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

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GBH Enterprises, Ltd.

101 THINGS THAT CAN GO WRONG ON A

PRIMARY REFORMER - BEST PRACTICES GUIDE

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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.



At this stage the furnace was modelled using CFD to determine theoretically whether we would expect the unusual flow patterns observed during the dry powder plant trials. The result of this model is shown below,

As can be seen the model predicts that there is up flow at the walls and there is cross flow from the outer lanes to the inner lanes at the top of the furnace. The root cause of the problem was a mismatch between the burner capacity, outer lane sizing and the outer coffin sizing. As can be seen the flow patterns predicted by the CFD model match that seen on the plant. GBHE has a range of solutions to resolve this problem and these are discussed in more detail in reference 5.



3.1.4 Foster Wheeler Furnaces

3.1.4.1 Fluegas Fan Effect Fluegas mal distribution is not limited to top fired furnaces, it can also occur on Foster Wheeler furnaces. A European Methanol Plant suffered from the ‘Camel Hump’ effect due to the position of the fluegas fans. In this furnace the two fluegas fans where mounted on top of the convection section and these preferential drew fluegas towards them. This is shown below,

The position of the fans should be noted.



This gave high fluegas flowrates directly below then and low fluegas flowrates in between the fans. This was seen as high tube wall temperatures directly below the fans and lower tubes wall temperature between the fans as illustrated below,

3.1.4.2 Flow Maldistribution between Cells It is possible on Foster Wheeler and Side Fired furnaces to achieve differential process gas flows to each reformer. This will give differential exit temperatures exit the furnace. A similar effect can be seen on the fuel system and the table below illustrates this,

Name Units Poorly Balanced Well Balanced Cell 1 Cell 2 Mixed Gas

Fuel Flow % 55 45 100 100 Exit Temperature °C 869 810 840 842 Exit Slip mol % 1.5 3.7 2.6 2.3 ATE °C 11 11 16 12



3.1.5 Tests for Mal Distribution Flue gas mal distribution can be checked for in two ways, the first is to inject dry powder from a fire extinguisher through either the peepholes or the burner ignition ports. The second method is to injection potassium bicarbonate through the burner ignition ports. The peephole tests allow for checks on up flow at the walls and in the centre of the furnace whilst the burner ignition port checks allow for tests on cross blow and flame shape. A combination of both of these tests allows for practical determination of the fluegas flow patterns within the radiant section. This combined with tube wall temperature measurements provides a powerful trouble shooting tool in analysing problems in the primary reformer. An example of the use of these tools is given in case study X4.

3.2 Coffins

3.2.1 Design of Coffin Roof There are two options for the design of the coffin roof; the first is with the roof being supported directly from the side walls as illustrated below,



This design of roof has lead to failures as the top of the side walls are only supported in the vertical direction and it is easy to move then to the left or the right. There is little margin before this movement causes the coffin roof to fall. An improved design recommended by GBHE is to have a stepped section at the top of the side wall – not only does this improve the strength of the side wall but increases the margin if the roof or side wall moves.

3.2.2 Effect of Damage to Coffins If the coffins are damaged in any way then this will cause a mal-distribution in the localized area around the damage. An example of this was the collapse of the coffins at a South American (a 576 tube reformer). The following picture illustrates this, The following figure illustrates the damaged area of the coffins and the effect that this damage has had on the flow distribution in the reformer,



This in turn causes a distribution of temperatures along the length of the coffins as shown below,



3.2.2.1 Movement of Tunnel Walls Another problem is the movement of the tunnel walls. If the movement of the tunnel walls is sufficiently great, then the tunnels can collapse. The following figure illustrates tunnels that have started to move,

3.2.3 Coffin Damage on Kellogg Furnaces At a Middle Eastern Plant (a Kellogg Ammonia plant), one of the side coffins had collapsed and the debris was pushing against the half headers. This caused the half head to be moved to one side and cause a bend to be formed in the tubes. This bend would raise the stresses on the tube and lead to early failure of the tube.

3.2.4 Removal of Coffins A number of plants have removed the coffins from the radiant section of the box because of either, • High pressure drop through the poses a limit to the plant rate. • Repair issues associated with damage to the coffins.



The following figure shows the fluegas flow patterns with coffins installed – as is clear the fluegas in passing down the furnace in plug flow,

The following figures shows that fluegas flow patterns with the coffins removed. As is clear there is a significant mal distribution of gas and much of that gas is flowing preferentially towards the fluegas extraction end.



The overall effect of this is to increase the tube temperatures at the fluegas extraction end of the furnace,

As can be seen the tube wall temperatures are much higher at the fluegas extraction end of the furnace, and these tubes may fail prematurely due to excessive creep.

3.2.5 Modification to Port Layout In an effort to reduce the pressure drop on the fluegas side of a furnace, some operators have increased the free area of the side wall of the coffins by removing some of the bricks. If the wrong number of bricks are removed r they are removed from the wrong position, then this can lead to flue gas mal-distribution similar to that discussed in section 0, albeit on a more localized level.

3.3 Effect of Wind on Box Stability Wind can dramatically affect the performance of a furnace and lead to large temperature drops on the side of the furnace facing the wind. On a European Ammonia Plant, plates were installed on the side of the furnace to reduce the effect of the wind on the pigtails. At Far Eastern Plant, the effect of the wind could drop the outlet temperatures by as much as 20°C. This will reduce the inlet waste heat boiler temperature, thereby reducing the amount of HP steam raised.



