Field Evaluation

Chapter from the Fired Heaters Design Guide.

FIRED HEATER FIELD EVALUATION

Field Evaluation Methodology

A fired heater field evaluation assesses heater performance, identifies efficiency improvement opportunities, and determines if the heater can meet current or increased duty requirements.

Data Collection Checklist:
1. Process Side: flows, inlet/outlet temperatures, operating pressure, fluid composition
2. Fuel Side: fuel gas composition (lab sample), fuel flow rate, fuel pressure
3. Stack Measurements: O2%, CO ppm, stack temperature, draft at multiple points
4. Firebox: visual inspection (flame pattern, refractory, tube condition), draft measurements
5. Skin Thermocouples: all readings, trend data
6. Convection Section: tube surface temperature (IR), fin condition
7. Design Data: original data sheet, P&ID, tube metallurgy, design duty

Key Calculations:
• Current duty: Q = m × Cp × ΔT (or enthalpy method for phase change)
• Current efficiency: indirect method from stack O2 and temperature
• Average radiant flux: duty / radiant surface area
• Excess air: from stack O2 (% EA = O2% / (21 - O2%) × 100)
• Bridgewall temperature: from energy balance or direct measurement

Common Findings and Recommendations

Finding	Frequency	Impact	Typical Recommendation	Priority
High excess air (>30%)	Very common	2-5% efficiency loss	Burner tuning, O2 trim control	High — quick win
Damaged refractory	Common	Hot casing, heat loss, safety	Repair during next turnaround	Medium
Fouled convection tubes	Common	Reduced convection duty, high stack T	Soot blowing, cleaning, fin repair	Medium-High
Tube coking (process side)	Moderate	High TMT, reduced throughput	Steam-air decoking or mechanical	High
Flame impingement	Moderate	Localized tube damage	Burner maintenance, tip replacement	High — safety
Tripped or offline burners	Occasional	Uneven heating, reduced capacity	Repair and return to service	Medium
Skin TC failures	Common	Loss of TMT monitoring	Replace during turnaround	Medium
Insufficient draft	Occasional	Poor combustion, CO formation	Stack damper adjustment, ID fan check	High

Source: Fired_Heaters_Design_Guide_v1.xlsx · sheet “Field Evaluation”

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