{"slug":"/standards/api-936","title":"API 936 Refractory Installation: Panels, Dryout and Cores","description":"API 936 refractory QC: applicator test panels, mixing water by weight, dryout ramps near 50 degF per hour and cores at 90% of published density.","h1":"API 936: Controlling a Monolithic Refractory Installation Before It Is Buried","answer":"API 936 is the quality-control standard for installing monolithic refractory: castable, gunned, pumped and rammed linings. It governs material receipt and storage, applicator and equipment qualification by test panel, mixing water control, anchor verification, curing, controlled dryout and post-installation testing by coring and hammer sounding. It sets no lining design at all; thickness, material selection and anchor design belong to the specifier.","expansion":"Refractory fails for reasons decided in the first eight hours of its life and paid for years later, which is why API 936 concentrates almost entirely on installation control rather than on the finished lining. Too much mixing water in a low-cement castable lowers density and strength, and raises the volume of water that must later escape as steam. A dryout ramp run too fast turns that trapped water into vapour pressure the dense lining cannot vent, and the result is explosive spalling that takes a cyclone or a heater wall out of service. Anchors of the wrong alloy, spaced off drawing, welded by an unqualified welder or left without tip coating produce cracking along entirely predictable lines. None of this is visible in a lining that looks acceptable, so the standard puts its acceptance on records, panels and cores.","source":"API 936, Refractory Installation Quality Control — Inspection and Testing Monolithic Refractory Linings and Materials; API 560, Fired Heaters for General Refinery Service; API 573, Inspection of Fired Boilers and Heaters; ASTM C133 (cold crushing strength and modulus of rupture), ASTM C704 (abrasion resistance), ASTM C113 (reheat change), ASTM C860 (ball-in-hand consistency), ASTM C865 (firing of specimens); ASME BPVC Section IX and AWS D1.1 for anchor welding qualification.","table":{"caption":"API 936 control points, the check behind each, and the usual acceptance basis","columns":["Control point","Typical check or test","Usual acceptance basis","How it fails in the field"],"rows":[["Material receipt and storage","Manufacturer's lot certification, bag condition, date of manufacture, storage off ground and dry","Within shelf life, typically six to twelve months for hydraulic-setting castables; certification traceable to the lot installed","Bags stored on a dock in Gulf humidity; partially set material installed and never detected until cores fail"],["Applicator qualification","Test panel installed with the actual crew, equipment, material and orientation, then cured, fired and cored","Cores meeting the specified density and cold crushing strength; panel free of laminations and rebound pockets","The contractor is qualified rather than the individual nozzleman and pump configuration; the crew changes and qualification is silently void"],["Mixing water","Water measured by weight or calibrated volume, potable, temperature controlled; recorded per batch","Within the manufacturer's stated range for the specific product; ambient and material temperature typically held between 10 and 32 degC","Water added by hose time and by eye to improve workability; density and strength fall together"],["Consistency","Ball-in-hand check per ASTM C860 for castable and gunned work","The ball holds shape with slight moisture at the surface and no slumping or free water","Skipped entirely because the mix looks right; wet mixes are placed to save vibration effort"],["Anchors","Alloy verification, spacing and projection against drawing, weld quality, hammer or bend testing on a sampled basis, tip coating","Correct alloy for hot-face temperature, spacing per drawing, sound welds, tips coated to permit differential expansion","Uncoated tips crack the lining along the anchor pattern; wrong alloy oxidises and lets the lining go"],["Curing and dryout","Moist cure for the specified period, then a controlled heat-up with recording thermocouples and hold points","A dryout performed to a written schedule with a chart trace showing ramp rates and holds achieved","Ramped fast to make schedule; steam pressure spalls a dense low-cement lining within hours"],["Post-installation testing","Hammer sounding, visual crack assessment, thickness check, cores tested for bulk density and cold crushing strength","Commonly 90 percent of the manufacturer's published values, or the values achieved on the qualified panel","Coring skipped because the lining looks sound; nothing is known about what is behind the surface"]],"note":"Values described as typical come from owner specifications and manufacturer data sheets. The governing numbers for any job are those written into the project specification."