JOTP-011
5 December 2014
APPENDIX G. MARGIN OF SAFETY CALCULATIONS FOR PRESSURE VESSELS
The test depicted in Figure G1 is designed with a consumer or Type I risk of 35 percent and
criterion level of 4.26489. As one can see from the figure, the motor must be better than the
criterion to have much chance of passing the test. Also, the criterion shows how the power of the
test to discriminate between good and bad units increases as the sample size is increased. The
curves in Figure G1 may also be used to estimate the level of extra safety that will have to be built
into the units to ensure a high probability of passing the test. For example, if 10 units are to be
used for testing (5 for burst pressure, 5 for maximum pressure) then to ensure an 80 percent chance
of passing the test, it would be necessary to build units with a pressure difference of approximately
6.75 times as large as the standard deviation of the estimate of the difference. On the other hand,
the pressure difference would only have to be approximately 5.50 times as large if 20 units were
to be used for testing.
G-4
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