Emerson Process Management M310/FSM, Rev AA Designing a Safety Function Using the Level Switch

Horizontal Float Switch

Functional Safety Manual

M310/FSM, Rev AA

January 2012

DESIGNING A SAFETY FUNCTION USING THE LEVEL SWITCH

Safety Function

Environmental Limits

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.

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 product data sheet IP101 for environmental limits.

Application Limits

Design Verification

NOTE:

For all product information and documentation downloads, see www.mobrey.com.

It is very important that the SIF designer checks for material compatibility by considering process liquids and on-site chemical contaminants. If the Mobrey 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 Mobrey level switch are specified in the product data sheet and the product reference manual (see Table 1 on page 5).

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.

A detailed Failure Modes, Effects and Diagnostics Analysis (FMEDA) report for the Mobrey magnetic level switch is available to download from the web sites www.mobrey.com and www.mobrey.com. The FMEDA report details all failure rates and failure modes as well as expected lifetime.

NOTE:

The FMEDA report is available from the Safety quick link at www.mobrey.com. In the right-hand panel, there are SIL documents including the FMEDA report.

The achieved Safety Integrity Level (SIL) of an entire Safety Instrumented Function (SIF) design must be verified by the designer using a PFDAVG 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.

Each subsystem must be checked to assure compliance with minimum Hardware Fault Tolerance (HFT) requirements. When using the Mobrey level switch in a redundant configuration, a common cause factor of at least 5% should be included in the safety integrity calculations.

The failure rate data listed in the FMEDA report is only valid for the useful lifetime of the Mobrey level switch. The failure rates increase after this useful lifetime period has expired. Reliability calculations based on the data listed in the FMEDA report for mission times beyond the lifetime may yield results that are too optimistic, i.e. the calculated SIL will not be achieved.