Introduction to Refractories for maintenance, operations and support teams.

Introduction to Refractories for maintenance, operations and support teams

Effectively I am saying that this is an introduction to refractories for those less acquainted with some important details of refractories. The objective is to give a very basic outline of Refractories in general, with more specific references made to Petro Chem. Reforming and the related auxiliary equipment. However the outlined principles are generic and applicable to almost all aspects of refractories over a wide scope of industries.

Content:

Chapter 1: What are Refractories?

Chapter 2: Why do we make use of Refractories?

Chapter 3: Types of Materials

Chapter 4: Types of Tests

Chapter 5: How to install refractories

Chapter 6: What they look like after service

Chapter 7: Risks and failures in service.

Chapter 1

What are Refractories?

Typically refractories would be used to insulate or protect assets thanks to their inherent capabilities which are:

• Resistance to chemical attack
• Resistance to mechanical damage &
• Ability to contain heat whilst maintaining process and equipment integrity.

To look at what Refractories are in a little more detail we have to ask and answer these points.

A) How should it be treated?

B) How are they classified?

C) What misconceptions are there about refractories?

1. 
   How should refractories be treated?

It should not be an after though which can be placed on the “to do list”

It’s an asset that requires attention to detail in all facets from planning, installing, curing, controlled firing, start-up and cool down

B)
How are they Classified?

Based on:

• Physical properties.
• Chemical composition.
• Method of manufacture and
• According to their refractoriness.

The wrong choice for a specific purpose can lead to down time, equipment failure and accidents that can place one in harms way.

Refractories are the backbone of general industry because without it, chemical industries, and various commodities & goods such as our cars couldn’t be created.

C)

Misconceptions

There are 5 basic misconceptions about refractories to get out of the way before we really get going.

1. It is not a generic product.

In other words there is no such thing as one type does all the work.

There is allot of science that goes into designing and producing the correct type for each condition which needs to match with process environment and installation method.

2. It will fail over time.

It does not last forever. Refractories are exposed to severe conditions they will wear and with time reach failure point by when they are repaired or replaced.

Factors such as – and including – a good maintenance schedule and detail inspection assessments will be discussed throughout as it adds to prolonging lifespan.

3. Water is not just water.

The wrong water quality or amount can damage the refractory and be detrimental to its performance.

WHY?

• Water affects bulk density.
• Reduces strength.
• Increases shrinkage.
• Promotes segregation and
• Can prevent good hydration.

Note the unique difference between site and safe (potable) drinking tap water.
Hint: Its not always about the Ph:

4. Similar materials from different suppliers are not compatible!

Refractory “similarities” stops on paper. Just as no 2 people are the same no 2 areas where material was mined or how it is processed delivers the same quality or grade of material.

Neither does every supplier add the exact same quantities of trace elements or additives into batches.

5. Weather does affect installation characteristics.

Cold conditions negatively affect bonding abilities, precautions must be taken during mixing, installation and storage!

Conditions that can affect the mix: Is it raining, is it humid is it extremely hot OR cold.

Extremely cold conditions can form free water ice crystals in monolithic materials. This leads to poor hydration due to expansion in the matrix. Resulting in a poor hydraulic bond. Ice is simply expanded H2o and it will leave voids.

“let’s warm the water up with that steam line” The one with years of rust, oil and gunk in it? Rather not!

If water is not in the optimum temperature range as stipulated by material manufactures often >5-28°C it could either under or over induce hydration and the various phases of the Calcium aluminates in a mix may not react as they are required.

What makes all of this so brutally important is a two fold matter:

1. Your material supplier/manufacturer can just say “let’s see the mixing sheet/log. Once the variables does not comply to their installation and handling instruction, well there goes your guarantee. Assuming you truthfully completed the paperwork.
2. More often than not it is millions worth of infrastructure you are working on, and maintaining equipment integrity goes hand In hand with best practice.

Chapter 2

Why do we make use of Refractories?

The correct Refractories:

• Allows protection of capital investment.
• Protection against process conditions.
• Avoids production loss.
• Minimizes power consumption figures for heating equipment.
• All the above said, the value of Refractories is far more than the monetary portion.

A hotspot will indicate gas or temperature bypass and is commonly caused by lining failure. Gas (or whatsoever the process/product contained) will always try to find the easiest route through or out. Thus it may not always follow the intended flow of process if there is an opportunity for bypass. It must be avoided at all cost as it can bring a unit out of operation!

An example of a hotspot during start-up after a refurbished lining, note the refractory “settles” then expands and disappears. This is not a failure.

