Engineering Practice Troubleshooting Tube-Deterioration Troubleshooting Tube-Deterioration Mechanisms in Direct-Fired Heaters A practical step-by-step guide for reducing future tube failure Babak Maghbooli and Hamidreza Najafi Farayand Sabz Engineering Co.
F
orced outage of a direct-fired heater is the worst nightmare for any operator in a petroleum-refining or petrochemical plant. Since fired heaters are not replaceable by spares, forced outage in many cases means emergency shutdown of the entire plant. In most cases, actual tube failure or even severe tube deterioration are responsible for outage of a direct-fired direc t-fired heater. Tube-deteriora T ube-deterioration tion mechanisms are not only an economica economicall concern, but also a matter of safety of plant and personnel. An unpredicted tube rupture may lead to disastrous explosions and severe human casualties. In order to prevent such unwanted events and keep the fired heater tubes functional as long as possible, many petroleum refineries and petrochemical plants have organized a preventive program, which according to the American Ameri can Petr Petroleum oleum Insti Institute tute [1] is referred to as a “reliability program.”
Reliability programs
Misunderstanding problem problem signs led to a dramatic internal coke build up, followed by high tube metal temperatures, creep and finally tube rupture in one of the pass outlets of a visbreaker plant heater. heater. Operators had unintentionally ignored high tube skin temperatures for one week FIGURE 1.
plex metallurgical phenomena, and inspection engineers with degrees or specialties in metallurgical engineering should evaluate and explicate them; but after the inspection or evaluation stage, the main root causes of such problems are mostly simple and can be explained to an operator with the aid of fact sheets or flowcharts. If the operators are being kept out of the loop, it is always possible for personnel to misinterpret a sign of tube deterioration. And since heater inspectors are not around all of the time, such misinterpretations could easily led to a disastrous event.
A good reliability program includes various detailed inspection programs performed both during heater operation and overhaul or occa- Dangers of misunderstan misunderstandings dings sional maintenance shutdown peri- As an exampl example, e, conside considerr Figure 1, ods. Continuous assessments of the which demonstrates a dramatic tube remaining lifetime of tubes and the rupture case that was the result of supervision of heater performance misunderstanding the problem signs are also included in the program of and lack of root-causes knowledge of many refineries. Although reliability operating personnel. In this particular programs have proven successful case, the pass outlet was near the during long periods of heater opera- internal header-box refractory linings. tion, many heaters are still victims of Because of this, the operators had tube-deterioration tube-deterio ration mechanisms mechanisms.. That unintentionally ignored high tube-skin is because heater operators or field temperatures and assumed that they engineers are usually being kept out were misreading the refractory wall of the loop of a reliability program. temperature by the infra-red pyromIt is true that deterioration mecha- eter eter.. If they were w ere aware of other signs nisms are mostly categorized as com- and root causes of internal coke build
An example of good good troubleshooting troubleshooting knowledge of operating personnel. Observation of a failed tube hanger triggered the alarm for possible roof tubes sagging and led to the heater shut down for maintenance and hanger repair FIGURE 2.
