Protection layers alarm systems

Control layer Protection layer that is used to maintain the process within the normal operating limits, such as standard operating procedures, basic process control system, and process alarms. 

Human factors in the layer of protection analysis with emphasis on alarm system management... 

Proper designing of the alarm system, as the layer of protection in hazardous plant, will contribute to reducing the hmnan error probability (HEP), what will result in decreasing the risk of potential accidents. The human and organizational factors should be also care fully considered in designing and operating of protections to eliminate or reduce probability of latent and active failures (Kosmowski 2007). 

Control System (BPCS), including functions of Supervisory Control and Data Acquisition (SCADA) system, the alarm system (AS) and Safety Instrumented Systems (SIS) performing defined Safety Instrumented Frmetions (SIF). Proper design of layers of protection is based on hazards analysis and risk assessment with consideration of human and organizational factors. It is essential to ensure required safety integrity level (SIL) for each of these layers. 

Figure 5 illustrates the relations between factors influencmg on possibility of latent error commitment which can induce active human errors. Relevant factors should be considered during analysis of protection layers, especially the alarm system and human machine interface (Scarborough et al. 2005). 

In paper a procedure is proposed and outlined, which include more important aspects of the layers of protection designing process in particular the alarm system with emphasis on treating the human and organizational factors. Nowadays issues concerning the... 

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. 

ANSI/ISA-84.01-1996 excluded systems where the operator was the sole means of returning the process to a safe state. ANSI/ISA-84.00.01-2004-1 did not specifically exclude this type of system, but did not explicitly include it either. ANSI/ISA-84.00.01-2004-1 Clauses 11.3.1 through Clauses 11.3.3 provide requirements where the operator is required to take specific action in response to safety critical alarms and diagnostic alarms. When the Hazard and Risk Analysis (H RA) identifies a critical alarm as a protection layer, the detection and response may include many different components, such as sensor(s), logic solver, operator HMI, and final element(s). It is important that all elements, including the operator, be capable of achieving the required risk reduction. ISA-TR84.00.04-1 Annex B provides additional discussion on this subject. 

Like sensor alarm annunciation also play a major role in safety instmmented system as protection layer. Now focus will be on alarm systems. 

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 ... 

Alarm Systems and Layer of Protection Analysis Implications... 

Factors for Alarm Systems as Independent Protection Layer... 

As with other protection layers, the alarm itself is only part of the protection layer. The full protection layer needs to include the alarm, the operator, the machine-operator interface, any communications systems (if communications between operators is required to deliver the required alarm function) and a final element. For the response to the alarm to be included as a protection... 

While an overflow detection system combined with a warning alarm and evacuation procedures may meet the requirements for an effective protection layer in considering the risk to an individual, it may not do so for the overall exposed population. 

For a non-SIL alarm function (in this context, a function that does not conform to the requirements of BS EN 61511 -1 for a safety instrumented function) an overall PFDavg of no less than 0.1 (see BS EN 61511 -1 Table 9) may be used. If, however, there is a view that there could be some increased time pressure on the operators, or other factor making the task conditions less favourable then a higher overall probability of failure may be considered. Note that a component of the protection layer may have a PFD lower than 0.1, but when combined with the rest of the system, it cannot result in an overall PFD lower than 0.1. 

BPCS is a system which handles process control and monitoring for oil and gas facilities. SIS is a system composed of sensors, logic solvers and final elements to keep people , environment , assets in safe conditions during operation periods often oil and gas facility. Physicd mitigation system is a system which consists of active fire protection and fire/blast walls. Among the various multiple protection layers, SIS is the most important and critical protection layer to prevent or reduce the risk of abnormal process condition which may be hazardous. A variety of SISs are installed in the oil and gas plants such as fire gas system, emergency shutdown system and process shutdown system. SIS is the next layer of protection following BPCS and alarm/operator intervention. 

Obviously we need to be sure that each safety layer has a suitable integrity to qualify as a protection layer. For example an alarm system would help with our example but we would have to analyze its effectiveness to be sure of its integrity. 

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. 

In general, the safety of a process relies on multiple layers of protection. The first layer of protection is the process design features. Subsequent layers include control systems, interlocks, safety shutdown systems, protective systems, alarms, and emergency response plans. Inherent safety is a part of all layers of protection however, it is especially directed toward process design features. The best approach to prevent accidents is to add process design features to prevent hazardous situations. An inherently safer plant is more tolerant of operator errors and abnormal conditions. 

Activation of layers of protection such as relief valves, interlocks, rupture disks, blowdown systems, halon systems, vapor release alarms, and fixed water spray systems... 

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