03 · Abnormal Operations
Loss of Feed
Loss of process fluid flow is one of the primary causes of tube failure in fired heaters. This page covers recognition, immediate response, trip criteria, and recovery — for operators responding to a loss of feed event.
Life Safety — Act Immediately
Loss of feed with continued firing is a leading cause of tube rupture and firebox fire.
At full fired duty, tube overtemperature can occur within minutes of flow loss.
Response time is critical — reduce firing before investigating the cause.
What Is Loss of Feed?
Loss of feed refers to any significant reduction or total interruption of process fluid flow through the heater tubes. It ranges from complete flow stoppage to partial reductions affecting one or more passes. Each type carries different risk profiles and response priorities.
Severity: Critical
Full Loss of Feed
Total flow stoppage to all passes. No process fluid moving through any tube.
Heat removal ceases entirely. Tube metal temperature rises rapidly.
Causes: pump trip, upstream block valve closed, suction loss
Severity: High
Partial Flow Reduction
Flow falls below safe minimum across all passes. Heat flux per unit of flow increases.
Film temperature rises even though flow is still occurring.
Causes: control valve fails closed, partial blockage, low tank level
Severity: High
Pass Imbalance
One or more passes running low while others appear normal. May not trigger a
total feed alarm. The low-flow pass is at risk of overtemperature.
Causes: pass control valve fault, fouling in one pass, restriction
Why It's Dangerous
The fundamental hazard is the mismatch between heat input and heat removal. The burners continue to fire at the same rate — releasing heat into the firebox — while the process fluid that normally absorbs that heat is no longer flowing. The tube wall is left to absorb the excess heat with no cooling.
Failure mechanism — the thermal chain
Tube metal temperature (TMT) = process fluid temperature + (heat flux × total thermal resistance). When flow stops, the thermal resistance term dominates and TMT rises rapidly.
Flow stops
→
Velocity drops
→
Film coefficient collapses
→
Thermal resistance spikes
→
TMT exceeds design limit
→
Tube rupture / fire
Time-Critical: Minutes Matter
At full fired duty, complete loss of feed can cause tube overtemperature within 2–5 minutes.
Do not wait for a high TMT alarm before acting. Reduce firing on first confirmed indication of flow loss.
Early Warning Indicators
These indicators should be monitored together. No single reading is definitive — correlate across instruments before concluding the cause.
Loss of Feed — Indicator Reference
| Indicator | What You See | Significance |
|---|---|---|
| Coil Outlet Temperature (COT) | Rising rapidly | Less fluid absorbing the same heat — first measurable sign |
| Pass Flow Meters | Low / zero / alarm | Direct measurement — confirm all passes, not just totals |
| Tube Metal Temperature (TMT) | Rising toward / beyond limit | If rising with low flow, tube is losing heat removal capacity |
| Pass Differential Pressure (ΔP) | Drops on low-flow pass | Useful for detecting pass imbalance when total flow looks normal |
| Feed Pump Discharge Pressure | Drop or loss of header pressure | Indicates pump trip or suction problem upstream |
| Fuel Gas Consumption | Unchanged | Continued firing with rising COT confirms heat absorption loss |
Immediate Operator Response
Reduce Firing First — Then Investigate
Do not spend time diagnosing the cause while the heater continues at full fire.
Cut fuel gas to minimum hold immediately. Every second of continued full firing
with no flow adds thermal damage to the tubes.
Loss of Feed — Immediate Response
01
Reduce fuel gas to minimum hold immediately
Cut firing to the minimum stable hold rate without delay. Do not wait for confirmation
of the cause. If the alarm is false, you can restore; if it is real, delay causes tube damage.
02
Confirm loss of feed — check individual pass flow indicators
Verify the flow on each pass independently. Distinguish between full loss (all passes low)
and pass imbalance (one or more passes low while others appear normal).
03
Notify shift supervisor immediately
Alert your supervisor and process team. Loss of feed is a condition requiring
immediate management oversight and potential escalation.
04
Identify and attempt to restore feed
Investigate the most likely cause: check feed pump status (running / tripped),
check upstream/downstream block valves (open / accidentally closed), check
pass control valve positions. Attempt restoration only with supervisor authority.
