Switch Off Safely—Better Proof Times

The targeted control of analog field devices is one typical application in process automation. Many of these devices are located in a hazardous area and are therefore subject to special requirements for intrinsic safety. Numerous electrical, electronic, and programmable systems must meet the functional safety requirements of the SIL 3 rating according to IEC 61508/IEC 61511 to minimize the risk of malfunctions. In the process industry, this mainly relates to the controlled supply of process media to critical reactive processes.

Regular checking for proper functioning of the inflow valves used for this purpose is not the only requirement. In the event of a process malfunction, safe shutdown must also be ensured at all times.

Functional safety according to SIL 3 is often not optional. It is a mandatory feature for many signal circuits in process plants. Especially when a chemical process is in a critical phase, the automation system must be able to stop the further supply of the process medium immediately to prevent the process getting out of control and resulting in extensive damage.

Minimal hardware effort alone is not sufficient to achieve this target. The inflow of process medium must also be interrupted reliable. Periodic proof testing should not adversely affect plant availability.

Pepperl+Fuchs meets the boundary conditions of functional safety according to SIL 3 without the use of special emergency shutdown valves and safety shut-off valves. Instead, the existing analog positioners are used in a discrete mode to a certain extent. They are either fully open or closed. The functional inspection according to SIL 3 is achieved by actuating these controllers with HART signals for partial stroke tests in addition to their actual function. These tests allow for functional inspection testing without shutting down the entire media supply and without redundant hardware. The KCD2-SCD-Ex1.ES SMART current driver, for example, is suitable for this function up to safety level SIL 3.

SMART current drivers according to SIL 3 are available for mounting on the DIN rail and termination boards. The periodically required proof tests are carried out in the form of partial stroke tests. These tests enable functional inspection without redundant cable routing or the entire process having to be shut down. Plant availability therefore remains unlimited with no restriction on its cost-effectiveness. Even in SIL 2 applications, the use of the devices makes sense: proof times can be extended significantly.

With this solution for functional safety according to SIL 3, Pepperl+Fuchs offers a complete portfolio of isolated barriers for all four signal types.

• Required proof tests without restricting plant availability
• Direct triggering of partial stroke tests (PST) on the inflow controller via HART signals
• Significant extension of proof times in SIL 2 applications by using SIL 3 isolated barriers
• Isolated barriers up to SIL 3 for all four signal types (AI, AO, DI, DO) with minimal hardware effort