Protection is achieved not only by the logic solver but the entire loop including process sensors and final elements (motor controllers, remote actuated valves). The entire loop is designed to sense potential process hazards and take action to reduce risk. High integrity depends on correct functioning of all the parts. This entire loop is called a safety instrumented function (SIF).

No automatic protection system can have perfect integrity but it is clear that higher risk hazards must have higher integrity designs. According to international standards this integrity is defined by “Safety Integrity Levels” called SIL. Four levels are defined with level four indicating the highest integrity and level one indicating the lowest.
With each SIL there are rules specified to reduce design error.

The rules for checking and testing get more stringent with higher integrity. With each SIL there is a maximum probability of hardware failure on demand (PFD). This probability of failure applies to the entire SIF-field equipment and logic solver.

The total PFD is calculated by combining the PFD sensor + PFD logic solver + PFD final element. A study done by exida, the safety consulting firm, has shown that on average a SIL certified logic solver contributes less than 10% of the PFD. The sensors contribute less than 30% and the final elements contribute over 60%.

That is why field instruments are now being designed to meet the tough requirements of IEC 61508. This helps field instruments achieve higher quality and integrity, like the IEC 61508 certified logic solvers.

In a good process design, hazards do not occur often. The safety valve may not move for a long time. Failure modes of the valve then include seat leakage, binding and breakage. Binding and breakage failures can prevent the valve from moving to a safe position. These failures can dominate the PFD of the SIF.

To improve safety and availability the valve must be checked periodically. This can be done with a full stroke test and leak test. Unless a special bypass valve has been installed, the process must be shut down. This testing can be costly.

Partial stroke testing (PST) is a technique that partially moves the valve. With pressure and position feedback sensors this technique can detect many of the actuator and valve failures.

According to studies by exida, PST can detect 50% to 80% of the dangerous failures depending on valve type and shutoff requirements. Using PST on a remote actuated valve can substantially reduce PFD and will reduce false shutdown by diagnosing future failures. PST can be initiated by the independent PST device or by the other software like asset management.

Asset management software can also help improve the safety and availability of a SIF. Proof testing can be done with less cost and more accuracy. Additional diagnostic information is automatically collected. Proof test records are automatically generated and stored. Additional data can be collected via digital communication with the field devices to obtain all the benefits of asset management for the safety system as well as the DCS. Yokogawa offers PRM as it's asset management solution.