Engineering Evaluation Case: The Scrubber Meets the Gas-Removal Target but the Downstream Fan Keeps Corroding
A wet scrubber may meet its measured gas-phase outlet requirement while downstream equipment still corrodes rapidly.
This can appear contradictory.
If the scrubber is removing the acid gas, why is the fan failing?
One possible explanation is that the fan is not being attacked primarily by residual gas.
It is being attacked by liquid droplets carried out of the tower.
Project Situation
Consider an acid-gas scrubber with an induced-draft fan downstream.
Gas analysis shows acceptable contaminant removal.
Yet maintenance reports:
- fan blade corrosion;
- wet deposits inside the fan housing;
- frequent coating repair;
- liquid in the outlet duct.
The first response is often:
“We need better acid absorption.”
The absorber may already be doing its job.
The mist-elimination system should be investigated.
Absorbed Acid Moves into the Liquid Phase
A successful wet scrubber intentionally transfers acid gas into the circulating liquid.
That liquid can therefore contain:
- acid;
- salts;
- reaction products.
If droplets leave the tower, the pollutant has not disappeared from the equipment train.
It has simply left in a different phase.
Stack Gas Analysis May Not Fully Reveal Droplet Exposure
Depending on the sampling method, gas-phase concentration and liquid aerosol may be measured differently.
A system can meet a gas-phase HCl or SO₂ target while still sending corrosive droplets toward the fan.
The performance specification should distinguish:
- gas removal;
- mist carryover.
The Demister May Be Overloaded
Possible causes include:
- gas velocity above design;
- excessive liquid entrainment from packing;
- high liquid circulation;
- fouled mesh;
- damaged or missing sections.
The separator may be functioning normally for its original design but operating outside its current duty.
Wall Bypass Is an Important Failure Mode
Gas can pass around the demister instead of through it when:
- perimeter sealing is poor;
- mesh does not fit the shell;
- sections separate.
This bypass gas can carry large droplets directly downstream.
Replacing the mesh with denser material will not solve a perimeter gap.
Drainage Can Cause Re-Entrainment
Captured liquid must drain against the gas flow.
If drainage is restricted, liquid accumulates inside the separator.
Gas then strips the retained liquid back into the outlet.
A separator can therefore have good clean collection efficiency but poor real performance because its drainage is failing.
Fan Location Makes the Consequence Severe
With the fan downstream, every escaped droplet reaches:
- fan inlet;
- impeller;
- casing.
High tip velocity spreads the liquid across metal surfaces.
Repeated wetting with acidic or saline droplets can accelerate:
- corrosion;
- coating damage.
Condensation Must Be Distinguished from Carryover
The fan may also receive condensate created after the demister if saturated gas cools in the outlet duct.
Therefore, inspect:
- gas temperature;
- dew point;
- duct wall temperature.
The fan corrosion can result from:
- true demister carryover;
- downstream condensation;
- both.
Deposit Analysis Can Identify the Liquid Source
If fan deposits contain the same:
- sodium;
- chloride;
- sulfate;
- other salts
as the scrubber liquor, liquid carryover becomes a strong possibility.
This is often more useful than visually inspecting the fan alone.
Higher-Efficiency Demister Is Not Always the First Solution
Before changing technology, confirm:
- actual gas flow;
- separator area;
- liquid loading;
- drainage;
- wall sealing;
- fouling.
A correctly repaired existing separator may solve the problem without a major tower retrofit.
Materials Still Matter
Even with good mist removal, some residual humidity or contamination may remain.
The fan material or coating should be appropriate for the defined outlet gas condition.
But upgrading the fan material without controlling heavy carryover can create unnecessary recurring cost.
Monitor Both Tower and Fan
Useful trends include:
- packing differential pressure;
- demister differential pressure;
- outlet duct drainage;
- fan corrosion pattern;
- actual airflow.
A sudden fan-corrosion increase after tower throughput rises can reveal a velocity-related separator problem.
Engineering Takeaway
Wet-scrubber performance does not end at gas absorption.
If the captured contaminant leaves as droplets, downstream equipment can still experience severe exposure.