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

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.