101 Things That Can Go Wrong on a Primary Reformer - Best Practices Guide

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
GBH Enterprises, Ltd.
101 THINGS THAT CAN GO WRONG ON A
PRIMARY REFORMER - BEST PRACTICES GUIDE
Process Disclaimer
Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
good faith, but it is for the User to satisfy itself of the suitability of the Product for
its own particular purpose. GBHE gives no warranty as to the fitness of the
Product for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE accepts no liability for loss, damage or personnel injury
caused or resulting from reliance on this information. Freedom under Patent,
Copyright and Designs cannot be assumed.

0 Introduction
1 Common Problems Affecting the Catalyst ......................................................9
1.1 Poisons .....................................................................................................9
1.1.1 Chloride Poisoning............................................................................10
1.1.2 Arsenic..............................................................................................11
1.2 Carbon Formation and Hot Tubes...........................................................11
1.2.1 Causes of Carbon Formation............................................................11
1.2.2 Effect of Carbon Laydown.................................................................13
1.2.3 Effect of High Hydrocarbons .............................................................13
1.2.4 Loss of Fuel ......................................................................................14
1.2.5 Purging of Feed System....................................................................14
1.2.6 Actions to Limit Carbon Laydown Down............................................14
1.2.7 Carbon Removal by Steaming ..........................................................15
1.2.8 More Severe Steaming .....................................................................15
1.2.9 The ‘Wind Down’ Effect.....................................................................15
1.3 Catalyst Breakage...................................................................................16
1.3.1 Effect of Trips....................................................................................16
1.3.2 Effect of Catalyst Design...................................................................16
1.3.2.1 Example of a Catalyst with Good Breakage Characteristics .......17
1.3.2.2 Example of a Catalysts with Poor Breakage Characteristics.......17
1.3.2.3 Up Flow Fluidization Problems....................................................18
1.3.3 Milling of the Catalyst........................................................................19
1.3.4 Effect of Water ..................................................................................19
1.3.4.1 Effect of Water Carry Over..........................................................19
1.3.4.2 Shattering of the Catalyst............................................................20
1.3.4.3 Condensation..............................................................................20
1.3.4.4 Passing Steam Valve..................................................................21
1.4 Catalyst Loading .....................................................................................21
1.4.1 Poor Catalyst Loading.......................................................................21
1.4.2 Effect of Voids...................................................................................22
1.4.3 Tube Expansion................................................................................23
1.4.4 In-Correct Catalyst Loading ..............................................................23
1.5 Reduction of the Catalyst........................................................................23
1.6 Ammonia Formation................................................................................25

2 Common Problems Affecting the Tubes .......................................................25
2.1 Hot Tubes ...............................................................................................25
2.2 Tube Failure............................................................................................26
2.2.1 Fundamentals of Tube Design..........................................................26
2.2.2 Tube Failure by Creep ......................................................................28
2.2.3 Failure due to General Overheating..................................................30
2.2.4 Thermal Cycling ................................................................................31
2.2.5 Failure due to Localized Overheating................................................32
2.2.5.1 Flame Impingement ....................................................................33
2.2.5.2 Tunnel Port Effect .......................................................................33
2.2.5.3 Single Tube Catastrophic Failure................................................35
2.2.5.4 Pigtail Nipping .............................................................................35
2.2.5.5 Domino Effect .............................................................................36
2.2.6 Loss of Feed .....................................................................................38
2.2.7 Tube Weld Positions .........................................................................38
2.3 Failure of Mixed Feed Pre Heat Coil.......................................................39
2.4 Boxing Up of Reformer............................................................................40
2.4.1 Storage of Tubes ..............................................................................41
2.5 Effect of Water ........................................................................................41
2.5.1 Effect of Water Carry Forward ..........................................................41
2.5.1.1 Effect on the Tube.......................................................................41
2.5.1.2 Effect on the Catalyst and Tube..................................................42
2.6 Stress Corrosion Cracking of Tube Tops and Bottoms...........................42
2.6.1 Tube Tops.........................................................................................42
2.6.2 Tube Bottoms....................................................................................43
2.7 Bowed Tubes..........................................................................................44
2.8 Tensioning of Tubes ...............................................................................45
2.9 Pigtails ....................................................................................................45
2.9.1 Failure by Creep ...............................................................................45
2.9.2 Failure by Cracking ...........................................................................46

2.10 Differential Tube Metallurgy’s...............................................................47
2.11 Risers...................................................................................................48
3 Common Problems Affecting the Furnace Box .............................................49
3.1 Fluegas Maldistribution ...........................................................................49
3.1.1 Top Fired Furnaces...........................................................................49
3.1.2 Injection through Side Wall Peepholes..............................................50
3.1.3 Injection through Burner Ignition Port................................................50
3.1.4 Foster Wheeler Furnaces..................................................................52
3.1.4.1 Fluegas Fan Effect......................................................................52
3.1.4.2 Flow Maldistribution between Cells.............................................53
3.1.5 Tests for Mal Distribution ..................................................................54
3.2 Coffins.....................................................................................................54
3.2.1 Design of Coffin Roof........................................................................54
3.2.2 Effect of Damage to Coffins ..............................................................55
3.2.2.1 Movement of Tunnel Walls..........................................................57
3.2.3 Coffin Damage on Kellogg Furnaces ................................................57
3.2.4 Removal of Coffins............................................................................57
3.2.5 Modification to Port Layout................................................................59
3.3 Effect of Wind on Box Stability................................................................59
3.4 Purging of the Box ..................................................................................60
4 Common Problems Affecting Burners...........................................................60
4.1 Operation and Maintenance of Burners ..................................................60
4.1.1 Burner Misalignment .........................................................................61
4.1.1.1 Cleaning of the Burner Tips ........................................................61
4.1.1.2 Damage to the Burner Quarls .....................................................62
4.1.1.3 Top Fired Reformers...................................................................63
4.1.2 Lighting Burners................................................................................64
4.1.2.1 Side Fired Furnaces....................................................................65
4.1.2.2 Foster Wheeler Furnaces............................................................65
4.1.3 Non Optimal Firing in Foster Wheeler Furnaces...............................66