},"facets":[{"q":"Does API 936 cover brick and ceramic fibre linings?","a":"No. API 936 addresses monolithic refractory: castables placed by casting, pumping, gunning or shotcreting, and plastic and ramming mixes. Brick and other preformed shapes, ceramic fibre blanket and modules, and insulating firebrick construction fall outside it, and those installations are controlled by the owner's specification and the manufacturer's instructions instead. Projects with mixed construction need both regimes written into the inspection and test plan, because API 936 alone leaves the fibre and brick work uncontrolled."},{"q":"What does an API 936 test panel have to demonstrate?","a":"That this crew, using this equipment in this configuration, with this material at this orientation, can produce refractory meeting the specified properties. The panel is installed under production conditions, cured, fired and cored, and the cores are tested for bulk density and cold crushing strength. Overhead and vertical panels are qualified separately from flat work because gunning rebound and pump placement behave differently against gravity, and a flat panel proves nothing about a roof."},{"q":"How much mixing water is allowed in a low-cement castable?","a":"Only what the manufacturer's data sheet permits for that specific product, and it must be measured by weight or calibrated volume rather than estimated. Low-cement and ultra-low-cement castables have narrow water windows, often under a percentage point wide, because their strength depends on tight particle packing. Excess water raises porosity, drops density and cold crushing strength together, and increases the free water that the dryout schedule must later drive off safely."},{"q":"Why do dense castables need polypropylene fibres?","a":"Low-cement and ultra-low-cement castables achieve strength through very low permeability, which is exactly what prevents steam escaping during heat-up. Polypropylene fibres blended into the mix melt at a few hundred degrees Fahrenheit and leave a network of fine channels that vent vapour before internal pressure exceeds the tensile strength of the material. Omitting the fibres, or specifying a dense castable and then running a conventional dryout ramp, is a reliable route to explosive spalling."},{"q":"What acceptance value applies to a core taken from the lining?","a":"Almost never the manufacturer's nominal data sheet figure, which is a laboratory value produced under laboratory conditions. Project specifications commonly accept cores at ninety percent of the published bulk density and cold crushing strength, or at the values demonstrated on the qualified test panel, whichever the specification names. Arguing acceptance against the nominal datasheet number produces failures on sound refractory and disputes that could have been avoided by writing the basis down."},{"q":"Does API 936 specify the dryout schedule?","a":"It requires that dryout be performed to a written procedure with recorded temperatures, but the schedule itself comes from the refractory manufacturer and the equipment designer, because it depends on the material, the lining thickness, the geometry and the vessel. Typical schedules ramp slowly to around 250 degF, hold for a period scaled to lining thickness, ramp again with a second hold, then proceed to service temperature. The chart trace is part of the acceptance record."}],"sections":[{"heading":"What API 936 controls, and what it leaves to the designer","paragraphs":["API 936 is an installation quality control standard. It covers the materials as received, the qualification of the people and equipment that will place them, the mixing and placement itself, curing, controlled dryout, and the inspection and testing that establishes whether the finished lining meets the specification. It exists because monolithic refractory is a material that is manufactured on site, by hand, under weather and schedule pressure, from a bag of powder and a hose of water. Almost every property that matters is decided during those few hours.","What it explicitly does not do is design anything. Lining thickness, material selection, hot-face and back-up layer configuration, anchor type, alloy and spacing, and the thermal design of the vessel all belong to the designer and appear on the drawings and in the specification. Nor does it cover brick, ceramic fibre or other preformed construction. Inspectors who arrive expecting API 936 to tell them whether a six-inch dual-layer lining is correct for a given service will not find it there; the standard tells them how to verify that what was drawn was actually built.","This division causes a specific, recurring problem. Where the specification is thin — no acceptance class for cracks, no core sampling frequency, no defined dryout responsibility, no stated acceptance basis for core properties — API 936 does not fill the gaps, and the contractor fills them instead. Closing those gaps before mobilisation is straightforward specification work, and it is a natural use of [ASNT Level III consulting](/consulting) alongside the refractory engineer, because the NDE and acceptance language is the part most often left undefined."]