Emerson Mobrey Series, Руководство по функциональной безопасности

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7
Functional Safety Manual
M310/FSM, Rev BA
Designing a Safety Function Using the Level Switch
April 2017
Designing a Safety Function Using the Level Switch
Section 3 Designing a Safety Function
Using the Level Switch
Note
For all product information and documentation downloads, see the on-line
Mobrey Magnetic Level Switch web page at
Emerson.com/Mobrey .
3.1 Safety function
A change in liquid level through the operating range of the float causes the switch to operate. It may be
used in high level or low level safety related applications. In either case, it is recommended to use the set
of contacts (A-A or B-B) which are Open in the Fail Safe State.
3.2 Environmental limits
The designer of the SIF (Safety Instrumented Function) must check that the level switch is rated for use
within the expected environmental limits. See the Mobrey Magnetic Level Switch
Product Data Sheet for
environmental limits.
3.3 Application limits
It is very important that the SIF designer checks for material compatibility by considering process liquids
and on-site chemical contaminants. If the level switch is used outside the application limits or with
incompatible materials, the reliability data and predicted SIL capability becomes invalid.
The construction materials of a level switch are specified in the product data sheet and the product
reference manual (see
Table 1-2 on page 3 ). Use the model code on the product label, and the ordering
information table and specification in these product documents, to find out the construction materials.
3.4 Design verification
A detailed Failure Modes, Effects and Diagnostics Analysis (FMEDA) report for the Mobrey Magnetic Level
Switch is available from Emerson. This report details all failure rates and failure modes as well as
expected lifetime.
Note
The FMEDA report is available from the Mobrey Magnetic Horizontal Float Switches web site page at
Emerson.com/Mobrey . In the Documents section, there are SIL documents including the FMEDA report
and this safety manual.
The achieved Safety Integrity Level (SIL) of an entire Safety Instrumented Function (SIF) design must be
verified by the designer using a PFD
AVG
calculation considering the architecture, proof-test interval,
proof-test effectiveness, any automatic diagnostics, average repair time, and the specific failures rates of
all equipment included in the SIF.