How Sudden Depressurization and Flashing Can Overload a Mist Eliminator

Mist eliminators are normally sized around defined operating conditions such as:

• normal flow;
• maximum continuous flow.

But some processes experience short transient events that look completely different from steady operation.

One important example is rapid depressurization.

When vessel pressure falls suddenly, dissolved or pressurized liquid can flash into vapor.

Gas volume can increase rapidly.

Liquid can become violently dispersed.

The mist eliminator may temporarily receive both:

• a large gas-flow surge;
• a large liquid-entrainment surge.

A separator that performs well during normal operation can therefore become severely overloaded during a depressurization event.

Why Flashing Occurs

A liquid can remain stable at high pressure because its boiling condition depends on pressure.

If pressure is reduced rapidly, the existing liquid may suddenly be above its new equilibrium boiling condition.

Part of the liquid vaporizes.

This is flashing.

The phase change can be rapid.

It can generate:

• vapor;
• turbulence;
• liquid droplets.

The separator is therefore exposed to a process condition that may not resemble ordinary spray mist at all.

Gas Volume Can Increase Very Rapidly

Mist eliminator velocity depends on actual gas volume.

During depressurization, newly generated vapor adds to the existing gas flow.

The instantaneous actual volumetric flow through the separator can rise sharply.

This creates:

• high face velocity;
• greater aerodynamic force on captured liquid;
• high differential pressure.

Even if the event lasts only a short time, the mechanical and hydraulic load can be significant.

Flashing Also Generates More Liquid Entrainment

The problem is not only increased gas flow.

Violent vapor generation can break liquid into droplets.

Bulk liquid surfaces become highly disturbed.

The separator may receive:

• large droplets;
• fine mist;
• intermittent liquid surges.

Therefore, both sides of its operating envelope move in the wrong direction simultaneously:

• more gas;
• more liquid.

This makes depressurization particularly demanding.

Normal Sizing Data May Not Capture the Event

A process datasheet may state:

• normal gas flow;
• design gas flow.

Neither necessarily describes emergency depressurization.

If the mist eliminator is expected to remain in place and survive the event, the transient should be reviewed separately.

It may not be necessary for the separator to meet its normal outlet efficiency during every emergency condition.

But it may need to avoid:

• collapse;
• displacement;
• unacceptable pressure loading.

The performance requirement and mechanical survival requirement should therefore be distinguished.

Differential Pressure Can Rise Quickly

A sudden gas surge increases separator DP.

At the same time, additional liquid holdup further increases resistance.

The total pressure difference across the separator can therefore rise much faster than under steady dry gas conditions.

This matters mechanically because:

F=ΔP×AF=\Delta P \times A

A large separator area multiplied by a transient pressure difference can create substantial total force.

Support and hold-down design should therefore consider realistic upset loads where relevant.

Wire Mesh Can Become Temporarily Saturated

Fine mesh contains many liquid-collection surfaces.

During a flashing event, incoming liquid may exceed drainage capacity almost instantly.

The mesh becomes heavily wetted.

Effective open area falls.

Gas accelerates through the remaining passages.

This can cause severe re-entrainment.

After the event, the separator may recover if:

• structure remains intact;
• drainage clears the retained liquid.

But if the event mechanically compresses or displaces the pad, permanent performance loss can remain.

Vane Separators Have Limits Too

Vane separators usually offer better tolerance for heavy liquid loading.

But very large flashing surges can fill:

• drainage pockets;
• channels.

Once these regions become flooded, liquid is exposed to the gas again.

Re-entrainment increases.

A vane pack should not be assumed immune simply because its passages are open.

The transient liquid-generation mechanism still matters.

Liquid Level Can Rise or Become Unstable

Depressurization can cause:

• foaming;
• flashing

inside the vessel liquid inventory.

The apparent liquid level may rise rapidly.

Disengagement space decreases.

Foam or bulk liquid can approach the mist eliminator.

This can create direct wetting that never occurs during normal operation.

The event therefore combines:

• phase equilibrium;
• vessel hydraulics;
• separator hydraulics.

Reverse Flow Can Occur in Some Systems

During trips or pressure equalization, gas flow direction can temporarily change in certain process systems.

A mist eliminator designed and mechanically restrained for one flow direction may then experience load from the opposite direction.

This is particularly important for:

• directional vane packs;
• lightly restrained mesh pads.

The complete depressurization scenario should therefore include expected gas-flow direction.

Drain Systems Can Become Unstable

Rapid pressure changes also affect drainage systems.

A seal pot or downcomer that works correctly at steady pressure may temporarily experience:

• different pressure balance.

Liquid can be pushed backward.

Gas may enter the drain.

The demister can then retain more liquid exactly when flashing has already increased its load.

Drainage behavior during transient pressure changes deserves review in critical systems.

What Can Happen After the Event?

Possible post-event conditions include:

• normal recovery;
• permanently compressed mesh;
• bent support grid;
• shifted segments;
• broken hold-downs;
• clogged drainage.

If plant performance changes immediately after a trip or depressurization event, inspect the mist eliminator rather than assuming the change is unrelated.

The event may have physically altered the separator.

How to Review a Depressurization Case

Useful questions include:

• How fast does pressure fall?
• How much liquid can flash?
• What peak vapor rate is expected?
• Does liquid level swell?
• Can gas flow reverse?
• What maximum transient DP can occur?

Process simulation may be required for severe or critical events.

The separator supplier alone cannot determine the flash load from vessel diameter.

The transient belongs to the overall process design.

Do Not Oversize Normal Operation Solely for an Extreme Event Without Review

An emergency event may be extremely short.

Designing the separator for full collection efficiency during that event could produce an unnecessarily large or restrictive unit during normal operation.

Instead, define what the separator must do during the upset:

• maintain structure?
• prevent complete blowout?
• meet a temporary carryover target?

The engineering response should match the real requirement.

Final Engineering Perspective

Rapid depressurization can create a mist eliminator duty far more severe than normal operation by simultaneously increasing:

• vapor generation;
• gas volume;
• droplet entrainment;
• mechanical load.

These events should be evaluated as distinct transient cases.

A separator designed for steady operation may still need mechanical and hydraulic protection against flashing, liquid swell, differential-pressure spikes, and possible reverse flow.