An FGD scrubber running at 85% efficiency instead of its designed 98% doesn't announce itself with an alarm. Scale builds in the spray headers, pH drifts a tenth of a point at a time, and outlet SO2 climbs quietly while every individual reading still looks close enough to normal. The daily walk exists to catch what a permit violation notice would otherwise catch for you. Sign up to log density, pH, and mist eliminator condition every shift and connect any drift straight to a work order.
Why It Matters
Slurry density affects pump performance, spray quality, dewatering, and absorber scaling — it should be controlled, not guessed. pH swings toward acidic conditions accelerate corrosion, while swings toward alkaline cut SO2 capture. Neither shows up as a single dramatic reading; both show up as a slow drift that a consistent daily walk is built to catch.
The Walk In Three Stops
Absorber Chemistry
Check slurry pH and density against target range, and confirm the reading is stable, not swinging shift to shift.
Pump & Recirculation
Walk each recirculation pump for vibration and seal condition, and confirm spray header flow and pattern look normal at the sight glass.
Mist Eliminator & Log
Check mist eliminator differential pressure and wash coverage, then log every reading with anything outside range flagged for the next shift.
The Daily Walk Checklist
Record absorber slurry pH against target range: most wet limestone systems target roughly 5.0 to 5.8 — drift toward acidic accelerates corrosion, drift toward alkaline cuts SO2 capture, and both matter equally
Check slurry density against target: density that drifts off target changes pump performance, spray quality, and absorber scaling all at once, so it deserves its own reading rather than being inferred from pump behavior alone
Inspect recirculation pump vibration and seal condition: impeller wear on a recirculation pump reduces slurry flow rate and cuts the liquid-to-gas ratio below design spec without triggering a single alarm
Verify spray header flow and nozzle pattern: blocked nozzles reduce gas-liquid contact area directly, cutting SO2 absorption efficiency well before outlet emissions data confirms it
Check mist eliminator differential pressure and wash coverage: scale buildup on the eliminator raises stack moisture carryover and can cause downstream duct corrosion — this is widely considered the most neglected component in FGD systems
Every Reading Trended, Every Drift Actioned Before A Violation
Oxmaint logs pH, density, pump condition, and mist eliminator readings against target range for every absorber, and raises a work order the moment a reading drifts out of compliance range. Sign up for a free trial to run it on your own FGD system, or book a demo to see it configured for your absorbers.
Why The Mist Eliminator Gets Overlooked
Every operator watches pH and density closely because they're control loop parameters displayed on the DCS. The mist eliminator sits downstream, doesn't drive a control setpoint, and typically only gets a formal visual inspection at planned outages — which is exactly why it becomes the most expensive secondary failure in most FGD audits. Scale accumulates on the eliminator blades gradually, raising differential pressure and letting droplet carryover increase long before anyone traces a downstream duct corrosion problem back to it. A daily differential pressure check and a look at wash coverage costs a few minutes and catches the fouling while a wash cycle can still fix it, instead of waiting for the next shutdown to find hardened scale.
What A Drifting Reading Usually Means
| Check Point |
Early Warning Sign |
Likely Cause |
| Slurry pH |
Drift below target range without load change |
Limestone feed rate misalignment |
| Recirculation pump |
Vibration trending up, flow trending down |
Impeller wear reducing L/G ratio |
| Spray nozzle pattern |
Uneven coverage or reduced flow at sight glass |
Scale or gypsum buildup blocking nozzles |
| Mist eliminator DP |
Rising differential pressure between washes |
Scale fouling on eliminator blades |
Frequently Asked Questions
Q
Why does pH matter this much if outlet SO2 still looks compliant?
Outlet SO2 can stay within permit while pH is already drifting, because the absorption chemistry has some margin before it shows up in emissions data. By the time outlet SO2 confirms a problem, the pH drift has usually been developing for hours or days already.
Q
Is a daily mist eliminator check really necessary if it's inspected every outage?
Yes — a formal visual inspection at outage catches what already failed, while a daily differential pressure check catches fouling early enough that a wash cycle can still clear it. Waiting for the outage means the scale has usually hardened past the point a wash alone can fix.
Q
Does a UK power station need a digital record of the FGD daily walk?
A digital, timestamped record gives a far stronger environmental compliance trail than a paper log and, more usefully, it's what connects a drifting pH or density reading straight to a work order instead of leaving it noted in a logbook until someone reviews it.
Catch The Drift Before It Reaches The Stack
Oxmaint tracks slurry pH, density, recirculation pump condition, and mist eliminator readings against target range, and turns a drifting number into a work order before it becomes a permit issue. Sign up for a free trial to build your own FGD daily walk checklist, or book a demo to see it configured for your absorber.
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