Performance Monitoring
Performance monitoring is the systematic tracking of key parameters over time to detect degradation before it becomes a safety event or a production problem. Thermal efficiency tells you where you are; performance monitoring tells you where you are heading.
The six core performance KPIs
These six parameters define heater health. Each is readable from the DCS or field instrumentation. Together they give a complete picture of combustion quality, process performance, and equipment condition. Values shown are representative typical targets.
Stack temperature - reading the trend
Stack temperature is the most accessible indicator of heater performance. Every 20C rise above a clean-baseline costs approximately 1% in thermal efficiency. A heater that drifts from 250C to 310C over six months has lost roughly 3% efficiency.
| Temperature vs. Baseline | Condition | Likely Cause | Action |
|---|---|---|---|
| Within plus/minus 10C | Normal | Operating at design conditions | Continue monitoring at scheduled frequency |
| +10 to +20C | Monitor closely | Early convection fouling; excess air increase; soot build-up | Increase check frequency; schedule sootblowing; verify stack O2 |
| Greater than +20C | Action required | Significant convection fouling; air ingress; refractory damage | Sootblow immediately; investigate air ingress; plan outage if trend continues |
| Sudden drop >30C | Investigate | Firing rate reduction; draft change; partial flame loss; instrument fault | Check firing rate, flame condition, and analyser calibration |
O2 and CO - reading combustion health together
Stack O2 and CO must always be read as a pair. Neither parameter alone gives the complete picture. The four diagnostic combinations below cover every combustion condition you are likely to encounter.
| O2 Reading | CO Reading | Combustion State | Operator Action |
|---|---|---|---|
| 2-4% O2 | <100 ppm CO | Optimal. Complete combustion at correct excess air. | No action. Maintain and log. |
| >5% O2 | <100 ppm CO | Too much excess air. Combustion complete but stack heat loss elevated. Efficiency penalty. | Trim air registers. Move in 0.5% O2 steps. Monitor CO after each adjustment. |
| <1% O2 | >200 ppm CO | Insufficient air. Incomplete combustion. Unburned fuel in flue gas - serious hazard. | Increase air immediately. Open air registers. Check for burner blockage. Add air before reducing firing rate. |
| 2-4% O2 | >200 ppm CO | Poor mixing. Total air adequate but distribution wrong. Burner or register fault. | Inspect burners at peephole. Check register positions. Suspect damaged tip or blocked port. |
Pass balance monitoring
Most process heaters have multiple parallel passes. Each should carry roughly equal flow and exit at the same coil outlet temperature (COT). When passes are out of balance, the hot pass has elevated tube metal temperatures - increasing tube failure risk. A spread greater than 10-15C between hottest and coolest pass is the threshold for investigation.
Recognising fouling progression
Internal tube fouling (coking) and external convection fouling (soot, scale) both reduce heat transfer and force higher firing rates to hold COT. Early indicators appear in the DCS well before the situation becomes critical.
What to check and when
(2-4 hrs)
Quick efficiency estimate - the field method
A simplified stack loss method using only stack temperature and O2 gives an estimate accurate to plus or minus 2-3% for daily trend tracking.
O2 = 2% K = 0.028
O2 = 3% K = 0.031
O2 = 5% K = 0.037
O2 = 7% K = 0.044
Thermal Efficiency = 100% - Stack Loss% - 3% (radiation / convection assumed)
Example: T_stack = 280C, T_ambient = 20C, O2 = 3%
Stack Loss = 260 x 0.031 = 8.1% Efficiency = 88.9%
Trusting your instruments - and knowing when not to
Fired heater instruments operate in a harsh environment and will drift or fail. A reading that contradicts two other instruments is usually the wrong one.
| Instrument | Common Fault | How to Spot It | Response |
|---|---|---|---|
| Stack O2 analyser | Cell degradation; sample line blockage; air ingress at probe | O2 high while flame looks rich; reading unresponsive to air register changes | Cross-check with portable analyser. Blocked sample line typically reads high. |
| Thermocouple (COT / stack) | Drift high (oxidised junction); open circuit | Diverging from adjacent TCs; sudden jump to full scale or zero | Cross-check with adjacent TC. Replace if open circuit. |
| Firebox pressure (draft) | Impulse line blocked with soot or condensate | Reading stuck at one value; unresponsive to damper changes | Purge impulse line. Cross-check with portable manometer at peephole port. |
| Tube metal temp (IR / TC) | View port fouling (IR pyrometer); TC pullout | Uniformly low readings; reading lower than COT (physically impossible) | Clean view port. Verify TC seating. Do not dismiss low TMT readings. |
Summary
Three habits define effective performance monitoring: read O2 and CO together at every round; trend stack temperature and efficiency weekly with the numbers written down; treat any instrument reading that contradicts the others as suspicious until confirmed. The log is the tool. The trend is the story.