Updates on improving refractory lining service life

03.01.2011|Maity, M., SABIC Technology Centre,Jubail, Saudi ArabiaTips help maintenance and operations care and maintain refractory products in high-temperature operations

Keywords:The hydrocarbonprocessing industry (HPI) utilizes high-temperature conditions and relies on the performance of refractory lining for various furnaces and vessels. The total installed cost of refractories in HPI facilities is negligible compared to the total cost of the entire complex.

However, refractory lining is very important and it plays a critical role in the total performance, profitability and reliable operation of the plant. There are several instances of refractory lining non-performances and premature failures resulted in unplanned emergency shutdown. We will discuss common problems and failures encountered in HPI facilities and some remedial measures to follow.

Background.

In the HPI, refractories are mostly installed on fired heaters, hydrogen reformers, ammonia primary and secondary reformers, cracking furnaces, incinerators, process gas boilers, catalytic cracking units, coke calciner, sulfur furnaces, utility boilers, air heaters, ducting, stacks, etc. Some of the listed equipment operate under high pressure, and operating temperature can vary from very low to very high (approximately 500C to 1,600C).

Refractories play a critical role for the total performance and reliable operation of high-temperature processing units. Refractories can be the controlling factor in the success or failure of a furnace and vessels service life, as well as the safe and profitable operation of the plant.

Most HPIfacilitiesoperate under continuous operating mode and will run for several years before a scheduled shutdown formaintenanceor turnaround. Therefore, the prime objective here isequipmentreliability. Also, in the HPI, the time between shutdowns and maintenance outages is increasing with the implementation of stringent quality control and preventive maintenance programs. The shutdown period is usually short in duration and is planned primarily for mechanical overhaul. The life and durability of refractory lining should not be the determining factor in the frequency and duration of shutdowns.

In spite of its importance, refractories are, in many cases, neglected, misunderstood and abused, and the uncared for refractory can cause several problems during regular plant operations.

The unexpected problems can cause an emergency shutdown or require longer maintenance time to rectify both the damages in the refractory lining as well as mechanical damages to unit equipment. Also, the sudden failure of the lining can pose a significant risk and threat to plant safety. Therefore, it is important for HPIfacilitiesto optimize the liningreliabilityand performance in various applications.

It may be possible to minimize refractory related problems and reduce unnecessary expenses by introducing sound engineering practices, following proven guidelines and sharing practical experiences while designing the lining and selectingconstructionmaterials, installation methods, etc. This article will discuss several common problems and failures of refractory lining and highlight important considerations to mitigate such problems.

Common problems and failures.

The performance of any lining in a furnace is considered to be reasonable when similar service lives are achieved on a regular basis. Premature lining failure may be defined as one that does not achieve normal or average performance and service life.

The furnaces and vessels in the HPI do not consume the refractory with corrosive liquid metal, slag, abrasion, impact, etc., which are common in metallurgical furnaces. Refractory lining problems and failures are mainly due to thermo-mechanical stresses, erosion and chemical attack. The most common refractory problems as experienced in the HPI are:Hot spots (higher casing temperature)Excessive crackingSpalling of lining (thermal, mechanical, structural)Erosion and thinning of liningChemical attack/corrosion from process gases (such as hydrogen, carbon monoxide, sulfur dioxide, alkalies), flue gases (sulfur, sodium, vanadium), steam, etc.Acid-gas dew-point corrosion of refractory and metallic partsPartial melting and degradation of liningExcessive shrinkage and development of gapsAnchor failure and detachment of lining from wallFailure of metal liner over refractoryExplosive spalling during dry outMechanical damages.

The extent of damages and failures may vary equipment to equipment. Sometimes the problems appear within a short time of operation or during commissioning, and this can become a major concern.

Most processing industries handle highly combustible hydrocarbons. Therefore, lining problems in critical pressure vessels and boilers are a major concern and, in many cases, causes immediate shutdown to avoid any accidents. HPI processing plants are complicated involving continuous chain reactions in the interconnected network of reactors, vessels and pipelines. Any problem in any particular vessel due to a refractory problem can result in a complete shutdown of the unit and/or the entire facility.

