Chemical Engineering May 2015 - 74

Table 1. Typical fired heaTer Tube -deTerioraTion mechanisms and rooT causes
Root Cause
Problem category
Hotspot
Fouling
Corrosion
Creep
Vibration
Erosion
Thermal fatigue
Mechanical
deterioration
Problem
sub category
--Internal
fouling
External fouling
Internal corrosion
External dew-point
corrosion
Minor creep
Bulging
* * *
* * *
Bowing and sagging * * * *
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*
* *
*
* * * *
* * * *
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*
* *
* *
* *
*
*
*
*
*
*
*
*
* *
*
*
*
* * *
*
*
*
* * *
Note: * In this article, the term " improper two-phase-flow regime " refers to " slug flow " regime only and complex two-phase-flow phenomena caused by poor design,
or improper operating conditions (problems like sudden flash evaporation or mist flow) are beyond the scope of the current study.
triggered an alarm, which led to the
shutdown of the heater for maintenance
and hanger repair.
A good troubleshooting guide can
demonstrate the fact that failed roof
hangers are caused by overheating
in the convection section. In this vein,
by monitoring burner flame heights
and burner fuel pressures (controlling
the amount of heat released), as
well as monitoring bridge wall temperature,
operators can prevent this
phenomenon in the first place.
Troubleshooting
Even if a company offers proper
training courses for personnel, and
has an effective reliability program
and an effective schedule of visual
inspections, these measures will not
reduce the importance of practical
troubleshooting knowledge. An
operator without proper knowledge
and understanding of root causes
and a troubleshooting sequence
72
would ignore or misunderstand vital
signs of a possible tube-deterioration
mechanism.
In this article we propose a simple
root-cause analysis and also a computerized
troubleshooting algorithm
that directs the heater operator from
one root cause of a possible tubedeterioration
mechanism to another,
in sequence. The algorithms indicate
how each of the improper performances
for a given cause is to be
corrected, and continues until all
possible causes for tube-deterioration
mechanism can be corrected.
Tube-deterioration mechanisms
According to the valuable references
listed at the end of this article [1,
3-4], as well as our field experience,
typical fired-heater tube-deterioration
mechanisms that may lead to
tube failure can be classified into the
following 12 main categories:
1. Local or longitudinal hotspots
2. Internal fouling
3. External fouling
4. Internal corrosion
5. Creep (general or minor creep)
6. Bulging
7. Bowing and sagging
8. Vibration
9. External dew point corrosion
10. Erosion
11. Thermal fatigue
12. Mechanical deterioration
Root-cause analysis
It should be noted that before entering
the root-cause-analysis phase,
one should not neglect the possibility
that the material of construction used
for the heater tube can be a possible
cause of some of the problems referred
to above. Thus, readily available
data for allowable stress and corrosion
rates for various tube alloys, as
a function of temperature, should be
made available from appropriate tube
manufacturers. These data should
ChemiCal engineering www.Chemengonline.Com may 2015
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Flame impingement
Heat flux imbalances
Improper fuel pressure
Heavy fuel-oil combustion problems
High content of impurities in fuel
Low process fluid flowrate
Upstream equipment malfunction
Heavy hydrocarbon molecules pyrolysis (coke buildup)
Mineral sediments
Thermal shock
High firing rate
Chemical composition of process fluid (critical species)
Improper two-phase-flow regime*
Air Leakage
Poor heater-casing design
Misplaced or failed tube support or hanger
High pressure drop across heater passes
Control systems malfunction
Increased heater charge rate
Cyclic heat-load variations
Faulty roll procedures or workmanship
Undue force to close fittings
Improper mechanical or steam air decoking procedures
Refractory damages or improper refractory repair

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Chemical Engineering May 2015

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