4.1.4 Fuel Usage........................................................................................67
4.1.5 After-Burning.....................................................................................67
4.1.6 Metal Dusting of Burner Tips.............................................................68
4.2 Flame Instability......................................................................................68
4.3 NOX .........................................................................................................68
4.4 SOX .........................................................................................................69
5 Common Problems Affecting the Fluegas Duct ............................................69
5.1 Too Much Excess Air ..............................................................................69
5.1.1 Leaks in Rotary Air Preheaters .........................................................69
5.1.2 Areas of Potential Air Leakage..........................................................70
5.2 Too Little Excess Air ...............................................................................70
5.2.1 Due to Insufficient ID Fan Capacity...................................................70
5.3 Fluegas Coiling Fouling...........................................................................71
5.4 Problems with Fans ................................................................................73
5.4.1 ID Fan Trips ......................................................................................73
5.4.2 ID Fan Close to Maximum Speed Pressure Boxes ...........................73
5.4.3 Governor Instability ...........................................................................73
5.4.4 Flue Gas Mal-Distribution – Effect on Box Pressure .........................73
6 Common Problems Affecting the Header Designs........................................74
6.1 Fuel and Fuel Header Designs ...............................................................74
6.1.1 Symmetry..........................................................................................74
6.1.2 Deposition of Particular Matter in Fuel Headers................................74
6.1.3 Fuel Valve Suction ............................................................................74
6.1.4 Purge CV Changes ...........................................................................74
6.2 Combustion Air Problems .......................................................................75
6.2.1 Poor Combustion Duct Design..........................................................75
6.2.2 Combustion Air Maldistribution .........................................................75
6.2.2.1 Due to Mechanical Failure ..........................................................75

6.3 Process Headers ....................................................................................77
6.3.1 Inlet Process Gas Header Design.....................................................77
6.3.1.1 Dead Legs and Low Points .........................................................77
6.3.1.2 Headers too Hot..........................................................................77
6.3.2 Exit Header Design ...........................................................................78
6.3.2.1 Exit Header Failure .....................................................................80
7 Common Problems Affecting Refractory.......................................................81
7.1 General Refractory Damage ...................................................................81
7.2 Tracking of Gas behind Refractory .........................................................81
7.3 Seals around Tube Inlets/Outlets............................................................81
7.4 Peephole Refractory ...............................................................................82
7.5 Cooling of Hot Reformer Casing .............................................................82
7.6 Damage to Refractory Anchors...............................................................83
8 Common Miscellaneous Problems................................................................84
8.1 Nickel Carbonyl Formation......................................................................84
8.2 On Line Analyzers...................................................................................84
8.3 Temperature Measurements...................................................................85
8.3.1 Exit Header Temperature Measurement...........................................85
8.3.1.1 M W Kellogg Furnaces................................................................86
8.3.1.2 European Plant Experience ........................................................87
8.3.2 Variations in Exit Temperatures........................................................87
8.3.3 Fluegas Temperature Measurements ...............................................88
8.4 Metal dusting of Waste Heat Boilers.......................................................89
8.5 Flowmeter Errors ....................................................................................89
8.6 Sample Shifting.......................................................................................90

8.7 Zinc Alloys...............................................................................................91
8.8 Power Failures........................................................................................91
9 Troubleshooting ............................................................................................92
9.1 Process Troubleshooting Guide..............................................................92
9.2 Mechanical Troubleshooting Guide.........................................................95
10 Conclusions ...............................................................................................98
11 GBHE INTERNAL References...................................................................99

0 Introduction
This paper details some common problems that can occur on primary reformer,
the associated convection section and the waste heat boiler. These problems
can lead to either a full plant shut down to effect repairs or to a loss of plant
efficiency. The problems have been grouped into and under the following
headings,
• Catalyst,
• Tubes,
• Furnace box,
• Burners,
• Fluegas duct,
• Header designs,
• Refractory
• Waste Heat Boilers.
Some typical examples include, but are not limited to,
• Carbon formation.
• Tube failure due to general overheating or overheating in a specific area.
• Fluegas maldistribution.
• Metal dusting of Waste Heat Boilers.
• Damage to coffins or coffin removal.
• Maintenance of burners.
• Combustion air maldistribution.
• Leaks in Rotary Air Pre-heaters.
• Flame impingement.
• Effect of water on tubes and catalyst.
Plant reliability could be defined by the following graph,

After plant start up, there are a number of problems formally associated with
commissioning, design issues and the operators learning about the plant.
Towards the end of the plants life, the problems are more associated with ageing
hardware, loss of corporate memory, changes in plant personnel and changes in
operating philosophy. It should be noted that many of these problems that have
occurred in the past are starting to re-occur again. This is a function of the above
issues and the reduction in plant personnel due to the effect of market forces on
fixed costs. See reference12 for further details.
For details on reformer design, references 14 and 15 are recommended reading.
1 Common Problems Affecting the Catalyst
1.1 Poisons
There are a large number of poisons that can affect primary reforming catalyst;
typical poisons include,
• Sulfur compounds such as hydrogen sulfide, COS, mercaptans and
thiophenes.
• Chlorides and halides.
• Mercury.
• Arsenic.
• Silica.
• Phosphates.
• Organo-metallic’s.

• Heavy metals.
• Alkali metals.
• Vanadium – this can be a problem with plants with a Vetrocoke system.
Sulfur can be moved by steaming as discussed in section 0. With the exception
of sulfur, once the catalyst has been poisoned, either the affected portion or all of
the catalyst will have to be discharged and replaced.
1.1.1 Chloride Poisoning
Chlorides are a particularly virulent poison. It should be noted that chlorides
have an unusual effect on zinc oxide as they react on the surface of the pellets to
form zinc chloride. This skin completely blocks off access to the internal volume
of the pellet, thereby dramatically reducing he sulfur absorption capacity. The
following figure illustrates this effect,
This means that if a chloride guard is not installed then chlorides can pass
through to the reformer very quickly and since the zinc oxide has been poisoned,
the reformer will also see high levels of sulfur.