},{"heading":"Applicator and equipment qualification: the test panel","paragraphs":["The centrepiece of API 936 is qualification by demonstration. Before production work begins, the installer builds a test panel using the actual material lot, the actual equipment in the actual configuration, the actual crew, and an orientation representative of the production work. The panel is cured and fired following the procedure that will be used in production, then cored, and the cores are tested for bulk density and cold crushing strength. Panels are also broken or sectioned to look for laminations, rebound pockets, voids behind anchors and poor consolidation.","Qualification is specific, and this is where most disputes start. It attaches to the nozzleman or the pump crew, to the gun or pump and hose configuration, to the material, and to the orientation. Swapping the nozzleman mid-campaign, changing hose length or nozzle type, switching to a different product, or moving from wall work to overhead work each raises the question of whether the qualification still applies. Contractors qualify a company; the standard qualifies a demonstrated capability, and the difference surfaces when a roof lining fails and the panel turns out to have been shot flat.","Orientation deserves particular emphasis in gunned work. Rebound behaves entirely differently overhead, and material that consolidates properly on a vertical wall can arrive at a ceiling with a fraction of the density and pockets of trapped rebound. Where the production work includes overhead or complex geometry — cyclone crowns, transitions, plenum roofs — insist on a panel that reproduces it. A flat panel that passes tells you nothing useful about the part of the job that will fail first."]},{"heading":"Materials: certification, shelf life, storage and water","paragraphs":["Every lot delivered should arrive with the manufacturer's certification showing the product, lot or batch number, date of manufacture and the tested physical properties for that lot. That certification has to be traceable to the material actually installed in a given area, which means bag control and daily records rather than a certificate filed once for the whole delivery. Where independent verification testing is specified, samples are drawn from the delivered lot and tested for the properties the specification calls out, commonly cold crushing strength and modulus of rupture to ASTM C133, permanent linear change to ASTM C113, and abrasion resistance to ASTM C704 for erosion service.","Shelf life and storage are the quiet failure mode. Hydraulic-setting castables have a limited life, commonly six to twelve months from manufacture, and they degrade further if stored damp. Material stored on the ground, under a tarpaulin, in a humid climate, will partially hydrate in the bag. It mixes, it places, it looks entirely normal, and it never reaches strength. Bags off the ground on pallets, in a dry and temperature-controlled store, with stock rotated by date, is unglamorous and prevents an expensive category of failure.","Water is the single most consequential variable on the job. It must be potable, temperature-controlled, and measured by weight or calibrated volume for every batch, within the range the manufacturer states for that specific product. Low-cement formulations have narrow water windows because their strength depends on dense particle packing, and a crew adding water to improve workability is trading strength and density for ease of placement. The ball-in-hand check to ASTM C860 takes seconds and catches wet mixes before they are placed; skipping it is common and expensive."]},{"heading":"Anchors: alloy, spacing, welding and tip coating","paragraphs":["Anchors hold the lining to the shell and accommodate the differential expansion between a steel shell and a ceramic lining that will not move together. The drawing specifies type, alloy, length, projection, spacing and pattern, and every one of those is an inspection point. Alloy selection follows hot-face temperature, with carbon steel acceptable for lower duty and austenitic grades such as 304, 309 and 310 used as temperatures rise. Positive material identification on delivered anchors is cheap and finds substitutions that a visual check never will.","Anchor welding is welding, and it is governed accordingly: qualified procedures, qualified welders, and a sampled verification programme. Practice is normally 100 percent visual examination plus hammer or bend testing on a defined proportion of anchors per area or per shift, with any failure triggering an increased sample. The recurring finding is anchors welded by whoever was available, on a procedure written for the shell material rather than the anchor alloy, with no record of who welded which area.","Tip coating is the detail most often omitted and most reliably punished. Anchor tips are coated — commonly with a bituminous or wax product, or wrapped — so that the anchor can expand without splitting the refractory around its tip. Uncoated tips produce cracking in a pattern that maps the anchor layout exactly, which makes the cause obvious afterwards and useless at that point. Verifying coating presence and thickness before the refractory goes on takes one walk-down, and it has to happen before placement because afterwards there is no way to check."]