3.4 Purging of the Box It is very important that the reformer box and auxiliary duct are purged thoroughly to remove any hydrocarbons prior to restarting the plant. This will prevent ignition of any residual hydrocarbons in an uncontrolled manner. This is normally achieved by starting up the combustion air fans to flush the system with air.

4 Common Problems Affecting Burners

4.1 Operation and Maintenance of Burners The importance of good burner operation in terms of good operation of the primary reformer and auxiliary duct firing must not be understated. Poor performance of burners can lead to poor process efficiency and premature tube failure. Reference 6 gives more details on the operation and maintenance of burners. The important areas that require checking are, • Tile to gun position.

• Tip port dimensions.

• Surface finish of holes.

• Centricity of gun.

• Condition of tile and security of fixings.

• Condition of metallurgy (oxidation). It is important to keep a good record of the vendor drawings of the burners to ensure that if they are ever removed for maintenance, that they can be re-installed with the appropriate dimensions for all components. The picture below shows some burner tips that have suffered from carbon laydown due to insufficient air,



4.1.1 Burner Misalignment Burner or flame misalignment can lead to tube damage particularly in top fired furnaces. Below are some examples of the causes of burner misalignment.

4.1.1.1 Cleaning of the Burner Tips It is important that the burner tips are kept clean since any depositions on the burners will lead to misalignment of the flames.



4.1.1.2 Damage to the Burner Quarls The quarl is the part of the burner that sits in the furnace wall and is exposed to the hot gases in the radiant section of the reformer. These should be checked visually on a regular basis for damage such as cracking and gaps. Below is a picture illustrating the cracks that can occur in a burner quarl,

Any gaps can lead to a mal-distribution of the fuel and air entering the furnace which will in turn lead to a misaligned flame.



4.1.1.3 Top Fired Reformers If a burner is misaligned on a top fired furnace, then the flame will not run parallel to the tubes and will play on the tube surface. This will lead to very high outside tube wall temperatures and eventual tube failure. The figure and picture below illustrates this,

A good example (see Ref. 4) was on a small methanol plant which has only 72 tubes. Catastrophic tube failure occurred in the late 1980’s. After investigation, the sequence of events leading to the failure was found (in summary) to be, • Serious burner problems on a significant number of burners; the burner quarls

were black, showing that the flame was not stabilized. Subsequently, many burners were found to have erosion at the tip, leading in the worst case to a hole. This gave rise to local overheating, which led to a small tube leak followed by catastrophic failure of a single tube. Burner problems of this type have been noted on several plants, and can be easily rectified by the choice of a suitable material for the burner tip.

• The single tube failure led to a plant trip. An attempt was made to restart immediately after this trip. The large leak on the failed tube resulted in reduced flow to the other tubes. This, coupled with control of the reformer using unreliable temperature measurements, gave rise to severe overheating of the furnace in general. Addition of the natural gas during the start-up led to quenching of the reformer tubes, causing many other failures, predominantly at upper tube welds. The plant shut down again, and had to be completely retubed.



4.1.2 Lighting Burners On start up, it is important that the burners are lighted in a sequence so that each tube receives an even heat flux and no tubes are over heated. A typical burner lighting pattern for a top fired reformer is highlighted below,

The principle shown above is can also be applied to Side Fired and Foster Wheeler furnaces. Failure to follow the required procedure can lead to overheating of the tubes and potentially causing early tube failure (see section 0). It should be noted that the majority of reformer burn downs do occur during plant start up and shut downs. Some operators do not light off individual burners using an ignitor (typically a pizo-electric device). Instead they light off a few burners as usual and then rely on the fact the reformer is above the auto ignition temperature before introducing fuel to the unlit burners. The main issues with this is that each time a burner valve is opened, fuel passes into the furnace and combusts – sometimes this occurs close to the tube causing flame impingement (see section 0). The second issue is that the sudden combustion of the fuel gas will lead to localized pressure increases which can stress the furnace casing.



4.1.2.1 Side Fired Furnaces Misalignment on a side fired furnace is not a problem from the tube perspective but can lead to refractory damage. The following figure illustrates the design of a typical side fired burner and how they are positioned in the furnace,

It should be noted that on side fired furnaces there are large number of burners, which increases the maintenance costs for the furnace and increases the probability of a problem occurring.

4.1.2.2 Foster Wheeler Furnaces In a Foster Wheeler furnace, the burner is angled such that the flames run parallel to the refractory. If the flames are not parallel, then the flame can impinge directly on the refractory and cause damage as illustrated below,



4.1.3 Non Optimal Firing in Foster Wheeler Furnaces It is possible to have the wrong split of firing between the two levels of burners in a Foster Wheeler furnace. By increasing the firing at the top level it is possible to increase the fluegas temperature and hence the amount of steam raised in the duct or by increasing the firing at the lower level it is possible to reduce the methane slip. The following table illustrates this effect, Firing Split % 45/55 50/50 45/55 Outlet Temperature °C 769 769 770 Exit Composition CH4 mol % 12.97 12.97 12.97 CO mol % 7.79 7.78 7.8 CO2 mol % 11.04 11.04 11.03 H2 mol % 63.54 63.5 63.54 N2 mol % 4.67 4.67 4.66 Catalyst Pressure Drop bara 1.2 1.2 1.2 Furnace Duty MW 66.1 66.1 66.2 Max. TWT °C 821 817 827 Min. TWT Margin °C 137 140 131 Flue Gas Temperature °C 988 1012 942



