings

and common sense”

Werner von Siemens

Berlin, in the year of 1880




Safety instrumented systems (SIS) are used to provide safety control functions for processes, e.g. emergency shutdown (ESD), fire detection and blow-down functions.

SIS typically are composed of sensors, logic solvers and final control elements.

Due to the critical nature of such systems, OSHA recognizes compliance with the standard ANSI/ISA S84.01 - Application of Safety Instrumented System for the Process Industries - as a good engineering practice for safety instrumented systems.

INTRODUCTION




(Hardware and Software)

Logic solver

Sensor Logic Solver Actuator

Scope of a Safety Instrumented System

INTRODUCTION




Today SIS Safety Instrumented Systems play an increasingly important role in many process plants. Safety standards such as IEC 61508, IEC61511 as well as ISA S84.01, are creating more stringent safety requirements for process plants.

INTRODUCTION




Depending on application use, other names used for SIS Safety Instrumented Systems are:

Emergency Shutdown Systems (ESD), Burner Management Systems (BMS), Fire and Gas Systems (F&G), Critical Turbo-machinery Control, Railway Switching, Semiconductor Life Safety Systems (SEMI S2), Nuclear 1E Safety Systems, High Integrity Protection Systems (HIPS), High Integrity Pressure Protection System (HIPPS)

INTRODUCTION




The operation of many industrial processes involve inherent risks due to the presence of dangerous material like gases and chemicals.

Safety Instrumented Systems SIS are specifically designed to protect personnel, equipment and the environment by reducing the likelihood (frequency) or the impact severity of an identified emergency event.

INTRODUCTION




PROBABILITY of

FAILURE




PROBABILITY of FAILURE

By understanding how components of a Safety Instrumented System can fail, it is possible to calculate a Probability of Failure on Demand (PFD).

There are two basic ways for SIS to fail. The first way is commonly called a spurious trip which usually results in an unplanned but safe process shutdown.

While there is no danger associated with this type of SIS failure, the operational costs can be very high.




The second type of failure does not cause a process shutdown or nuisance trip. Instead, the failure remains undetected, permitting continued process operation in an unsafe or dangerous manner.

If an emergency demand occurred, the SIS would be unable to respond properly. These failures are known as covert or hidden failures and contribute to the probability PFD of the system failing in a dangerous manner on demand.

The PFD for the Safety Instrumented System SIS is the sum of PFD's for each element of the system.





The governing standards for Safety Instrumented Systems SIS state that plant operators must determine and document that equipment is designed, maintained, inspected, tested and operated in a safe manner.

Thus, it is imperative that these components of Safety Instrumented Systems be tested frequently enough to reduce the PFD and meet the target SIL.





System Architecture




SYSTEM ARCHITECTURESafety Instrumented Systems

Operatorsystem

Maintenancestation

Engineeringsystem

Redundant/Non-redundant

Fail-safe fault-iterant and high availability

ET 200M

Standard/safety-related

Standard

Standard/safety-related

ET 200iSPRedundant & non-redundant

ET 200S

PROFIBUS

Industrial Ethernet

High-availability

Redundant/Non-redundantET 200M

ET 200iSPRedundant & non-redundant

ET 200S

PROFIBUS

Standard

Standard

Standard



Basic PLC System

PLC

CommunicationsInput interfaces

Output interfaces

Output devices/ final elements

(e.g. actuators)

Input devices(e.g. sensors / transmitters)

Power supplies

SYSTEM ARCHITECTURE




Basic PLC System - Problems

Not designed for safety applications

Limited fail safe characteristics

High risk of covert failures (undetected dangerous failure modes) through lack of diagnostics.

Reliability of software (also stability of versions)

Flexibility without security

Unprotected communications

Limited redundancy

SYSTEM ARCHITECTURE




Upgrading of Standard PLCs for Safety Applications

Improvement Steps Improve Software QA Add hot back up Add I/O diagnostic circuits Install dual redundant system components

Problems Remain Reliability Analysis Cost and complexity Risk inherent in custom designs Documentation control Need for certification

SYSTEM ARCHITECTURE




IEC 61508 / 61511 defines the following architectures:

1oo1

The architecture consists of a single channel for the safety function; any hazardous failure causes the failure of the safety function, if this is demanded.

