IEC
61508 Frequently Asked Questions
- What is a process plant Safety Instrumented
System (SIS)?
- What are the industry standards for SIS?
- Are SIS requirements new?
- Has Rosemount supplied products for SIS
loops?
- What are the process plant requirements for
Emerson Process Management devices used in SIS loops when IEC 61508 is
applied?
- Will Rosemount provide reliability
assessment results?
- Does this mean the Rosemount 3051C,
Rosemount 3051T,
and Rosemount 3144P are "certified for use" per SIL 2?
- Can the Rosemount 3051 or Rosemount 3144P be SIL approved by TÜV, FM,
or exida.com for SIS applications?
- How do I order a Rosemount Transmitter
for an SIS application?
- What is IEC 61511?
- Will other Emerson Process Management devices be
available for use in IEC 61508 applications?
1. What is a process plant Safety
Instrumented System (SIS)?
Any process loop identified as a safety system is referred
to as a Safety Instrumented System (SIS). In the past, these
loops were defined as Emergency Shutdown (ESD) or Safety
Shutdown (SSD). The purpose of an SIS is to reduce the risk of
an accident.
SIS loops defined: "An SIS is a
distinct, reliable system used to safeguard a process to
prevent a catastrophic release of toxic, flammable, or
explosive chemicals." (Ref. ISA S84.01)
SIS loop scope: "System composed of sensors,
logic solvers, and final
control elements for the purpose of taking a process
to a safe state, when predetermined conditions are
violated." (Ref ISA S84.01 and IEC 61508)
Thus, SIS is composed of the entire control loop. This
loop will likely consists of pressure and temperature
transmitters, Control Valves, DCS (or PLC and SLC), plumbing,
wiring, power supplies, and other loop equipment.
2. What are the industry standards for
SIS?
In the past, there were many different standards covering
SIS application and use. The dominant standards are ISA S84.01
"Application of Safety Instrumented Systems for the
Process Industry " and DIN 19250/19251.
In an effort to harmonize under one
standard, and to address the increased use of micro-processor
based logic solvers (PLCs) and SMART field devices in SIS
loops, the IECTC65 developed two new standards:
IEC 61508 "Functional Safety of Electrical / Electronic /
Programmable Electronic Safety-related Systems."
This standard is for suppliers of microprocessor-based
instrumentation to Process Control, Medical, and Avionics
industries.
IEC 61511 "Functional Safety: Safety Instrumented Systems
for the process industry sector." This
standard is for end-users and engineering firms detailing
the requirements for design and implementation of SIS for
Process Control Industry.
The purpose of these standards is to assist users in
reducing risk. Each of these standards detail different levels of safety
referred to as Safety Integrity Levels 1-4 (SIL 1-4). (NOTE
DIN 19250 uses AK levels 1-8 with similar meaning). The higher
the SIL level the more critical the safety and therefore an increased need to reduce risk.
The basic objectives of the standards are to ensure availability
of the safety loop. Another way of stating this is to ensure
that as many loop failure modes as possible are detectable
such that the loop can execute its shutdown strategy or alert the operator before an accident occurs.
One additional note: Discussions are currently
underway between the ISA and IEC groups to standardize on IEC
61511 as the global SIS standard. When completed, IEC 61511 will become a global
requirement for all process control industries.
3. Are SIS requirements new?
No. SIS has been implemented for many years in all process
industries. The issue has recently gained importance as
government agencies have pushed regulatory codes for safety
management. The emergence of microprocessors and critical
software in logic solvers and field instruments is also a
factor.
Each process plant already has in place a detailed Company
Procedures and Policies for SIS loop identification,
engineering, specification, commissioning and
maintenance. When a process plant decides to implement
IEC 61511, additional requirements will be needed
for compliance. These requirements are primarily in the
methodologies and documentation of how the safety loop was
designed and how it will be maintained through the life of the
plant.
4. Has Rosemount supplied products for SIS
loops?
YES! Rosemount has enjoyed the largest market share
of pressure and temperature measurement points in both process
and SIS applications. Rosemount Model 1151 and 3051 pressure transmitters
and Rosemount 3144 temperature transmitters are preferred due to their high reliability,
performance, and fast response time.