Chapter 3

Types of REFRACTORY materials.

The wrong choice/type/grade can lead too down time, equipment failure and accidents that can place one in harms way.

• Ceramic Fiber
• Metallic Anchors
• Gunning
• Casting
• Bricks and special shapes
• Mortar
• Oil paper (not a refractory but crucial to aid design)

Each of these noted above will enjoy more elaboration in the balance of this document, however for this section I will touch on some important aspects… and later on there will be more subject specific information.

Ceramic Fiber:

Modules, Paper, Blanket, Bulk, Webbing, rope, and custom vacuum formed products are also available. Commonly used in gas fired furnaces and expansion allowance areas.

Safety note:

Interim results from animal toxicity studies conducted by TIMA, Inc. on laboratory animals were released in October 1990. The studies indicate that when inhaled, the health effects of unfired refractory ceramic fibers (RCF) are similar to those for asbestos, I would like to add that fired fibers carry additional contaminant and a range of related risks.

New tech. however has the advantage of being bio-soluble—it dissolves in bodily fluids within a few weeks and is quickly cleared from the lungs, just check your MSDS to make sure you got the good stuff.

Grades of fiber:

• Alumina silicate wool (ASW) Is typically a 50:50 ALO203 & SIO2 mix is rated 1260°C .
• Polycrystalline wool (PCW) is a >70% ALO2O3 inclusion rate 1400°C but the higher the ALo2o3 content the higher the rating – typically up to 1600°C.

The low Silica levels in PCW ensure that there is no possibility of Cristobalite formation after exposure to high temperature. As well as the mitigation of Silica for leaching and deposits which can cause pitting corrosion on downstream or tube sheet metallics, where it condenses and causes concerning degrees of damage (in HPi reforming industries specifically).

There are other products in the Fibrous range, but these are the ones we predominantly deal with and are worth noting. Also understand that ALo2o3 is more volume stable the higher the grade, given the temperature range is found around its ability to stay within its original dimensions (2% for rigid shapes and 4% for fibers) between cycled heating and cooling in a 24hour period.

Cross and top section views of good installation.

Note how there are overlaps from the front to the back. This is in order to better prevent gaseous bypass to the shell and anchor backs.

“Toilet rolls” a roll of CFB around the Metallic anchor locking mechanisms helps add protection from the process.

Metallic Anchors

For the purposes of this discussion I am not referring to ceramic anchors.
Anchors are required to restrain movement of refractories. They are capped with a plastic cap or coated with bitumen. This is to accommodate for the variance between refractories and metallic expansions, as a metallic anchor EXPANDS more than the refractory under heat, and the capping or coating disintegrates with heat.

Anchors for Brick retention and Monolith linings .

Anchors for fibre BLANKET  linings

Anchors are diverse and is cause for numerous debates when one considers:

• Actual need – often debated on tubes sheets for example – given metallic ferules can be argued to aid anchorage of systems in question. Or even behind bricked linings given self-supporting structures. It does however help during installation preventing slumping and relative material restraint in service. Excessive restraint could however induce mechanical failure in some instances floating anchors are a logical step in certain instances.
• Amount Required: The area in question and type of material restrained for example light or dense applies when calculating qualities. The rule of thumb is 2 to 3 times the distance apart of the lining thickness on a vertical wall. 1:1 or 1:1.5 on an overhead area. 1:3 on a floor. But each owner and specification and application has its unique set of requirements.
• Orientation: Diamond, squared or staggered (see anchor installation in How to install refractories Chapter 5)- if the tips are at least an inch apart and not pointing towards each other it helps to reduce, or avoid all together any micro cracking/crack propagation during thermal cycling.
• Quality or grade is atmosphere/environment dependent: 304 stainless steel anchors do not react well in oxidizing environments cycling near 1000 deg C, even if it is protected by the insulation around it the risk still exists. It also acts bad in carburizing environments and lining failure may lead to critical failure. 304 stainless steel anchors may be applicable to the cold side of a WHB. However it is more common than not that 310 or higher grades stainless are used throughout, even though the application area specific justifies said grade.
• Type of welding whether stud or hand weld. Make sure your applicator/equipment can stud overhead otherwise the installation will likely be scrapped! And don’t cross welds!

Castable and gunning material + equipment.

These refractories are mixed with water or other liquids. Water specs for optimal performance or maintaining guarantees are pH = 6 – 8 and Chlorides < 50 ppm but refer to the section about the misconceptions mentioned earlier.

