API 936 thermocouple placement for refractory cure control

API 936 Refractory Installation Quality Control: Thermocouple Placement for Cure Schedules links workmanship inspection with the evidence needed to heat refractory safely. A poorly positioned thermocouple can report a satisfactory temperature while another zone remains wet, under-cured or exposed to damaging heat.

The standard provides a quality-control framework, while the refractory supplier, heater manufacturer and project engineer normally set the material-specific dry-out or cure curve. Thermocouple layout therefore turns that curve into verifiable measurements rather than an assumption based on furnace casing temperature.

Scope and project responsibilities

API 936 principally addresses refractory installation in fossil-fuel-fired heaters, not line pipe manufacture. For Australian LNG and gas-processing projects around Perth and Gladstone, procurement packages may include both requirements; this line pipe guidance explains the separate API 5L scope and helps teams avoid applying the wrong acceptance criteria.

Several sensor arrangements can support cure verification:

Arrangement	Primary measurement	Main advantages	Main limitations
Embedded thermocouple	Refractory body temperature	Closest representation of the cured material	Must be planned before closure and installed without defects
Shell-mounted thermocouple	Casing or outer-surface temperature	Simple installation and permanent monitoring	Can lag the refractory and reflect metal temperature
Retractable probe	Local refractory temperature through an access port	Flexible and useful during commissioning	Limited to accessible areas and requires consistent positioning
Permanent fixed thermocouple	Selected zone during furnace operation	Continuous trending and alarm capability	A useful operating sensor may not represent the original cure condition

Select the arrangement through the approved heating plan, material supplier's instructions and heater OEM requirements. No method is automatically suitable because it is easiest to install.

Selecting representative monitoring locations

Place points where the refractory is likely to lead the heating cycle: high heat-flux zones near burners, corners, shell penetrations, supports and zones expected to heat slowly. Identify these on an approved monitoring drawing and label each channel with its exact location, depth and orientation.

Use enough points to distinguish governing zones, but do not choose a count by rule of thumb. The thermal model and cure curve define coverage. Avoid direct flame impingement, open expansion joints and supports that short-circuit heat; protect probes from erosion where gas flow is severe.

Installing and protecting thermocouples

Embedded probes can measure the refractory body, while shell-mounted sensors primarily indicate casing temperature. Embedding requires an approved method that avoids voids, weak planes and density defects. Select thermocouple type, sheath and protective arrangement for the maximum temperature, atmosphere and service life.

Before closure, verify calibration over the working range, channel identity, polarity, insulation and cold-junction compensation. Route and secure extension leads away from sharp edges, hot surfaces and vibration, with strain relief at both ends. Protect the probe during later trades and confirm that every connection remains accessible.

Running a controlled cure schedule

A controlled dry-out normally progresses through slow moisture removal, burnout of volatiles, a material-specific densification or sintering hold, and controlled cooling. Heating rates and temperatures come from the refractory supplier and project procedures; generic values can cause condensation, steam spalling, cracking or permanent damage.

Record actual refractory temperatures at the specified interval, calculate ramp rates from successive readings and start a hold only when the governing zone reaches its minimum temperature for the full duration. Casing or ambient readings cannot prove the refractory achieved the hold. Pause escalation when an abnormal trend appears and obtain an approved disposition.

Inspecting workmanship and deviations

The API 936 inspection plan should coordinate workmanship checks with hold and witness points for materials, anchors, dimensions, density where applicable, sensor positions and records. Sensor checks do not replace visual inspection or other refractory tests; together they show whether the installed system matches the approved plan.

Record deviations such as an incorrect location, damaged lead, failed calibration, excessive ramp rate or missed hold as nonconformances for engineer review. Disposition should consider the refractory supplier's advice, repair requirements and effects on the cure. Do not relocate or conceal a point without traceable approval.

Commissioning and handover

After cure, functionally test the permanent instrumentation and compare the recorded profile with OEM acceptance criteria and operating limits. Many permanent shell thermocouples cannot reproduce the original refractory measurement, so cure evidence must remain a separate, traceable record.

Commission alarms and data logging under representative burner firing, establish an operating baseline, and issue an as-built monitoring plan with locations, serial numbers, calibration certificates, cure curves, trends and punch-list closure. This handover package lets operations distinguish a furnace condition from an installation or cure anomaly.

Training and controlled access for Australian teams

Training should explain why each ramp and hold exists, not merely ask technicians to copy numbers. A practical cooking process guide demonstrates a useful discipline for any process with irreversible timing: identify the governing condition, hold it long enough and verify completion rather than inferring success from elapsed time.

Across Perth and Gladstone, project teams often coordinate API documents with AS/NZS references, site procedures and contractor inspection plans. Australian summer ambient conditions make stable cold-junction compensation and data logging important, while remote travel and restricted site access increase the value of clear electronic records.

Download API 936 from Document Bays as an immediately accessible PDF, with multiple currencies and major payment methods supported. As a digital purchase, it avoids freight and customs entry; retain the invoice and controlled copy, confirm the edition through the project document register, and use it to brief the installation and cure team.