SYSTEM ARCHITECTURE




Risk of covert failure due to failures of : Input circuits I/O comms Processor Program cycle Output circuits

Basic PLC architecture without diagnostics Type: 1oo1

Input Circuit

CPU OutputCircuit

SYSTEM ARCHITECTURE




Characteristics of Safety PLCs (1)

Automatic diagnostics continuously check the PLC system functions at short intervals within the fault tolerant time of the process.

>99% faults are notified for attention and repair. Redundant hardware options available to provide

uninterrupted operation even if one channel has failed.

On line hot replacement of failed modules can be provided.

All application software updated transparently to redundant channels.

SYSTEM ARCHITECTURE





Provides a predictable and safe response to all failures of hardware, power supplies and malfunctions of system software.

Fault injection testing is applied to all of the system

I/O subsystems continuously checking all I/O signals

Bus communications self-checking with group isolation on fault.

High security write protection on communication ports

SYSTEM ARCHITECTURE





Software QA applied on life cycle principles

Program flow control checking

Data verification checks validity at all times

Operating system and application program tools by same vendor.

Application software use limited variability language.

SYSTEM ARCHITECTURE




Single Channel Safety PLC Architecture With Diagnostics Type: 1oo1D

Fail safe operation: (single fault tolerance) Independent output opens on detection of faults in

Input circuits I/O comms Processor (self test or watchdog) Program cycle Output circuits

Input Circuits

Diagnostic Protection System

Control Module

Output Circuits

V+




Dual redundant processor option in single channel module

Single Channel Safety PLC Architecture With Dual CPUType: 1oo1D

Input Circuits


Control Module

Output Circuits

Control Module

V+





1oo2The architecture consists of two sensors and two final elements of which only one set is required for shut-down.

This system is safer than 1oo1 but has twice as many nuisance trips.

SYSTEM ARCHITECTURE





2002

The architecture consists of two sensors and two final elements, both of which are required for shut-down.

This system is less safe than 1oo2 but has fewer nuisance trips.

SYSTEM ARCHITECTURE




Input Circuits


Control Module

Output Circuits

V+

Input Circuits


Control Module

Output Circuits

Both channels must operate to trip output . Reverts to 1oo1D if module fault is detected. Diagnostics must check other CPU.

Dual Redundant Channel Safety PLC Architecture With DiagnosticsType: 1oo2D




Input Circuits


Control Module

Output Circuits

V+

Input Circuits


Control Module

Output Circuits

Calculate mode is on-line, verify mode is off-line

1oo2D Safety PLC with mode switching

Verify

Calc

Using alternating modes reduces risk of common cause Hw/Sw errors




Input Circuits


Control Module

Output Circuits

V+

Input Circuits


Control Module

Output Circuits

Calc

Verify

1oo2D Safety PLC with mode switching

Calculate mode is on-line, verify mode is off-lineUsing alternating modes reduces risk of common cause hw/sw errors





2oo3

The architecture consists of three redundant channels that are connected with a majority output device. The output state does not change if the result of only one channel does not conform to the other two.

SYSTEM ARCHITECTURE




Triple Modular Redundant PLCs (TMR)

No single point of failure. High safety integrity. High availability.

INCPU

OUT

INCPU

OUT

INCPU

OUT

2oo3

Comparators & 2oo3 voters at each stage




Strong need for communications between the SIS and

the plant control systems.

a) For operator information and co-ordination with control system.

b) For tidy up of DCS or PLC controller states or sequences arising from action of the SIS.

c) For event recording. d) For I/O status and status of the SIS itself.

Security required in communications to prevent incorrect writing of data into the SIS.

Communications and data formats need to be compatible with DCS/PLC vendor standards or open standards.