1.1.2 Arsenic
If the catalyst is poisoned by arsenic, then not only does the catalyst have to be
discharged but the inside of the tubes have to be cleaned to remove any residual
arsenic. If this is not done, then this residual arsenic will leach out of the parent
metal and poison the replacement catalyst.
1.2 Carbon Formation and Hot Tubes
Carbon formation is normally highlighted by the formation of hot bands on the
reformer tubes as highlighted by the following figure,
1.2.1 Causes of Carbon Formation
Carbon formation occurs when one of the following occurs,
• The plant is operated at a low steam to carbon ratio; this typically occurs
during a plant transient such as shut down or start up.

• The feedstock composition changes such that the feed includes more heavy
hydrocarbons; this is occurring more often as many gas wells are
approaching the end of their useful life.
• The catalyst activity drops such that the inside tube wall and/or the process
gas temperature becomes high enough that carbon formation rate exceeds
the carbon gasification rate; this typically occurs at the end of the catalyst life
or if the catalyst has been poisoned. The latter problem is occurring with
more regularity as many gas wells are approaching the end of their useful life.
• The catalyst has poor heat transfer characteristics which cause an increase in
tube wall and process gas temperatures.
• Insufficient purging of the plant to remove residual hydrocarbon prior to
restart.
• Collection of liquid hydrocarbons in dead legs or low points.
• Complete loss of steam whilst all or some of the feedstock is still being
passed to the reformer. In the latter case, this cannot be removed even with
steam (see section 0). Typically, this can be caused by a passing valve or a
lack /poor instrumentation.
It should be noted that once carbon is laid down, a viscous circle is formed; this
is because the carbon lay down causes,
• A decrease in inside tube wall heat transfer coefficient.
• A decrease in the inter pellet heat transfer coefficient.
• A decrease in catalyst activity as the active nickel sites are covered by
carbon.
• An increase in resistance to flow through the affected tube, thereby
decreasing the heat sink available.

1.2.2 Effect of Carbon Laydown
These all cause an increase in inside tube wall and process gas temperature and
hence an increase in the rate of carbon deposition which then increases the
effects of the above.
Eventually the outside tube wall temperature is increased such that it glows with
the typical orange color that is a sure sign of carbon laydown. If nothing is done
to halt the progress of the carbon formation, then eventually the tube wall
temperature will increase such that it reaches the design tube wall temperature
and hence becomes a plant limitation.
1.2.3 Effect of High Hydrocarbons
It is well known that slugs of high hydrocarbons can lead to hot banding if the
steam to carbon is not adjusted accordingly. Such incidents are well known and
relatively common.
Once such incident occurred on a South American plant. The upstream LNG
plant has two stages of condensate removal, the first operating at 35°C and the
second at –35°C. Both stages were subject to trips and shut downs and when
they were out of service, large amounts of higher hydrocarbons were not
removed from the natural gas and therefore passed to the steam reformer. This
lead to excessive hot banding of the reformer.
The following figures illustrate some typical hot bands as observed on this
reformer,

The reformer was steam out and was successful since the hot bands were
removed,
1.2.4 Loss of Fuel
If the fuel is lost to the furnace, then the exit reformer and fluegas temperatures
from the furnace will start to drop very quickly. This latter effect causes a loss of
feed pre heat and steam generation. If no action is taken, then it is possible for
carbon formation to occur due to the reduction in steam to carbon ratio.
1.2.5 Purging of Feed System
If the front end of the plant is not purged adequately enough, then CO and CO2
can be methanated to form CH4. On restart this can crack thereby depositing
carbon on the surface of the catalyst.
1.2.6 Actions to Limit Carbon Laydown Down
Increasing the steam to carbon ratio and the hydrogen recycle rate is
directionally the correct action to take once carbon formation has been detected.
This will only reduce the rate of carbon formation slightly. In reality it will not help
gasify carbon that has already been laid down.

1.2.7 Carbon Removal by Steaming
The only method to ensure that carbon is removed is to steam the catalyst. The
following is a set of guidelines that should be followed when it is necessary to
steam the catalyst,
1. The steam rate shall be set at a minimum of 50% of the design steam rate.
2. The reformer exit temperature shall be as high as possible and shall be in
excess of 700°C.
3. The steam out shall be performed for at least 12 hours.
4. The gas exit the reformer shall be tested for methane and carbon dioxide; it
should be noted that there will be little carbon monoxide since the water gas
shift reaction favors the formation of carbon dioxide. The results of the test
shall be trended as a measure of the progress of the steaming.
5. The exit reformer gases shall also be tested for hydrogen sulfide. An
alternate is to test the process condensate for sulfites and hydrogen sulfide
(in some cases a small test is adequate for detecting this).
6. If the gas sample is taken down stream of the process condensate knock out
pot, the nitrogen shall be added at the mixing tee to act as a carrier gas.
Further details are available in Ref. 1 and 2.
1.2.8 More Severe Steaming
If normal steaming as detailed above, fails to remove the carbon from the tubes,
then hydrogen can be added to speed up the process. If this fails, then air (or
oxygen can be added to help remove the carbon by burning. If this fails, then the
only option is to replace the catalyst.
1.2.9 The ‘Wind Down’ Effect
If a hot tube or hot spots develop, then it may often happen that the local firing
around the affected tubes is reduced, to lower the tube temperature. In order to
maintain the overall production rate, however, it is then deemed necessary to
increase the general level of firing. This has been known to lead to more hot
spots - so the local firing is reduced, and the general firing increased, as before.
This process can lead to a vicious circle, ending with many damaged tubes, and
reduced overall firing efficiency.