up (fouling), such as back pressure in the specified pass, maybe serious course of actions would have been considered before raising the tube metal temperature to values that had eventually caused severe creep and tube rupture. Explaining the root causes of the deterioration mechanisms mechanisms to the operating team can prevent deterioration in the first place. It is important that the inspection team arrange meetings in which operating personnel take a walkthrough the chain of events and performance imbalances that lead to a tube failure. As anoth another er exam example, ple, the situa situation tion shown in Figure 2 was dealt with before the shield section tubes experienced serious sagging, which could easily lead to tube rupture and possible explosion. As the operators were aware of the possible threat of roof tubes sagging (in case of fallen or failed tube hangers), observing the failed hanger at the floor of the heater
TABLE 1. TYPICAL FIRED HEATER TUBE DETERIOR ATION MECHANISMS AND ROOT CAUSES
Root Cause
s m l e e l b u o f r p n i n s o i e i t i e t s r r u s u t e u b p n c s e n s m i e o m r m a f p c e l l o i g a b l e o t n i u l n m f p I t e e x u n m u r e f i o l p e f o y c t v h m a r a p g a e e i l F H m I H H
n o i t c n u e f t l a a r w m o t l n f e d i m u l p i f u s q s e e c m o r a e p r t w s o p L U
Problem category
Problem sub category
Hotspot
---
•
•
•
•
•
Internal fouling
•
•
•
•
•
External fouling
•
•
Internal corrosion
•
Fouling
Corrosion
Creep
•
•
Minor creep
•
•
•
Bulging
•
•
•
Bowing and sagging
•
•
•
---
Erosion
---
Thermal fatigue
---
Mechanical deterioration
---
•
s t n e m i d e s l a r e n i M
•
•
•
•
•
•
•
•
e g a k a e l r i A
•
•
•
•
•
•
•
n o i t c n u f l a m s m e t s y s l o r t n o C
e t a s r n e o i g r t a a i h r c a r v e t d a a o e l h t d a e e h s a c i e r l c c y n I C
p i h s n a m k r o w r o s e r u d e c o r p l l o r y t l u a F
s g n i t t i f e s o l c o t e c r o f e u d n U
•
•
•
s e s s a p r e t a e h s s o r c a p o r d e r u s s e r p h g i H
•
•
•
n g i s e d g n i s a c r e t a e h r o o P
r e g n a h r o t r o p p u s e b u t d e l i a f r o d e c a l p s i M
s e r u d e r c i a o r p p e r g y n r i k o o t c c a r e f d e r i r a r e m p a o r e t p s m r i o r l o a s c i e n g a a h m c a e d m y r r e o p t c o r a r p f e m I R
•
•
•
•
k c o h s l a m r e h T
•
•
External dew-point corrosion
Vibration
•
) s e i c e p s l a c i t i r c ( d i u l f * s e s m e i c g o r e p r f w o l o n f o e i t i s s a o h p p e t m o a r o w c t g l r n i a e r i c i p f o m h e r g h p i H C m I
) p u d l i u b e k o c ( s i s y l o r y p s e l u c e l o m n o b r a c o r d y h y v a e H
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
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 vain, 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 phenomena in the first place.
signs of a possible tube deterioration mechanism. In this article we have proposed a simple root-cause analysis and also a computerized troubleshooting algorithm that directs the heater operator from one root cause of a possible tube-deterioration mechanism to another, in sequence, indicating how each of the improper performances for a given cause is to be corrected, and continues in this vain until all possible causes for tube-deterioration mechanism can be corrected.
Troubleshooting Even if a company follows 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 would ignore or misunderstand vital
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 be checked before taking any step for understanding the root causes of tube-deterioration mechanisms.
A
Troubleshooting algorithm for hotspots on the tubes FIGURE 3.
Hotspots on tubes can be the result of internal or external problems. First check external problems which are mainly caused by faulty burner performances.
Do you experience flame impingement on tubes ? Solve the problem according to the instructions given in burner troubleshooting guide found in Ref. 2. Yes
Yes
Is your problem solved?
Caution: Since the flame of fuel gas is hardly visible, flame impingement can be easily ignored by the operator. Inspect the firebox carefully and use sodium salts to make sparks and turn the flame pattern into a visible form.
No
Yes
No
Solve the problem according to instructions given in burner troubleshooting guide found in Ref. 2.
Yes
One of the main external reasons that leads to hotspots is heat-flux imbalances, mainly caused by non-asymmetric burner-firing patterns along the firebox.
Is your problem solved?
Do the burners frequently go out and stay ignored by the operator? No
No
Adjust the fuel pressures to the same value on each burner, also adjust the air and draft amounts accordingly.
Yes
No
Are the fuel pressure gauges on every burner showing the same value?
Caution: If by doing so flame patterns were corrupted or misshaped, based on the problem refer to burner troubleshooting guide found in Ref.2.
Yes
After solving the problem, adjust fuel pressures. Is your problem solved?
No
Yes
Solve the problem according to instructions given in burner troubleshooting guide found in Ref. 2. Is your problem solved?