SYSTEM ARCHITECTURE





SAFETY PLC




This is a specially designed class of PLC which provides high reliability and high safety through the use of special electronic design, special software and pre-engineered redundancy.

The Safety PLC has I/O circuits which are designed to be fail-safe with built-in diagnostics.

SAFETY PLC




The processor of the Safety PLC has built in diagnostics for memory, processor operation, watchdog timer and all communications systems.

One-out-of-one with diagnostics (1oo1D) architecture uses specific diagnostic circuits to convert dangerous failures into safe failures by de-energising the output.

SAFETY PLC



The TRICONEX system is designed with a fully triplicated architecture throughout, from the input modules through the Main Processors to the output modules.

Every I/O module houses the circuitry for three independent legs.

SAFETY PLC



Process Control, Instrumentation and SafeguardingSafety Instrumented

SystemsELCO Connectors for I/O Termination

PowerTermination

TriBus

DualPowerRails

MainProcessorsA, B, & C

Right I/O Module*

Left I/O Module*

Communication Module

*Either the left module or right module functions as the active or hot-spare module.

Typical Logical Slot

Channel AChannel B

Channel C

Channel AChannel B

Channel C

CommBus

I/OBus

Backplane of the Main Chassis

Terminal8trip81

Terminal8trip82

PowerSupply

#1

PowerSupply

#2

SAFETY PLC



Each leg on the input modules reads the process data and passes that information to its respective Main Processor.

The three Main Processors communicate with each other using a proprietary high-speed bus system called the TriBus.

SAFETY PLC




SIMPLIFIED TRICONEX SYSTEM

SAFETY PLC




The individual input table in each Main Processor is transferred to its neighbouring Main Processors over the proprietary TriBus.

During this transfer, hardware voting takes place.

The TriBus uses a Direct Memory Access programmable device to synchronize, transmit, vote and compare data among the three Main Processors.

SAFETY PLC




If a disagreement is discovered, the signal value found in two out of three tables prevails, and the third table is corrected accordingly.

One-time differences, which result from sample timing variations can be distinguished from a pattern of differing data.

The three independent Main Processors each maintain data about necessary corrections in local memory.

Any disparity is flagged and used at the end of the scan by the built-in Fault Analyser routines to determine whether a fault exists on a particular module.

SAFETY PLC




SAFETY PLC




Major Systems




MAJOR SYSTEMS

YOKOGAWA PROSAFE – RS SYSTEM




Yokogawa / ProSafe-RS Achieving absolute integrity between distributed

control systems (DCS) and safety instrumented systems (SIS) for plant automation has traditionally raised complex design and integration issues.

Conventionally, two separate monitoring and operating environments were required for a DCS and SIS. Different communications and distinctive hardware architectures had to be set up.

MAJOR SYSTEMS




Yokogawa / ProSafe-RS

Plant managers striving to optimize process operations have taken it as a given that project time and expenses would escalate.

Now Yokogawa puts an end to DCS-SIS incompatibility with the new ProSafe-RS, the world’s first truly integrated "safety PLC" for the process industries.

MAJOR SYSTEMS




Emerson - DeltaV SLS-

1508

MAJOR SYSTEMS




Emerson Process Management / DeltaV SLS-1508

Emerson extends the proven innovations of their PlantWeb® architecture to safety applications.

The resulting smart safety instrumented system provides an integrated approach to complete safety loops - from sensor to logic solver to final control element.

It also uses digital intelligence and diagnostics to enable more automated safety loop testing and other features that increase system availability while reducing life-cycle costs and easing regulatory compliance.

MAJOR SYSTEMS




Emerson Process Management / DeltaV SLS-1508

As a key component of this smart SIS solution, the DeltaV SIS system takes advantage of the PlantWeb architecture's digital communications and smart diagnostics within field devices to increase the availability of the whole of the Safety Instrumented Function.

Scheduled partial-stroke testing of final control elements can improve the safety level, reduce the number of risky personnel trips into the field, and increase the mandatory proof test interval.