Besides water other typical mixing chemicals in the trade are:

• Colloidal Silica
• Potassium Silicate
• Sodium Silicate
• Phosphorus pentoxide

It is important to note that not all mixing chemicals are conducive for all environments, or with all aggregates and additives. Also important – is that material must be packaged in weatherproof bags typically 20-25kg – NOT A SHOPPING BAG!

In short – because this is an elaborate subject – Materials are a combination of refractory grains, sieved and included at a specific ratio in terms of granular size. It also has a suitable bonding agent that, after the addition of a proper liquid, is installed into place to form a refractory shape or structure that becomes rigid due to a chemical reaction with water or its mix liquid or in some instances heat.

Rotary paddle mixer.

They must be kept clean, old material will cause havoc in new mixes! The paddles should be at least 5mm from any side (Floor, shaft or pan side.) or else it won’t mix the material and cause build up on the sides. The pan should be round and the sluice gate should open easily. It should be water tight with no leaks at the gate. DO NOT CLEAN IT IF IT IS STILL PLUGGED IN (is energized)!

Guniting.
Certain principles noted above apply to gunning as well– besides that gunning gets special additives, given it mixes at the tip of a nozzle for a mere split second and not in a mixer for 3-7 munities. It must also stick to a surface – immediately while maintaining its structure in the process of being built up progressively.

Dry gunning is old tech but very commonly used – shot creting or wet gunning should become more prominent in times to comes, but for smaller jobs conventional gunning should remain for some time to come.

Conventional rotary gunning machine                                     

You should see Rotary gunning machine plates around as they require regular maintenance and can deteriorate fast with coarse aggregate material

                                 Plates                                           Rotor & pot

Even though technology progressed, application techniques have been consistent. PS: Keep hoses clean and empty hopers after a shift!

Bricks and precast shapes

Hot face / process side requires special attention. … Calcined high alumina refractories have exceptional creep resistance. Corundum or tabular alumna is typically used for the hot face in secondary reformers. The bricks for these hot face applications are specifically built to allow for structural stability as well as its refractoriness.

Apparent porosity will for example always be noted as it implies a material resistance to spalling, cracking, impregnation of process gas or in other applications slags

Insulation bricks are porous and can easily be cut with a hand saw blade.
Air is the best insulator! Bubble alumina is used in these applications and Apparent porosity is less critical given it is protected by the dense working lining.

Many vessels require radius bricks i.o.w a brick with a sharp and a slow side.

• Sharp is the narrower or process/internal side
• Slow is the back – or the wider side shell side.

Bricks sizes.

The small size assists in better mechanical stress management per brick unit.

Why are general bricks roughly sized 230mm x 114mm x 76mm ?  One can better manage and work with it.

Special SHAPED REFRACTORIES

It is generally for Areas that require detail fitment where casting is not the best option.

Special shapes (and bricks) can be cast and then dried at high temperatures, but often they are pressed, fused or sintered  all depending on the required final product and serviceability it must offer.

Mortar

The “binding areas” or joints has the potential to let process temperatures and product through therefor it is required that the mortar must be of the same type but a higher grade than the material it is binding. Eg:95% alumina brick and 98% alumina mortar.

Mortar may NOT be used to compensate for poor brick dimensions or workmanship.

Mortar joints is (or should) generally be <2mm

No window framing is allowed – it traps air and allows for cavities within the area required to have a hermetic seal. It creates easy passage for whatever requires containment.

Oil paper

Oil paper will burn out when the unit is in operation, the purpose is to create a slip joint between layers thus preventing different materials binding.

Oil paper is generally secured to the underlying lining with good wall paper glue or staples where insulation material is applicable.

Good practice call for sheets to overlap by an inch.

Chapter 4

Types of lab tests (Physical).

There are primarily 8 Factors to considered when deciding on materials.

1 Bulk density

2 Cold crushing strength

3 Modules of rupture

4 Apparent porosity

5 Thermal shock resistance

6 Plc (Permanent linear change)

7 Thermal conductivity

8 Abrasion resistance

For the purpose of this paper I am not referring to chemical analysis, as this is primarily identified in the Data sheet. For independent testing one could also add Loss on ignition with chemical analysis to help verify that you got what you asked for without chemical manipulation.

Note API 936 for details of test methods noted below.