Here are some of the common examples of refractory damage (Figs. 112). The reduction in lining thickness in the catalytic cracking unit is caused either by cracking and spalling due to heavy thermal shock or erosion by catalyst particles and subsequent hot spot or partial exposure of shell plate. Also, mechanical damage in the cyclones due to erosion of the lining and plate may disturb the unit operation. Failure of the lining due to inadequate anchor system is very common for all kinds of lining. Explosive spalling may be caused during the initial dry out due to uncontrolled heating.

Fig. 1.Typical refractory lining failuresCrack in castable lining.

Fig. 2.Typical refractory lining failuresCastable wall detached.

Fig. 3.Typical refractory lining failuresRoof lining collapse.

Fig. 4.Typical refractory lining failuresCastable damage and anchors exposed.

Fig. 5.Typical refractory lining failuresLoose bricks hanging.

Fig. 6.Typical refractory lining failuresCF module fallen from roof.

Fig. 7.Typical refractory lining failuresCasing plate corrosion with perforation.

Differential movement of the shell and lining due to a mismatch inexpansionbehavior, uncontrolled heating-cooling, mechanical stresses, etc., can cause various problems in the lining. In many cases, localized hot spots or high temperatures are controlled by steam impingement to reduce the casing temperature and continue plant operation. Reductions in thermal efficiency, as well as associated risks and plant safety, are major concern for such cases.

Failure analysis and corrective actions.

Refractory-lined equipment function as a system. There are several interacting physical and chemical effects that may be ongoing, progressive, cyclic, etc., that will definitely control the performance of refractory. Therefore, in most of the cases, it may be very difficult to conclude a single reason for nonperformance and premature failure of the lining. Also, there are numerous precommissioning factors related to design, material selection, installation, etc., as explained previously that directly affect performance.

In some cases, the poor quality material or installation workmanship of the refractory may contribute to the problem. But it is also possible that a good quality refractory or installation can give unsatisfactory performance because of a combination of other factors. The analysis of any refractory problem should consider numerous factors and identify the main root cause for the problems and select appropriate recommendations.

Identifying the actual reason of non-performance for the lining is a difficult task, and this involves systematic study and analysis of the problem. Reviews of background information particularly design engineering, quality and source of material, installation procedures and records, operational records, post-service inspection and maintenance history are important for the root-cause analysis. Awareness of the operational parameters and potential degradation mechanisms that can lead to failure of the lining is essential to understand the problems and remedial measures. A thorough system analysis should result in a better understanding of the various factors that control the performance of the lining and yield in sound basis for corrective remedial measures and actions. Therefore, clues and relevant facts of failure should be gathered, analyzed, explored and studied to make a meaningful conclusion. Collecting samples and selective laboratory testing should be part of the failure analysis, if required. Important factors responsible for the performance of the lining are briefly explained here.

Important factors governing refractory lining performance.

Whenever there are some problems in lining, we mostly conclude that either the refractory material was bad or the installation was poor. However, in reality, the problems are probably due to a combination of multiple factors and may not be solely just one factorpoor installation or inferior refractory quality. Clients as well as theprojectcontractors, suppliers and others dont always recognize that their actions and oversights can directly affect the performance of the lining.

To address the problems associated with refractories, it is necessary to recognize the main factors that are involved and contribute refractory related problems. These factors are:Design of furnaces/vesselsDesign of refractory lining and detail engineeringSelection of refractory materials and specificationQuality of refractory materialsInstallation of liningCuring, startup and maintenance of liningOperation practicesInspection and maintenance practices.

It is important to give attention to these listed factors to avoid and minimize unexpected and premature failure or problems of lining.