MAJOR SYSTEMS




Ivensys Triconex / Tricon

MAJOR SYSTEMS





The TRICON is a state-of-the-art fault tolerant controller based on a Triple-Modular Redundant (TMR) architecture. TMR employs three isolated, parallel control systems and extensive diagnostics integrated into one system. The system uses two-out-of-three voting to provide high integrity, error-free, uninterrupted process operation with no single point of failure.

MAJOR SYSTEMS





Setting up applications is simplified with the TRICON, because the triplicated TMR system operates as a single control system from the user's point of view.

The extensive diagnostics are inherent and transparent to the programmer. All diagnostic information is stored in system variables and annunciated with Light Emitting Diode (LED) indicators.

MAJOR SYSTEMS




Ivensys Triconex / Tricon The Tricon controller can interface with Modbus

masters and slaves, Distributed Control Systems (DCS), external host computers on Ethernet networks and other Tricon systems on a Peer-to-Peer network.

The TriStation 1131 Developer's Workbench is an integrated tool for developing, testing, and documenting safety and critical process control applications for the Tricon and Trident programmable logic controllers. The programming methodology, user interface, and self-documentation capabilities make the system superior to traditional and competing engineering tools.

MAJOR SYSTEMSSafety Instrumented Systems



TYPICAL QUESTIONS &

ANSWERS




TYPICAL Q & A

1.What is a SIS?

A SIS is a Safety Instrumented System. It is designed to prevent or mitigate hazardous events by taking the process to a safe state when predetermined conditions are violated.

A SIS is composed of a combination of logic solver(s), sensor(s), and final element(s). Other common terms for SISs are safety interlock systems, emergency shutdown systems (ESD), and safety shutdown systems (SSD). A SIS can be one or more Safety Instrumented Functions (SIF).




2. What is a SIF?

SIF stands for Safety Instrumented Function.

A SIF is designed to prevent or mitigate a hazardous event by taking a process to a tolerable risk level. A SIF is composed of a combination of logic solver(s), sensor(s), and final element(s).

A SIF has an assigned SIL level depending on the amount of risk that needs to be reduced.

One or more SIFs comprise a SIS.

TYPICAL Q & A




3. What is SIL?

SIL stands for Safety Integrity Level.

A SIL is a measure of safety system performance, or probability of failure on demand (PFD) for a SIF or SIS.

There are four discrete integrity levels associated with SIL.

The higher the SIL level, the lower the probability of failure on demand for the safety system and the better the system performance.

TYPICAL Q & A




3. What is SIL?

It is important to also note that as the SIL level increases, typically the cost and complexity of the system also increase.

A SIL level applies to an entire system.

Individual products or components do not have SIL ratings.

SIL levels are used when implementing a SIF that must reduce an existing intolerable process risk level to a tolerable risk range.

TYPICAL Q & A




4. What does functional safety mean?

Functional safety is a term used to describe the safety system that is dependent on the correct functioning of the logic solver, sensors, and final elements to achieve the desired risk reduction level.

Functional safety is achieved when every SIF is successfully carried out and the process risk is reduced to the desired level.

TYPICAL Q & A




5. Why were the ANSI/ISA 84, IEC 61508, and IEC 61511

standards developed?

The standards were a natural evolution for the need to reduce process risk and improve safety through a more formalized and quantifiable methodology.

Additionally, and specifically for IEC 61508, as the application and usage of software has evolved and proliferated, there was an increased need to develop a standard to guide system / product designers and developers in what they needed to do to ensure and “claim” that their systems / products were acceptably safe for their intended uses.

TYPICAL Q & A




6. When do I need a SIF or a SIS?

The philosophy of the standards suggests that a SIS or SIF should be implemented only if there is no other non-instrumented way of adequately eliminating or mitigating process risk.

Specifically, the ANSI/ISA-84.00.01-2004 (IEC 61511 Mod) recommends a multi-disciplined team approach that follows the Safety Lifecycle, conducts a process hazard analysis, designs a variety of layers of protection, and finally implements a SIS when a hazardous event cannot be prevented or mitigated with something other than instrumentation.

