Free Practice Questions for API API-936 Exam

Creepin refractories is the slow, time-dependentdeformation of a material under constant stressand high temperature. It is critical in applications like reformers and furnaces where the refractory is under load for extended periods. The amount of deformation is typically measured over 50–100 hours in standardized tests such as ASTM C832.

Creep is not the same as thermal expansion (which is reversible), or cracking (a sudden failure).

Permanent linear change (PLC)measures the irreversible dimensional change of a refractory specimen after it is subjected to a high temperature for a specified duration. It is expressed as a percentage of the original length. This property helps evaluate the stability and thermal compatibility of refractory materials during service.

This is distinct from:

Modulus of rupture (A): a strength test under bending.

Modulus of elasticity (C): a measure of stiffness.

Pyrometric cone equivalent (D): a test for melting behavior.

According to API Standard 936, the correct specimen dimensions for cold crush strength testing are:

“Test specimens for cold crushing strength shall be2 in. (50 mm) cubesamples prepared in accordance with ASTM C133.”

—API Std 936, Section 6.4.2​

This directly invalidates option A, which incorrectly states the specimen size as 4-1/2 inches.

For the other options:

Option B – Density determination:

API 936 states that density shall be measured using the same specimens used for linear change and abrasion resistance:

“Bulk density shall be determined using the same specimens prepared for the linear change and abrasion resistance tests.”

—API Std 936, Section 6.4.3​

“These specimens include cubes, linear change bars, and erosion plates.”

—Appendix A, API Std 936​

Option C – Abrasion resistance specimen size:

“Two 4-1/2 in. × 4-1/2 in. × 1 in. (114 mm × 114 mm × 25 mm) specimens shall be prepared for abrasion resistance testing.”

—API Std 936, Section 6.4.4​

Option D – Permanent linear change specimen size:

“Permanent linear change (PLC) is determined with a bar 2 in. × 2 in. × 9 in. (50 mm × 50 mm × 230 mm) cast in steel molds.”

—API Std 936, Section 6.4.1​

Therefore, Option A is the only false statement.

According to API TR 980, specific practices are required during the installation of erosion-resistant refractory linings:

“Thin-layer linings shall be installed so that overfill is trimmed flush to the hexmetal...”

“Mixing procedures, including mix times and water ratios, shall be defined during material qualification testing…”

“The refractory shall be fully compacted in and around the anchor supports.”

— API TR 980, Section 4.3.2​

Additionally, slicking (Option A) is not permitted as it interferes with mechanical bond development.

The API 936 terminology section defines "green refractory" as:

“Green Refractory: A monolithic refractory that has been placed but has not yet been exposed to curing or dryout temperatures.”

— API Std 936, Section 3: Definitions​

Thus, “green refractory” is the technically correct term for the unheated, freshly installed condition.

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TR 980 outlines curing and dryout best practices for monolithic refractories. For high-purity low-cement castables, gradual heat-up is necessary to avoid thermal shock and cracking:

“A minimum 24-hour air cure at ambient temperature should be followed by a controlled dryout ramp not exceeding 50°F (28°C) per hour up to 600°F (316°C).”

— TR 980, Section 6.4.2​

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According to API Std 936, the quality control program includes several essential elements to ensure compliance with material specifications and installation procedures. These elements include:

"Key quality control activities include, but are not limited to: material qualification testing, initial inspection, installation monitoring, and as-installed testing."

— API Std 936, Section 4.1, Quality Control Program​

Although documentation is essential, report writing is not listed as a core quality control element within this section. Therefore, A, B, and C are the correct answers.

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Apparent porosityis a measure of the volume of open pores (voids that can absorb water) within a refractory sample, relative to the total volume. It is always expressed as apercentage. Testing is done using standards such as ASTM C20, which API 936 references. This metric is crucial for understanding a material’s permeability, thermal shock resistance, and mechanical integrity.

Hydraulic bonded castables, such as those using calcium aluminate cement, cure through ahydration reactionthat requires the presence of water. Premature drying due to wind, sun, or heat can halt the hydration process, weaken the structure, and cause surface cracking. API 936 mandates thatno part of the exposed surface of the castable shall be allowed to dry more than 3 hoursbefore a membrane curing compound is applied.

This statement ensures:

Retention of sufficient surface moisture for ongoing hydration.

Prevention of microcracking, which can later act as stress risers during service.

Uniform strength development across the lining.

The curing compound forms a film that limits evaporation, allowing for continued hydration over the next 24–48 hours.

API Std 936 specifies in Section 6.6.1.1 that the test panels for pneumatic gunning must be cast to a minimum thickness of 2 inches with an acceptable tolerance:

"The minimum gunned thickness shall be 2 in. (50 mm), and the final thickness shall be within ±0.25 in. (6 mm) of the target."

— API Std 936, Section 6.6.1.1​

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