Functional Safety 2016

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7 Proof testing categories
7.1 Partial testing
Partial testing is the term used for tests which do not reveal all possible failure modes and are therefore
tests with PTC <100%.
A partial test example is partial stroke checking of a valve, this would involve closing / opening a valve
from its fully open / close position by a small percentage between 10 and 20%. This test would likely be
credited of a PTC of <100%.
The failure modes of the equipment, for example a valve arrangement, must be analysed to see if Partial
Stroke Testing would detect the failure. shows an example for a typical valve arrangement.
Table 4 - Valve Arrangement analysis for partial testing
Failure Mode
Failure Rate
(per year)
Detected by
partial stroke
testing?
Solenoid fails to
vent 6.00E-08 Yes
Valve sticks open 7.20E-07 Yes
Valve doesn’t fully
close or passes 1.80E-07 No
Other unknown
failures 1.08E-06 No
TOTAL 2.04E-06
Without partial testing is can be seen that with a 4-year test based on the failure rate data i
that the PFD of the valve assembly is:
PFDavg =
λdu∗Ti
2
PFDavg =
2.04E−06∗4
2
Therefore, PFDavg = 4.08E-06
By applying a 3-month partial test on the valve assembly based on the data contained within
the remainder of the test staying at 4 years, the PFD can be seen to be:
PFDavg =
λdu(pst)∗Ti(pst)
2 +
λdu(npst)∗Ti(npst)
2
PFDavg =
7.2E−07∗0.25
2
+
1.26E−06∗4
2
PFDavg = 9.0E-08 + 2.52E-06
Therefore, PFDavg = 2.62E-06
This example demonstrates that with this data that the PFDavg is approximately 1.5 times better with
partial stroke testing taken from HSE guidance OG-00054 Appendix 3 shows the effect on
the PFDavg when partial testing is implemented.