Design of furnace/vessels.The performance and stability of the refractory lining depends on the structural design of the furnace and its configuration. Sometimes adequate attention is not given to the refractory lining and its engineering related issues during early plant equipment design and detail engineering. One of the most common observations is that the refractory designers or specialists are involved at the final stage ofprojectimplementation or during installation. This may lead to several compromises with refractory-lining design and engineering practices as there is limited scope for change in vessel design, operating conditions and the process to reduce the impact of these factors on lining performance.

The problem of refractory lining may be due to insufficient combustion volume. The heat released within the system is more than absorbed by the process and is dissipated through walls or exhausted with flue gases. In such cases, there are possibilities of the lining approaching the flame temperature and causing several problems. The burner type, its design, location, flame shape, possibilities of flame impingement, flowing pattern of flue gases, etc., may affect the lining. In many cases, limited vessel dimensions, inaccessibility and complicated configuration restricts the best lining practices during initialconstructionas well as subsequent maintenance and repair.

Design of refractory lining and detail engineering.The design and detail engineering of the lining for a furnace and vessel should be done on the basis of careful analysis of service conditions, availability of refractory materials, thickness requirements, anchorage, ease of installation and future repair andmaintenance. Adequate knowledge on operating conditions that are activesuch as temperature, pressure, chemical attack, thermal shock, abrasion, erosion, furnace gas composition, mechanical movement, vibration, etc.should be very useful for the optimum lining design and selection of refractory. Chemical attack may occur from gases such as steam, hydrogen,carbonmonoxide, alkalies, sulfurous gases, etc.; these acid gases can initiate various problems in the lining, which are explained in the literature. All of the important operating factors and any other criteria specific to the process under consideration should be verified for their possible effect on the performance of the lining.

Thermal calculation is essential for any lining to ensure design casing temperature, temperature gradient in the wall and heat losses. Thermo-mechanical FEM analysis may be carried out for critical vessels and load bearing refractory structures to predict temperatures, stresses and displacements in the lining. The FEM analysis is a reliable tool to investigate the spalling mechanisms and to develop ways on improving the lining behavior.

Anchors are used for almost all types of refractory applications. These are mostly metallic type. Lining failures due to inadequacies in the anchoring system are very common (Figs. 6, 9 and 10). Selecting the proper metallurgy, anchor dimensions, configurations, and spacing are very important to achieve the maximum service life of the lining. Where metal liners are used over the lining, the mechanical design should be sound and allow free movement of the liner on one end from its fixed positions.

Fig. 8.Typical refractory lining failuresAcid condensation below lining.

Fig. 9.Typical refractory lining failuresOxidation of anchor.

Fig. 10.Typical refractory lining failuresSurface spalling and exposed anchor.

While designing a new lining for a vessel it is important to consider ease of future maintenance and repair. However, this aspect of the lining design is compromised in many cases because a lining system that is maintenance friendly may be more expensive with respect to initial materials and installation expenses as compared to a lining that is adequate to meet the initial contractual requirements.

Details of the lining layout structure, thickness, dimension, shape and sizes of individual bricks and other shaped items, their laying and bonding patterns, provisions forexpansionallowances, support of brick-wall, etc., should be part of the detail engineering for each piece of equipment.

Finally, practical experiences and experience-based judgments are very important for successful and reliable design of any refractory lining. Therefore, involvement of experienced engineers from the design stages to final implementation is one of the essential parameters to get the optimum performance of lining.

Selection of refractory materials and specification.The majority of refractories used in the HPI are aluminasilicate and high alumina varietiesboth insulating and dense types. Mainly bricks, monolithics, ceramic fiber items, different types of insulating blocks, etc., are used for lining. Bricks and monolithics are available for both dense and insulating types with a wide range of properties and each material has an application that is more suitable.

Selecting materials should always be based on properties and specifications suitable for the specific application and operating conditions. Most refractory materials react and change during service according to the principle of thermo-chemistry. It is important to know the furnace atmosphere, presence of any major or minor chemicals and their possible effects on the lining. Selecting materials solely based on price and ease of installation should be avoided. Very often, monolithic lining system is selected in the lining design of critical vessels where brick lining or some other design may be more suitable. The selection may be due to cheap and easy availability of monolithics and easier installation than brick lining and to avoid preparation of too many engineering drawings for the complicated brick shapes and laying details. In many cases, recommendations of refractory manufacturers are biased and based on their available product ranges, which may not be appropriate for the required conditions.

