SMAR LD400-SIS and TT400-SIS: a valuable aid to safety systems

Aiming at completing its pressure transmitter line, SMAR developed recently two equipments for SIL (Safety Integrity Level) applications: the LD400-HART-SIS and the TT400-HART-SIS. Both have hardware with high MTBF (Mean Time Between Failure) and advanced diagnostics.

The LD400-HART-SIS is an intelligent pressure transmitter for measurements of differential, absolute and gauge pressures, and level and flow applications. The equipment 4-20mA output signal is equivalent to the applied pressure. This information is sent to a PLC and is shown on the display, or monitored remotely via HART communication. The LD400-SIS is TÜV - certified for safety applications

The TT400-SIS, also with TÜV certification, is a transmitter mainly for temperature measuring with RTD or thermocouples. These equipments accept up two sensors and can operate in any of the following modes:

• Simple measuring with only one sensor providing a current output, or via communication;
• Differential pressure, two sensors of the same type providing a current output or via communication;
• Backup measuring, two same type sensors providing a current output or via communication;
• Maximum, minimum or median measuring with two similar sensors providing a current output, only in HART® mode.

The SIS equipments have advanced diagnostics. In the event of a failure, if it is impossible to measure the pressure or the temperature compliant to their specifications, the HART protocol will report what is happening. The process may be put in a safe state, avoiding that this condition affects the loop current. Thus, the failure turns into a safe condition, using the high SFF (Safe Failure Fraction) value, and the HART protocol helps improve this fraction in safe and protected measuring.

Safety systems aim at minimizing risks at acceptable levels, so, the SIL level for a control loop can be determined by the analysis and identification of process risks. The SIL level can be verified by the PFD (Probability of Failure under Demand) test.

The IEC 61508 standard defines the requirements for a system´s functionality and integrity. These functionality requirements are process-based, whereas the integrity requirements are oriented to trust ability, which is defined as the Safety Integrity Level (SIL). There are four discreet levels with these important properties:

1. Applicable totally to safety;
2. The higher the SIL level, the more strict are the requirements;
3. Applicable to technical and non-technical requirements.

The SIL evaluation has grown mainly in chemical and petrochemical applications. To Cesar Cassiolato, SMAR Marketing, Quality, Project and Services Engineering Director, the need for the SIL level may be expressed by the probable impact on the plant and the community. “This impact can be small to the industry and extremely harmful to the community, but when well applied on the process, this measure will offer protection to both”, he explains.