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Fault Tree Analysis. Part 5: Digraph-Based Fault Tree Synthesis Procedure (Multiple Loops). HEAT EXCHANGER WITH MULTIPLE FEEDBACK LOOPS. V 1. 10. 1. 2. A. O. 3. 4. HOT. T. 9. 5. 6. SET PT. TRC. V 2. A.C. AIR SUPPLY. 8. COLD. 7. T3. T3. - PowerPoint PPT Presentation
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Fault Tree Analysis
Part 5: Digraph-Based Fault Tree Synthesis Procedure (Multiple Loops)
HEAT EXCHANGER WITH MULTIPLE FEEDBACK LOOPS
HOT
V 2 A.C.
COLD8
9
6
3
T
4
5
SET PT.
AIR
SUPPLY
1021
7
TRC
V 1
A. O.
T3
T3
MULTIPLE FEEDBACK LOOP OPERATOR
IF the Output Variable Is On Two NFBLs of Equal Power and Speed
Output (Value)
OR
Large or
Fast Disturbances
Off both Loops
Loops
Pass
Disturbances
A
AND
OR OR
Inputs (Value)
(Off both NFBLs)
Inact Both Loop I Causes
Loop II Normal
Loop I Normal
Loop II CausesAND
OR
Inact Loop I
OR
Inact Loop II
AND
EOR
Loop I
Causes
Loop II
OK
(prob = 1)
AND
Loop I
OK
(prob = 1)
EOR
Loop II
Causes
Disturbances Inactive Loops
A
OR
BOTH
CAUSE
Loop I Inact.
Loop II Cause
Loop I Cause
Loop II Inact.
AND
EOR EOR
Loop I
Causes
Loop II
Causes
AND
EOR OR
Loop I
Causes
Loop II
Causes
AND
EOR
Inact.
Loop II
OR
Inact.
Loop I
T4(+1)
OR -1
T3(+1)
OR -2
OR -3
Fire atHx(+10)
T2(+10)
OR -14
T1(+10)
T9(+10)OR -15
T8(+10)
2 NFBL
AND -16
OR -4
(page 1)
(page 2)
(page 3)
AND -16
OR -17
Fire atHx (+1)
T2(+1)
OR -20
T1(+1)
T9(+1)
OR -21
T8(+1)
OR -18
AND -19
-9Loop IInactive
-11Loop IIinactive
-6Loop Icause
-7Loop IIcause
(page 2)
(page 3) (page 3)
(page 3) (page 3)
OR -4
AND -8 AND -10 AND -5
EOR -6 EOR -7
M2(+1)(page A)
M9(-1)(page B)
-7 OR -9
OR -12
V1stuck
TRCstuck
TRC onmanual
Sensorstuck
-6
OR -11
OR -13
V2 stuck
TRCstuck
TRC onmanual
Sensorstuck
(page 3)
2M ( 1)
OR
OR
1M ( 10)
1VFalls
Open
(+1)
AND
OR
1M ( 1)
-23
-12
-25
-26
EOR
1V
Reversed7
P ( 1)
OR
OR
Set
Point
(+1)
Air
Press.
(+10)
TRC
Falls
High
(+1)
AND
Air
Press.
(+1)
OR
TRC
Stuck
TRC
On
Manual
Sensor
Stuck
EOR
TRC
Reversed5
P ( 1)(page C)
-22
-24
-27
-28 -29 -30
-31
1 NFBL
1 NFBL
(page A)
9M ( 1)
OR
OR
8M
(-10)
2V
Falls
Closed
(+1)
AND
OR
8M(-1)
EOR
2V
Reversed
6P ( 1)-13
-39
-38
-36
-40-37
OR
OR
Set
Point
(+1)
Air
Press.
(+10)
TRC
Falls
High
(+10)
AND
Air
Press.
(+1)
OR
TRC
Stuck
TRC
On
Manual
Sensor
Stuck
EOR
TRC
Reversed5
P ( 1)
-32
-45
-44
-43
-41
-42
1 NFBL
1 NFBL
(page B)
(page C)
(page 3)
OR
OR EORAND
Temp.