4.1.4 Fuel Usage Burners are designed to operate on specific ranges of fuels and any deviation from this range of fuels may cause the burner to operate in such a way as to cause problems. Therefore, prior to any significant change in fuel gas composition, the burner vendor should be contacted to ensure that the burner has been designed to accept that fuel composition. A simple to check to determine if a gas change is acceptable to check the Wobbe number; if the Wobbe number of the new gas is similar to that of a gas that the burner has been designed for, then it is likely that the new gas will also give acceptable combustion. The Wobbe number is defined by, Wobbe = LCV /(ρ)0.5 If burners have insufficient air supplied to them, then the flame shapes and length may deviate from the design requirement which can lead to unstable flames, misaligned flames or after-burning. Insufficient combustion air can be attributed to, • Combustion air mal-distribution – see section 0. • Insufficient capacity on Forced Draft1 (FD) or Induced Draft2

(ID) fans – Sec 6.2.1

4.1.5 After-Burning After-burning is caused by incomplete combustion in the top of the furnace which allows fuel to move down until it mixes with oxygen, at which point the fuel combusts giving the classic observation of flames licking around the tubes in the bottom half of the furnace. To resolve this problem, additional combustion air needs to be supplied to the area affected by the after burning.

1 Combustion Air Fan. 2 Flue Gas Fan.



After-burning can also occur in the flue-gas duct on plants where there are auxiliary burners or flue-gas from fired heaters is fed into the duct. This will be heard clearly as a rumbling which is indicative of multiple detonations. It should be noted that such after-burning causing local pressure rises (typically from one to eight bar) and it is this that causes the distinctive rumbling.

4.1.6 Metal Dusting of Burner Tips Metal dusting of burner tips has been observed on methanol plants if they operate mainly on purge gas from a methanol loop. We have just had a very similar enquiry from a South American Methanol Plant, as they are suffering corrosion on burners in all 3 of their plants. The tip can be just inside the metal dusting region in the 450 - 500°C temperature range even though the purge normally contains only 1% CO and CO2 it does lie in the carbon forming side of the equilibrium. The corrosion you have highlighted on the final photo is classic metal dusting pitting corrosion. Often burner tips are made from alloy 800 or a similar cast alloy. Alloy 800 is the WORST alloy for metal dusting resistance and the material could (should) be changed for one with less risk of metal dusting.

4.2 Flame Instability At a South American Methanol Plant, there were serious problems with flame roll over around the burners leading to damage to the burner tips and the tiles (see ref. 11). In order to minimise the problem, John Zinc modified the tip design to reduce the deviation from the vertical of the flames.

4.3 NOX The legal limits on NOX emissions have become tighter and tighter over the last few years. This pressure will increase and this will lead to more operators using low NOx burners.



There have been a number of problems with low NOx burners, most notably at an Asian Plant. The main issue is around the flame shape and the flame stability. At an Asian Plant it was noted that the one row of low NOX burners installed in the reformer was very unstable leading to flame impingement.

4.4 SOX Due to environmental considerations, SOX emissions are becoming more of a problem. It is normal that the top up natural gas used on the reformer is taken off from the feed gas close to battery limits. However, one plant has modified his take off point to after the zinc oxide bed to minimise the sulphur being passed to the reformer and hence limit the SOx production.

5 Common Problems Affecting the Fluegas Duct

5.1 Too Much Excess Air Many plants operate with very high levels of excess air (more than 10%) in order to overcome operational difficulties such as poor combustion air distribution or high coil temperatures. Excess air does not cause any operational problems per se, but it does represent an inefficiency.

5.1.1 Leaks in Rotary Air Preheaters A customer had noticed that the efficiency of his plant had been gradually reducing over the previous six months. GBHE conducted reformer survey covering both the radiant and convection sections. Detailed flowsheeting of the front end of the plant showed that the plant was generally operating as would be expected; however, there was a heat imbalance across the combustion air pre heater; it should be noted that the pre heater was of a rotary design. Further modelling of the combustion air pre heater indicated that there appeared to be a very large air leakage between the combustion air and the fluegas side. The plant checked the oxygen levels throughout the convection section and the air pre heater and it was found that there was 14% oxygen in the fluegas exit the air pre heater compared to 3 % inlet the air pre heater. This leak caused a plant inefficiency that is worth US$ 500,000 per year.



5.1.2 Areas of Potential Air Leakage Other than the air preheater there are many other places where air can leak into the radiant and convection section of the reformer; these include, but are not limited to, • Peephole doors. • Radiant and convection section construction joints. • Header box joints. • Tubes entry and exit points through the radiant box casing. • Burner attachment to the reformer casing. • Explosion protection plates.

5.2 Too Little Excess Air

5.2.1 Due to Insufficient ID Fan Capacity During a routine reformer survey of a world scale Methanol customer’s primary reformer, it was noted that there was severe after burning was occurring in the centre of the radiant box. After-burning is caused by incomplete combustion in the top of the furnace which allows fuel to move down until it mixes with oxygen, at which point the fuel combusts giving the classic observation of flames licking around the tubes in the bottom half of the furnace. It was noted that no after burning was seen in the outer lanes. It was noticed that the oxygen measurement exit the radiant box was at 1.5%; this was rechecked and confirmed by the plant operator. It was also noted that the fluegas fan was operating at its maximum speed and that the box pressure was close to being positive. Inspection of the combustion air duct showed that after the air was pre heated, the air was split into two ducts which passed along the side of the convection section and then to the midpoint of the radiant section. At this point both ducts turned through 90° and ran vertically alongside the radiant box until they reached the level of the penthouse. At this point both ducts were split and as shown in the figure below, ran along the side of the penthouse.



At the ends of the penthouse all the sub ducts turned through 90° and passed along the ends of the penthouse. From here headers pass the combustion air along the burner rows and to each burner.