Selection should be based on the desired service life and cost considerations. Initial cost of refractory lining should not be the selection criteria but rather the service life of lining under operating conditions. It is better to develop the material specification for any application based on discussions with the manufacturers to ensure it is more practical and realistic. The specification should be regularly reviewed and updated based on actual performance of used materials and current industry practices.

Quality of refractory materials.Refractory materials are heterogeneous, and quality varies both as manufactured and as installed. Materials should be procured against specifications most appropriate to the specific application. For critical applications, purchasing of materials based on a comparison of product datasheet or catalogue specifications or equivalent principles should be avoided. The actual performance references and records for specific products or brands should be verified against similar applications. Reviews of manufacturingfacilitiesand quality-control program, and random inspection and testing of important properties are essential.

Lining installation.Unlike other engineering subjects, there are very few well-established and recognized engineering standards, design and installation guidelines for refractory jobs to ensure quality installation of the lining and its subsequent satisfactory performance. In many applications and contracts, the only quality assurance is limited to warranty and guarantee of material and lining for a limited operating period. Installations of refractory rely upon manpower. Because of the human element, care should be taken to ensure involvement of only experienced manpower in the installation. Developing job specific installation procedures, quality plans and acceptance criteria of the installed lining, prequalification materials, and installation crew are some of the important factors that companies must address before any job.

API 936 guidelines, developed by the American Petroleum Institute (API) are very useful tools for quality control of monolithic lining. There are also some specifications and standards developed by Process Industry Practices (PIP) especially for process industries. Prequalification of materials, installation procedures, machinery and crew, testing of as installed samples, ambient condition monitoring, acceptance criteria for installed lining, involvement of neutral inspection agency for quality monitoring, etc., are important requirements of these standards. Many clients and licensors have started recommending compliance to these guidelines for critical applications. Possible quality control for brick and ceramic fiber lining in similar lines are expected to improve quality for the total installation job and compliance to engineering practices.

Using common standards and guidelines will help the industry to mitigate installation problems. This also helps in developing quality installation manpower that are actually executing the jobs in the field, particularly crews, masons and supervisors.

Dry out and heat up.The heat dry out of a new lining, particularly monolithic lining, is a critical step when considering the total quality for the installation. Slow and controlled removal of free and chemically bonded water from the lining system is essential before actual startup of the unit. Explosive spalling or cracking may occur in lining when quick and uncontrolled initial heat up or dry out of the refractory is done.

Also, alkali hydrolysis is a major concern for monolithic lining in tropical and subtropical weather conditions. Dry-out needs to be done at the earliest to prevent damage. When delays in dry out or complete dry-out are not practical, suitable sealants may be used on monolithic lining to reduce alkali hydrolysis reaction and damage. Also, natural-air circulation should be maintained within the furnace to avoid hot and humid conditions.

Developing job specifications for the dry out schedule is essential instead of following general guidelines from the supplier. Burner size and location, exhaust location, air volume, velocity, temperature-control locations, etc., need to be properly addressed. Permanent burners or special external burners may be used for dry out depending on requirements. Permanent burners have limitations of inadequate temperature control at the initial stage. Many specialized dry-out agencies are available to carry out this job in most professional manner.

Unit operation.Production and operation personnel should be aware of the process parameters that may affect the service life of the refractory lining for furnaces or reactors. Minor changes in operating conditions and processes may strongly influence the performance of any lining. Abnormal changes in burner operationsuch as flame impingement on refractory surface, incomplete burning of fuel causing change in furnace atmosphere, changes in temperature, pressure, fuel quality (dirty fuel), heating and cooling rate, etc.have direct effects on the refractory lining. Operating at a higher temperature than specified in the design can reduce the service life of the refractory. High limit thermocouples should be located at strategic positions for monitoring and controlling temperatures within the system.