It is probably advisable to live with the slight loss of efficiency caused by NOT
increasing the general level of firing in the first place.
This is a particular problem if the original cause of the hot spot is due to carbon
formation since it does mean that the tubes that have their firing increased will
become hotter and therefore will be more susceptible to forming carbon.
1.3 Catalyst Breakage
Catalyst breakage can be caused by carryover of water (see section 0),
excessive trips or poor catalyst design.
1.3.1 Effect of Trips
Excessive trips cause expansion and contraction of the tubes; the contraction of
the tubes cases large stresses to build up on the pellets and these stresses can
only be relieved by movement of the catalyst axially in the tube or pellet
breakage. In reality, only the catalyst at the top of the tubes can move and the
catalyst towards the bottom of the tube, where the temperature changes will be
the greatest, are locked in position. Therefore, the only possibility is for the
catalyst to fracture.
1.3.2 Effect of Catalyst Design
If the catalyst has been designed such that on breakage, it forms a large number
of small fragments, the pressure drop will rise rapidly. An example of this
phenomenon is given below.

1.3.2.1 Example of a Catalyst with Good Breakage Characteristics
Comp J four hole catalyst is an example of a catalyst with good breakage
characteristics, in that when it does break it forms large fragments which means
that the pressure drop is relatively small. This is because,
• Pressure drop is inversely proportional to effective pellet diameter – therefore
if the fragments formed are large, then the effective pellet diameter only
increases marginally,
• Pressure drop is related to voidage by the following term (1-e)/e³ and
therefore any decrease in voidage will cause large increases in pressure drop
1.3.2.2 Example of a Catalysts with Poor Breakage Characteristics
An example of a catalyst with poor breakage characteristics if that of the Comp U
Wagon Wheel (the extended Wagon Wheel – EW, with thicker ligaments may be
better) and Comp H’s seven hole catalyst,

Breakage of the catalyst in a tube will lead to a high resistance to flow and
therefore, the flow through the tube will be low. This will cause the tube to
operate hot – a similar effect is caused by variability in the loaded voidage (see
section 0).
1.3.2.3 Up Flow Fluidization Problems
The majority of reformers have the process gas flowing downwards and hence
there are no issues associated with fluidization of the catalyst, however, there are
a number of up flow circular reformer. If the design of the reformer is poor or the
plant has been uprated, then is it possible to achieve process side velocities that
are sufficiently high to fluidize the catalyst. This will lead to catalyst attrition and
breakage which will cause excessively high pressure drop and fouling of
downstream equipment by catalyst dust.
A potential solution to this problem is to install a hold down device with sufficient
mass to resist the fluidization force. A typical design is shown below.

1.3.3 Milling of the Catalyst
Milling of the catalyst can occur if the tube inlet is incorrectly designed. Typical
designs of inlets are shown below for a side and top entry.
Both designs are acceptable, however the separation between the inlet and the
catalyst surface must be sufficiently large to ensure that catalyst damage does
not occur. It should be noted that for side entry pigtails, the separation shall be a
minimum of 100 mm and for top fired, a minimum of 200 mm.
At a European Plant, the customer complained of a high pressure drop and when
the tubes where opened, it was found that the catalyst had been milled into
spherical particles. In this reformer, the separation distance was only 100 mm
and the jet of gas leaving the pigtails rolled the catalyst around.
1.3.4 Effect of Water
1.3.4.1 Effect of Water Carry Over
A further problem is water carry over from the steam drum, where the liquid is not
fully disengaged from the steam. If this liquid is not vaporized in the steam
superheater, then it is possible for boiler salts to be carried over to the reformer
where it can be poisoned or a crust of salts can be formed on the catalyst.

1.3.4.2 Shattering of the Catalyst
Recently on an Ammonia plant in South America, the operator managed to fill the
bottom section of the reformer tubes with water. Upon restart, the pressure drop
across the reformer was high and this lead to a shut down. After discharging the
catalyst it was found to have had the edges sheared off as shown below,
The cause of this was when the catalyst was heated up, the water could not
escape from the centre of the ligaments, which represents the thickest part of the
catalyst pellet, before it was vaporized. As soon as the water vaporized, there
was a huge volume expansion which caused these sections to break away from
the rest of the pellet.
1.3.4.3 Condensation
On a plant trip it is very possible that steam can condense and sit in dead legs or
low points in the feed header system. On a plant restart, it is possible that the
water is carried forward on to the catalyst. The catalyst is normally hot at this
stage, and as the cold water hits the hot catalyst, the catalyst will be rapidly
cooled and the stresses induced can shatter the catalyst.
This problem can be prevented by eliminating low points and dead legs during
the design of the plant – it is usual that this kind of problem will be picked up
during the plant HAZOP review. Suitable positioning of drains and correct start
up procedures will also help in minimizing the risk.

1.3.4.4 Passing Steam Valve
If the process steam valve passes during a shut down or whilst the plant is shut
down, then it is possible for water to condense on the catalyst. On restart this
can lead to a number of problems such as shattering of the catalyst and potential
formation of concrete.
1.4 Catalyst Loading
1.4.1 Poor Catalyst Loading
Ensuring a good catalyst loading is fundamental in ensuring efficient operation of
the primary reformer. Any deviations
in resistance to flow through the tubes
will result in differential flows between
tubes and this in turn will lead to tube
wall temperature differences as
illustrated to the right,
A good catalyst loading will cause
even process gas distribution and
hence even tube wall temperature
distribution as shown below

Another effect is that there will be process gas exit temperature spreads on the
reformer which will artificially increase the methane slip from the reformer. The
effect of this effect is illustrated below.
The industry has developed a number of pressure drop measurement devices,
one of which is called the PD Rig which allows for tubes pressure drops to be
measured at various points during catalyst loading. The results of this allow the
operator to determine which tubes have a low resistance to flow (a low pressure
drop) which need further vibration and those with a high resistance to flow (a high
pressure drop) which need reloading.
Also the method of loading is very important. The traditional sock loading, can
when applied correctly, give a very good catalyst loading. However, the more
modern Unidense method can give a loading where little or in some cases no
remedial action is required during and after catalyst loading to achieve a uniform
catalyst loading.
1.4.2 Effect of Voids
Furthermore, a poor loading can give rise to localized voids within the tube which
will be seen as hot spots on the tube. This can then limit the reformer
performance since to keep these tubes cool; the firing around these tubes with
hot spots has to be reduced.