On inspection of this system it was clear that there was insufficient pressure differential between the duct and the box to force enough air through to the centre of the furnace. This caused there to be a lack of combustion air in the centre of the furnace, and hence there was incomplete combustion and fuel passed into the bottom half of the furnace. The outer lanes of burners had an excess of air, and therefore there was

high excess oxygen content in the fluegas. Some of this excess oxygen then mixed with the excess fuel in the centre of the furnace and this resulted in the after burning noted. This effect was costing the plan operator approximately US$ 383,000 per year in lost production. A further hidden cost was that the tubes affected by the after burning would be operating with high tube wall temperatures for short periods of time and therefore their overall life would be shortened.

5.3 Fluegas Coiling Fouling There have been many cases of fouling of the fluegas duct coils due to, • Insufficient combustion air resulting in carbon formation in the radiant section

or downstream of tunnel or auxiliary burners. • Refractory dust.



This fouling causes two problems, the first is that it increases the resistance to flow through the duct which causes the ID fan to become power limited, which can eventually limit plant rate. The second problem is that the fouling blocks up the gaps between the fins on the coils (where applicable) and coats the surface of the coils. This increases the heat transfer resistance and therefore reduces the heat picked up in the affected coils and thereby raises the temperature of the downstream coils. This will raise the suction temperature of the induced draft fan, reducing the gas density – alternatively, this can be considered to be an increase in the actual volumetric flowrate. This increase the pressure drop across the pressure drop across the coils and thereby increases the load on the ID fan. This is illustrated below as a movement of the operating point on the fan curve,

Eventually the fan will operate at its maximum speed and any further increase in fouling will require a plant rate reduction.



5.4 Problems with Fans

5.4.1 ID Fan Trips If an ID fan fails then it is possible to keep the plant on line, albeit at a lower rate (typically about 60%). On a top fired furnace, the flue gas is forced through the duct by the flow of freshly combusted gases. On side fired and Foster Wheeler furnaces, the same effect is seen but is coupled with buoyancy of the fluegas trying to rise. The same is true if the ID fan governor fails or mis-operates.

5.4.2 ID Fan Close to Maximum Speed Pressure Boxes If the ID fan is close to its maximum speed then the box pressure can become variable leading to safety issues such as localized zone of positive pressure which can lead to flames issuing from the reformer peepholes.

5.4.3 Governor Instability In many cases the ID fan speed is controlled by the box pressure (see Ref. 11). It should also be noted that it there are pressure variations within the reformer and the box pressure measurement is taken in one of these zones, it is possible for the governor to hunt and in the worst case, to cause a gross instability leading to a plant trip.

5.4.4 Flue Gas Mal-Distribution – Effect on Box Pressure If there is mal-distribution in the reformer box, then it is possible to have variable/transient pressures throughout the box, and in the worst case, zones of positive pressure with the potential for flames to issue from the peepholes.



6 Common Problems Affecting the Header Designs

6.1 Fuel and Fuel Header Designs

6.1.1 Symmetry Fuel headers should be design such that all burners receive the same amount of fuel. This is normally means that the fuel header is design to be as symmetrical as possible. If the design is not symmetrical, then some tubes can receive more heat than others and some tubes less heat. This can lead to wide temperature variations which will be observed as high approaches to equilibrium and high methane slips. The hot tubes will also have their life reduced.

6.1.2 Deposition of Particular Matter in Fuel Headers During a reformer survey at a Far Eastern Plant, it was noted that many burners were out of service. Discussions with the plant personnel found that the burners became dirty and needed regular cleaning. Part of the problem was thought to be that dirt and oil in the feed gas was deposited in the fuel headers.

6.1.3 Fuel Valve Suction During a reformer survey at Far Eastern Plant (a Kellogg furnace), a transient was observed where the temperature measurements on one of the manifolds suddenly rose by 49°C; the temperatures of the other manifold thermocouples dropped by only 2-3°C. The root cause was determined to be that the main fuel valve to one row of burners suffered from suction. This valve would suddenly open, pulling more fuel in from the main fuel header and raising the exit tube temperatures. The other rows of burners then received less fuel and this caused the tube exit temperatures to drop.

6.1.4 Purge CV Changes Care should be taken on plants where the calorific value of the natural gas or purge gases sent to the fuel headers. If the CV of the fuel gas does rise, then the total furnace duty will rise, and this can lead to an increase in the exit header temperature. Unless the exit temperature is used to control the fuel gas rate automatically, then this can be a problem – alarms should be set with a suitable range to pick this up.



6.2 Combustion Air Problems An area that is often over looked is the combustion air pre-heater. However, any problem in this area is inextricably linked to the performance of the radiant section, burners and the operation of the combustion air and fluegas fans.

6.2.1 Poor Combustion Duct Design Combustion air duct design headers should be design such that all burners receive the same amount of combustion air. This is normally means that the ducting is designed to be as symmetrical as possible. If the design is not symmetrical, then some tubes can receive more combustion air and hence will be cooler than; the converse is true. This can lead to wide temperature variations which will be observed as high approaches to equilibrium and high methane slips. The hot tubes will also have their life reduced. After burning may also be an issue.