Yes
In case of fuel oil firing, you may suffer poor atomization problems causing misshaped flame patterns and firing rates along the firebox.
No
Do you have poor atomization problems?
No
If there is no external reason for the hotspots in the firebox, your tube may be suffering from an internal fouling phenomena.
Go to Point B in Figure 4
Seek engineering consultancy. End trouble shooting procedure.
In order to analyze each deterioration-mechanism category and its sub categories in the most concise and efficient way, a root-causeanalysis table has been proposed (see Table 1). By understanding the root causes of tube-deterioration mechanisms, one has passed through the primary step of tube damages troubleshooting. Now what matters the most is the proper sequence of troubleshooting actions, which are provided by comprehensive algorithms described in the next section.
Troubleshooting algorithms The comprehensive troubleshooting algorithm referred to above is based on the logic depicted in Figures 3 to 14, and has been successfully used to both recognize what the tubedeterioration mechanism might be, indicate the cause of the problem and correct the heater defect, so as to achieve the proper tube operating lifetime.
A practical example Let’s assume that an operating team tries to prevent a future tube
failure like the one shown in Figure 1. In order to achieve this goal, it is obvious that they need to troubleshoot the deterioration mechanisms that are responsible for this event. First of all, an inspection team should identify the nature of tubedeterioration mechanisms that have led to such a disaster. A preliminary analysis would demonstrate that severe tube creep accompanied by internal fouling (coke build up) are to be blamed. This particular heater is in visbreaker service, which can be classified as a thermal cracking pro-
B
FIGURE 4.
Troubleshooting algorithm for internal fouling
Low process-fluid flowrate is one of the main causes of internal fouling. Low flowrate can cause high skin temperature and coke buildup as a result of pyrolysis can occur. Did you experience low flowrate in one or multiple passes of your heater?
Internal fouling may be caused by coke formation or other material sediments (usually minerals salts dissolved in crude oil). Follow the questions below to be able to differentiate between the two phenomena.
If the problem is not caused by human error, then in most cases, a malfunction of the control system is responsible. In such cases, perform immediate controlsystems maintenance or even replacement procedures.
Yes
No
Go to Point DC
Consider coke buildup with high probability. Solve the problem according to the instructions given in the burner troubleshooting guide found in Ref. 2.
Did you experience any fire-side (external side) hotspot problems (mentioned in Figure 3) like over firing or flame impingement for long periods of time?
Yes
End trouble shooting procedure.
No
The fouling is 90% caused by mineral sediments.
Yes
Is your fired heater in atmospheric crude distillation preheater service or is it located downstream of a de-salter? No
Fouling is 90% caused by mineral sediments or even small particles carried out by fluid stream into the furnace tubes.
Yes
Did the fouling happened right after overhaul or upstream equipment s malfunction?
Consider removing and replacing the tubes with supervision of inspection department.
’
No Yes
If your furnace is in heavy fluid service, like vacuum-distillation-tower charge heater or visbreaker service or any vaporizing heavy feed service, consider your fouling problem as coke buildup.
Follow spalling and decoking with steam and air procedure according to vendors instructions.
DC
Caution: Coke buildup in gas reactive services like olefin plant furnaces is beyond the scope of this algorithm, look at the proper references.
No
Is your problem solved?
Follow spalling with steam procedure according to the vendors instructions. No
Is your problem solved? Yes
cess. In thermal cracking of heavy petroleum cuts, coke formation is an unwanted side reaction that cannot be eliminated, but the reaction rate and duration can be controlled and reduced by maintaining proper operating conditions. In some cases the interval between decoking processes can be extended by up to three to four years. In this particular case, unwanted coke deposition had happened during the first year of an operating period. This event was unpredicted and surprising for the operators. Internal fouling, caused by rapid local coke formation, made an insulating layer, which in turn led to higher tube skin temperatures for long periods
of time. In this vain, high tube-skin temperature led to sever creep and eventually disastrous tube rupture. In order to solve this problem and prevent it from ever happening again, the main cause that is internal fouling should be clearly understood and analyzed. By taking a look at Table 1, one can find the root causes of internal fouling, which are as follows (not necessarily in order of priority): 1. Flame impingement 2. Heat flux imbalances 3. Improper fuel pressure 4. Heavy fuel-oil combustion problems 5. Low process fluid flowrate 6. Upstream malfunction of equipment
7.
