Why Mist Eliminator Performance Tests Can Be Wrong Without Proper Droplet Sampling

Mist eliminator performance is often summarized with a precise number:

• outlet carryover;
• removal efficiency;
• liquid concentration.

But the quality of that number depends completely on how the droplets were sampled.

Mist is not always distributed uniformly across a duct or vessel.

Droplets also have inertia.

A sampling probe can therefore collect too much or too little liquid if its:

• location;
• orientation;
• gas velocity;
• temperature

are not appropriate.

A poor sampling method can make a good demister look bad—or make a failing separator appear acceptable.

Performance testing should therefore evaluate the sampling system as carefully as the separator itself.

Why Droplet Sampling Is More Difficult Than Gas Sampling

Gas molecules generally follow the gas flow easily.

Liquid droplets have inertia.

When gas enters a sampling nozzle, droplets may not follow the gas in exactly the same way.

Large droplets are especially sensitive.

If the velocity entering the probe differs significantly from the surrounding duct velocity, the sample may no longer represent the real gas stream.

This creates sampling bias.

What Is Isokinetic Sampling?

In simplified terms, isokinetic sampling aims to draw gas into the sample probe at approximately the same velocity as the local gas stream.

This reduces aerodynamic distortion of the droplet trajectories.

If sample velocity is too low or too high, certain droplet sizes can be:

• overrepresented;
• underrepresented.

The exact sampling standard depends on the measurement purpose and process, but the engineering principle is universal:

the probe should not artificially separate the droplets before they are measured.

Large Droplets Are Particularly Sensitive

A very fine aerosol tends to follow gas movement relatively well.

A large droplet has more inertia.

When gas bends into the sample nozzle, the droplet may:

• continue forward;
• miss the probe.

Alternatively, under another sampling condition, the probe may preferentially collect large droplets.

This means one poorly designed sample can produce a liquid concentration that is significantly different from the actual average duct condition.

Sampling Location Matters

The gas immediately downstream of a mist eliminator may not be perfectly uniform.

One region may have more carryover because of:

• local overload;
• edge bypass;
• segment gap.

Another region may be very dry.

A single-point sample taken in the driest region can exaggerate performance.

A sample taken near a local wet spot can make the separator look much worse than the average.

For large ducts or vessels, representative sampling may require multiple locations.

Outlet Nozzles Can Distort the Measurement

Gas accelerates toward vessel outlet nozzles.

A sample taken too close to this region may see a different velocity and droplet distribution from the main separator outlet area.

The measurement can therefore contain effects from:

• nozzle acceleration;
• wall flow.

A sampling location should be chosen to represent the performance boundary being evaluated.

Condensation Inside the Sample Line Can Create False Carryover

This is one of the most important errors.

Suppose the gas leaving the demister is:

• hot;
• saturated.

It enters a cooler sampling line.

Water vapor condenses inside the line.

The collected liquid is then measured as though it passed through the mist eliminator as droplets.

The result overestimates carryover.

The separator is blamed for liquid that was actually created by the measurement system.

Sample-line temperature and condensation control are therefore critical.

The Opposite Error Can Also Occur

Droplets may deposit on:

• probe walls;
• tubing

and fail to reach the collection device.

The test then underestimates outlet carryover.

This is especially important for large droplets or long sample lines.

Every surface between the process and measurement device can alter the sample.

The complete sampling train should therefore be considered.

Sampling Before and After the Demister Requires Consistent Methods

To calculate separation efficiency:

η=Cin−CoutCin\eta=\frac{C_{in}-C_{out}}{C_{in}}

where:

• CinC_{in} = inlet liquid concentration;
• CoutC_{out} = outlet liquid concentration.

But if the inlet and outlet samples use different:

• probe conditions;
• temperature control;
• collection efficiency,

the calculated separator efficiency may be meaningless.

Both measurements need compatible methods.

A precise formula cannot correct inconsistent sampling.

Very High Efficiency Is Especially Difficult to Verify

Suppose the separator removes most of the incoming liquid.

The outlet concentration becomes very small.

Measurement uncertainty now represents a larger fraction of the result.

Small quantities of:

• condensation;
• contamination;
• sample loss

can materially change the calculated efficiency.

Performance claims near very high removal levels therefore require careful test design.

A percentage with many decimal places does not automatically indicate accuracy.

Flow Basis Must Be Defined

Outlet liquid concentration may be reported as:

• mg/m³;
• g/Nm³;
• another volumetric basis.

The gas volume must be defined at:

• actual;
• normalized conditions.

Otherwise, two tests may appear to disagree simply because they use different flow bases.

Temperature and pressure should be recorded.

Test During Stable Process Conditions

A demister cannot be evaluated fairly during:

• startup;
• shutdown;
• foam upset;
• temporary wash cycle

unless that condition is specifically the subject of the test.

For a normal-performance acceptance test, process conditions should be as stable as practical.

Record:

• gas flow;
• liquid circulation;
• temperature;
• pressure;
• demister differential pressure.

The test result should always be tied to the operating condition at which it was obtained.

Separate Downstream Condensation From Separator Carryover

Even with a correct sampling probe, the measurement point may be too far downstream.

If gas cools between the demister and sample location, real condensation can occur in the duct.

This liquid exists in the process, not only in the sample line.

But it still did not pass through the demister as droplets.

A carryover test therefore needs to define clearly:

Are we measuring demister outlet performance or total downstream liquid?

These are different questions.

Compare More Than One Operating Load

Testing at:

• normal;
• high gas load

can reveal whether the separator approaches re-entrainment.

If outlet concentration remains low at normal flow but rises sharply near maximum flow, hydraulic capacity may be the issue.

One test point cannot show this operating trend.

Multiple conditions can provide much more engineering information.

Field Observation Still Has Value

Formal sampling is valuable, but it should be compared with:

• downstream drain rate;
• visible wetting;
• DP trends;
• inspection findings.

If a test reports almost zero carryover while the downstream duct contains large quantities of process liquid, investigate the test method.

Measurement should agree reasonably with physical evidence.

Why Supplier and Buyer Should Agree on the Method First

A performance guarantee becomes difficult if the measurement method is defined only after startup.

The buyer and supplier may disagree about:

• sample location;
• test conditions;
• condensation correction;
• concentration basis.

For critical projects, these items should be agreed before fabrication or commissioning.

A good acceptance criterion includes both:

• required result;
• how that result will be measured.

Final Engineering Perspective

A mist eliminator performance number is only as reliable as the sampling method behind it.

Droplet inertia, nonuniform flow, probe velocity, sample-line deposition, condensation, location, and gas-volume basis can all distort the result.

Reliable performance testing therefore requires control of both the separator operating condition and the measurement system.

The question is not only:

“What carryover number was measured?”

It is also:

“Did the sampling method measure the real carryover?”