6.2.2 Combustion Air Maldistribution

6.2.2.1 Due to Mechanical Failure During a reformer survey on a Western European Ammonia plant, an unusual temperature distribution between two halves of the primary reformer was noted as highlighted in the figures below,



On site inspection of the layout of the reformer and feed/fuel and combustion air ducts coupled with detailed discussions with the plant engineers eliminated many of the causes of this temperature mal distribution such as process feed variations, firing differences, inlet/outlet header asymmetry. The only possible cause left was that there were differential amounts of combustion air being feed to the two halves of the reformer; this was possible since each half of the reformer was fed from its own combustion air duct. During a subsequent plant shut down, inspection of the air dampers in each half of the duct found that one was wide open and the other was stuck. Repairs were conducted and after the plant start up, GBHE performed a further reformer survey from which the following temperature plots were generated.

As can be seen the mal distribution has been eliminated and the tube wall temperatures are very even throughout the reformer. This problem has been shown to have cost the plant approximately US$500,000 per year.



6.3 Process Headers

6.3.1 Inlet Process Gas Header Design The design of the inlet header is key to ensuring even gas distribution to the tubes. The design should be as symmetrical as possible so that each tube receives the same flow of process gas which should ensure an even tube wall temperature profile throughout the furnace. Failure to ensure such symmetry can lead to variations in exit temperatures and hence differential methane slips from the tubes and a higher than expected methane slip.

6.3.1.1 Dead Legs and Low Points The inlet headers and associated pipework from the mixing tee to the tube inlet shall be designed such that there are no dead legs where condensate (feed or steam) can collect. If there are low points then drains should be installed such that this condensate can be removed. Operations procedures should clearly state that these drains are opened during start ups.

6.3.1.2 Headers too Hot If the headers are to hot due to over performing pre heat coils or high fluegas temperatures inlet the feed preheat coil, then the inlet headers can fail due to lack of strength.



6.3.2 Exit Header Design The same principles apply to the exit header as apply to the inlet header as noted in section 0 above. The following picture illustrates a design of exit header that is not symmetrical,

As can be seen, process gas will try and flow through the left hand end of the reformer since the flow path through the reformer is the shortest. It should be noted that just like reformer tubes, the exit headers on a reformer operate in the creep regime. As such, it should be expected that the headers will fail due to creep. Another failure mechanism is the high stresses that can be raised in the exit headers due to the high thickness required by the high operating temperatures and differential pressures.



An example of this is from a European Methanol Plant; the following figure illustrates the results of the stress analysis on the exit headers for the original design,

Note the high stresses raised as highlighted by the red coloured zones. The redesigned headers were manufactured using modern production methods (spun cast) and modern alloys (36X and XM) which allowed for the headers to be reduced in thickness. This reduced the stresses in the headers significantly as highlighted below,



Reference 13 gives more detail on this.

6.3.2.1 Exit Header Failure The following figure shows cracking observed on the exit header of a European Methanol Plant reformer.



7 Common Problems Affecting Refractory

7.1 General Refractory Damage Refractory can be damaged in a number of ways, such as, • Rapid cooling/heating or the refractory during transients.

• Poor installation of the refractory.

• Poor design/specification of the refractory.

7.2 Tracking of Gas behind Refractory If the refractory moves away from the casing due to the failure of the refractory anchors, then a gap can form between the casing and the refractory wall. Gas can then track between these two surfaces, and this will raise the temperature of the cold refractory facing and the casing. This can lead to rapid failure of the casing wall.

7.3 Seals around Tube Inlets/Outlets To allow for free movement of the tubes when the reformer is heated up and the tubes expand, a small gap between the tubes and the reformer casing is required to allow the tubes to expand. This gap is normally filled with refractory rope to prevent ingress of air into the reformer, thereby, increasing the excess air in the reformer. The following figure illustrates the design of such a seal,



7.4 Peephole Refractory A common problem associated with peepholes is that the refractory is not fully sealed around the peephole. This causes the casing and in some cases the peephole door temperature to rise, which causes the paint on the refractory casing to become hot.

7.5 Cooling of Hot Reformer Casing When the casing of the reformer become hot, it is usual to cool the casing using either a water curtain or a steam lance. Care should be taken around and below areas where steam lances are being used since the condensate can be quite hot. It should be noted that the reformer casing is normally painted with a heat sensitive paint which highlights the areas that are becoming too hot. If the casing does fail, then it is possible to continue to operate. Additional air will be draw into the furnace which will cause a reduction in plant efficiency. Clearly care should be taken to ensure that the damage to the casing does not progress – use of a steam lance is required and personnel safety is important – the area should be cordoned off.



Care should be taken to ensure that excessive water curtains are not used on the reformer – in one case, moss was found to be growing where the water curtain was applied.

7.6 Damage to Refractory Anchors On Far Eastern Ammonia Plant during operation it was noted that refractory was falling off from the walls of the reformer (Foster Wheeler). During a plant shut down, inspection showed that there was significant damage to the refractory anchors as illustrated below,

It was found that the wrong material was used for the refractory anchors and that these failed due to excessive over heating followed by oxidation. The root cause of this is that if the cover of cup lock is not firmly secured or not thick enough to drop the temperature, the pin underneath oxidises and fails, and this is what has been seen in the pictures. Furthermore, GBHE would not recommend the use of the blanket system of this type, in a FW furnace, as the turbulence is too great and the cups are very easily damaged, along with the lining. It is more normal to have a brick lined system or vacuum formed system for the lower halve, and then the upper sections are installed with a blanket system which is less susceptible to erosion.