1. Bulk density (The ratio of weight to volume)

Densities are generally as stated as:

Ceramic fiber: Generally 96 or 128 kg/m³

Monoliths and preformed or brick:

Insulation: <1200kg/m³

Medium weight: 1200-1800 kg/m³

Dense: 1800-3000 kg/m³

With Monolithic’s it helps to use or consider as installed results as oppose to dried to 110°C densities from a lab when calculating quantities. Quantities are typically slightly higher and can affect estimations negatively. So, add a percentage considering the application and method + area of application for waste and as installed densities when calculating quantities.

Most compensate by estimating significant percentages of waste to the lab data sheets value. However rather often the argument between contractor and client occurs as to “where did the material go?” Even though tonnages were monitored.

One has to consider that site application varies from lab scenarios and 5 – 10% cast waste goes “missing” not to mention 10 – ≥30% gunned waste. Gunning’s increased Bulk densities and individual artisans skill, air pressure etc… affects these numbers too.

One could catch the rebound and cutback in a canvas during pre-qualification mock ups and get a good idea of costs related to waste.

Note the difference below between average LAB and as installed avg results.

The CCS values on site is also higher corelating to the higher densities.

2 – Cold crushing strength. CCS

This is where the maximum stress is measured to mechanically destroy a refractory.

Note that the samples your installer take on site is critical in this sense.

To much water will influence the results negatively given material is displaced for liquid and the resulting air has zero resistance in this test. Poorly formed samples (not perfectly cubed) will crush irregularly and give false readings. Samples must be “as installed” and not as an after-thought.

Cross sections of fired Bulk Density test specimens.

Note the interesting discoloration visible from the hydraulic bond to the ceramic bond on the higher temperature sample.

    Sample dried to 110˚C                                Sample dried to 1000˚C

3- Modules of rupture.

The goal is to identify the strength of the bonding system in a refractory. A slab is spanned across two points and pressure applied over the middle to breaking point. It is done at room temp however Hot M.O.R is also done in certain cases.

Not to be confused with R.U.L (refractories under load) tests. Testing at a constant load and increasing temperature to note “softening” range.

4 Apparent porosity

It’s a test allowing one to evaluate a materials physical resistance to process infiltration or migration through the exposed pore structure. True porosity considers the totality of a sample. As a result lower porosity can increase resistance to gas, slag or chemical ingress. In the case of reforming the atmosphere risks are gaseous in nature.

Even though it is an important aspect to understand the determination of the apparent porosity at 105 °C does not apply to phos-bonded or plastic materials.

5  Thermal shock resistance

This test method indicates the ability of a refractory product to withstand the stress generated by sudden changes in temperature. It’s simply when a sample is cycled through heating and cooling and as per spec placed under M.O.R / CCS tests to deduce the loss in a sample’s strength or bonding ability over several cycles.

Heating for a period, then cooling is done intermittently while soaking in water (in certain instances) or exposing to ambient conditions, which it’s repeated for a certain number of times. Testing varies from the spec as the specific test method does not consider the temperature range or morphological changes in various materials. Given the specifics of a materials requirement some tests vary to accommodate the specific intention.

Search “Line-ox 1100° C Quench in cold water tests” to see how well a stable synthetic high Alumina performs, however seemingly not as per ASTM standard but impressive.

6 Plc

(Permanent linear change)

7 Thermal conductivity

Thermal conductivity is dependent on factors such as- but not limited to – chemical and mineralogical composition, temperature, porosity, the extent of sintering, the environment eg: Pressure …

It is the amount of heat moving from the hot/process to the cold face. There are a number of standardized test each for various refractory types. Modern aggregates aim to reduce the Conductive values for increased energy conservation, and it makes for interesting development in this part of the refractory game.

8 Abrasion resistance

In laymans terms when a sample is exposed to a rugged material such as silicone carbide grains at an angle with a consistent pressure and feed to record lining loss. The specific result is given with a test done at room temperature.

Note that there are more tests that could be carried out in making the correct choice – these are the basics. It all depends on your specific application.

Chapter 5

How to install refractories.

In order to cover the basics of installation, some important factors preceding the actual installation must be considered:

• Demolition,
• Shell preparation and
• Type of Anchorage.

Because the final result is only as good as the preparation.

The various areas of a secondary reformer – naturally this applies to each unique equipment as each requires its own approach of every aspect from demo. to prep. Installation.

Note that this is a general description – some terms are proprietary – some sections may not be relevant to each plant. Terms may also differ. The point is to illustrate that there are many areas each with their own requirement.

Demolition requires the contractor to take care and among other ensure that they DO NOT damage the shell/base. They must also use equipment that is sufficiently powerful to dislodge the material.