In fact, there have been instances where Rosemount has lost
a transmitter project to a competitor, yet the project
management choose to purchase Rosemount transmitters for the
SIS loops due to the need for high reliability and fast
response time.
5. What are the process plant requirements
for Emerson Process Management devices used in SIS loops when IEC 61508 is
applied?
When a plant implements IEC 61511 they will be required to
use specific methodologies to document that the loop design
meets the intended SIL level 1-4. These methodologies are
quantitative and require specific data on each device
per IEC 61508.
The specific data required to determine Device SIL 1, 2, and
3 are documented in IEC61508 as follows:
- Safe Failure Fraction (SFF) (Diagnostic Coverage Factors)
- Safe Detected
- Safe Undetected
- Dangerous Detected
- Dangerous Undetected
- Mean time to repair
- Frequency of Functional Check Requirements
Rosemount publishes the
required IEC 61508 data for their devices so that
customers can calculate the SIS instrument loop value.
6. Will Rosemount provide reliability
assessment results?
Rosemount will post the the reliability evaluations on
their website, www.rosemount.com.
3051S Pressure Transmitter,
Report No.: Ros 01/12-04 R210 (PDF
@575Kb)
3051C Pressure Transmitter, Report No.: Ros
03/10-11 R100
(PDF @460kb)
3051T Pressure Transmitter, Report No.: Ros
01/12-01 R210
(PDF @510kb)
3144P HART Temperature
Transmitter, Report No.: Ros 01/06-01 R110 (PDF
@580kb)
644 HART Temperature Transmitter, Report No.: Ros 03/05-11 R001
(PDF @580kb)
8732C
Magnetic Flow Transmitter, Report No.: Ros 03/07-26, R001
(PDF @520kb)
8800C Vortex Flowmeter, Report No.: Ros 02/07-14 R001
(PDF @540kb)
7. Does this mean the Rosemount 3051C,
Rosemount
3051T, and Rosemount 3144P are "certified for use" per SIL 2?
No. The design of the Rosemount 3051C, 3051T and 3144P transmitters
have PFD and SFF
values that are in the SIL 2 category. However, to receive SIL
2 certification, the design must meet specific
interpretations of IEC 61508.
However, a device can be reviewed and approved for
"applicability or use" in a SIL 1-4 SIS loop.
IEC 61511 allows plants to use devices designed per IEC 61508
or to use devices proven-in-use. The Rosemount 3051C and
3144P are proven-in-use by our customers, and therefore to use these
devices only the data described in Section 5
will be required.
8. Can the Rosemount 3051 or Rosemount 3144P be SIL approved by TÜV,
FM, or exida.com for SIS applications?
No -- a single measurement device cannot be approved or
certified per a SIL level. The reason is that the SIL level 1-4
applies to the entire SIS loop (Transmitter + Logic Solvers +
Valves + power supply + other loop equipment.) as defined in ISA
S84.01 and IEC 61508.
The end user must define the level of SIS required for the
application and determine the level of documentation
required. What we can say is the Rosemount 3051 and the
Rosemount
3144P transmitters are suitable for
use in SIS applications due to our quality control, quality
design, and history of high reliability in actual field
conditions, MTBF data, and the TÜV or exida.com review.
9. How do I order a Rosemount
Transmitter for an SIS application?
- Specify the Rosemount 3051C, Rosemount 3051T, or
Rosemount 3144P transmitter as you would have
previously for an SIS application.
- Request "Option Code QS." This option
code will ensure the Rosemount IEC 61508
Certification papers are sent with the transmitter.
10. What is IEC 61511?
Discussions are currently underway between ISA and IEC to
develop a single global standard for process control SIS applications
defined as
IEC 61511. When completed, IEC 61511 will become a global
requirement for all process control industries. The
completion target date for IEC 61511 has not yet been set.
11. Will other Emerson Process
Management devices
be available for use in IEC 61508 applications?
YES. Fisher Controls has already submitted the Model
DVC6000 for review. Other devices scheduled for review
include the Emerson Process Management DeltaV System.
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Emerson Process Management,
Rosemount Division, 8200 Market Blvd.
Chanhassen, MN USA 55317
North America: 1-800-999-9307 International:
952-906-8888 Fax: 952-949-7001
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