Why Wash-Water Quality Matters in Mist Eliminator Cleaning Systems

A mist eliminator wash system is installed to prevent fouling.

But the wash water itself can become a source of fouling.

If cleaning water contains:

• dissolved salts;
• suspended solids;
• hardness;
• process contaminants,

repeated washing can leave deposits on the separator.

The plant may believe it is cleaning the demister while gradually introducing new material that:

• blocks mesh;
• restricts vane drainage;
• increases pressure drop.

Wash-system engineering should therefore consider not only how much water reaches the separator, but also what is in that water.

Clean-Looking Water Can Contain Dissolved Minerals

Water can appear visually clear while containing significant dissolved material.

Examples include:

• calcium;
• magnesium;
• silica;
• chlorides;
• sulfates.

When wash water remains on a hot mist eliminator, part of it can evaporate.

Dissolved minerals remain behind.

Repeated wash cycles gradually build a deposit.

This is similar to scale formation in:

• heat exchangers;
• boilers;
• spray nozzles.

The demister becomes another surface where water chemistry matters.

Hard Water Can Produce Scale

Calcium and magnesium salts can precipitate under suitable:

• temperature;
• concentration;
• pH conditions.

Fine wire mesh is particularly sensitive because its passages are small.

A thin scale layer on every wire can reduce open area significantly.

Drainage becomes more difficult.

Differential pressure rises.

The cleaning system then becomes part of the plugging mechanism.

Suspended Solids Can Be Deposited Directly

Wash water may come from:

• recycled process water;
• scrubber blowdown;
• utility-water systems.

If suspended solids are present, the spray can deliver them directly onto the demister.

The wet mesh captures the particles efficiently.

Instead of removing deposits, washing adds more solids.

This can be particularly severe in recirculating wash-water systems without adequate filtration.

Recycled Process Liquid Requires Care

Using process liquid for washing can reduce fresh-water consumption.

It may also introduce:

• dissolved salts;
• reaction products;
• solids.

If the liquid becomes more concentrated over time, its cleaning effectiveness can deteriorate.

At some point, washing a crystallizing demister with near-saturated salt solution may provide little benefit.

Fresh or lower-concentration water may be needed periodically.

The correct strategy depends on process chemistry.

Chloride Content Can Affect Metal Components

Wash-water chemistry also affects corrosion.

A stainless-steel demister may be chemically compatible with the main gas service but exposed to concentrated contaminants during repeated washing and drying.

Chloride-containing wash water deserves attention where corrosion sensitivity exists.

Material selection should consider the complete environment, including:

• maintenance fluids.

Wash Water Can React With Existing Deposits

Some deposits dissolve easily in water.

Others may react.

For example, changing:

• pH;
• ionic concentration

can cause a previously dissolved species to precipitate.

Cleaning chemistry should therefore be compatible with the specific contaminant.

The assumption that “water always removes salt” is too broad.

Solubility depends on what salt is present and under what conditions.

Evaporation After Washing Is Important

Suppose the demister is washed while hot gas continues flowing.

Wash water reaches the mesh.

Part drains away.

Part evaporates quickly.

Any nonvolatile material in that remaining fraction stays on the separator.

Repeated online washing can therefore concentrate contaminants directly on the wires.

The cleaner the wash water, the lower this risk.

Nozzle Plugging Is Another Consequence

Poor wash-water quality affects not only the demister.

Small wash nozzles can become plugged by:

• scale;
• suspended solids.

Spray distribution becomes uneven.

One region receives adequate cleaning.

Another receives almost none.

Fouling becomes nonuniform.

The demister may then show localized pressure-drop or carryover problems even though the wash system is operating.

Nozzle condition should therefore be inspected as part of demister maintenance.

Filtration Can Improve Wash-System Reliability

Where recycled water contains solids, filtration may reduce deposition and nozzle blockage.

The required filtration level depends on:

• nozzle size;
• contaminant type;
• desired cleaning quality.

The objective is not necessarily ultra-pure water.

It is water that does not introduce more fouling than the wash system removes.

Conductivity Can Provide a Useful Trend

In some salt-bearing systems, water conductivity can provide a simple indicator of dissolved-ion concentration.

A rising conductivity trend in recycled wash water may indicate increasing salt concentration.

This does not identify the exact chemistry, but it can help operators recognize when wash water is becoming increasingly contaminated.

More detailed chemistry analysis may be needed for critical systems.

Wash Temperature Can Affect Cleaning

Warm water may dissolve some deposits better.

But higher temperature can also influence:

• precipitation;
• corrosion;
• evaporation.

Wash temperature should therefore be selected based on:

• deposit chemistry;
• separator material;
• operating conditions.

Hotter is not automatically better.

After Washing, Flush and Drain Matter

If cleaning uses a chemical solution, a final rinse may be required.

Otherwise, cleaning chemicals remain on the separator and enter the next operating cycle.

The system should provide sufficient drainage after washing.

Standing contaminated liquid can:

• concentrate;
• crystallize

as the separator dries.

How to Diagnose Wash-Water-Induced Fouling

Clues include:

• deposits appearing soon after wash-system installation;
• scale concentrated near spray impact regions;
• clogged nozzles;
• demister fouling despite frequent washing.

Analyze both:

• demister deposit;
• wash water.

If their composition is similar, the cleaning system may be contributing to the problem.

Wash Strategy Should Be Evaluated by Net Result

A successful cleaning system should produce:

• slower DP increase;
• longer operating run;
• reduced fouling.

If frequent washing does not improve these trends, simply increasing wash frequency may make the problem worse.

The water source, chemistry, spray coverage, and drainage should be reviewed.

Final Engineering Perspective

Mist eliminator washing is a material-transfer process.

The water removes contamination—but it can also introduce contamination.

Water hardness, dissolved salts, suspended solids, chemistry, filtration, temperature, and drainage all influence the net cleaning result.

The best wash system therefore uses not merely enough water, but water of suitable quality for the deposit and separator material.