• The cross section of the lining can be inspected during and after the demolition process.
• Carbon build-up (which is common in reforming), penetration, cracking, discoloration, and the condition in general should be assessed.

Shell preparation

Before cleaning                                              Cleaned     

• Sand or grit blasting and
• Power wire brush cleaning is the conventional means.
• HP water cleaning and ice blasting is also applied in some areas.

Anchorage

The type of anchor used is chosen for its atmospheric condition resistance and the temp. rating is always decided by the hot face temp.

The method of installing a monolithic also decides the anchor design: eg Dual layer…

Consider the anchors condition when demolishing then evaluate if the type installed is correct and if it is required to reinstate a new set. Verify anchor length, the trade “rule of thumb” is 2/3 to 3/4 of the full lining thickness.

Besides spacing, the pattern is also critical, one does not want anchor tips to be pointed in a line this will promote cracking.  Anchor tips should also be at least an inch apart form one and other as micro crack propagation can become problematic at a stage.

• Once installed testing occurs by bending an anchor over then back into position again -check if the weld failed.
• Make sure the weld is adhered to the shell and the anchor!
• Check that it was not overheated while welding. Do a magnetic test where applicable if you are not sure.
• Carry out an acoustic hammer test. You want to hear a Ping sound to be sure it is adhered firmly.
• If the anchors are bolt into place and not welded ensure they are firmly secured prior to installing the monolithic material.

Nozzles and protrusions into the refractory lining must also be wrapped in ceramic fibre paper to allow for expansion and covered in a waterproof manner to prevent absorbsion. Do not overapply waterproofing!

Ceramic Fiber Paper – Wrapping thickness typical requirements … (check your spec):

50 mm Ø < Nozzle < 125 mm Ø: 2 mm

125 mm Ø < Nozzle < 300 mm Ø: 3 mm

300 mm Ø < Nozzle < 600 mm Ø: 6 mm

The gunning or mixing station and or installation area should be fully equipped with:

Thermometers ,Measuring jugs
Stopwatch, Blok brushes, clean buckets
discharge bin, Clean water/supply
A scale, backup electrical /pneumatic equipment
Sample boxes, Quality control documentation
…
trowels, wood floats
straight edges, mason level
spirit levels, brick and claw hammers
shuttering  …
  and what ever specifically for the purpose for the application…\

You simply cannot put someone random at a gunning machine or refractory mixer then give them a measuring jug and a stopwatch and “let them loose” to mix and install the refractory.

One need to employ the senses – literally to see feel and hear – often even smell.

Experienced staff is non negotiable!

Casting

Material is dry mixed to reverse segregation, its then mixed with a strict amount of liquid all depending on the immediate means and area of application the correct amount of water/liquid is critical! Low or no cement material are very sensitive to water addition – they are not as easy to manipulate as high Cement materials.

Traditionally Casted material is installed by means of pouring it into place at the correct consistency, often it may be pumped or packed by hand.

Shuttering and stop ends must be firmly secured, flush, plumb and level with the shell. It must also be sealed to prevent dehydration of material into the wood – assuming wood is used as oppose to steel.

Clean working equipment must be in place (and kept clean to avoid contamination or flash setting).

Backup equipment should also be readily available.

Mixer paddles must be positioned correctly to ensure sufficient mixing.

A paddle an inch from any side, deems the mix wasted.

Material must be available with its data  sheets as well as batch specific  test certificates.

From a safe working perspective the material safety data sheets must also be available.

Water supply confirmed as potable must also be checked for temperature.

Safe water and ambient parameters are between 5˚C and 28 or in some cases 32˚C  for a wide variety of materials.

Verify material specific parameters.

Ball in hand test
ASTM c860

Prior to the actual installation the ideal water content must be achieved and one method of testing it is “the ball in hand test”. These test are done in order to verify the consistency of the material

B.i.h is applied to conventional high cement refractories; it’s not applicable to Low cement (<2.5%) or self-flow materials. However the action of having it in hand helps assess its workability to an extent in any event.

                   To wet                               To dry                        100%

Pouring or non pneumatic placement requires vibration compaction in order to consolidate the mass adequately. Less if any vibration is needed for self flow material.

Over vibration for compaction results in:
– Material segregation.
The coarse and fine material sets apart. The bind and insulating qualities are compromised
– Lamination within the lining
– Air escapes excessively causing for a denser lining than designed.
– Potential flash setting of material around the poker as it heats up in the lining.

After installation it is cut back to the correct profile and thickness.