1.4.3 Tube Expansion
Care should be taken to allow for the effect of tube expansion. Sufficient catalyst
must be charged into the reformer tube when cold to make sure that when
operating, and therefore hot, the catalyst does not settle down so far as to
expose empty space at the top of the reformer tube.
1.4.4 In-Correct Catalyst Loading
Another problem can occur if a two tier catalyst combination is being loaded with
the top catalyst being potash doped. Unless the catalysts are a different shape
or size, it is easy to load the catalyst the wrong way around. This means that
there is no protection against carbon formation in the top of the tube and carbon
will be a problem if the conditions as outlined in section 0 are fulfilled.
1.5 Reduction of the Catalyst
As with many catalysts, primary reforming catalysts are supplied in the oxide
form and therefore require reduction. Unlike the majority of catalysts, there is
normally no hydrogen to reduce the reforming catalyst.
It is normal practice therefore to start the plant up on steam and natural gas and
allow the reduction to be performed by the cracking of natural gas. Once the
plant is operating, it is possible to recycle hydrogen from the back end of the
plant to complete the reduction.

This initially incomplete reduction can lead to the observation from a customer
that the tubes are hot and therefore the catalyst is not active. Another potential
problem occurs with reformers where the inlet temperature is too low. Reformer
catalysts are required to be at a sufficiently high temperature in order to be
reduced – the required temperature is a function of the catalyst support as shown
below,
If the inlet temperature is less than these figures, then the catalyst will not be
reduced and the un-reduced section of the catalyst will remain until the operating
temperature at that point in the tube exceeds the minimum reduction
temperature. Since catalysts containing magnesium oxide require a very high
temperature before they reduce, they are normally supplied with the top 15% as
pre-reduced. This however, only good for the first start up – thereafter, all
restarts which must a trip and subsequent oxidation, will suffer from the problem
outlined above.

1.6 Ammonia Formation
Ammonia will be formed over primary reformer catalyst by combination of
nitrogen from the feedstock and hydrogen formed within the reformer. Ammonia
formation is favored by high temperatures and therefore the bulk of the ammonia
is formed at the tube exit – this is also where the hydrogen content of the process
gas is its highest. Nitrogen formation rate is also proportional to the nitrogen
content of the process gas and the activity of the catalyst. This means that at
start of run, the ammonia formation levels will be their highest, typically 30% of
the equilibrium value and at end of run, they will be at their lowest, typically 10%
of equilibrium.
2 Common Problems Affecting the Tubes
2.1 Hot Tubes
There are a number of forms of hot tubes, each with a different cause; the
following figure illustrates the different forms,

The causes of this are,
• Hot spots due to localized high voidage or catalyst poisoning (0).
• Hot tubes due to low flow caused by a high loaded density (low voidage).
• Giraffe necking,
• Tiger tailing.
• Speckled tubes due to small zones of high voidage where the catalyst is not
touching the inside wall of the tube.
These hot zones on the tube can lead to a reduction in tube life and
consequently, premature tube failure.
2.2 Tube Failure
There are many causes of tube failure within primary reformers of which some
are discussed below. Some of these failures must be expected and others that
can be deemed as premature.
2.2.1 Fundamentals of Tube Design
Due to the operating conditions of a primary reformer, that is high temperature
and moderate pressure, the reformer tubes are operated in the creep regime; in
this regime, the tubes are gradually being stretched and hence the tube loses
strength and thickness. This process is similar to that which affects glass; if one
where to look at an old glass window, it appears to be of variable thickness and
appears ‘wavy’.
This fact has been accounted for in the design of the reformer tubes and it is
typical that reformer tubes are designed to last for 100,000 hours (with an
expectation that 2.5% of the tubes will fail before this time is reached). In many
cases, tubes have lasted much longer than this due to over design of the tubes
or operation at less than the design temperature and pressure.

The process used to design the reformer tubes is to determine the hoop stress
that is applied to the tube due to the differential pressure between the process
gas and fluegas sides of the furnace. The Larsen-Miller plot (as shown below) to
determine the maximum allowable operating temperature.
The reverse procedure can also be used where the design temperature is set
and then the maximum allowable stress is calculated from the Larsen-Miller plot,
which then allows the tube thickness of be determined.

2.2.2 Tube Failure by Creep
Failure of the tube is normally due to creep damage that occurs from the inside of
the tube wall to the outside of the tube wall, as illustrated below,
The typical progression of creep damage at the micro level is shown below,

The left hand figure shows the development of isolated creep voids between the
grains. The middle figures shows how these develop into fissures between the
grains and the right hand figures shows these fissures joining up and developing
into cracks. Typical tube failures are shown below,
Failures can also occur at the welds in the tubes,
This was a common problem with older tubes since the weld material was
somewhat weaker than the parent tube material. However, modern weld
material, if properly applied will actually be stronger than the parent material and
so this problem is now less common on modern furnaces.

2.2.3 Failure due to General Overheating
This situation is where either all of the majority of tubes have failed in a reformer
due to operation at high temperatures. It is typical that this problem occurs
during start ups or shut downs of the primary reformer. One of the root causes of
such failures is that the process parameters are very different from the normal
operating conditions and it is not normally obvious to the plant operators that
there is a problem.
A classic example is the complete ‘burn down’ of the tubes in a Canadian primary
reformer. The plant was tripped due to loss of feedstock, however the feedstock
isolation valve did not close fully and feedstock continued to be passed forward
to the reformer. The set point on reformed gas pressure not reduced and the
reformer continued to be operated at 16 bar. Steam was introduced for plant
restart at reduced rate and all the burners were lit (a deviation from operating
procedure). At this time the steam reformer tubes "looked normal" but there was
nearly three times the amount of fuel going to burners than there should have
been. Also the fuel gas had a higher than normal calorific value which increased
the heat release by a further 15%.
At this point the first tube started to rupture and the oxygen level in the furnace
dropped to zero since the feedstock was now combusting in the furnace.
Normally the high pressure furnace trip would have been activated but this was
being bypassed. Flames were observed issuing from the peepholes and a visual
inspection of the reformer found that the tubes were “white hot and peeling
open”. The following are photos of the reformer after the plant was shut down,