},{"heading":"Curing and dryout: where linings are actually destroyed","paragraphs":["Hydraulic-setting castables need moist curing before they are heated, typically at least 24 hours above about 50 degF with the surface kept damp under wet burlap or plastic sheeting. Curing develops the hydraulic bond, and heating an uncured lining destroys it. Cold weather placement extends the requirement and may need heated enclosures; hot, dry, windy conditions demand more aggressive moisture retention, because a surface that dries out in the first hours will never develop strength no matter how long it is left.","Dryout is the highest-risk operation in the whole sequence. The lining contains free water plus chemically combined water, and it has to be driven off slowly enough that vapour escapes faster than it accumulates. Typical schedules ramp at modest rates — often in the region of 50 degF per hour — to around 250 degF, hold for a period scaled to lining thickness, ramp again to an intermediate hold, and then proceed to service. Thermocouples and a chart recorder document the actual trace, and that trace is part of the acceptance record, not a commissioning nicety.","Dense low-cement and ultra-low-cement castables are the dangerous case, because the low permeability that gives them their strength also prevents steam escaping. Polypropylene fibres are blended in precisely to melt out and leave vent channels. Where the fibres were omitted, or the ramp was accelerated to recover schedule, the result is explosive spalling: sheets of refractory blown off the hot face within hours, in a vessel that is now on the critical path. The commercial pressure to shorten dryout is enormous and it is the single decision that most often destroys an otherwise sound installation."]},{"heading":"Acceptance testing: cores, sounding and the ninety percent rule","paragraphs":["Post-installation inspection begins with the things that can be done without damage. Hammer sounding across the lining locates hollow areas, debonding and voids behind the surface, and it is genuinely skilled work — an experienced ear separates a hollow behind the hot face from a change in lining thickness. Visual assessment covers cracking, and the specification should state what is acceptable: fine craze cracking is normal in curing and dryout, while cracks over a stated width, cracks that follow the anchor pattern, and cracks with displacement are not. Thickness is verified against drawing, and finished surface profile is checked where flow or erosion performance depends on it.","Cores supply the numbers. Sampling frequency is set by the specification, commonly per area or per shift per crew, and cores are tested for bulk density and cold crushing strength. The acceptance basis is the point to settle before work starts: project specifications typically accept 90 percent of the manufacturer's published values, or the values achieved on the qualified test panel. Comparing cores directly against nominal data sheet figures produced under laboratory conditions creates failures on perfectly serviceable refractory and arguments that nobody wins. Core holes are repaired to the same specification as the surrounding lining.","Records tie it together. The complete package is material certifications by lot, panel qualification records with core results, daily placement records showing batch water and ambient conditions, anchor verification records, curing records, the dryout chart trace, sounding and visual reports with a marked-up drawing, and production core results. That package is what an auditor reads and what the next inspection outage will be compared against, so keeping it in a searchable [inspection data management system](/inspection-data-management-system) rather than a project folder pays back on the first re-line."]},{"heading":"Misreadings that turn into audit findings","paragraphs":["The most common is treating the manufacturer's data sheet as the acceptance criterion. Data sheet values are laboratory results on ideally prepared specimens, and field cores from a real lining will not match them. Where the specification has not stated a basis, the argument runs for weeks with sound refractory in the balance. The second is qualifying a company rather than a crew and equipment configuration, so that a nozzleman substitution on nightshift silently voids the qualification behind an entire area of lining.","The third is treating dryout as somebody else's problem. Refractory contractors frequently demobilise before dryout, operations run the heat-up, and no one produces the chart trace as a QC record. Where the lining later spalls, there is then no evidence of what ramp was actually run. The fourth concerns repairs: patch and re-line work carries the same qualification, material control and dryout requirements as new installation, but it is routinely executed as maintenance with none of them. The fifth is skipping cores because the lining looks good, which forgoes the only direct measurement of what the material actually achieved.","Where a refractory dossier arrives incomplete after the fact — a common situation when an owner inherits a completed installation or takes over from a contractor mid-project — the practical route is a structured review of what does exist against what the specification required, so that the gaps are identified and the risk quantified before the unit is fired. That is exactly the shape of an [independent report validation](/report-validation) exercise, and it produces a defensible position far faster than re-examining a finished lining."]