The finished surface is checked for irregularities/steps from panel to panel. Thicknesses must be within tolerable limits. Eg: over- or undercast/applied. Note the Dogleg joint @ the edge of panels to help prevent gas-bypass.

Gunning

Material is placed in a machine that pneumatically transports it to the area of installation were mili –  seconds before application it is mixed with water at a controlled consistency (which may differ from applicator to applicator).

It differs form Shot-creting (wet gunning) in that shot-creting conveys Hydraulicly, and neither mixing nor water addition occurs at the end of the pipe. It is not dusty either. 

Gunning is a fast but not as consistent a means of application due to pressure and density variances during the process, a few inches closer or further back can manipulate physical properties such as giving higher densities and aggregate segregation (rebound) …

…it comes back to the correct means of application given the equipment as well as skill.

Shadowing must be avoided. It’s a cavity behind an anchor, the method of installation is lacking in such cases.

A serrated steel trowel is used to cut back the installed lining and a wood float is used to “finish” the lining. Metallic tool finishing will draw out fines and segregate the lining, also sealing the front face causing a barrier making it hard for vapor to escape as a result spalling or popping the face of in process. Months or years of a lining can be lost as a result

Poor workmanship can result in cavities within the lining.

Critical and unique to gunning is air and water pressure. Should these be insufficient the material will not adhere to the applied surface. It could simply bounce of (excessive rebound)  or not compact well enough.

Insufficient air pressure will not consolidate the lining up against the shell – remove asap!

Poor cross section with permeable porosity and bypass through to the shell caused by insufficient vibration/ consolidation up against an edge of a former…. or poor gunning technique.

On completion of a lift or panel the open end or top is best doglegged or tongue and grooved (application dependent).

It is to prevent straight joints to the shell, the next mix or batch of material will fill the area . As a result making it less susceptible to gas bypass.

But beware – changes in the form of a joint can cause early breakage … thus many rather prefer a rounded tongue and groove instead of a 45 or 90 degree dogleg because it is less likely to shear of or crack as there are not sharp corners.

Hammer testing is done on cured linings to verify its acoustic qualities:
One can determine a sound lining and detect laminations, “rat holes” / cavities, use the ball pin side of your inspection hammer with a finger on the stem to feel inconsistencies.

Any anomalies on the surface must be ground down or filled up with fines. Especially if bricking will occur on top of it.

Rebound material form gunning or Material that fell on the floor during cast activities may not be re-used!

Samples of as installed work must be made during each shift and for every batch

Samples must be identified and recorded.

Brick and precast shapes

• These have already been fired (in some cases only dried – make sure!) during manufacturing thus there’s no need for bricks or shape to be dried out if its fired.
• Damaged brick may not be used, i.o.w cracked or broken edge bricks follow, the table below is a fair but very lenient guideline and ASTM C134 for specified adherences.
• Keep an eye out for Iron spotting or anomalies in the quality of the bricks such as warpage.

Warped or cracked bricks and shapes are not to be used! If one brick fails it is a total failure.

In advance of brick work that may start – get prepared for mortar preparation. Phosphorus bound mortar require up to 12 hrs aging and should be applied within 24 hrs. of preparing. At this stage the wall paper glue can be prepared, some contractors use staple guns.

Oil paper must be placed between linings.

Changes in brick color on the face is not uncommon however the contractor must inform a qc to verify that any such variances are not potentially detrimental.

Tolerances are extremely strict and must be check continuously.

BOND

The word bond, when used in reference to refractories brick work, may have three meanings

Structural Bond:

• The method by which individual brick units are interlocked or tied together to cause the entire assembly to act as a single structural unit. ie: Tongue and groove.

Pattern Bond:

• The pattern formed by the brick units and the mortar joints on the face of a wall. The pattern may result from the type of structural bond used. Ie: Offset/half brick joints

Mortar Bond:

• The adhesion of mortar to and between the brick units.

A hermetic seal is desired, but will only really be achieved at operating temperatures once expansion gaps and allowances are closed up, this is to prevent gas and or heat transfer to the shell or to lower laying insulating layers of refractory.

Expansion joints.

• Expansion allows (as the word says) for expansion , it naturally occurs when the refractory heats up under process conditions. This is built into the lining.

Example of 3mm expansion:
It is maintained with polystyrene in this case, often cardboard is also used, as long as it something that can incinerate and not impact the area as an obstruction.

Ceramic fiber (or paper) is commonly used in built in expansion joints. It may be compacted (as example) from 25mm to 15mm.