Pyrolysis of heavy hydrocarbon molecules (coke buildup) 8. Mineral sediments 9. High firing rate With the exception (in some cases) of root-cause Number 9 (high firing rate) [2], all the other causes can be treated by the stepwise troubleshooting procedure outlined in this article. In order to solve the internal fouling problem, the operator can start with the procedure given in Figure 4. This procedure starts with the simplest solution, which is solving low process-fluid flowrate in heater passes. Low process-fluid flowrate can cause high tube-skin temperatures that can lead to internal fouling. It should be noted that human error
C
FIGURE 5.
There are two types of external fouling, metallic oxides and external scaling. Follow the questions below to be able to differentiate between the two.
The particles are metallic oxides proceed to point MO.
Yes
Troubleshooting algorithm for external fouling
End trouble shooting procedure.
Do the external particles respond to a magnet?
Seek engineering consultancy.
No
Yes
No
Because of possible metallurgical changes, some particles do not respond to a magnet therefore be sure that tube's surface is not scratched or thinned in the particle position. Does the tubes thickness decreased in the particle position?
No
Metallic oxides are mainly caused by over firing or high fuel gas temperature ranges. Do you experience uneven heat distribution or over firing conditions?
No
Yes
Solve the problem according to instructions given in burner troubleshooting guide found in Ref. 2.
Yes
Is your problem solved? In case of gas firing, external scales (other than oxides) are caused by complex heat transfer phenomena that needs advanced engineering consultancy and determining the reason is not a straight forward step.
No
The external fouling is soot or scale. Do you use fuel oil in burners?
Yes
MO
Yes
There is a possibility that your fuel oil has a high content of vanadium. In high-temperature ranges or unexpected thermal shocks, vanadium oxide would attack tube material rapidly causing dangerous corrosion and reducing tube thickness. Optimize your firing conditions and revise fuel composition if it is possible. Is your problem solved?
No
Seek engineering consultancy.
Solve the problem according to instructions given in burner troubleshooting guide found in Ref. 2.
Yes
Is your problem solved?
Yes
Main cause of soot formation in fuel oil combustion is poor atomization or combustion issues with fuel oil. Do you experience burner poor performance? No
There is a possibility that your fuel oil has a high content of sulfur (in form of mercaptans, H2S or any bound or unbound form). Analyze your fuel and in case of high sulfur content consider fuel change or upstream process modifications. Is your problem solved?
No Yes No
No
Do you frequently experience thermal shocks or high firing rates in the fire box?
Seek engineering consultancy.
Yes
and control equipment malfunction are the main causes of low processfluid flowrate. It is mandatory to prepare a regular program for checking control equipment accuracy. Also, restrain inexperienced personnel from controlling sensitive equipment like fired heaters. If the problem is not related to this cause, the operator is asked to check for burner firing problems, like flame impingement, heat-flux imbalances, improper fuel-oil pressure or heavy fuel-oil combustion problems.
If any of these problems have been observed for long operating periods, one can almost be certain that the internal fouling is due to coke buildup. In order to solve any burnerfiring problem, the operator can refer to the detailed troubleshooting algorithms described in Ref. 2. Mineral sediments fouling, which is not the case in this example, may happen in crude charge heaters that are placed after de-salters and heat exchanger networks in most refineries. Apparently, if the heater is in
crude charge service, checking the de-salter’s malfunction should be the next step in the trouble shooting algorithm. If this is not the case, upstream equipment malfunction can be the next cause. Small particles carried by a fluid stream can plug heater tubes and reduce flowrate locally or in a whole pass. Similar to low process-fluid flowrate, this can lead to severe internal fouling. Operators should be warned about the possibility of this problem, specifically after overhaul or upstream equipment
D
FIGURE 6.