In many cases, the problem or initiation of deterioration in lining due to operational issues may not become visible immediately. Therefore, it is important to gather information on operational information and records while studying the problem.

Inspection and maintenance.Regular inspection of the lining and condition monitoring should be part of the operating plan for critical equipment. The frequency of inspection may be decided based on historical problems, severity of operating conditions, complexity of design and other factors. Timely identification of problems and corrective actions may lead to longer life of the lining. Temperature-sensitive paints are widely used to monitor casing temperature and locate hot spots. Infrared thermography is an important tool for online-temperature measurement, condition monitoring of lining, predicting problems, and maintaining equipment uptime during a problem.

Thermography is very useful for locating and monitoring effected areas in case of any operational upset or localized problem in the furnace, and thus allowing the inspectors insight into what is happening inside the lining. This allows the plant to make appropriate decisions in planning the shutdown schedule and maintenance repairs and estimating the total materials needed for these repairs.

The maintenance strategy for refractory linings should be based on cost-effective proactive systems rather than on conventional reactive systems. The probable reasons and mode of failure should be ascertained before redesigning or repairing a lining. Change in lining design and installation practices without proper analysis of factors limiting the service life of the existing lining may not be a long-term solution. All repair and maintenance jobs should be treated like a new job with proper quality control. It is important to inspect and record all repairs to maintain a proper trend and database.

Other factors.There are many other factors that may directly or indirectly affect the performance of the lining. In mostconstructionsites, refractory installation is one of the last activities. With any delay in other pre-activities, there is always pressure on shortening and possibly compromising the refractory installation schedule to make up for earlier delays. This attitude of getting the job done fast may have major adverse effects on quality.

Ambient temperature and working conditions for workers have direct effects on the quality of the installation job. Implementing cost-cutting measures, purchasing refractory and selecting the contractor solely on the basis of commercial issues with less importance on quality, services, etc., may be the contributing factors to poor performance over the longer term.

Fig. 11.Typical refractory lining failuresBrick wall bulged.

Fig. 12.Typical refractory lining failuresBrick wall collapsed and hexmesh detachment.

Observations for quality.

Refractory is a diverse class of materials that are used to insulate and protect industrial furnaces and vessels. The properties of the refractories are tailored to specific applications by varying the composition of raw materials. Thetechnologyof refractories is making remarkable progress recently. Developments in refractory lining design system and performance specifications with as-built quality requirements are very important for this specialized field. With every lining failure, there is a degree of uniqueness that results from variability in design and application, complexity of service conditions and material behavior. Awareness of the potential degradation mechanisms of lining and operating parameters is essential to mitigate such problems. Implementing various quality control programs, advanced installation procedures and online inspection can contribute to satisfy results.HPBIBLIOGRAPGHYAmerican petroleum Institute,API RP 936, Refractory installation quality control guidelines-inspection and testing monolithic refractory linings and materials.Biglin, J., RefractoryMaintenanceand repair,The American Ceramic Society Bulletin,Vol.75, No. 5, May 1996.

Crowley, M. S., Design better vessel linings,Hydrocarbon Processing,December 1979.

Devera, D., After installation the importance of a controlled dry-out for castable refractories,The Refractories Engineer, July 2003.

Gardner, A., FCC Cyclone Refractories,TodaysRefinery,November 1998.

Hanley, R. M., Refractories utilization in the hydrocarbonprocessing industries,The Refractories Engineer,July 2000.

Hancock, J. D., Practical Refractories, Cannon & Hancock CC.

Heard, N. E., Quality Control Practices in the USPetrochemicalIndustries, Unitcer, 1989.

James, S., The Fundamentals of Refractory Inspection with Infrared Thermography.

Schacht, C. A.,Refractory Linings,Marcel Dekker, Inc.

Semler, C. E., Overview of refractory problems in industry,Interceram,Vol.40, No.7, 1991.