Skin Thermocouples

Chapter from the Fired Heaters Design Guide.

SKIN THERMOCOUPLE INSTALLATION & INTERPRETATION

Skin TC Purpose and Types

Skin thermocouples (skin TCs) measure the outer wall temperature of heater tubes to monitor tube metal temperature (TMT) and detect overheating.

Types:
• Welded pad: TC junction welded to a pad that is welded to tube — most common, reliable
• Clamp-on: mechanical clamp holding TC to tube — easy to install but less accurate
• Peened: TC junction peened into a drilled hole — good contact but more installation effort

Standard TC Type: Type K (Chromel-Alumel) for most refinery heaters (range to 2,280°F)
Type N: preferred for long-term accuracy above 1,000°F (less drift than Type K)

Number and Placement:
• Per API 560: minimum of 2 per pass at the highest flux zone
• Typically 6-12 per heater, concentrated in the hottest zone
• Radiant section: middle and lower (highest flux area)
• Return bends: critical for horizontal tube heaters (highest TMT)
• Convection: first 2-3 rows (shield section) if operating near metallurgical limits

Interpretation Guidelines

Condition	Reading Pattern	Likely Cause	Action	Urgency
Normal operation	Stable, within design TMT	Expected operation	Monitor	Routine
Gradual TMT increase (all)	All skin TCs rising uniformly	Fouling/coking inside tubes	Plan cleaning, check flux	Monitor — plan maintenance
One TC high, others normal	Single TC significantly above others	Local flame impingement or coke deposit	Investigate burner, check tube	High — could be localized damage
TMT oscillating	Cycling 50-100°F periodically	Two-phase flow instability, slug flow	Check flow distribution, fluid conditions	Moderate — investigate cause
TMT suddenly drops	Rapid decrease in one TC	TC failure, bond failure, or eruption of coke	Verify with IR gun, replace if failed	Investigate — may indicate TC issue
All TMTs above design	All readings above maximum allowable	Overfiring, low flow, high fouling	Reduce firing, increase flow	HIGH — risk of tube failure

Source: Fired_Heaters_Design_Guide_v1.xlsx · sheet “Skin Thermocouples”

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