TL;DR
Drum level hunting is nearly always a control loop tuning issue or a faulty drum-level transmitter. Check shrink/swell behavior during load changes and re-tune the three-element control loop.
What you might see
- HP drum level oscillating or hunting above and below setpoint
- feedwater control valve cycling at high frequency
- drum level trips on high or low during load transients
- steam quality degrading with water carry-over at high level
Likely causes
Feedwater control valve PID tuning too aggressive (high gain or too little integral time) for the current drum operating pressure
Faulty or miscalibrated drum-level transmitter giving erratic signal to the controller
Swell and shrink dynamics at the drum during fast load changes confusing a single-element level controller
Feedwater supply pressure fluctuating, changing valve gain and making the tuned response wrong
Required tools
- DCS trend function (drum level, feedwater flow, valve position)
- Feedwater supply pressure instrumentation
- PID tuning access in the DCS
Safety first
- Drum water carry-over to the steam turbine can cause catastrophic water induction. If level is persistently high and uncontrolled, notify the shift supervisor immediately and reduce generation load.
- All HRSG work involving valve manipulation requires shift supervisor authorization.
Procedure
- 1
Review all three drum level transmitters (if installed) on the DCS and compare readings. Divergence between transmitters points to a faulty primary element or reference leg.[1]
- 2
Trend the feedwater control valve position, drum level, and feedwater flow simultaneously. Hunting valve at constant demand confirms the controller is oscillating, not the process.
- 3
Check feedwater supply pressure upstream of the control valve for fluctuation. A varying pressure head changes the effective valve gain.
Warning: HRSG systems operate at elevated temperature and pressure. Do not open any valve or manual bypass during a control instability event without first consulting the shift supervisor. - 4
If using a single-element control strategy, consider enabling the three-element mode (drum level + steam flow + feedwater flow) if the system supports it, since three-element is more stable during load transients.
- 5
Retune the PID parameters during a stable load period. Reduce proportional gain by 20% and observe whether oscillation frequency decreases. Iterate slowly.
- 6
After retuning, perform a step load test to validate swell and shrink response and confirm no trip conditions are reached.
Sources
Vogt Power Vogt-Nemag HRSG (Heat Recovery Steam Generator) general technical documentation, Vogt Power
HRSG drum level control, three-element feedwater control tuning, general power generation references (general)
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