TL;DR
Rising differential pressure across the mist eliminator means gypsum scale is blocking the chevron blades. Verify that wash nozzles are firing on schedule and not plugged. Plan a high-pressure water wash during the next outage.
What you might see
- differential pressure across mist eliminator rising
- slurry droplets visible in stack or duct downstream of absorber
- mist eliminator wash cycles completing but not reducing pressure
- gypsum deposits visible on mist eliminator blades at inspection
Likely causes
Wash nozzle plugging preventing the mist eliminator from being cleaned between cycles
Wash cycle frequency too low for the gypsum carry-over load
Slurry mist loading higher than design due to over-spray from absorber sprays
Slurry density too high, causing sticky deposits on the chevron blades
Required tools
- DCS wash cycle log and differential pressure trend
- High-pressure water lance for manual cleaning
- Confined-space entry equipment and SO2 gas monitor
Safety first
- Mist eliminator inspection and cleaning requires confined-space entry procedures with continuous SO2 monitoring.
- Wear chemical-resistant PPE during all slurry contact. Gypsum deposits are abrasive and the wash water is mildly acidic.
Procedure
- 1
Confirm that mist eliminator wash cycles are executing on schedule by reviewing the DCS wash cycle timer log.[1]
- 2
Increase the wash cycle frequency temporarily (from once per hour to once per 30 minutes, or per OEM guidance) and observe whether differential pressure decreases.
Warning: Increasing wash frequency adds water to the absorber. Monitor absorber level and slurry density to avoid diluting the reagent below effective operating range. - 3
During a scheduled minor outage or low-load window, inspect the wash nozzles from the access platform. Plugged nozzles will not spray and should be removed, cleaned with warm water, and reinstalled.
- 4
If differential pressure is critically high and wash cycles are not reducing it, request a manual high-pressure water wash during a planned unit outage.
- 5
During the outage, direct a high-pressure water lance at the mist eliminator chevron blades from top to bottom. Do not exceed the pressure recommended by the mist eliminator manufacturer to avoid blade distortion.
Warning: Confined-space entry with SO2 gas-testing is required before entering the absorber. Wear chemical-resistant PPE and use a supplied-air respirator. - 6
Return to service after confirming differential pressure is back within the normal band.
Sources
Mitsubishi-Hitachi Power MHPS Wet FGD FGD (Flue Gas Desulfurization) general technical documentation, Mitsubishi-Hitachi Power
Wet FGD mist eliminator maintenance and wash system troubleshooting, general power plant emission control references (general)
More guides for Mitsubishi-Hitachi Power MHPS Wet FGD
How to remove gypsum scaling in the absorber on a Mitsubishi-Hitachi Power MHPS Wet FGD
Gypsum scaling in the absorber comes from slurry density running too high or oxidation air being insufficient. Reduce slurry density to the design band and verify oxidation air flow before attempting mechanical cleaning.
How to restore low SO2 removal efficiency on a Mitsubishi-Hitachi Power MHPS Wet FGD
SO2 removal falling short of the permit target is usually a slurry pH problem from poor limestone quality or a blocked spray level. Check limestone reactivity and slurry pH first.
Stop fixing the same fault twice.
Dovient turns guides like this into your team's shared playbook, with AI that catches recurring issues before they break the line.