Sensor
Fails Low
(+1)
-33 -34 -35
-322 NFBL
(page C)
P5(-1)
T3(-1)No reversegain
[Example] TANK PRESSURIZATION PROBLEM
This process separates a two-phase stream (stream 1) into vapor (stream 2) and liquid (stream 3) using a flash tank. Level in the tank is controlled by a negative feedback loop through a level controller. A pressure sensor monitors the tank pressure which is relayed back to the control room. Should the operator see a high pressure on the indicator, he is instructed to manually open valve V1 which drains the tank and reduces the pressure. The relief valve is designed to vent the vapor portion of the tank mixture when high tank pressure is encountered.
P
1
L
V1
RV
7
2
5
LC
4
3 6 A.O.
• TOP EVENT:
• Normal Conditions:
Flow in stream 1, 2, and 3. Tank 50% full. Level controller on automatic. Relief valve and V1closed.
• Equipment Behavior:
Level Sensor : P5 increases when level increases. The sensor has stuck during operation.
Level Controller: P4 increases when P5 increases. The controller set point may be changed. The controller may be switched to manual operation.
Control Valve : Increasing P4 causes the valve to open.
Valve V1: The valve is manually operated. It may stick.
Relief Valve : The valve may fail shut. If the relief valve is full of liquid, it will not vent the system fast enough.
)10(TANKP
PLUGIN LINE 2
2P
1P
OPERATORACTION TK
P RVP
0 ( RV FAILS CLOSED)
LOOP II
0 ( L = +10)
70 (P +10)
+10+1
+1
TK+1 (P = +10)
V1P
0 (PRESSURE SENSOR) INOPERATIVE
0 (O
PER
AT
OR
OP
EN
S
WR
ON
G V
AL
VE
)
0 (V
1 S
TU
CK
)
+1
L
(OPERATOR DOES NOTSEE PRESSURE INDICATOR)
1M
5P
+1
+1
4P
(LEVEL SENSOR STUCK)
0
+1
LEVEL SETPOINT
+1
-1
0
(CO
NTR
OLLER
ON
MA
NU
AL)
3M
6M
+1
+1
-1
-10
LOOP I
LOOP III
+1 )10( TKP
-1
0
LOOP I
LOOP I
LOOP II
LOO P III
TKP ( 10)
OR
AND OR(See Page 2)
OR OR
(Loops Pass
Disturbance)
RVP (-10)L (+10) AND
OR
TKP ( 10)
(See Page 2***)
Loop II
cause
Loop I
cause
OR
RVP (0)
TKP (0)
L (+10)RV
Falls
Closed
7P ( 10)
L (0)
OR
VIP (0)
OR
Plug In
Line 2
V1
Stuck
Operator
Action (0)
Operator Opens
Wrong Valve
OR
Pressure Sensor
Inoperative
Operator does not See
Pressure Indicator
2P ( 10)
1P ( 10)
2 NFBL
No
Uncontrollable
Disturbances!
( Loops Cause )
Loop II
inactive
Loop I
inactive *
**inconsistent
(page 1)
( Loops Cause )OR
AND
L (+10)
OR
( Done )
*** L (+10)7
P ( 10) RV
Falls
Closed
RVP (0)
OR
AND
OR VIP (0)
( Done )
**( See Page 1 )
1M ( 10)
3M ( 10)
OR
6M ( 10)
4P ( 10)
OR
Level
Set Pt.
(+10)
5P ( 10)
L (-10)
AND
L (0) RVP (-10)
( Done )
*( See Page 1 )
TKP ( 10)
AND
L (+10)
( Done )
***
RVP (-10)
TKP ( 10)
TKP (0)
(page 2)
***1 NFBL
Isolation valveClosed in error
Loop I causeLoop II inactive
Loop I inactiveLoop II cause
Both cause