Internal corrosion is mainly influenced by the chemical composition of the process fluid. Do you have sulfur compounds present in your process fluid composition?
No
Troubleshooting algorithm for internal corrosion
End trouble shooting procedure.
Yes
No
Seek engineering consultancy.
DE
Yes
Its possible that the inlet process fluid contains particulate matter. Particulate matter can increase the corrosion rate by stripping away protective scale and exposing the fresh metal to continuous corrosion process (see Ref. 1). Inspect upstream process equipment and resolve any deficiency that can cause particulate formation in process stream. Is your problem solved?
Do you experience localized or general tube-wall thinning?
Yes
No
No
Yes
Sulfidic corrosion may have happened inside the tube walls, beware of chemical components like chlorides and hydrogen that can aggravate the corrosion rate. Also local flame impingement and high skin temperatures can increase the corrosion rate.
Control the chemical composition of the process fluid and lower critical species concentration. In case of frequent flame impingements, solve the problem according to instructions given in burner troubleshooting guide (Ref. 2).
Did you experience high corrosion defects after a recent overhaul or upstream equipment malfunction? Mostly de-salter malfunction or low performance?
Yes
Is your problem solved?
Go to Point DE
Yes
Two-phase flow currents with high linear velocity in either of the phases, or slug flow regimes can cause local or overall corrosion and even enhance the corrosion rate caused by other factors. Consult an expert engineering team and try to figure out possible improper flow regimes in boiling regions of tubes. Is your problem solved?
No
No Yes
Do you have naphthenic acids present in your process fluid composition?
No
Is your fired heater in atmospheric or vacuum towers charge heating service?
No
Yes
Do you experience tube wall thinning in turbulent areas, such as bends or pass outlets?
No
Yes No
Naphthenic acid corrosion may have happened inside the tube walls. Beware that high skin temperatures can increase the corrosion rate.
Control the chemical composition of process the fluid and lower critical species concentration if possible.
Yes
Go to Point DE
Is your problem solved?
malfunction and maintenance. As the next step, the nature of the process should be noted. For visbreakers or any service in which heavy feed is vaporizing, coke buildup is a certain possibility. As coke formation in these kinds of heaters is highly anticipated, controlling operating conditions, especially the items mentioned in previous steps of this algorithm, is very important and vital. By following the right steps, operators may control — and even in some cases eliminate —
coke buildup phenomena. Other steps of this algorithm are dedicated to the decoking and spalling practices that should be followed, depending on the nature of the internal fouling problem. Following this procedure would usually eliminate and even prevent future internal-fouling problems, as for the case history shown in Figure 1. Surveying the operating history of the heater proved that long-time local flame impingement was the main cause of this event, and tun-
ing the draft and excess-air amounts could have solved the problem in the first place. As a final note, we should add that our experiences have proven that in similar case histories, increasing the awareness and knowledge of operating teams about the initial signs of internal fouling phenomena is as important as knowing the troubleshooting sequences. For example, in a similar case that happened in a vacuum charge heater, operators were too much dependent on
FIGURE 7.
E
Troubleshooting algorithm for creep ( general case or minor creep)
If the problem is not caused by human error, then, in most cases, a malfunction of the control system is responsible. In such cases, follow immediate control system maintenance or even replacement procedures.
Yes
Low process-fluid flowrate is one of the main causes of internal fouling. Did you experience low flowrate in one or multiple passes of your heater? No
Seek engineering consultancy.
No
Yes
Solve the problem according to instructions given in burner troubleshooting guide (Ref. 2).
Do you experience flame impingement on tubes? Yes
Is your problem solved?
Caution: Since the flame of fuel gas is hardly visible, flame impingement can be easily ignored by the operator. Inspect the firebox carefully and use sodium salts to make sparks and turn the flame pattern into a visible form. No
Yes
Adjust fuel pressure in all burners. If firing problems happened, solve the problem according to instructions given in burner troubleshooting guide (Ref. 2).
Yes
Is your problem solved?
Local over firing can cause creeping phenomena. Check individual burners fuel pressure. Do you have high pressure in some burners? No
No
Seek engineering consultancy.
