Safety integrity level availability

Results of the evaluation typically include a number of safety integrity and availability measurements. Most important, the average probability of failure on demand (PFDavg) and the safe failure fraction (SFF) is calculated for low demand mode. Probability of failure per hour is calculated for high demand mode. From charts, the SIL level that the... 

Often, time to failure data is not available for a collection of components. Incomplete data can be used to estimate failure rates imder these circumstances but one must be very careful, especially when estimating failure rate data to be used for probabilistic SIF verification. Results that are not conservative can lead designers to believe that safety integrity levels are higher than they really are. 

SIL 4 is the highest safety integrity level. While some SIL 4 certified components such as controllers and sensors are available, achieving a SIL 4 rated process applicaton is ui extremely complex endeavor and the author knows of no SIL 4 process applications. Some experts are of the opinion that if the process or apphcation is such that it requires SIL 4 to ensure Scdety integrity then one should reconsider whether that process or application should be done in the first place. 

In Chapter I, risk matrix has been discussed. Risk graph is similarly used to categorize risks additionally it can be used to determine SIL in process industries (especially when there many hazards to analyze). The discussions are presented naturally in a generalized way, however for each application one needs to calibrate the same. In this approach, there are four sets of parameters, viz. C, F, P, and W (see Fig. VII/1.3.4 1), each arranged as a column. All of these parameters describe the nature of the hazardous situation when safety instrumented systems fail or are not available. One parameter is chosen from each of four sets and the selected parameters are then combined to decide the safety integrity level allocated to the safety instrumented functions (Annex Dlof IS/IEC 61511 3 2003). Let an example with exposure to radioactive radiation case be analyzed. 

The safety evaluation tool is a free online calculation tool for the lEC 62061 and ISO 13849-1 standards, and offers valuable support for the implementation of safety functions on machines. The third party approved online tool guides are available for step-by-step support from definition of the safety system stmcture through selection of the components, all the way to determination of the achieved safety integrity level (SIL/PL) [12]. The following are the major benefits of it ... 

Table 11.2 Typical Safety Integrity Levels (SIL) SIL Availability...

Safety integrity level 1 equates to a simple non-redundant single path designed to fail safe with a typical availability of 0.99. Level 2 involves a partially redundant logic structure, with redundant independent paths for elements with lower availability. Overall availability is in the range of 0.999. Level 3 is composed of a totally redundant logic structure. Redundant independent circuits are used for the total interlock system. Diversity is considered an important factor and is used where appropriate. Fault tolerance is enhanced since a single fault of an ESD system component is unlikely to result in a loss of process protection. 

Dependability and safety parameters in Europe railway domain are specified in standards (CENELEC, 2000, CENELEC, 2003). According to standard (lEC, 1990) dependability is the collective term used to describe the availability performance and its influencing factors reliability performance, maintainability performance and maintenance support performance . Safety levels are characterized by tolerable hazard rate for Safety Integrity Levels (SIL) (CENELEC, 2003). 

The upset in Event 4 is similar to the upset in Event 1. Operator intervention can stop this runaway by starting the steam turbine driven water pumps, or adding Shortstop. While this operator action was judged to be very effective, no risk reduction credit was taken because of operator availability. The analysis shown in Table 7 led to safety integrity level 3 for SIF S-1. 

Pulsed electronic logic can offer high safety integrity. However, some functions are not available with pulsed solid state systems or electronic logic, such as calculation capability, higher-level communications, and networking. 

W. Goble, J.C. Grebe, White Paper Hybrid Fault Tolerant Architecture New Levels of Performance, Availability and Safety Integrity, August 27, 2008. Exida RTP 3000 System Report No. RTP 06/10el6 R002 Version VI, Revision R2. 

Interface). The former tends to be used where high levels of functionality and data rates are needed whereas the latter, which is based on the controller area network (CAN) protocol, is used in applications where there are lower functionality and simple input/output requirements. The manufacturers of these fieldbus systems have worked on developing them for use in safety applications, mainly to incorporate appropriate levels of fault tolerance or safety integrity. This has led to the availability of the Profisafe and AS-Isafe fieldbuses. In addition, PILZ has developed the SafetyBUS fieldbus for safety applications, which is again based on the CAN protocol, and the Open Devicenet Vendors Association has developed a safety version of the DeviceNet fieldbus called DeviceNet Safety. 

If no specific safety integrity criteria apply for the application under consideration then the aim should be to ensure that the safety integrity achieved is not inferior to that which would have been achieved in similar situations using a non PES based system (ie similar level of hazards, frequency of access etc). In this case no specific safety integrity criteria is available and so a comparison of this type with similar situations is necessary. 

System safety is more than the sum of the parts. In most situations, safety is achieved via the integration of a number of systems/sub-systems/ components, which rely on a variety of technologies (be they mechanical, hydraulic, pneumatic, electrical, electronic, programmable electronic, etc.), which is then put into an environment where it has to function safely as an operational system. It is this environment (i.e. physical installation as well as operational application) which highlights a deficiency in the DEF STAN 00-35 definition of a system, which does not clearly differentiate the various system levels available. 

Electrical Isolation. These two circuits are integrated to create one circuit through a safety barrier. The purpose of this safety barrier or protective interface is to ensure electrical isolation so that the higher levels of energy available in the non-hazardous circuits cannot be transmitted to and through the circuits in the hazardous area. 

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