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Furnace Tube Skin Temperature - Radiant Section

Dittus-Boelert · API 530

Fired heater · skin temp prediction via Dittus-Boelert correlation

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TimestampT1 MeasT2 MeasT3 MeasT4 MeasProcess TFire (MW)Flow (kg/hr)HottestResidualStatus
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Dittus-Boelert Correlation

Tube skin temperature is predicted using the Dittus-Boelert forced-convection heat transfer correlation, a standard in fired heater design (API 560, API 530).

Delta_T = k * (Fire / Fire_ref)^0.8 * (Flow_ref / Flow)^0.8

where:

  • k = tube-specific offset (K) accounting for radiant position (T1 bottom 80K, T2 65K, T3 50K, T4 top 35K)
  • Fire = current firing rate (MW)
  • Fire_ref = reference firing rate (25 MW default)
  • Flow = current process flow (kg/hr)
  • Flow_ref = reference flow (80,000 kg/hr default)

Predicted skin temp = Process outlet + Delta_T

Limits (API 530, ASME PTC-4)

Advisory: 446°C - investigate; no immediate action.

Alarm (PASCC): 475°C - process automatic shutdown cascade; action required.

Trip: 550°C - emergency shutdown; system protection.

Alert for residual >5 K signals potential correlation drift or sensor error.

Scope

Reference-grade PoC. Not a control system, not SIS. Not a substitute for a formal API 530 thermal design acceptance test.

Engineering Manual - Furnace Skin Temperature

1. Purpose & Scope

Radiant section tube skin temperature monitoring and prediction for fired heaters using Dittus-Boelert forced-convection correlation per API 560 and API 530.

2. Key Standards

API 560 - Fired Heaters for Refinery Service. Thermal design guidelines for radiant section duty and skin temp limits.

API 530 - Tubular Heaters with Cylindrical Furnaces. Operational limits: advisory 446°C, alarm 475°C (PASCC), trip 550°C.

ASME PTC-4 - Performance Test Code on Indirect Heat Transfer Equipment. Methodology for thermal acceptance tests.

3. Input Tags (6 variables)

Timestamp, Skin T1, Skin T2, Skin T3, Skin T4 (measured °C), Process outlet T (°C), Firing rate (MW), Process flow (kg/hr).

4. Correlation & Calculations

Delta_T_i = k_i * (Fire/Fire_ref)^0.8 * (Flow_ref/Flow)^0.8

T_predicted = T_process + Delta_T

k_i: T1=80K (bottom, hottest), T2=65K, T3=50K, T4=35K (top, coolest).

Residual = T_measured - T_predicted. Alert if |residual| > 5 K.

5. Alert Tiers

Watch: Any residual >5 K (measurement drift or correlation deviation).

Advisory: Any T_measured >= 446°C.

Alarm: Any T_measured >= 475°C (PASCC triggers).

Trip: Any T_measured >= 550°C (emergency shutdown).

6. Assumptions

Correlation exponents fixed at 0.8 (valid for radiant furnaces). Process outlet temp forward-filled if missing (4-hour window). Default fire ref=25 MW, flow ref=80,000 kg/hr.

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