The failure is mainly attributed to the fixing becoming over heated and oxidizing away. A rough guide to the oxidation resistance some materials. • Carbon Steel 450°C • Chrome Moly 580-640°C • 18 Cr 8 Ni (SS 304) 870°C • 17 Cr 10 Ni (321) 950°C • 25 Cr 20 Ni (310) 1040°C • 35 Cr 25 Ni (800) 1040-1100°C

8 Common Miscellaneous Problems

8.1 Nickel Carbonyl Formation Much of the above details potential issues with the mechanical equipment and its operation, however, there is a less common problem that does not fit in either category, that of nickel carbonyl formation. Nickel carbonyl is high toxic and can kill at levels greater than 0.05 ppm and cause hospitalization at levels greater than 2 ppm. Its formation is favored by low temperature and moderate pressure with a reaction occurring between a CO containing gas and metal containing nickel. It should be noted that much of the front end of a synthesis gas generation plant is constructed from alloys containing nickel. At ambient temperatures, nickel carbonyl is a liquid that is indistinguishable from water.

8.2 On Line Analyzers One issue that reoccurs with regularity is that on line analysers are not as accurate as laboratory analysers and have a tendency to drift. This can lead to false compositions being used in data set analysis and can affect the accuracy of the fit and in some cases, the actual results of the fit. This is particularly important for primary reformers, where a small change in the methane slip can have a dramatic effect on the exit temperature and apparent catalyst activity. Where possible it is recommended that we always receive the analysis of samples from the laboratory.



8.3 Temperature Measurements

8.3.1 Exit Header Temperature Measurement On a primary reformer, it is typical to have thermocouples installed at various points between the exit of the tubes and the next equipment items (either a secondary reformer or waste heat boiler). There will be heat losses from the various components (pigtails, sub headers, headers and transfer mains), which will cause the measured temperature to be lower than the real tube exit temperature. Some typical guidelines are for the temperature drop based on various thermocouple locations is as follows, • Exit tubes : 1-3°C. • Sub headers : 3-10°C. • Main headers : 5-15°C. • Inlet secondary,

• Top Fired Reformers : 10-20°C . • Foster Wheeler Reformers : 15-35°C.

• Inlet WHB, • Top Fired Reformers : 10-20°C. • Foster Wheeler Reformers : 15-35°C.

These numbers are indicative only and depending on the state of the insulation, the numbers can be significantly higher. When asking a customer to specify an outlet temperature, the position of the thermocouple should always be checked. Programs such as VULCAN CERES / VULCAN TP3 and VULCAN REFSIM can be used to determine the extent of this temperature loss. Use of the measured exit temperature when attempting to simulate the reformer can lead to a high methane slip.



8.3.1.1 M W Kellogg Furnaces The one type of reformer where the exit tube temperatures can be believed is the M W Kellogg furnace. Here the thermocouples are positioned at the point where the half headers meet and the riser is located as illustrated below,

Since this is all contained within the furnace, there are no heat losses and therefore, the measured temperatures can be believed. It should also be noted that it is typical that the secondary inlet temperature can be generally believed. This is because as the process gas passes up the riser it picks up heat from the fluegas, normally the temperature rises by about 15°C. This equates to the usual temperature loss seen down the transfer main to the secondary reformer.



8.3.1.2 European Plant Experience A classic example of this problem occurs at a European Plant; after replacing the catalyst, the operator was complaining of high observed approaches to equilibrium and this inferred that the catalyst had a low activity. A reformer survey was conducted and the simulated equilibrium temperature was found to be 853°C; a good match was found between the simulated and corrected tube wall temperatures indicating no loss of catalyst activity. The operator was adding 34°C to the measured exit temperature based on historical heat losses from transfer mains. In discussions with the operator, it was found that during the recent turn around, the maintenance department had replaced the insulation on the bottom of the tubes and on the pigtails. This meant that the heat losses had been reduced from their historical values such that the assumption that 34°C could be added to the measured reformer outlet temperature was not valid. The operators subsequently increased the measured outlet temperatures and then methane slip dropped to the expected values. Reference 8 has more details on this in the form of a full case study.

8.3.2 Variations in Exit Temperatures If a reformer suffers from a variation in exit temperature, then this will cause the overall approach to equilibrium and methane slip to rise. This is due to the nature of the methane equilibrium curve which has a concave shape as illustrated below,



If we take an extreme case as an illustration, with 50% of the reformer tubes having a high exit temperature and 50% of the tubes having a low exit temperature, this operating points can be plotted on the equilibrium graph as shown below, As can be seen the approach to equilibrium is very much higher than would be expected. Typical causes are, • Poor catalyst loading – see section 0. • Carbon formation – see section 0. • Fluegas mal-distribution – see section 0. • Poor maintenance of the reformer.

8.3.3 Fluegas Temperature Measurements

One area where it is typical to see large errors is in the measurement of fluegas temperatures. There are two prime causes of this, firstly the thermocouples are operating at very high temperatures and either complete failure or drifting is common.



The second cause depends on the actual position of the thermocouple in the thermosheath; consider the following figure,

If the thermocouple is position such that it is touching the side of the thermosheath facing the coil (in this case steam raising but in reality this is irrelevant), then the thermosheath will receive less radiation and will typically read lower than one where the thermocouple is touching the opposite side of the thermosheath.

8.4 Metal dusting of Waste Heat Boilers Metal dusting of waste heat boilers is a common problem and can lead to failure of the boiler tubes. It is normal to insert ferrules (a metal insert) into the tubes at the hot end as a sacrificial protection system.

8.5 Flowmeter Errors Flowmeter error is a common problem, and can cause a significant error to occur in the measured steam to carbon ratio. The steam to carbon ratio is determined from the steam rate divided by the feedstock flowrate, therefore, • If the steam flowmeter is reading higher than reality, the actual SC ratio will

be lower than as measured, • If the steam flowmeter is reading lower than reality, the actual SC ratio will be

higher than as measured,



• If the feedstock flowmeter is reading higher than reality, the actual SC ratio will be higher than as measured,

• If the feedstock flowmeter is reading lower than reality, the actual SC ratio will be lower than as measured.