05
Trip the heater if flow is not restored within ~2 minutes
Do not hold at minimum fire indefinitely waiting for feed. If the cause cannot be
identified and corrected quickly, initiate a heater trip. See trip criteria below.
06
Trip immediately if TMT is rising toward design limit
If tube metal temperatures are rising and approaching their design maximum, do not
wait for the 2-minute window — trip without delay. See Operating Limits for TMT
limits by tube material.
07
Monitor COT and TMT continuously throughout
Even at reduced fire, COT and TMT must be tracked until flow is confirmed restored
and stable. If either continues to rise at minimum fire, trip immediately.
08
Document the event timeline
Record times of: initial alarm, firing reduction, cause identified, feed restored
(or heater tripped). This log is required for post-event investigation.
When to Trip the Heater
Trip Without Delay — Do Not Wait for More Alarms
Any one of the conditions below is sufficient reason to trip. You do not need all
conditions to be met simultaneously. When in doubt, trip the heater.
01
Complete flow loss to any pass
Zero or near-zero flow confirmed on any individual pass. One dry pass at full fire
is sufficient for rapid TMT escalation.
02
Flow below minimum design rate
Total or per-pass flow below the minimum safe operating velocity specified in
site procedures. Typically < 25–30% of design rate.
03
TMT approaching design limit
Tube metal temperature rising toward or exceeding the design maximum for the
tube material and service. See Operating Limits page for material-specific limits.
04
Cause unknown after ~2 minutes
Feed not restored and cause of loss cannot be identified and corrected within
site-defined hold period. Do not continue to hold at minimum fire indefinitely.
Recovery After Feed Restoration
Do Not Re-Fire Before Verifying Tube Condition
If the heater was tripped during the loss of feed event, do not re-light until
tube condition has been assessed. Tubes exposed to overtemperature may have
sustained creep damage that is not immediately visible.
Post-Event Recovery
01
Confirm feed is fully restored and stable on all passes
Verify flow on each pass individually. Do not rely on total feed flow alone.
Pass balance must be confirmed before any attempt to re-fire.
02
Review TMT readings — confirm all within normal range
Check all tube metal temperature readings. If any remain elevated after flow
has been restored, do not re-fire — consult site engineering or inspection team.
03
Visually inspect tubes through peepsites for damage
Look for tube bowing, localised bulging, discolouration, or scale loss.
Any visible deformation is a mandatory stop — do not re-fire.
Engage inspection team before proceeding.
04
Identify and rectify the root cause
Do not re-fire until the cause of the loss of feed has been confirmed and corrected.
If the cause is unknown, escalate to engineering before restart.
05
Obtain supervisor authority and re-fire per startup procedure
All re-fire decisions after a loss of feed event require shift supervisor
or site authority. Follow the standard startup procedure — do not shortcut the
purge and pre-ignition sequence.
06
Complete incident report and post-event review
A formal incident report is required for all loss of feed events, whether or not
tube damage occurred. Include timeline, cause, response actions, and duration
of reduced/zero flow.
Controlled Turndown vs. Uncontrolled Loss
A planned low-flow turndown during a process rate reduction is fundamentally different from an uncontrolled loss. Understanding this distinction helps operators calibrate the urgency of their response.
| Factor | Controlled Turndown | Uncontrolled Loss |
|---|---|---|
| Firing level | Reduced proportionally with flow | Unchanged at time of flow loss |
| Heat flux per unit flow | Maintained at design ratio | Spikes — heat input exceeds fluid capacity |
| Minimum velocity | Maintained above design minimum | May drop below minimum instantly |
| Advance notice | Planned — operator prepared | No notice — response must be immediate |
| Operator response | Monitor and maintain heat balance | Emergency — reduce firing immediately |
| Tube risk | Low if procedure followed correctly | High — escalating risk with time |
Minimum Flow Velocity
Each heater has a minimum process fluid velocity specified in design documentation —
below this velocity, film boiling or coke deposition risk increases significantly.
This threshold, not just total flow volume, defines safe low-flow operation.
Confirm the site-specific minimum with your engineering team.