Failure Fraction

CRF chronic renal failure fractional excretion of urea... 

The extent to which faults lead to a safe condition or can be detected by diagnostics so that a specified action can be taken. This capability is termed the safe failure fraction of the device ... 

Instrumentation in safety applications (SIS) utilises vendor information on diagnostics and safe failure fraction (SFF) as well as performance information collected from the applications to calculate the probability of failure on demand (PFD). 

The reference fuel performance models account for the effect of partially failed fuel particles on the fission gas and fission metal releases the particles with failed SiC but intact OPyC coatings retain fission gases but not metals. The peak predicted core-averaged fuel particle failures from all mechanisms are given in Table 4.2-17, which shows the failure fractions used to predict both the fission gas and fission metal releases. Time histories of particle failures are shown in Figures 4.2-21 and 4.2-22 for the fissile and fertile particles, respectively. 

At each reload, the fuel failure fraction in the core decreases due to the replacement of the oldest one half of the fuel in the core with fresh fuel. The results indicated that the predicted fuel particle failure due to temperature effects was very small even at the high temperature points (at the outer boundary of the active core in the proximity of the control rods) since such high fuel temperatures are maintained for only short periods of time. The predicted pressure vessel failure was negligible. Thus, the overall particle failure was predicted to be caused by manufacturing defects, primarily by particles with missing buffer layers. 

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... 

If these restrictions are met, only one transmitter or valve is needed for a S1L2 SIF to meet this requirement. Alternatively the charts of lEC 61508 may be used for field devices. Given the lack of definition as to what "prior use" really means, the authors prefer to use the tables from lEC 61508 which are more flexible, provide at least the same level of "exception" for products with sufficient design quahty and are clearly justifiable. The disadvantage of these charts is that the safe failure fraction must be calculated for the field devices. 

The use of this chart requires the calculation of a measure called the Safe Failure Fraction (SFF). This chart is completely equivalent to the same chart in lEC 61508. 

Safe Failure Fraction Hardware Fault Tolerance ... 

To determine the architectural requirements, the SFF number is calculated. This applies to each SIF subsystem, i.e., sensor, logic solver, and final element. To calculate the Safe Failure Fraction for the pressure switch we must first calculate and. ... 

Component Failure rates (1/hr) Architectural Constraint Type Safe Failure Fraction... 

These can be retained by intact coating layers, then these are released from failed particles. Noble gases exist in the gas phase and can be removed by primary coolant purification system. Though some of them have condensable daughter FPs, this effect is not large at failure fraction of HTGR fuel. 

The through coating and SiC coating failure fractions are assumed to be 4x10 and 1.6x10, respectively, which are one order lower than expected values in the HTTR. The maximum fuel temperature is assumed to be 1300 C and SiC layer thickness is 35 pm. 

The fractional release of cesium from fuel can be reduced as low as lO" -10 by minimizing failure firaction, however, diffusive release from intact particle should be reduced to attain lower fractional release than 10. For silver, since diffusive release fraction from intact particle is higher than that of cesium, the fractional release does not depend on the failure fraction. The countermeasures to decrease diffusive release from intact particle are reduction of fuel temperature (the maximum temperature 1100 °C) and/or adoption of diffusive resistant material as coating layer (ZrC layer). 

To obtain data on fission product release from a fuel element with a known fuel failure fraction, in particular to confirm the effects of high pressure and high flow on release. 

The required SIL is shown with the relationship between hardware fault tolerance (HWFT) and safe failure fraction (SFF) for two types in Table 4. 

Installation 1 is more than 20 years old. Barrier data have been reported since 2004. The fraction of failures for isolation valves on risers has been above the NCS average in all years, except 2007. Also for Downhole Safety Valves (DHSV) and Blowdown valves (BDV) are average values as well as values in 2007 for Installation 1 above the average for NCS. For the DHSV failure fraction, the trend is increasing over the years, possibly indicating that this is an age dependency. 

It may be observed from Figure 6 the failure fractions for riser ESV are above the industry average, except in 2007. It may further be observed that failure fractions for Downhole Safety Valves are above the industry average from 2003 and onwards, and the trend is clearly increasing. 

Failure fractions for Blowout Preventer have also been above the industry average in 2004 and 2005, but have been sharply falling. The reliabihty of the reporting schemes have been imcertain imtil recently, for this barrier element in particular. 

Company X has several relatively old installations, the somewhat higher failure fractions for some of the isolation valves may be influenced by be age dependency. 

The fraction of the overall failure rate of a device that results in either a safe fault or a diagnosed (detected) unsafe fault. The safe failure fraction includes the detectable dangerous failures when those failures are annunciated and procedures for repair or shutdown are in place. 

Method Execution time on a silicon graphics CRIMSON workstation (qm s/U3 moves) Fraction of geomdric failures Fraction of accepted moves (based cm total moves)... 

ANSI/ISA-84.00.01-2004-1 requires the use of minimum fault tolerance (i.e. device redundancy) to ensure that adequate protection is provided. The required fault tolerance is related to the device complexity. It is important to note that the device s safe failures tend to drive the process toward the safe state, whereas the safe failure fraction is based on the safe failures and the dangerous detected failures. Thus, there is an implicit assumption in the safe failure fraction that the requirements of ANSI/ISA-84.00.01-2004-1, Clause 11.3, are met. Refer to ISA-TR84.00.04-1, Clause K.4, for more information concerning the safe failure fraction. 

Clauses 11.3 and 16. Othen/vise, the safe failure fraction becomes excessively optimistic. The owner/operator should account for this when developing operating and alarm response procedures. Refer to ISA-TR. 00.04-1, Annex B, for more guidance on operator response as part of an SIF. 

In other words, the safe-failure fraction is a measure that indicates the probability of a subsystem failure being either safe or detected by diagnostics. The measure is applied to each major subsystem in a safety instrumented function (sensor, logic solver, final element) separately. 

The random hardware failure results are also used to calculate the Safe Failure Fraction (SFF) of the device and the PFDavg or Frequency of Failure for continuous-mode devices. The PFDavg provided by the manufacturer is based upon specific proof-test intervals and the mean time to repair specified by the manufacturer. For different assumptions, the user can use the fundamental failure rates provided and determine a PFDavg for their specific application. Details on SFF and PFDavg are included in Annex K. 

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