Alarm systems independent protection layer

According to layer of protection analysis (LOPA), alarm is one of the independent protection layer (IPL) used. So in SIS importance of alarm system is quite obvious. In order to have brief but comprehensive discussions on alarm system entire discussions have been sub-divided into ... 

Factors for Alarm Systems as Independent Protection Layer... 

This upset initiates a runaway reaction that can catastrophically rupture the reactor. The impact of this event was judged to be extensive, which, as discussed in Table 6 Note 1, leads to a tolerable frequency of 10 /year for a single scenario. Several failures in the control system could cause this upset, with operating experience indicating that this type of upset occurs about once every 10 years. Protection per Table 5 was the Shortstop addition, but the runaway reaction may be too fast for the operator to respond to an alarm. This protection layer is not included for risk reduction. The area is normally occupied, so it was assumed that personnel could be impacted by the event. The pressure safety valves (PSVs) are only estimated to be 90% effective, since plugging is a common problem in this service. Since the PSVs share a common relief line, they are conservatively considered to be a single Independent Protection Layer. This led to an intermediate event likelihood of a 10 per year. Per the conservative assumptions used in this example, only the PSVs qualified as an IPL. The PHA team reviewed all the process safety risk issues and decided that a SIF was appropriate. As shown in Table 7, this requires a SIL 3 SIF. 

The first two layers are described in section 3.1 and 3.2. Apart fi om these control layers the plant is protected against excursions outside the operating boimdaries by an Alarm Management system and an Instrumented Protective System (IPS). The Alarm Management system warns the operators to take manual action in case the plant moves outside the allowed operating window. The IPS system is fully independent fi om the control and alarm system and can automatically shut down (parts ol) the plant in a safe manner. 

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