10. FMEA PROCESS
Componen
t
Potentia
l Failure
Mode
Potentia
l Failure
Effects
S
E
V
Potential
Causes
O
C
C
Current
Controls
D
E
T
R
P
N
Recommende
d
Actions
What is
the
Input?
What can go
wrong with the
Input?
What is the
Effect on
the
Outputs?
How
bad?
How
Often?
What are the
Causes?
How can
this be
found?
How
Well?
What can
be done?
10
11. SEVERITY
Effect Severity of Effect Ranking
Hazardous –
W/O Warning
11
Very high severity ranking – Affects operator, plant or
maintenance personnel, safety and or affects non-compliance
with government regulations, without warning.
10
Hazardous –
With Warning
High severity ranking – Affects operator, plant or
maintenance personnel, safety and/or affects non-compliance
with government regulations with warning.
9
Very High Downtime of more than 8 hours or the production of
defective parts for more than 4 hours.
8
High Downtime of between 4 and 8 hours or the production of
defective parts for between 2 & 4 hours.
7
Moderate Downtime of between 1 and 4 hours or the production of
defective parts for between 1 and 2 hours.
6
12. SEVERITY
Effect Severity of Effect Ranking
Low Downtime of between 30 minutes and 1 hour or the production
12
of defective parts for up to 1 hour.
5
Very Low Downtime of between 10 and 30 minutes but no production of
defective parts.
4
Minor Downtime of up to 10 minutes but no production of defective
parts
3
Very Minor Process parameter variability not within specification limits.
Adjustment or other process controls need to be taken during
production. No downtime and no production of defective parts.
2
None Process parameter variability within specification limits.
Adjustment or other process controls can be taken or during
normal maintenance
1
13. OCCURENCE
13
Probability
of Failure
Criteria: No. of
failures within
Hrs of operation.
Criteria: The reliability based on
the users required time. Ranking
Failure Occurs
every Hour
1 in 1 R(t) <1 %: MTBF is about 10% of the
User’s required time.
10
Failure occurs
every shift
1 in 8 R(t) = 5%: MTBF is about 30% of
User’s required time
9
Failure occurs
every day
1 in 24 R(t) = 20%: MTBF is about 60% of
the User’s required time.
8
Failure occurs
every week
1 in 80 R(t) = 37%: MTBF is equal to the
User’s required time.
7
Failure occurs
every month
1 in 350 R(t) = 60%: MTBF is 2 times greater
than the User’s required time.
6
14. OCCURENCE
14
Probability
of Failure
Criteria: No. of
failures within
Hrs of operation.
Criteria: The reliability based on
the users required time. Ranking
Failure occurs
every 3 months
1 in 1000 R(t) = 78%: MTBF is 4 times greater
than the User’s required time.
5
Failure occurs
every 6 months
1 in 2500 R(t) = 85%: MTBF is 6 times greater
than the User’s required time
4
Failure occurs
every year
1 in 5000 R(t) = 90%: MTBF is 10 times greater
than the User’s required time.
3
Failure occurs
every 2 years
1 in 10,000 R(t) = 95%: MTBF is 20 times greater
than the User’s required time.
2
Failure occurs
> 5 years
1 in 25,000 R(t) = 98%: MTBF is 50 times greater
than the User’s required time.
1
15. DETECTION
Detection Criteria Ranking
Very Low Design or Machinery Controls cannot detect a potential cause
15
and subsequent failure, or there are no design or machinery
controls.
10
Low Design or Machinery controls do not prevent the failure from
occurring. Machinery controls will isolate the cause and
subsequent failure mode after the failure has occurred.
7
Medium Design controls may detect a potential cause and subsequent
failure mode. Machinery controls will provide an indicator of
imminent failure.
5
High Design controls may detect a potential cause and subsequent
failure mode. Machinery controls will prevent an imminent
failure and isolate the cause.
3
Very High Design controls almost certainly detect a potential cause and
subsequent failure mode, machinery controls not required.
1
17. Boiler pressure part
Component Potential Failure Mode
Potential Effect(s)
of Failure
Sev
Potential Cause(s)/
Mechanism(s) of
Failure
Occ
tube
Preheater Fire side corrosion
Tube leak,gas side p.
drop, low eff.
acid dew point
ECO. FAC tube leak 5 parameter model
Evap/Wall FAC tube leak 5 parameter model
Underdeposit Corrosion tube leak
high heat flux, low flow, high
debris water
Short Term Overheat tube burst low water flow
SH/RH tube Graphitization Tube burst mis mat'l, high temp.
High Temp. Corrosion tube burst mat'L, corrosive media.,temp.
Long Term Overheat tube burst
low flow, inside oxide thk.,
high heat flux
Type IV Crack tube burst service condition, weld mat'l
Dissimilar Weld tube burst shaffer diagram.
Pipe
MSP Weld Defect pipe leak poor joint fitup & weld control
RH Weld Defect, Type IV Crack pipe leak poor joint fitup & weld control
Bypass Thermal Fatigue pipe leak
poor design, operation high
cycle,mat'L suscept
Hdr
ECO T Way FAC leak 5 parameter model
Final SH Crack dissimiilar weld leak
18. Introduction to Power Boiler &
Causes of failures in boiler system
Combine Cycle Power Plant
Thermal Power Plant
Hoz. flow Ver. flow
Sub. Cri
Pressure
Sup. Cri
18 Pressure
19. Causes of failures in boiler
system
Corrosion Crack Degradation
- Water Side - Weld Defect - Graphitization
FAC Lack of Fusion - Creep
Under deposit Undercut Weld Creep -> IV Crack
- Fire Side Base Metal Creep
High Temp. - Spherodisation
Low temp.
Erosion
SCC
Reference
Nalco Guide
21. DISCONTINUITY POSSIBLE CAUSES
Excessive Convexity Slow travel speed that allows weld metal to build up
Welding currents too low
Insufficient Throat A combination of Travel speed to fast and current too high
Improper placement of weld beads when multiple pass welding
Undercut Amperage too high
Arc length too long increasing the force of the arc so that it cuts into corners
Improper weld technique causing the corners to be left unfilled or cut into
Groove joint not completely filled and overlapped
Insufficient Leg Size Using the wrong electrode angle causing the weld to be deposited to heavily on one side
Using the wrong angle on multiple pas welds Causing the welds to overlap incorrectly
Poor Penetration Amperage too low
Travel speeds too fast
Using too large an electrode for the root of the joint
Improper electrode angle at the root of the joint
Improper weave technique
Using the wrong electrode for the desired joint penetration: (using E-6013 instead of E-6010)
Poor Fusion Amperage too low
Travel speeds too fast
Improper electrode angle at the sides of the joint
Improper weave technique that does not allow enough time at the sides of the joint
Using the wrong electrode for the application
Overlap Amperage too low and /or travel speed too slow
Electrode too large with low currents
Porosity Dirty base metal painted or galvanized surfaces
Arc length too long especially with E-7018 Electrodes
Moisture in low hydrogen electrodes
Wind or fans strong enough to break down the shielding gas
Slag Inclusions Improper manipulation of the electrode especially with E-6013
Improper cleaning and slag removal between multiple pass welds
Cracks Using the wrong Electrode for the application
Using Excessively high amperage on some metals
Excessive Spatter Amperage too high
Electrode angle too extreme
Arc length too long
