Fisher Fisher - On the issue of not having a SIS on the production unit or storage tank farm

On the issue of not having SIS on the production plant or storage tank farm, share your opinions:

First of all, we have to figure out what SIS is for, in fact, SIS is not as mysterious as imagined, its function is the same as safety valves, rupture discs, etc., is a safe protective layer. So what is the difference between SIS and other protective layers, in fact, SIS is used to eliminate residual risks that cannot be eliminated by independent protective layers such as BPCS, safety valves, and rupture discs. If you understand this concept, you will understand the conditions under which you need to equip the SIS system.

For example, there is a reactor, nitrification reaction, the risk is that when the raw material ratio is uneven, the risk is that when the ratio of raw materials is uneven, if the protection of BPCS and safety valve can not reach the expected risk acceptable value, this process needs to install a SIS system. When the SIS system is installed, the interlocking logic of the reactor is that when the temperature or pressure reaches a certain value, the SIS issues a command to interlock the opening valve to eliminate the risk of overpressure explosion.

But if this reactor has a pipeline, straight through the exhaust gas treatment, there is no shut-off valve, as long as there is overpressure, energy can be released from this pipeline, then such a process, do we still have to dogmatically require the SIS system?

The shut-off valve also has a "failure probability when required", and there is no shut-off valve to directly release energy, but after being equipped with SIS, the safety level is reduced, which is a typical snake painting.

There are a lot of articles on the Internet that spray IEC61508 standards and specifications, I personally feel that this standard is still very scientific, many domestic standards and specifications for SIS, GB/T20438, GB/T21109 are basically directly translated, GB/T50770 and SH3018 are also IEC61508 localization.

In the IEC61511, SIS is evaluated according to the SIL level, and the SIS is only required when the required safety level is reached. That is, we often call LOPA analysis, first determine an acceptable risk value, and then find the independent protective layer, the product of all independent protective layers is the "probability of intermediate events", and the target acceptable value is divided, this is the SIL level, which is very simple and very scientific. If SIL1 and above are reached, the independent protective layer is insufficient and a SIS system is needed to eliminate the residual risk. In addition, the former State Administration of Work Safety Order No. 40 has requirements to constitute a major source of danger at the first and second levels, containing liquefied gases, toxic gases and highly toxic liquids, and requires safety instrumented systems.

To sum up: What conditions do you need to install a SIS system?

1. Equipped with an independent safety instrumented system (SIS) for primary or secondary major hazard sources involving toxic gases, liquefied gases, and highly toxic liquids.

2. If the instrument circuit level is SIL1 and above, an independent SIS is required to eliminate safety risks. (This article is controversial because in the text of GB/T50770, SIL1 should be independent of the process control system, that is, independence is recommended but not mandatory, but in the interpretation of the article, it is said that SIL1 should not be implemented in the process control system.)

Fisher Fisher - On the issue of not having a SIS on the production unit or storage tank farm

Fisher Fisher - On the issue of not having a SIS on the production unit or storage tank farm

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