When customers then compare the methane slip from one charge to another, they will see that the exit temperature is the same but the methane slip is higher. Therefore, it is recommended that VULCAN CERES is used to rationalize out plant data and checks are made on the measured and predicted steam to carbon ratio to check whether this is a problem.

8.6 Sample Shifting When sampling the reformed gas, particularly when the sample is taken at high temperature, the sample must be cooled very quickly to prevent significant shifting of the sample. Typically, this is highlighted by the measured CO being lower than that predicted by the GBHE fitting programs whilst the measured CO2 is higher than predicted value. The following table details a typical difference between the measured and predicted values,

Reformed Gas Composition Changes on Sampling Component Units Measured

Dry Measured

Wet Predicted

Dry Predicted

Wet CH4 mol % 8.8 5.1 9.1 5.2 CO mol % 8.6 5.0 9.6 5.5 CO2 mol % 11.6 6.8 10.8 6.2 H2O mol % 0.0 41.7 0.0 42.7 H2 mol % 70.4 41.0 70.0 40.1

Although these differences appear small, they have a large effect on the approach to equilibrium as shown in the following table,



Calculated Equilibrium Values

Name Units Measured Predicted Methane Steam Kp bara2 133.5 131.6 WGS Kp n/a 1.34 1.05 Methane Steam Eqm Temp. °C 790 789 WGS Eqm Temp. °C 733 794 Actual Exit Temp. °C 784 795 WGS Approach °C -61 +1 Methane Steam Approach °C -6 +6

This problem can cause difficulties in estimating exit temperatures and also will lead to rises in methane slip. More details on this are given in reference 9.

8.7 Zinc Alloys When cleaning the reformer tubes, alloys containing zinc should not be used since this will lead to a reduction in the strength of the reformer tubes and hence to premature failure of the tubes.

8.8 Power Failures Another common cause of problems on steam reforming plants is that of power failures. These normally cause the plant to trip which can then lead to, • Breakage of the catalyst due to rapid temperature changes,

• Condensation of feed/steam and then subsequent movement of the

condensate into the reformer.



9 Troubleshooting

9.1 Process Troubleshooting Guide The following table should be used as a guide to trouble shooting some common problems that occur on primary reformers. Problem Cause Actions Hot bands Poisoning of the catalyst.

Operation at low steam to carbon. Aged catalyst. Localized overheating. Incorrect catalyst loading.

Reduce localised firing. Steam catalyst. Steam catalyst. Replace catalyst. Reduce localized firing. Check burners. Check that potash doped catalyst loaded at inlet to tube. Replace as required.

Excessive Pressure Drop

General catalyst breakage. Carbon formation due to low to steam carbon operation or poisoning. Catalyst breakage at inlet to tubes. Catalyst breakage at outlet of tubes. Water leaching support material and causing formation of cement.

Reduce plant rate. Replace catalyst (all or a portion). Reduce localized firing. Steam catalyst. Check design of inlet to tube for jet impingement on catalyst. Check if there is water droplet carry over. Check is water can be carried over on a trip or start up. Check if condensate can form in dead legs or low points during a shut down. Check if there has been water ingress into the bottom of the tube. Clean the inside of the tubes with brushes or frapper to prevent a in reduction inside heat transfer coefficient.

Hot Tube Low catalyst voidage due Replace catalyst in affected



to breakage or over vibration on loading.

tubes.

Hot Patch Flame impingement. Localized over firing.

Reduce localized firing. Check burners. Reduce localized firing. Check burners.

Shimmering on Tube

Flame impingement. Reduce localized firing. Check burners.

Hot Spots3 Localized low voidage. Reload affected tube. Hot Patch at Top of Tube

Settling of catalyst. Poor reduction of catalyst at top of tube.

Reload or top up affected tubes. Add hydrogen to reformer feed to reduce this catalyst.

Tube Failure Flame Impingement. Localized overheating. Excessive creep. Catastrophic cracking. General overheating – burn down. Localized overheating at bottom of tube.

Repair burner and either nip or change tube. Nip or change tube. Check burners. Nip of change tube. Check other tubes for excessive creep. Nip or change tube. Check for water ingress. Replace tubes and review operating procedures. Nip of change tube and check for tunnel port effect. Install high heat transfer/activity catalyst.

Catalyst Changed Colour

Catalyst is blue/green or blue-green.

Catalyst has been overheated.

Collapsed Tunnel

Poor design. Repair next turn around. Check bottom of tubes for excessive temperatures.

Cracking of Tube Tops

Stress corrosion cracking. Lag tube tops to prevent condensation and vaporization.

Cracking of Tube Bottoms

Stress corrosion cracking. Check design of tube outlet. Redesign to eliminate cold zones.

Bowed Tubes Excessive firing from one side. Poor tensioning.

Reduce firing. Review and redesign tension system.

3 This is a small patch on the tube that appears hotter than the rest of the tube. As such it differs from a hot band.



Pigtail Failure Failure by creep. Failure by cracking.

Replace pigtail. Replace pigtail. Check for excessive stress on pigtail.

Fluegas Maldistribution

Flame impingement, high ATE’s.

Use dry powder or K2CO3 to check for flow patterns. Check design of outer lanes, burners and coffins. Modify as appropriate.

Camel Hump Effect4

High peak temperatures below fluegas fans.

Introduce more pressure drop in duct.

