过程行业中的 SIL

Product datasheets, certificates and model calculations for functional safety use a series of reference data and terms. Here are the most important ones for the SIL calculation:

• λ (failure rate ), the following classifications apply: S for the overall rate of safe failures; SD for the rate of safe, detected failures; SU for the rate of safe, undetected failures; D for the overall rate of dangerous failures; DD for the rate of dangerous, detected failures; and DU for the rate of dangerous, undetected failures.

• Device types: A is the code for a device for which the failure behaviour of all components used and the failure characteristics are adequately determined, e.g. through operational reliability. Device type B, on the other hand, means that the failure behaviour of at least one component used and the behaviour in the event of a failure are not adequately determined.

• HFT (hardware failure tolerance): the ability to continue to execute the required function in the event of faults and deviations. With HFT0, a single fault can result in the loss of the safety function (e.g. in 1oo1 circuits). With HFT1, a safety loss only occurs if at least two faults occur simultaneously (e.g. in 1oo2 circuits). With HFT2, at least three faults must occur simultaneously (e.g. 1oo3 circuits).

• High demand: operating mode with a high frequency of demands or continuous demands to activate the safety system. It operates continuously or its activation is demanded more than once a year.

• Low demand: operating mode with a low frequency of demands to activate the safety system. It must not be activated more than once a year.

• MTBF (mean time between failures): the mean time between two successive failures.

• PFD (Probability of failure on demand): the probability that a safety function will fail in low-demand mode (demands/year < 10) = low demand.

• PFH (Probability of failure per hour): the probability that a safety function will fail during continuous use (demands/year > 10) = high demand.

• SFF (safe failure fraction): the proportion of safe failures out of the total number of failures.

A SIL circuit generally consists of three segments:

• Sensors (e.g. pressure sensors, temperature sensors and level gauges)
• Evaluation and output unit (e.g. safety PLC)
• Automated process valve unit comprising solenoid valve, actuator and process valve

The distribution of the failure probabilities to the subsystems of a safety function is as follows for single-channel systems: the greatest weight is given to the SD failure rate of the actuators.

All failure probabilities required for the SIL calculation can be found in the manufacturer’s declarations or certificates (highlighted in blue). You can use these to calculate the total probability of failure (the values ​​highlighted in grey) according to the SIL.

The higher the required safety level of a system, the higher the level of independence that the standard requires for the body assessing the functional safety. According to IEC 61511, manufacturer’s declarations are perfectly adequate up to SIL2. For SIL3 and above, the certificate must be issued by an independent organisation such as TÜV or Exida.

Safety integrity level – assessing body

SIL1 – independent person

SIL2 – independent department

SIL3 – independent organisation

SIL4 – independent organisation

You can find all SIL certificates and SIL manufacturer’s declarations for Festo products on the relevant product detail page in the "Certificates" category of the "Product Support" section.

Festo can offer you the right redundant control for every safety requirement:

Redundant NAMUR block (1oo2, 2oo2): the NAMUR block enables two solenoid valves with a NAMUR connection pattern to be installed, which are wired redundantly via the NAMUR interface. The blocks are available in fail-safe function (1oo2) or with increased availability (2oo2). You can mount the block directly on quarter turn actuators using the interface. Separate installation with suitable piping is also possible.

Redundant inline valves (1oo2, 2oo2): in these compact systems, Festo uses the tried-and-tested VOFD valve technology. The valve’s redundant circuit ensures a redundant fail-safe function (1oo2) or provides increased availability (2oo2) for automated process valves. Thanks to the Ematal coating, these valves meet the highest safety standards in process engineering and can withstand the toughest of ambient conditions.

Combined valve block (2oo3): the 2oo3 system combines both technologies, therefore providing the highest level of safety and availability. This valve block is an in-line variant that is integrated into your system. The installed standard valves are defined and mounted on the block via the NAMUR interface in accordance with VDI/VDE 3845. This means that the block is installed once; only the valves are replaced via the interface according to a service life/safety lifecycle plan. With this system, you can also bypass the functions of the four valves so that maintenance can be performed during operation. The pressure indicators mounted directly on the valve block always give a reliable and swift indication if a valve is pressurised.