Fractional dead time

Failure probability The probability-a value from zero to one-that a piece of equipment will fail on demand (not to be confused with fractional dead time) or will fail in a given time interval. 

Fractional dead time The mean fraction of time in which a component or system is unable to operate on demand. 

If the failure rate. A, is the number of occasions per year that a protective system develops a fault (yr ) and the time interval between tests of the device is t years, then intuitively the device on average fails halfway between tests. The probability that the device is inactive (or fractional dead time) is then approximately... 

Figure 6.3. NEMA phantoms for PET performance tests, (a) This NEMA body phantom is used for evaluation of the quality of reconstructed images and simulation of whole body imaging using camera-based coincidence imaging technique, (b) This phantom is used for measuring scatter fraction, dead time, and random counts in PET studies using the NEMA NU 2-2007 standard, (c) Close up end of the sensitivity phantom, (d) Set of six concentric aluminum tubes used in phantom (c) to measure the sensitivity of PET scanners. (Courtesy of Data Spectrum Corporation, Hillborough, NC)...

The simplest technique for measuring the resolving time t of a nonparalyzable counting system uses a method of matched samples. Two samples of similar counting rates are counted separately and then together. The combined sources should give about 20% fractional dead time, obs t. From the difference between the measured count rate of the... 

The probability that the high and low temperature trips and agitator trips will not stop the feed of A is given by their fractional dead time (fdt). This is defined as the fraction of time that the trip is inactive—that is, the probability that it will fail to operate when required. The fdt depends on the failure rate of the trip components and the testing frequency. The following values have been used ... 

The fractional dead time, fdt, which is the percentage of time when the protective device (for example safety valve) does not operate effectively. This is made up of at least two components ... 

Given the above, the probability of the unwanted incident occurring will be D X fdt events per year. The probability can be reduced by installing additional safety devices, say a second safety valve or a pressure switch which deactivates the source of pressure, but each of these will have a fractional dead time, and the incident rate will be reduced to... 

For given values of 0 and, the fractional dead-time and spurious trip rate are controlled through the test interval t, and the mean repair time respectively. 

Target value for Fp is the tolerable risk frequency Ft PFDavg is also known as FDT Fractional Dead Time... 

The alternative name for PFD is the fractional dead-time, FDT. The meaning is clear the fraction of time that the safety system is dead ... 

Recalling the term PFDavg, this is the probability that the protective system will fail to operate when required. The PFDavg is also termed fractional dead time (Fdt) from the description that it is the fraction of time that the protective system is inactive. In summary the relationship is simply ... 

System Fail Safe Fault Rate Faults/Year Fail to Danger Fault Rate Faults/Year Fractional Dead Time... 

The reliability of these protective systems was quantified as the probability that the protective system is in a failed state when a demand is placed upon it. This was a dimensionless number between 0 and 1 also known as the fractional dead time. The reliability of the protective systems was calculated using component failure data, Teesside Power Station proof testing procedures, and generic human error rates for plant operators. 

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