Why Direct Wash-Nozzle Impingement Can Turn a Demister Wash System Into a Mist Source

A mist eliminator wash system is intended to reduce fouling.

Water or cleaning liquid is sprayed onto the separator to remove:

• salts;
• slurry;
• soluble deposits;
• loose solids.

But washing can create an unexpected problem.

If wash nozzles are positioned incorrectly or operated at excessive pressure, the cleaning spray itself can become a new source of mist.

Instead of simply washing deposits downward, the nozzles may:

• atomize liquid into fine droplets;
• strike the separator aggressively;
• splash liquid back into the gas;
• temporarily overload separator drainage.

The plant can therefore experience increased downstream carryover during or immediately after washing, even though the wash system was installed to improve demister performance.

A Wash System Adds Liquid to the Separation Duty

During normal operation, the mist eliminator already handles process entrainment.

When washing begins, additional liquid is introduced.

The total separator load becomes:

Ltotal=Lprocess+LwashL_{total}=L_{process}+L_{wash}

If the demister drainage system has limited spare capacity, wash liquid can push the separator into temporary hydraulic overload.

The problem is especially severe when the separator is already partially fouled.

Fouling has already reduced:

• open area;
• drainage capacity.

The wash cycle then adds extra liquid at exactly the time when drainage is weakest.

Nozzle Impact Can Break Liquid Into Smaller Droplets

A spray nozzle converts liquid pressure into velocity.

Depending on:

• nozzle type;
• pressure;
• orifice;
• spray pattern,

the wash water can form droplets ranging from relatively coarse to very fine.

If the wash system creates a large fraction of fine droplets, some can be carried directly into the gas.

The wash system has now created a new mist population.

This is particularly important if the nozzles spray:

• upward;
• across the gas stream;
• toward an exposed downstream surface.

Cleaning water should ideally reach the separator surface without unnecessarily generating an aerosol that is difficult to recapture.

Direct High-Energy Impact Can Cause Splashing

Suppose a high-pressure jet strikes a vane blade or support bar directly.

The liquid does not simply form a calm film.

It can:

• splash;
• rebound;
• break into secondary droplets.

Those droplets enter the gas.

If the nozzle is located on the downstream side of the active separator, the newly generated droplets may have no second collection stage.

They travel directly toward the outlet.

A plant may therefore observe a short but significant spike in downstream liquid whenever washing occurs.

Wire Mesh Can Become Temporarily Saturated

Wash water penetrates a wire mesh pad.

If the applied wash rate is greater than the rate at which liquid drains away, the pad becomes increasingly wet.

Void space available for gas decreases.

The gas is forced through the remaining open passages.

Local velocity rises.

This can strip liquid from the mesh.

The wash water is now being re-entrained.

Increasing wash intensity beyond the drainage capacity can therefore reduce rather than improve separation during the wash cycle.

Vane Packs Can Also Be Overloaded

Vane systems generally handle higher liquid loading than dense mesh.

But their:

• hooks;
• drainage pockets;
• channels

still have finite capacity.

If wash nozzles fill these drainage features faster than they empty, the collected liquid can overflow into the main gas passages.

Gas then strips liquid from the blade surfaces.

Even a well-designed vane separator can temporarily lose performance if the wash system ignores its drainage geometry.

Spray Direction Matters

A wash nozzle should not automatically be aimed perpendicular to the separator at the highest possible pressure.

The ideal direction depends on:

• separator type;
• deposit location;
• gas direction;
• drainage path.

For example, cleaning a deposit may require sufficient surface shear.

But unnecessary jet energy can:

• damage fine mesh;
• atomize liquid;
• drive deposits deeper into the media.

The wash objective is deposit removal—not maximum impact force.

Nozzle Distance Affects the Spray

The same nozzle behaves differently depending on its distance from the separator.

Too close:

• impact can be concentrated;
• coverage can be narrow.

Too far:

• droplets may disperse excessively;
• gas can carry part of the spray away before it reaches the separator.

The correct spacing should provide enough overlap for uniform coverage without creating excessive airborne spray.

There is no universal nozzle-to-demister distance suitable for every system.

Washing From Only One Side Can Create Uneven Wetness

A large mist eliminator may receive wash water from one side only.

The regions directly under the nozzles become heavily saturated.

Remote areas remain partially dirty.

The heavily washed zones develop higher wet resistance.

Gas shifts toward the drier regions.

This creates temporary gas maldistribution.

Therefore, wash coverage should be evaluated not only by whether every surface becomes wet but by whether the separator receives an excessively uneven liquid load.

Continuous Washing Is Not Always Better

Some systems use intermittent washing.

Others may consider continuous spray.

Continuous washing can prevent deposit accumulation but permanently increases liquid loading.

If the separator was originally designed around process entrainment only, continuous wash water can reduce hydraulic margin.

The operating strategy should therefore consider:

• deposit-growth rate;
• drainage capacity;
• pressure-drop allowance.

Frequent short washes may be preferable to continuous high liquid loading in some services.

DP Spikes During Washing Provide Useful Evidence

If differential pressure rises sharply whenever the wash system is activated, the separator is becoming wetter.

A small temporary increase may be expected.

A large increase accompanied by outlet carryover can indicate:

• excessive wash rate;
• poor drainage;
• blocked sections.

Recording DP before, during, and after the wash cycle provides useful diagnostic information.

The recovery time also matters.

If DP remains high long after washing stops, the separator may not be draining adequately.

Wash Water Can Carry Removed Deposits Into the Mesh

Cleaning dislodges:

• scale;
• solids.

These materials have to leave the separator.

If wash flow pushes loose solids deeper into a fine mesh instead of flushing them away, the cleaning operation can redistribute fouling rather than remove it.

A pad may appear cleaner on the surface while internal passages become more restricted.

This is one reason visual appearance alone is a poor measure of cleaning success.

How to Recognize Wash-Induced Carryover

Useful clues include:

• downstream liquid spikes during wash cycles;
• DP rising immediately after wash starts;
• carryover disappearing after wash water drains;
• wet downstream ducting correlated with nozzle operation.

If carryover occurs only while the wash system is active, the main mist eliminator may not be the root problem.

The wash hydraulic load needs review.

What Should Be Defined in Wash-System Design?

Important variables include:

• nozzle type;
• flow per nozzle;
• spray pressure;
• spray angle;
• nozzle distance;
• washing sequence;
• total wash rate;
• separator drainage capacity.

The system should also consider whether washing occurs:

• online;
• offline.

Online washing requires particular attention because gas continues moving through the separator while additional liquid is introduced.

Final Engineering Perspective

A mist eliminator wash system is another gas-liquid contact system.

If poorly designed, it can generate the same problem it is supposed to prevent: mist.

Successful washing therefore requires more than sufficient water volume.

The system must remove deposits while avoiding excessive atomization, local saturation, drainage overload, and secondary re-entrainment.