It should be noted that the reformer exit gas temperature on panel never
exceeded 700°C (1290°F); it should be noted that the exit temperatures from the
primary reformer during transients should not be used as a guide to tube
temperature.
2.2.4 Thermal Cycling
During the life of a reformer tube, it will experience a number of full thermal and
pressure cycles caused by plant start-ups and shut-downs. The cumulative effect
of these cycles can be very damaging, and lead to accelerated creep cracking.
The tube life is crudely related to the number of cycles it has seen, and possibly
also the tube wall thickness.
Thick tubes (typically made from HK40 and similar alloys) may be defined as
those in which the OD:ID ratio is greater than 1.35 (e.g. 17 mm (0.7 inch) wall
thickness for a 100 mm (3.94 inches) bore tube are significantly less tolerant to
thermal cycling than thin tubes. Fortunately, the availability of stronger alloys
(such as 36X and XM) in recent years, leading to thinner tubes, has reduced the
significance of this problem in new plants.

2.2.5 Failure due to Localized Overheating
There are many mechanisms for localized tube overheating which cause a single
tube or a group of tubes to fail catastrophically. One method of determining that
the tubes have been subjected to is the color of the catalyst; at high
temperatures, the catalyst support will be affected and spinel formation will start
to occur.
The effect of this is to change the color of the catalyst as shown in the following
picture from a Caribbean Plant,
Typical color changes are highlighted below,

Additional information can be found in reference 10.
2.2.5.1 Flame Impingement
There are a number of causes of flame impingement on the tubes, for example,
misaligned burners, fluegas mal-distribution (see section 0) and poor burner
maintenance. The effects of these are discussed in the relevant section
elsewhere in this document. The effect of these on the tubes is precisely the
same, in that a small section of the tube will become overheated and eventually
fail due to excessive localized creep.
2.2.5.2 Tunnel Port Effect
It had been noted that a number of large methanol reformers had suffered
premature tube failures in the bottom section of the tubes; NDT had shown that
the effect was limited to a length of 100-150 mm of the tube in the zones
opposite the portholes.
Checks using a surface contact thermocouple, and both an optical and gold cup
pyrometer were made on the tube temperatures in this zone and they were found
to be significantly higher than expected as illustrated below,

It was noted that the tubes had failed almost directly opposite the ports in the
tunnels, as shown below,
Theoretical checks where then made using a Monte Carlo simulation to
determine the paths that radiation would take from the inside of the coffin. This
shows that a beam of radiation did escape from the tunnels and impacted on the
tubes causing the tube temperature to be significantly greater than would be
expected,

It is typically found that this effect can raise the temperature of the tubes in this
zone by between 10 and 18°C which equates to a reduction in tube life of
between 25 and 45% which explains the premature failure of the tubes. The
short term solution is to install an insulated sleeve around this area; this does
increase the methane slip and causes a short term plant inefficiency. A longer
terms solution is to install a high activity/heat transfer catalyst in this zone to
reduce the tube wall temperature.
2.2.5.3 Single Tube Catastrophic Failure
If a tube does fail, then it is still possible to continue to operate the furnace.
Checks should be made to ensure that the jet issuing from the failure point is not
impinging on another tube, which could lead to localized overheating of and
premature failure. This check should be repeated at regular intervals and if the
jet dies impinge on another tube, then the tube shall be nipped as quickly as
possible.
2.2.5.4 Pigtail Nipping
If a tube or pigtail does fail, then it is
possible on appropriately designed
furnaces to nip the tube using a pig tail
nipper; the following picture illustrates
a tube that has been nipped (the
yellow tube) ,
This tube will be significantly hotter
than the other tubes since it is still
receiving full heat flux from the burners, but there is no flow of process gas
through the tube to cool it. Eventually the tube will fail as highlighted in the next
picture,

Note in this picture the collapsed coffin in the distance.
2.2.5.5 Domino Effect
A North American Plant operator of a large reformer in the USA suffered a
significant number of tube s failure in the 1990’s. The root cause of this problem
was their policy of fuel management after nipping failed tubes. As with many top
fired furnaces, NA Plant operator had the capability to nip tubes.
After a tube failure, it was nipped, however, the NA Plant did not reduce the fuel
to the burners around the failed tube – it should be noted that it is normal practice
to reduce the fuel firing around a nipped tube. By not reducing the firing around
the nipped tube, the adjacent tubes received the heat from the burners
associated with them and also from the burners next to the failed tube.
This increased their temperature significantly and lead to some of these adjacent
tubes failing; the following figures illustrate what happen - note newly failed tubes
are highlighted as red and previously failed tubes are highlighted as black.

This effect then propagated down the row with the failed tubes as illustrated
below,
As the number of tubes that failed increased, the tubes tube in the opposite row
became to hot, and eventually lead to the failures jumping across to the adjacent
rows. This then causes the adjacent tubes in these rows to fail, and the failures
then started to propagate along the adjacent tube row as shown in the following
figures,

By the time the plant was shut down, approximately 25% of the tubes in the
reformer had failed.
2.2.6 Loss of Feed
If the feed to the reformer is lost, then the operators face two potential problems.
The first is that the fuel rate to the reformer must be reduced since there is no
longer the steam-reforming reaction to keep the tubes cool. If the fuel rate is not
reduced quickly enough then the tubes will be overheated and in the worst case,
then the tubes will fail due to generalised overheating (see section 0).
2.2.7 Tube Weld Positions
When reformer tubes are manufactured they are produced in sections that are
between 3,500 and 5,000 mm long. These sections are then welded together to
produce the required reformer tube length. In the early days of tube
manufacture, the weld material used was significantly weaker than the parent
material and therefore represented a potential localized failure point. It was
therefore common practice to ensure that the welds were not placed at the point
of highest heat flux – on a Top Fired reformer this meant about one third of the
way down the tube. With more modern alloys this is less of a problem since the
weld material is now stronger than the parent material. However, it is still good
practice to ensure that the weld is position away from such places in case of a
weld defect.

