API RP 936: Quality control and installation of monolithic refractories

API RP 936 establishes precise guidelines for the quality control and installation of monolithic refractories, ensuring that heat-resistant materials meet durability and safety standards in boilers and high-temperature equipment. This standard regulates everything from the selection of refractory materials to drying procedures, inspection, and technical documentation.

Scope of API RP 936

API RP 936 focuses exclusively on monolithic refractories, such as mortars, concretes, and gunning mixes. It does not apply directly to prefabricated bricks or blocks. Its objective is to ensure that the installed linings:

• Resist high temperatures and thermal cycles.
• Meet the required mechanical and chemical properties.
• Are applied and cured correctly, preventing premature failure.

In industrial experience, effective application reduces premature repairs by up to 40% and protects the investment.

Regulatory references

The standard refers to multiple ASTM standards applicable to:

• Density, compression, and hardness testing.
• Core extraction for compaction control.
• Chemical and physical properties of refractory materials.

These references ensure uniformity and reliability in test results and meet the requirements of API 936 certification.

Terms and definitions

Some concepts include:

• Monolithic refractory: Material placed on site to form a continuous lining.
• Curing/drying: Controlled removal of moisture prior to operation.
• Qualified inspector: Professional certified to supervise installation according to API 936.
• Installation checklists: Documents that record step-by-step installation and quality control procedures.

Quality control elements under API RP 936

Se incluyen los siuientes:

• Verification of refractory material certificates and test results.
• Evaluation of density, compaction, and uniformity through core extraction tests.
• Control of drying and curing.
• Anchors and fastening systems: review of type, spacing, integrity, and correct installation to ensure mechanical adhesion between the refractory material and the metal substrate.
• Substrate preparation and condition: cleaning, removal of oxides, corrosion, paint, and application of primers where appropriate; verification of geometry and tolerances.
• Batch control and traceability: segregation by batch, verification of markings on packaging, and registration of material certificates for each supply.
• Mixing and dosing control: mixing records (water/powder ratio, additives), verification of homogeneity and pot life.
• Water and additive quality: water analysis (dissolved solids, pH, contaminants) and control of chemical additives according to manufacturer’s specifications.
• Control of joints and discontinuities: design and verification of working joints, expansion joints, and weak points that require reinforcement or special treatment.
• Mockups and pilot tests: execution of pilot tests (application samples) to validate installation procedures before full-scale implementation.
• Acceptance and rejection criteria: limit values for density, compressive strength, porosity, moisture levels, and surface appearance.
• Use of installation checklists that document each stage.
• Recording and management of non-conformities: procedures for handling and documenting repairs, rework, and corrective actions.

What does the core extraction test determine?

This test extracts cylindrical samples from the lining to evaluate density, cohesion, and compressive strength, ensuring that there are no voids or compaction deficiencies that compromise the integrity of the refractory.

Responsibilities

1. Owner: Must ensure that procedures comply with standards, provide plans, operating conditions, and coordination with the contractor.
2. Contractor: Responsible for the correct installation of refractory materials, applying approved installation methods and complying with quality control requirements.
3. Inspector: Must be qualified under API 936, supervise installation, perform tests, verify test results, and ensure that documentation is complete.
4. Manufacturer: Supplies refractory materials with verified physical and chemical properties, along with material certificates and applicable ASTM test results.

Inspector qualifications

The most important duties of an API RP 936 certified inspector include:

• Supervising installation and drying methods.
• Interpreting ASTM core extraction and density tests.
• Ensuring compliance with installation checklists and complete documentation.

Refractory materials

• Physical property requirements: High resistance to extreme temperatures and thermal cycles.
• Suitable mechanical properties: density, compression, and hardness.
• Storage: Avoid moisture and contamination, segregate by batch and type to ensure traceability.
• Packaging and marking: Clear labeling with batch, properties, and date of manufacture, protection against damage during transport.
• Anchors: Ensure correct dimensional arrangement of anchors and their adhesion to the metal substrate to prevent detachment.
• In industrial experience, adhesion and compaction problems are the most frequent causes of failure. Selecting high-quality materials from certified sources minimizes risks.

Qualification and testing

Tests and test procedures:

• Core extraction, density, and compressive strength.
• Hardness, water absorption, and chemical property tests.

Qualification tests prior to shipment

• Verification of physical and chemical properties prior to dispatching the material.

Qualification of installation procedure and personnel

• Confirm that installation methods comply with standards and that personnel are certified.

Sampling and testing during production

• Evaluation of the refractory as installed, ensuring uniformity and adhesion.

Sample preparation

• Procedures for extracting representative cores without damaging the lining.

Installation/Execution

Surface preparation: Clean rust, grease, and contaminants.

• Water quality: Water free of solids and contaminants for mortar or concrete mixes.
• Contaminated refractory: Do not install; drying, cleaning, or replacement is required.
• Preparation for installation: Adjustment of anchors, homogeneous mixing, and temperature control.
• Application temperature: Avoid applications in conditions that affect curing or adhesion.
• Spraying (Gunning): Uniform application, avoiding segregation or bubbles.
• Curing: Controlled procedure to remove moisture without cracking.
• Repairs: Inspection and correction of defects before commissioning.

Drying process

Drying procedure

• Approved drying curve, defining the rate of temperature increase, maintenance points, and total cycle duration. This curve ensures controlled removal of free moisture, preventing failures due to thermal shock or steam overpressure.
• Constant monitoring of temperature and humidity.

Drying schedule

• Avoid sudden temperature changes.

Main risk of improper drying: Crack formation, coating detachment, and reduced service life.

Best installation practices

• Correct selection of refractory types according to temperature and chemical composition.
• Personnel certified under API 936.
• Use of installation checklists to document each stage.
• Performance of applicable ASTM tests and core extraction tests.
• Complete documentation with material certificates and test results.
• Controlled drying to prevent premature failure.
• Periodic inspection and preventive maintenance of the lining.

Conclusion

API RP 936 provides a comprehensive framework for quality control and installation of monolithic refractories, ensuring heat-resistant materials, safe procedures, and reliable documentation. Proper implementation, along with API 936 certification and installation checklists, guarantees extended service life, durability, efficiency, and safety in boilers and high-temperature equipment.

Referencias

1. American Petroleum Institute. (2024). API RP 963: Control de calidad de la instalación de refractarios: inspección y ensayo de revestimientos y materiales refractarios monolíticos (5.ª ed.).
2. Organización Internacional de Normalización. (2012). ISO 1927-1:2012 Productos refractarios monolíticos (sin moldear) — Parte 1: Introducción y clasificación.
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