End trouble shooting procedure.
No
Yes
Many fired heaters are operating under over-duty conditions. In this cases permanent over firing and high fluegas temperatures can cause creep in tubes. Revise firing conditions or consider replacing current tubes with tubes that can tolerate such high temperature conditions. Is your problem solved?
F
Do you experience flame impingement on tubes ? Yes
Caution: Since the flame of fuel gas is hardly visible , flame impingement can be easily ignored by the operator. Inspect the firebox carefully and use sodium salts to make sparks and turn the flame pattern into a visible form.
Solve the problem according to instructions given in burner troubleshooting guide (Ref. 2). Is your problem solved? No
No
Local over firing can cause bulging phenomena. Check individual burners fuel pressure. Do you have high pressure in some burners?
No
Yes
Adjust fuel pressure in all burners . If firing problems occured, solve the problem according to instructions given in burner troubleshooting guide (Ref. 2). Is your problem solved?
No
Yes
Seek engineering consultancy.
End trouble shooting procedure.
Yes
G
Low process-fluid flowrate is one of the main causes of internal fouling. Did you experience a low flowrate in one or multiple passes of your heater?
FIGURE 9.
No
Yes
Troubleshooting algorithm for bowing and sagging
Many fired heaters are operating under over-duty conditions. In this cases permanent over firing and high flue gas temperatures can cause local or overall bowing in tubes. Revise firing conditions or consider replacing current tubes with tubes that can tolerate such high temperature conditions. Is your problem solved?
Yes
No
If the problem is not caused by human error, then in most cases a malfunction of the control system is responsible. In such cases, follow immediate control systems maintenance or even replacement procedures.
Make sure that all the tube hangers are installed according to general arrangement documents. Any misplaced hanger can cause sagging or bowing problems in longtime operating periods. Is your problem solved?
Yes
No
Make sure that tube supports or hangers have not failed during operation . Improper suspension is one of the main causes of bowing problem. Is your problem solved?
Yes
No
No
Uneven tube metal temperature distribution is another cause of bowing and sagging problems. Measure the tube metal temperature and check for hotspots. Do you have hotspots? Yes
Go to Point A in Figure 3
Seek engineering consultancy.
End trouble shooting procedure.
H
Control or measure the pressure at pass inlet and outlet and compare it with the allowed pressure-drop limit. Also compare the pressure drop in different passes. Do you experience back pressure?
No
Yes
If your furnace is in any vaporizing feed service, you may consider your vibration problem as a result of improper two-phase flow regime. Slug flow can be a serious problem causing hammering effect in some cases. This problem must be solved by the aid of advanced engineering team. Does the engineering team diagnosed your problem as improper two-phase flow regime? No
Control instruments and also control valve. It might be an instrument malfunction. Make sure the instruments are working properly. Is your problem solved? No
Yes
In some cases loose tube supports can cause tube vibration. Inspect fire box and convection section for loose tube supports. Tighten loose supports. Is your problem solved? No
The back pressure is not the result of instrument malfunction, but the result of internal fouling that could be very dangerous if back pressure and vibration is significant. Go to point B in Figure 4 to solve your problem. FIGURE 10.
Troubleshooting algorithm for vibration
Seek engineering consultancy.
End trouble shooting procedure.
Yes
Yes
I
FIGURE 11.
This kind of corrosion usually happens in cold ends of fired heater, that is, header boxes, tube pass outlets or last rows of convection section. The main reason for this kind of corrosion is tube metal temperature lower than acid-gas dew-point temperature at those locations. Follow the questions below in order to troubleshoot or prevent this phenomena.
No
Troubleshooting algorithm for external dew-point corrosion
Heavy fuel-oil combustion is the main reason for SO3 and SO2 formation in flue gas, which leads to sulfuric acid formation and condensation in cold ends. If sealing the air leaks doesn t solve your problem completely, consult with an engineering team in order to revise fuel oil composition. Is your problem solved? ’
Yes
No
Do you burn heavy fuel oil (Type 4 and above) in your fired heater?
