Feedwater and drum level instability: the failure signature
Low-water cut-offs and high-level alarms are safety events, but the cause is usually visible minutes earlier in the relationship between steam flow, feedwater flow and level. A level that dips when steam demand jumps behaves differently from a level that falls because feedwater cannot keep up, and both differ from a level transmitter that is simply wrong.
Illustrative pattern · simulated, not plant data
Diagnose this with AI
Try Free Boiler Diagnostic Upload a trend export (CSV or Excel) or a screenshot of the trend around the event. InduQT lines up which signal moved first and compares it with these variants.
Where it shows up
- Low-water alarm or low-water cut-off trip
- High-level alarm, carryover or wet steam complaints
- Feedwater valve hunting or pump cycling
Signals to trend
| Signal | What it does in this failure |
|---|---|
| Drum / water level | The symptom. Note whether it swings around setpoint, drifts away from it, or jumps. |
| Steam flow | A level swing that starts with a steam-flow step is shrink and swell; the level control must ride through it. |
| Feedwater flow | If feedwater flow is at its maximum while level falls, supply is the limit; if it oscillates with level, the loop is the problem. |
| Steam pressure | A sudden pressure drop makes level swell briefly, then shrink: a common cause of nuisance level alarms. |
| Firing rate | Large firing-rate changes have the same effect on level as pressure changes. |
Variants: what moves first, and what it points to
1Shrink and swell on load changes
- Points to it
- Level rises briefly when steam demand jumps, then falls below setpoint; feedwater responds late.
- Check
- Review the level control strategy. The DOE sourcebook notes that two- and three-element control (adding steam flow and feedwater flow) handles abrupt load changes better than level-only control.
2Feedwater supply shortfall
- Points to it
- Feedwater valve near fully open and flow flat while level falls; deaerator level or feed pump pressure low.
- Check
- Check feed pump discharge pressure, deaerator level and valve capacity during the event.
3Hunting level loop
- Points to it
- Level and feedwater flow oscillate together at steady load.
- Check
- Have the level loop tuned; check the feedwater valve for stiction.
4Level measurement fault
- Points to it
- Transmitter disagrees with the gauge glass, or the level jumps with no change in flows.
- Check
- Compare with the gauge glass and blow down the level column per site procedure.
How to read the trend
- Plot the three flowsTrend level, steam flow and feedwater flow together for the event.
- Find the triggerDid steam flow or pressure step first, or did feedwater flow stop following demand?
- Check the measurementConfirm the transmitter against the gauge glass before tuning anything.
- Act on the right partSupply problem: pump, deaerator, valve. Control problem: strategy and tuning. Measurement problem: level column and transmitter.
Evidence to collect
- Trend of drum level, steam flow and feedwater flow
- Level control type (single, two or three element)
- Feed pump and deaerator data
- Gauge glass vs transmitter readings
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