TYPICAL Q & A




7. What is a proof-test interval?

Proof testing is a requirement of safety instrumented systems to ensure that everything is working and performing as expected.

Testing must include the verification of the entire system, logic solver, sensors, and final elements.

The testing frequency varies for each SIS and is dependent on the technology, system architecture, and target SIL level. The proof-test interval is an important component of the probability of failure on demand calculation for the system.

TYPICAL Q & A




8. What is a Process Hazard Analysis (PHA) and who conducts this?

A PHA is an OSHA directive that identifies safety problems and risks within a process, develops corrective actions to respond to safety issues, and preplans alternative emergency actions if safety systems fail.

The PHA must be conducted by a diverse team that has specific expertise in the process being analyzed. There are many consulting and engineering firms that also provide PHA services. PHA methodologies can include a What-If Analysis, Hazard and Operability Study (HAZOP), Failure Mode and Effects Analysis (FEMA), and a Fault Tree Analysis.

TYPICAL Q & A




9. What voting configurations are required for each SIL level?

Obtaining a desired SIL level is dependent on a multitude of factors. The type of technology employed, the number of system components, the probability of failure on demand (PFD) numbers for each component, the system architecture (e.g., redundancy, voting), and the proof testing intervals all play a significant role in the determination of a SIL level.

There is not a standard answer for what voting configurations are required for each SIL level. The voting architecture must be analyzed in the context of all the factors noted above.

TYPICAL Q & A




10.Will a SIL rated system require increased maintenance?

SIL solutions are certainly not always the most cost-effective solutions for decreasing process risk. Many times, implementing a SIL solution will require increased equipment, which inevitably will require increased maintenance.

Additionally, it is likely that the higher the SIL level, the more frequent the proof testing interval will be, which may ultimately increase the amount of system maintenance that is required. This is why the standards recommend a SIL based solution only when process risk cannot be reduced by other methods.

TYPICAL Q & A




11. Can a F&G system be a SIF or SIS? A Fire and Gas (F&G) system that automatically

initiates process actions to prevent or mitigate a hazardous event and subsequently takes the process to a safe state can be considered a Safety Instrumented Function / Safety Instrumented System.

It is absolutely critical in a F&G system to ensure optimal sensor placement. If there is incorrect placement of the gas / flame detectors and hazardous gases and flames are not adequately detected, then the SIF / SIS will not be effective.

Correct sensor placement is more important than deciding whether a F&G SIF / SIS should be SIL 2 or SIL 3.

TYPICAL Q & A




12. What is SIL 4? SIL 4 is the highest level of risk reduction that can

be obtained through a Safety Instrumented System. However, in the process industry this is not a realistic level and currently there are few, if any, products / systems that support this safety integrity level.

SIL 4 systems are typically so complex and costly that they are not economically beneficial to implement. Additionally, if a process includes so much risk that a SIL 4 system is required to bring it to a safe state, then fundamentally there is a problem in the process design which needs to be addressed by a process change or other non-instrumented method.

TYPICAL Q & ASafety Instrumented Systems



13. Can an individual product be SIL rated?

No. Individual products are only suitable for use in a SIL environment.

A SIL level applies to a Safety Instrumented Function / Safety Instrumented System.

TYPICAL Q & A




14.What type of communication buses or protocols are applicable for SIL 2 or SIL 3 systems?

The type of communication protocol that is suitable for a SIL 2 or SIL 3 system is really dependent on the type of platform that is being used. Options include, but are not limited to: 4-20 mA output signal, ControlNet (Allen Bradley), DeviceNet Safety (Allen Bradley), SafetyNet (MTL), and PROFIsafe. Currently, the ISA SP84 committee is working on developing guidelines for a safety bus, to make sure that the foundations comply with IEC 61508, and IEC 61511 standards.

TYPICAL Q & A


Section-20 Safety Instrumented Systems

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