Expansion built into linings  must be strictly monitored for adherence to tolerances. Expansion is strategically placed so as to be ADJACENT TO areas where dimensional change is prevalent.

Cutting of bricks must be precise! Bricks cut to small will force mortar joints above the 2mm tolerance. Such bricks will be discarded  or allocated aside to be used as closure or key bricks

Safety is critical when using brick cutting equipment!

it must be well maintained to avoid (as example) the water pump during wet cutting from clogging up due to dust/mud thus overheating.

Cutting requires qualified staff as some cut are intricate !

Key bricks will not be smaller than half the original brick size! In part to avoid compressive failure. This may vary from spec to spec as some require 75% to be the minimum cut size of a brick.

Refractory hammers must be used as a standard rubber mallet will bounce of bricks and not place them securely.

Mushroom/ diffuser.

The diffuser must be level. The base must be sound and level within 2mm or less.

The height and level of each ring must be verified and recorded during the build.

Target tiles.

• Impact tiles (solid tiles) are placed below the burner.
• Building this requires starting in the middle and working towards the edges.
• The same principal as with brickwork applies where no size smaller than half the original may be installed.
• Typical target tiles are manufactured of Corundum Alumina, Chrome Alumina Spinel or Zirconia stabilized with either Calcium or Magnesium.

Tube sheet.

Prior to any refractory work the plant must evaluate the quality of the tube sheet surface. The tube sheet could be cast with a high alumina self flow castable or in some cases built using Hex ferules.

Dual layer tube sheets has a bubble alumina backing and the dense hotface self flow.

Given the impact a failed tube sheet may have on loss of production and down time it is critical that the highest standards of quality, materials and workmanship be incorporated in this portion of the unit.

Often its less than ½ a ton material installed on the tube sheet – but the most critical ½ ton !

The ferrule  extending between the inner surface of the refractory material and the entrance of the tube is important as it: (The principle applies to modular hex ferules as well besides not having castable between tubes for points 3 and 5 below.)

1. Protects the weld seam from process.
2. It effects a smooth flow of hot gases
3. Prevents possible damage of the castable   cross section caused by erosion.
4. Prevents gas bypass to the tube face via its passage from front to back.
5. And also (however debatably) serves an anchor.

A failed tube/weld(s) may be plugged, but given consistent failure and plugging down time cost are incredibly high for Waste Heat Boiler replacement.

A cast tube sheet requires its form work managed by qualified staff, poor formwork, bracing and large lifts/panels will result in a poor final result.

Severely overcast tube sheet will set schedules back by days.

Fiber

It’s used as the primary lining in a gas fired furnace, and also applied in expansions within the ATR/GHHER

Once anchors have been paced and tested for durability, blanket is pushed over and secured in place with lock washers.

Layers must overlap in order to prevent any bypass.
The washers and anchor tips are exposed to process conditions which is why they are often wrapped with what is referred to as toilet rolls.

Questions if installations succumb to green state failure.

Why didn’t the material set off?

The material crumbled it had no mechanical strength?

Why did it crack?

• It can be that to much water was added.
• Vibration was either to much or to little
• An incorrect mixer may even have been used
• The formwork was either removed to soon or it had not been sealed… the list goes on …
• Point being it is more than just following instructions which was often acted out in near perfect condition and not site scenarios… it is about knowledge as well.

Cracks in a furnaces castable and bricks in general.

Ceramic fibre is used to fill cleaned cracks lightly but if a crack is 3mm and narrower and firmly filled /caulked/ pointed with fibre it will cause aggravated cracking in service as expansion still requires release. Cracking is a way of material finding its own expansion lines.

Dry out.

• This is considered the most important part of refractories by many specialists.
• In the same breath it can be said that it is also the part where many people push for schedule!
• It is critical to carry out a time & temperature dependant dry out!
• Curing is the process of the chemical & hydraulic  bond of materials attained.

The method for ideal curing may vary from types of materials:

Heat Setting (P₂O₅):

1. Cure between 10^oC and 32^oC without covering.

Chemically Bonded Material (Colloidal SiO₂ or Na₂SiO₃ / K₂SiO₃):

1. Leave uncovered and free from moisture during curing period.

Hydraulic Bonded Material (CaO):

1. Apply curing compound (membrane-type) to all exposed surfaces before the surface is dry to touch.  Utilize curing pigmentation that allows for complete visual inspection of coverage; or
   1. Wet surface with water spray at 2 hour intervals; or
   1. Cover exposed surfaces with polyethylene/damp cloth/wet bags within 2 hours of installation; or
   1. Shuttering to remain in place for 24 hours.