No
Yes
Dew-point corrosions in cases of only fuel gas firing is very rare. If such phenomena has happened to your heater consult an engineering team to revise fuel gas composition and detect undesired sulfur source in fuel gas ring and resolve it. Is your problem solved? No
Yes
As the last step, revise heater (specially convection section) design with the aid of a professional designer team.
Is there any detectable air leakage on the header boxes or tube pass outlets? Yes
Follow regular inspection procedures to locate air leaks or even open door header boxes in your heater. Seal the leakages immediately and ask the operators to keep the header boxes closed at all times. Is your problem solved?
End trouble shooting procedure.
Yes No
J
Has the charge rate of your heater increased? Yes
No
Do you experience flame impingement on heater tubes or bends?
There is a high possibility that metal loss and thinning phenomena that has occurred in your tubes is mainly caused by corrosion mechanisms. Go to point D in Figure 6 to solve your problem.
No Yes
Solve the flame impingement problem according to instructions given in burner troubleshooting guide (Ref. 2).
The high linear velocity of feed through fired heater coils is the main reason for internal erosion. The mass flux of feed through heater coils has a fixed range that is determined by the designer and is dependent on many variables like heater service, and boiling regime. If you need to increase heater charge rate, first discuss the possibility of this action with a professional designer team. Consider revamp options as well. FIGURE 12.
Troubleshooting algorithm for erosion
End trouble shooting procedure.
K
FIGURE 13.
Do you see the cracks on locations like tube welding points, convection section tubes with fins or studs, bracing or weld attachments?
Troubleshooting algorithm for thermal fatigue
There is a high possibility that cracks are not caused by thermal fatigue but other mechanisms like creep are responsible for them. Consult an inspection engineer to determine nature of cracks.
No
Yes
Yes
Did these cracks appear after heater startup or shutdown? No
Yes
Do you experience cyclic heat load variations in your heater? No
Seek engineering consultancy.
End trouble shooting procedure.
Cyclic temperature variations can cause crack development in areas where two metals with different coefficients of expansion are joined by welding. Try to avoid cyclic temperature variations and also avoid rushed start up or improper shutdown practices.
L
Yes
Leakage in tube rolls is one of the main causes of mechanical deterioration. Such leakages may appear because of faulty roll procedures or workmanship. Follow proper personnel training and supervision during overhaul and maintenance periods to avoid such mistakes. Is your problem solved?
Follow proper timing in mechanical decoking procedures. Do not let the cleaner to operate in one place for so long that tube metal disruption and metal loss happen. Is your problem solved?
Yes
No
Yes
Undue force to close fittings may result in developing cracks on various parts of fittings or U-bend seats (see Ref. 1). Follow proper personnel training and supervision during overhaul and maintenance periods to avoid mistakes like improper cleaning of ground surfaces or mis-mating of plugs to return bends. Is your problem solved?
Control steam and air rate during steam-air decoking procedures. Also monitor and control tube skin temperatures carefully. Improper air to steam ratios or high tube skin temperatures could lead to serious oxidation or other deterioration mechanisms . Is your problem solved?
No
Yes
Enough space should be provided for thermal expansion of tubes. Improper refractory repair work or very tight fittings can cause unwanted stress that eventually leads to deformation or cracking of the tube and fittings. Is your problem solved?
No
No
Yes
No
End trouble shooting procedure.
Seek engineering consultancy.
Troubleshooting algorithm for mechanical deterioration FIGURE 14.