},{"heading":"Where API 936 sits among the other documents","paragraphs":["API 936 is invoked rather than self-standing. API 560 for fired heaters in general refinery service references refractory construction and installation requirements, and owner specifications for FCC units, reformers, sulphur plants, incinerators and boilers routinely call API 936 into the contract. For in-service condition assessment rather than installation, API 573 covers inspection of fired boilers and heaters, including the refractory. The two documents cover different phases of the same asset and should not be substituted for one another.","The test methods come from ASTM: C133 for cold crushing strength and modulus of rupture, C704 for abrasion resistance — where erosion-resistant castables in FCC service are commonly specified at very low volume loss after firing — C113 for reheat change, C860 for ball-in-hand consistency, and C865 for firing specimens prior to testing. Anchor welding qualification runs to ASME Section IX or AWS D1.1 depending on the construction code of the vessel. Personnel performing the inspection are typically certified under the API refractory personnel programme, and visual examiners working to the owner's written practice hold qualification under SNT-TC-1A or ISO 9712 through [NDT training and certification](/training) routes.","One further connection is worth making. Refractory condition drives shell temperature, and shell temperature surveys are how in-service refractory degradation is usually found. Where hot spots are being tracked across a heater or a reactor over successive campaigns, capturing lining geometry and shell as-built condition through [3D laser scanning](/3d-scanning-services) gives a repeatable baseline that thermographic surveys can be registered against, instead of comparing photographs taken from wherever the previous surveyor happened to stand."]}],"faq":[{"q":"Is API 936 a design standard?","a":"No. API 936 controls installation quality only. Lining thickness, material selection, layer configuration, anchor type, alloy and spacing, and the thermal design of the equipment all come from the designer and appear on the drawings and in the project specification. API 936 verifies that what was designed was actually built, using qualified crews, controlled materials and documented curing and dryout. Where the specification is silent, the standard does not fill the gap."},{"q":"Who is qualified to inspect refractory installation?","a":"Refractory inspection is normally performed by personnel certified under the API refractory personnel certification programme, supported by NDT personnel qualified under the owner's written practice for any visual or other examination performed. Anchor weld examination follows the qualification requirements of the applicable welding code. These are separate qualifications covering separate scopes, and the inspection records should show clearly which person performed which activity under which certification."},{"q":"Are hairline cracks in a new lining a rejection?","a":"Usually not. Fine craze cracking is a normal consequence of curing and dryout shrinkage in monolithic refractory and is generally accepted. What matters is the specification's stated crack criteria: width, length, whether the crack follows the anchor pattern, and whether there is displacement or hollow sound alongside it. Cracks that map the anchor layout point at uncoated tips or an anchor problem, and those are investigated rather than accepted on width alone."},{"q":"Does refractory work need a hold point on dryout?","a":"It should. Dryout is the operation most capable of destroying an otherwise sound installation, it cannot be undone, and its only evidence is the chart trace. Placing a hold point on the review of the dryout procedure before heat-up, and a second on the review of the recorded trace afterwards, costs almost nothing and captures the two moments at which a schedule-driven acceleration would otherwise pass unrecorded."},{"q":"What records make up a complete API 936 dossier?","a":"Material certifications traceable by lot to the areas installed, applicator and equipment qualification records with panel core results, daily placement records showing batch water quantities and ambient conditions, anchor material and weld verification records, curing records, the dryout procedure with its recording thermocouple trace, hammer sounding and visual inspection reports on a marked-up drawing, production core test results, and the disposition of every non-conformance raised."}]}