Calcium Silicate/Portland Cement Based:

1. Cover with polyethylene/damp cloth/wet bags within 24 hours of installation.

Drying of refractories occur for a few reasons:

1. To allow “free water” to escape, it has to escape the lining as vapour, steam under pressure will crack or explode the lining.

Depending of altitude it could occur from 94deg C, however the hot face is kept at 110 -120 to allow heat to seep through the lining over a period. Thickness dependent.

2. To allow Chemically bonded water escape which occurs at approximately 330 deg C.

This is the water that binds with the materials  on  molecular basis.

Each phase could account for 4-8% each of volume loss. Imagine 10% water with 10 tons installed it’s a ton of water that in part hydrates either upon curing or setting and heating up, and then it has to vaporize controlled in each of the above stages.

Thermocouples are place strategically inside the unit in order to monitor the temperature on the hot face and atmosphere.

Often insulated thermocouples are also placed on the outside if water jackets are not a factor.

A typical heat up schedule is monitored on the hot face.

Thermal gradient as anticipated @ 200˚C with a 50mm insulated outer shell for extreme cold conditions.

On completion of dry out it is advised to verify that the lining re-adjusted back to its installed dimensions.

Movement during dry out is inevitable as the hot face will see high temperatures (500-600˚C) in order to seep to the insulation CASTABLE at the back.

Chapter 6: What they look like after service

Saturated and heavy fiber can fail and fall through the anchorage.

It is critical that the correct thickness and density is installed it prevents condensation on the shell or between layers. (process dependent)

If required USE A VAPOUR BARIER SUCH AS Asphaltic coating on the wall or Stainless steel foil between ceramic fiber material layers.

However it may be said that reforming processes does not have any or for that matter concerning PPM levels of sulphides.

Ceramic fibre has excellent thermal shock resistance, but it does not give good mechanical wear resistance.

It may be rigidized with Colloidal Silica, (for the start up heater but not in the Reformer) An Alumina Coating,  or a Zirconia Based Solution among other can be applied.

A start-up heater for reforming’s burner tiles:
Burner quarls / tiles tend to separate on their joints. These must be re-aligned to the correct radius and sealed during maintenance preventing flame passage / impingement.

Flame impinged, Cracked or both to this extent deems replacement. A segment may fall into the burner in operation.

Expansion in a ring can all move to one area. Not always perceived a concern, but worth proper assessment.

Carbon deposits or formation on anchor tips to this degree is generally not concerning in the intermediate layers. It should however be recorded and tracked.

Cyclic – thermal mechanical spalling can generally not be repaired. Patch repairs generally do not adhere and pop off. The need and validity of such “repairs” can be questioned at times. The need to temporarily seal an area may apply. If securing expansion allowance paper/fibre in place through a vacuum phase it may help prevent “blowout/sucking out” until the lining secures itself via expansion.

Overhead plastering in a waste heat boiler can cause for patched plaster in place to fall of and potentially enter into or block tubes.

Chapter 7

Brief outline of causes of refractory failure in service.

Refractory materials is only one component in the overall design of process equipment. It is sensitive (and influenced) by time, temperature and operational factors as well.

When refractory fails the assumptions are almost immediately that it was either:

– bad design
– bad installation
– bad materials
or
– The dry out.

Improper installation can very well be they cause because we will always sit with the human element, so most of the time poor installation is relative easy to identify.

It then becomes a more detailed engineering factor which may include various factors from design to dry out to in- service practices…

Drawings, material sample records and dry out schedule will be evaluated.

It must also be said that if the material has performed well for a reasonable period of time then the above mentioned factors are generally not the cause.

For a detailed assessment one needs the variable production AND process Histories:

• Changes/variances/inconsistencies on temperature trends.
• Feed rates and/or product changes.
• Steam & Pressure trips and related condensation into linings with sudden thermal and pressure spikes thereafter.
• Unplanned thermal cycles, poor stoichiometric periods. Burner issues. Leaking boilers or questionable water usage.
• Hotspots steam or air application – sudden appearances and disappearances of hotspots.
• With everything available an informed decision can be made

All refractory linings will fail at some point so the most important is to learn from failures to improve refractory life.

It’s essential for refractory maintenance teams and Production to have a healthy relationship in order to share knowledge and changes in linings and operations.

It’s important to note that while refractories are incredibly robust materials they are also sensitive to process variations.

If the best material is selected and installed as best as possible the final responsibility is with the owner and production to look after their asset (refractories) as best as possible.