Afterburning Localised burning on tubes near bottom of box.

Insufficient combustion air supplied to some burners. Increase combustion air to these burners.

High O2 Levels at Stack

Too much excess air in box. Poor tube to casing seal. High air leakage into convection/radiant sections. Large air leakage in combustion air preheater.

Reduce combustion air rate to reformer. Repair existing or install new sealing system. Check for air leaks and repair. Check heat balance and O2 inlet/outlet air preheater and repair.

High Duct Temperatures

Fouling of duct coils. Clean coils. Check excess air levels. Repair refractory where damaged.

Hot Refractory Casing

Damage to refractory. Failure of anchors.

Use steam lance or water to cool casing. Repair at shut down. Use steam lance or water to cool casing. Repair at shut down.

4 Foster Wheeler Furnaces only.



9.2 Mechanical Troubleshooting Guide The following table is taken from reference 14 and is a guide as to what to inspect on a steam reformer.

Waste Heat Boiler Item Inspection

type Number Reason

Shell external

Visual All Freedom for thermal expansion to no binding or fouling To ensure no lagging on the WHB side extends to

DP or US of transition weld

All Looking for fabrication /creep damage

Refractory Visual Thermograph

All Evidence or refractory damage Looking for hot spots and early breakdown

Tube Plates

Visual inspection

100% Introscope insp. of ferrules looking for evidence or corrosion

Nozzles Visual inspection

100%

Water side Visual inspection

Where accessible

Evidence of build up of BFW solids behind the tube plates via blowdown branches. Evidences of external pitting of tube O/D near the high heat flux zone



Convection section Item Inspection

type Number Reason

Casing Visual All Looking for hotspots Visual All Looking for Structural defects Refractory Visual Where

accessible Looking for breakdown/spalling

Ceramic fibre

Visual Where accessible

Looking for breakdown / Shrinkage

Coils Visual of coils Supports. Visual /thickness survey of return bends Coil finning

All All Where accessible

Looking for breakdown of the supports sagging of coils Looking for internal corrosion Looking for mechanical damage and fouling

Primary Reformer

Item Inspection type

Reason

Casing Visual online Visual offline

All All

Looking for hotspots Looking for Structural defects and air ingress points.

Refractory walls Roof Tunnels

Visual off line Visual off line Visual off line

All All All

Looking for Shrinkage , spalling, look at panels Discoloration. Loose panels Looking for debris on floor, loose tiles and nosing tiles. Looking at tunnel movements, cracking of bricks and tunnel tops.

Burners Visual on line Visual off line

All All measure

Looking for consistent flame shape and equal combustion General condition – cleanliness and correct burner settings

Roof Visual All Looking for debris on floor



Reformer tubes SCC Reformer Tube Top SCC Reformer Tube Bottom Creep Rupture of Reformer Tube Butt Welds Diameter measurements Manual / LOTISTM

Eddy Current Ultrasonic’s Radiography Visual Bulging Bending

Dye Penetrant Dye Penetrant Radiography Measurement Visual Visual

20% 20% suspect welds only All All All %/Suspect areas. All All

Material at top in dead space is Austenitic and susceptible. Crack detect. Tubes fitted with 'Hot Bottoms'. No condensation possible These welds made by manual metal arc welding are weaker than the parent material. The lack of tube bowing reduces the risk Looking of creep growth Looking for out of alignment excessive bending

Exit Headers Item Inspection

type Number Reason

Exit headers (Hot)

Dye Penetrant butt welds. Girth Measurements DP examine the support and guide attachment welds

All Selected points All

Looking for cracking Looking for creep growth Looking for signs of thermally induced creep along with creep growth

Transfer main (cold) External Dissimilar

Visual Visual DP UT

All All All Selected

Review all transfer main movements Review thermographic paint for hot spots Looking for cracks Check thickness of shell based on hot spots and cold areas outside of the



welds Shell Internal

Visual All design temperatures Inspection of all refractory surfaces.

Pigtails Item Inspection

type Number Reason

Outlet Pigtails Cracking of Pigtails at Header or Tube Connections

Measure girths DP examine

All at agreed points All fillet welds

Looking for signs of creep Failures on other plants occur by fatigue (creep ratcheting), or by creep tearing when pigtails sag under their own weight. DP of the pigtail/header welds has shown no cracks

Inlet pigtails Visual all Check condition of lagging. Not in creep range

10 Conclusions As can be seen from the above there are many problems that can occur on a primary reformer and these can affect all the varied components of a reformer.



11 GBHE INTERNAL References Ref. 1. Safe Start Up and Operation of Steam Reformers Ref. 2. Carbon Formation and Removal in Primary Reforming Process Ref. 3. The Tunnel Port Effect : Validation by the Monte Carlo Simulation Ref. 4. Failure Mechanisms, Inspection Techniques and Repair Methods: Ref. 5. Fluegas Maldistribution Ref. 6. Combustion System Design, Operation and Maintenance for Primary

Reformers. Ref. 7. Replacement of Reformer Outlet Headers at a European Methanol

Plant Ref. 8. Modern Techniques for Optimization of Primary Reformer Operation Ref. 9. Reformer Surveys Ref. 10. Presentation Introduction to Reforming Ref. 11. History of Reformer and Waste Heat Boiler Problems at a Far Eastern

Methanol Plant Ref. 12. Safety Aspects of Ageing Plants Ref. 13. Replacement of Reformer Outlet Headers at a European Methanol

Plant Ref. 14. Steam Reformer Design, Maintenance and Inspection Ref. 15. Primary Reformers – Theory and Operation