Key: A
Hotspot
D
Internal Corrosion
G
Bowing and Sagging
J
Erosion
B
Internal Fouling
E
Creep
H
Vibration
K
Thermal Fatigue
C
External Fouling
F
Bulging
I
External Dew-Point Corrosion
L
Mechanical Deterioration
8,500 8,000
— Pass A
— Pass C
— Pass B
— Pass D
Authors
7,500 7,000 d / l b b , e t a r w o fl s s a p r e t a e H
6,500 6,000 5,500 5,000 4,500
Early warnings of internal fouling in pass A (rapid decrease in feed flowrate, accompanied by 80 psig pressure increase in pass inlet)
4,000 3,500 3,000
0
4
8
12 16 20 24 28 32 36 40 44 48 52 56 60 Time, days
64 68 72 76 80 84 88 92
This graph shows the operating history of the process fluid flowrate in four passes of a vacuum distillation unit charge heater. Ignoring early signs of internal fouling caused by rapid coke build up, led to disastrous tube rapture, like the one depicted in Figure 1
Babak Maghbooli is a member of the research and development team of Farayand Sabz Engineering Co. (No.117, Somaye Street, Tehran, Iran; Email:
[email protected]). With more than six years of experience in the field of tuning process fired heaters and troubleshooting their problems, he has provided consultancy services for more than five petroleum refineries in the field of troubleshooting, maintenance and analyzing mechanical and thermal behavior of fired heaters and boilers. He also has practical and professional experience in the field of process simulation and developing root cause analysis procedures for chemical process problems. He is an advanced VBA programmer and has developed many process simulation solutions in MS-Excel for various consulting projects. He holds a B.Sc.Ch.E. degree from Arak University and He is an associate member of IAChE and CSChE.
FIGURE 15.
infra-red pyrometer measurements ments will be needed. of tube-skin temperatures. That It should not be forgotten that the misreading of the skin temperature main cost saving and also safety made them ignorant to the actual benefits of this program will resulting problem that was taking place. As from the following: depicted in Figure 15, the process Fewer forced direct fired heater fluid flowrate had decreased during outage events due to tube failure coke buildup and simultaneously, Elimination of disastrous events, the heater pass inlet pressure insuch as a heater explosion caused creased 80 psig. These two events by unpredicted tube ruptures are very obvious early warnings Extending the operating lifetime of of internal fouling in the specified heater tubes heater pass (Pass A of Figure 15). The possibility of increasing The operators had simply related throughput and decreased down the decreased flowrate to the contime for decoking operations ■ trol equipment malfunction and igEdited by Gerald Ondrey nored it. If the operators were aware of other warning signs of possible Acknowledgment internal fouling, this problem may The authors wish to thank the help not have endured for such a long and contribution of Alan Cross. He operating period that would led to is always a mentor to us whom gives tube rupture. This is another reason us the courage and motivation for for the necessity of an operator-in- preparing and submitting our experivolved reliability program in refineries ences in engineering papers. in addition to good troubleshooting knowledge and training. References •
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•
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Computerized algorithm A computer program (computer wizard) can easily be developed based on logic described in Figures 3 to 14. By using this program in industrial environments, like petroleum refineries and petrochemical plants, operators will be able to understand and correct the root causes of a possible deterioration mechanism in more efficient ways. As result less maintenance operations or mandatory tube replace-
1. American Petroleum Institute, “Inspection of Fired Boil ers and Heaters,” Recommended Practice No. 573, API, Washington, D.C., Feb. 2003. 2. Maghbooli B., Najafi H., Bakhtiari A., Correcting Improper Performance of Direct Fired Heaters: A practical, stepby-step approach for finding the root cause and troubleshooting burner problems, Chem. Eng., May 2013, pp. 39–46. 3. American Petroleum Institute, “Damage Mechanisms Affecting Fixed Equipment i n the Refining Industr y,” Recommended Practice No. 571, API, Washington, D.C., April 2011. 4. Patel S., Typical Fir ed Heater Problems and Root Causes, Hydrocarbon Process., March 2007, pp.68–69.
Hamidreza Najafi is head of the research and development team of Farayand Sabz Engineering Co. (same address as above; Email:
[email protected]). He is a professional object oriented programmer and his main interest is developing simulator packages for industrial processes. With more than 12 years of experience, he has practical and professional experience in the fields of thermal radiation especially for design and simulation of thermal cracking furnaces and process fired heaters and also development of chemical kinetics and thermodynamics framework for simulator software. He has provided numerous solutions for process problems in these fields for various oil and petrochemical companies. He has a B.Sc.Ch.E. degree from Sharif University of Technology and a M.Sc.Ch.E. degree from Shiraz University and He is an associate member of IAChE and CSChE.
