Accidental Closure of the Feed-Water Control Valve: A 350MW Unit's Emergency Plan and Real-World Lessons

The feed-water control valve is the throat of the boiler feed circuit. A forum emergency-planning thread (17 replies) plus practitioners' additions give a very clear picture of handling the high-risk scenario of an accidentally or automatically closed feed-water valve.

1. The Unit

350MW; two 50% turbine-driven feed pumps plus one 30% motor-driven standby pump; the feed-water control valve sits downstream of the high-pressure heaters, maintaining the differential pressure for superheater spray water. Scenario: at full load with the valve in manual, operator error or a positioner fault closes the valve wrongly or automatically.

2. The Original Plan

1. Switch both turbine and boiler master stations to manual immediately, trip one coal mill, run the remaining two at minimum, and close the turbine control valves somewhat to cut evaporation

2. If the valve is fully or nearly closed: try to reopen it manually at once — do not yank it wide open against the huge differential (risk of water hammer); trip the second mill, start oil guns to stabilize combustion, and pull the load down fast

3. Start the motor-driven pump promptly per the turbine-pump situation — even if it merely rides along

3. Field Lessons from the Replies (worth more than the plan)

• Know your hand in advance: how much load can the unit carry with the valve fully closed and the bypass wide open? With that number fixed in mind, attention shifts to drum level and steam temperature
• A real 19-second case: on a 300MW-class unit at 300MW, "level acted 19 seconds after the valve closed; I tripped just one mill and I have never regretted it — our coal has 9% volatiles". Good coal buys you restraint; poor coal demands decisiveness
• "Trip mills firmly, but control the rate": one mill not enough, trip another — buy time for the rest of the handling
• Start the standby pump early; the turbine-pump recirculation valve should normally be open
• Protection differences: older units lack low-feedwater-flow protection — reopening the valve in time saves you; whether the flow-protection timer beats you depends on valve leakage: "ordinary control valves leak, and rarely seat fully — emergency measures may still work"
• Two sharp questions to answer beforehand: how long can the drum level hold with the valve shut? Can the valve even reopen against the full differential?
• Extreme cases: one plant actually tripped over this and survived by hand-cranking the feed-pump outlet valve while trimming pulverizer firing — the operators were commended; another plant simply removed the main feed-water control valve for good, at the cost of losing the control means

4. Takeaways for I&C Engineers

The two sources of wrong closure — human error and positioner fault — point to the same defenses: strict operating tickets for local/remote switching; catching positioner degradation early through command-vs-feedback deviation trends (see our article on acceptable deviation); and turning the emergency plan into concrete numbers — loadable MW, mill-trip timing, pump-start timing — rather than leaving it on paper.