FIGURE 2 shows an improved arrangement for separating water from the
hydrocarbon liquid for the process in question 2. Two simple level
control loops, LC, have been added to regulate flow control valves on a
pumped system. Extra protection is also provided in the form of high
level alarms, LA, one to each side of a baffle plate fitted in the tank.
It has been determined that estimating the probable frequency of
hydrocarbon passing to the water drain will permit judgement as to
whether this would be an acceptable risk.
FIGURE 3 is the
cause treefor this TOP event, showing the conventional
'AND' and 'OR' gates together with typical fail-to-danger fault
frequencies
for the control and alarm equipment, including estimates for
electricity and instrument air failure. Also shown are
probabilities(3%)
for operator failure to respond to the alarm system which is rated at 97%
reliable.
(a) Complete the conversion of this cause tree into a FAULT tree,
allowing for testing the alarm system once per week. (Use the simple
fractional dead time formula.)
(b) State which pieces of equipment have the greatest effect on the top
event frequency.
(c) If the reliability of the level detector and controller can be improved
from a fault frequency of 2.36 and 1.64 respectively to 0.5 per year,
determine the new top event frequency.