2.3 Failure of Mixed Feed Pre Heat Coil
A European HYCO plant operator suffered a catastrophic tube (8 tubes out of 24)
and coil failure. The root cause of this failure was due to poor design of the
mixed feed preheat coil, where one of the passes (out of a total of 11) received
less flow than the other
passes within that coil. The
diagram to the right illustrates
the layout of the coil.
It is unknown whether there
was a blockage in this pass or
whether since this was the
last pass, the pass received
less flow than the others.
However, it is clear that this
pass did see high
temperatures and that the
pass failed due to exposure to
high temperatures.
This caused the mixed feed to pass through the failed tube into the fluegas duct,
leading to high box pressure; the high pressure trip did not activate due to a relay
failure. The reformer tubes then saw low flow and this lead to the tube
temperature increasing such that they failed as illustrated below,
This in turn led to a fire in the radiant box and in the penthouse leading to
significant amounts of damage.

2.4 Boxing Up of Reformer
At a South American Methanol plant, on a plant trip, the operating procedure
dictated that all firing be stopped and all fans immediately shut down. A ten
minute steam purge was allowed but then all process flows were stopped hence
there was no flow through the tubes. The furnace was then left to soak in a hot
atmosphere.
The furnace is ceramic fibre lines with brick tunnels and brick floor. Therefore,
the walls at 1050°C radiated to the tubes and the tubes warmed up a little for
most of the length. As the ceramic fibre has a low density, the fibre cooled down
to the tube temperature without heating the tubes up very much. However, the
tunnels are also at 1050°C but have a large mass, warmed up the tubes in the
tunnel region to quite high temperatures. This is its own right was not a problem
as the tubes had little pressure inside them at this time.
As the furnace lost heat through the ceramic fibre lining, the upper section of the
tubes cooled down, but the tubes at the bottom did not as the tunnels retained a
lot of heat. The biggest problem was then on restart when a lot of steam flowed
into the tubes. This flowed down the top 10m of tube which was at say 600°C
and then flowed into the bottom 2m of tube that was still at say 950°C and
created thermal shock of the tubes by cooling them from the inside too rapidly.
The overheating during the first stages after the trip could have used up some life
if pressure was retained or the plant was re-pressured quickly. In the early days,
of this South American Methanol plant, had a very poor power supply so many
trips would have been rapidly restarted as there was no plant breakdown or
repairs required, simply wait for the power to come back on.
The result is that the tubes in the top part of the furnace, which operated a lot
cooler than design had very little creep, but the bottom of the tubes had up to 6%
creep, and it was a very sharp change from the low creep to high creep which
corresponded to the level of the tunnel tops. The problem with all this was that
the South American Methanol plant, had run a crawler down the tubes after 10
years or so and pronounced them to be fine with max. creep at 1.5% or so. The
crawler did not go down between the tunnels as it was too large.
The plant then restarted and some tubes failed and were found with large levels
of creep, so they blamed this on 4-hole.

2.4.1 Storage of Tubes
Spare tubes should always be stored in a dry and clean warehouse to prevent
damage. The following picture illustrates what should not be done,
2.5 Effect of Water
2.5.1 Effect of Water Carry Forward
If water is carried forward either from a saturator or from the process steam, it is
possible to generate an extreme thermal shock due to the quenching of the
inside of the reformer tubes. This creates both a high tensile stress on the inside
of the tubes, and reduced ductility leading to sudden, deep cracking, or even
shattering of tubes.
2.5.1.1 Effect on the Tube
Such a situation occurred in a Western European modern 1350 mtpd ammonia
plant which was successfully commissioned, and shown to be capable of running
well both at and above flowsheet rates. However, after less than a year in
operation, a tube failed. This was followed by seven further tube failures in the
following 8 months. On examination, non-destructive testing (NDT) revealed
widespread cracking of tubes, particularly at welds. The tubes had generally
failed by longitudinal splitting.

The tubes split some 1.2-1.5 metres (4 - 5 feet) down from the top of the roof,
with the split being typically 0.61 metres (2 feet) in length. In all cases, cracks
had originated from inside the tube bore. Deep craze cracking was found to be
common around the vicinity of the split, which were all brittle fractures which is
typical of thermal over-stressing. Further creep of the remaining much reduced
wall thickness led to final failure of the tubes over a period of time.
2.5.1.2 Effect on the Catalyst and Tube
In some cases where the catalyst has been wetted, the support material can be
leached out and deposited on the inside of the tube walls. When this residue is
dried out, a hard coating is formed on the inside of the tube wall which is very
difficult to remove. A device known as a ‘frapper’ can be used to remove this
coating; this device consists of a pear shaped metal head attached to a high
speed rotating shaft by a hinge. This problem occurred at Koch nitrogen at
Sterlington in the late 1990’s and took three days to clean out.
2.6 Stress Corrosion Cracking of Tube Tops and Bottoms
Stress corrosion cracking (SCC) has been seen on a number of reformers
around the world. Condensation (with associated concentration of impurities in
condensate on re-evaporation) can occur, leading to premature tube failure due
predominantly to stress corrosion cracking.
Careful design of tube ends, and suitable start-up and shutdown procedures to
avoid the dew-point of steam being reached, are needed. This problem appeared
to have receded, but has recently re-emerged, with several plants experiencing
cracks at the tube tops.
2.6.1 Tube Tops
It should be noted that the tube tops, do protrude above the furnace roof and
therefore is it recommended that the tube tops are insulated to keep them hot
and prevent condensation.

