©2016 Engineering Safety Consultants Limited
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.
Table 4shows 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
n Table 4that the PFD of the valve assembly is:
PFD
avg
=
λdu∗Ti
2
PFD
avg
=
2.04E−06∗4
2
Therefore, PFD
avg
= 4.08E
-06
By applying a 3-month partial test on the valve assembly based on the data contained within
Table 4,the remainder of the test staying at 4 years, the PFD can be seen to be:
PFD
avg
=
λdu(pst)∗Ti(pst)
2
+
λdu(npst)∗Ti(npst)
2
PFD
avg
=
7.2E−07∗0.25
2
+
1.26E−06∗4
2
PFD
avg
= 9.0E
-08
+ 2.52E
-06
Therefore, PFD
avg
= 2.62E
-06
This example demonstrates that with this data that the PFDavg is approximately 1.5 times better with
partial stroke testing
. Figure 6taken from HSE guidance OG-00054 Appendix 3 shows the effect on
the PFDavg when partial testing is implemented.