2.6.2 Tube Bottoms
In some reformer designs, such as the original design for a European Methanol
Plant, the tube bottoms have a cold catalyst discharge end. The following figure
illustrates the original Methanol plant tube bottom design,
In order to prevent this occurring again, GBHE has access to a hot bottom tube
design which prevents SCC at the tube bottom and this is illustrated below,

2.7 Bowed Tubes
Bowing of tubes can be caused by differential heating between the two sides of
the same tube. The bending stress produced is proportional to the deflection
from the vertical, and increases with the degree of top tensioning. If, therefore,
tubes are bowed, then the sum of the combined stress due to pressure,
tensioning and bowing may be such that the allowable stress is exceeded,
leading to shorter tube life. Since on many older plants, the welds are frequently
weaker than the parent material, the location of welds on bowed tubes must be
taken into account.
Excessive bending of the tubes can prevent easy movement of the tubes through
the casing of the reformer. It this occurs then the tubes cannot expand axially
and will be compressed increasing the stresses on the tube.
Furthermore, the tubes will tend to bend even more. This will lead to a reduction
in tube life. Once a tube is bent then even after cooling, the tube will stay bent
and even after being reheated, the tube will still stay bent. If the tube is bowed
such that it deviates by more than one diameter from the tubes in the row, then it
is recommended that the tube is replaced at the earliest opportunity possible.
This is because the bending on the tube induces stresses which are sufficiently
high that the tube could fail prematurely.

2.8 Tensioning of Tubes
Almost all reformer tubes are top tensioned. This tensioning produces
longitudinal stress in the tube which must be added to the longitudinal stress
caused by the pressure. Additional stresses can also be generated, for example
by tube bowing.
If the tube is over tensioned due to poor set up or design of the spring hangers
(or similar support systems), then additional stresses can be generated in the
tubes which can lead to the failure of the tube. If the tube is under tensioned,
then the tube will exert a force on the exit headers and this can reduced the life
of the exit header.
2.9 Pigtails
Outlet reformer pigtails operate in the creep regime, and can fail either by creep
of cracking of the pigtail welds.
2.9.1 Failure by Creep
Creep generally shows itself as bulging/ballooning of the material. This can be
accurately measured at shut-downs using GO,NO GO gauges or circumferential
(vernier) tapes. GO,NO GO gauges are manufactured from carbon steel frame
shaped like a ‘G’ clamp with tungsten carbide tips. The gap is set at the tubes
outside diameter for the material purchased plus 2½%. Therefore, if the exit
pigtail is 38 mm (1.5 inches) OD then the gauge will be set at 38.95 mm (1.53
inches).
This can then be used to quickly accept or reject exit pigtails that have suffered
excessive creep. It is also useful to manufacture a similar gauge set at 1% and
2%. These can then be used to assist in the decision making for future pigtail
replacement planning. A pigtail is generally deemed unfit for service when 2½%
creep has been achieved. This figure has been used by GBHE for many years
and was developed from tests during the development of the “Pigtail Nipper”, to
ensure the nip is successful. However, if pigtails are not going to be nipped it is
not uncommon for this figure to be as high as 4% before replacement is
necessary.

2.9.2 Failure by Cracking
The other mode of failure is cracking of the weld. This can be caused by a
number of external sources, i.e. movement between the reformer tube and the
outlet header or thermal gradients at the junctions. Depending on the type of
reformer/pigtail configuration, the profile of the weld is extremely important.
Foster Wheeler type reformers with short pigtails are particularly susceptible to
weld problems if the profile is not correct. With an incorrect profile the life in
cycles can be as little as 55 increasing to 250 with the correct profile. An
example of a poor weld is given below,
Note the two problems here, the first is that the pigtail has not fully penetrated
into the sockolet leaving a gap and the second is that weld is not complete –
again notice the triangular gap. An example of such a failure is shown below,

An example of this occurred on a European Methanol Plant in 1994.
A leak had been detected during routine check but was deemed to be small
enough that the plant could continue to operate. Four days later, the plant
tripped due to high box pressure and a fire was seen around the location of the
failed pigtail.
Further details on this failure are given in reference 7.
2.10 Differential Tube Metallurgy’s
An Asian operator of an Uhde reformer had placed some of the tubes in one area
of the reformer using a 36X equivalent; the original tubes where HK40. To take
advantage of the improved metallurgy, the firing in this area with the 36X tubes
was increased. This lead to an observed high temperature spread across the
furnace.

2.11 Risers
Risers are only used in M W Kellogg furnaces as illustrated in the figures below,
On a South American plant, (an M W Kellogg methanol plant), the risers suffered
from significant cracking around 30% of the way down the riser. The cause of
this was thought to be due to flame impingement.
The short term fix was to insulate the upper part of the riser, this would however,
cause a marginal reduction in radiant box efficiency. The longer term solution
was to replace all the risers in the reformer.

3 Common Problems Affecting the Furnace Box
3.1 Fluegas Maldistribution
Mal distribution occurs on many furnaces to some extent, however, in some
cases this can cause the methane slip to rise above the expected value. Below
is a discussion of some of the worst effects seen?
3.1.1 Top Fired Furnaces
This phenomenon was noted during a reformer survey on the Canadian
Methanol Plant primary reformer. An unusual tubewall temperature profile was
noted,
As can be seen the outer rows are significantly cooler then the inner rows. At
this stage some design problems were observed in the coffins by GBHE and
recommendations were made to the plant operators who rectified these issues at
a shut down. However after the shutdown, there still was a significant mal
distribution – tests with injecting fire extinguisher powder into the furnace
highlighted the following effects,

Injection through Side Wall Peepholes
As can be seen the fluegas near the wall is flowing upwards.
3.1.2 Injection through Burner Ignition Port
The following pictures illustrate the injection of dry powder through the burner
ignition ports,
As can be seen the fluegas is flowing across the furnace roof and is impacting on
the tubes.

101 Things That Can Go Wrong on a Primary Reformer - Best Practices Guide

101 Things That Can Go Wrong on a Primary Reformer - Best Practices Guide

Recomendados

Recomendados

Mais conteúdo relacionado

Mais procurados

Mais procurados (20)

Destaque

Destaque (20)

Semelhante a 101 Things That Can Go Wrong on a Primary Reformer - Best Practices Guide

Semelhante a 101 Things That Can Go Wrong on a Primary Reformer - Best Practices Guide (20)

Mais de Gerard B. Hawkins

Mais de Gerard B. Hawkins (20)

Último

Último (20)

101 Things That Can Go Wrong on a Primary Reformer - Best Practices Guide