Engineering Evaluation Case: Rainwater Enters an Outdoor Scrubber Stack and Overloads the Demister and Sump

Outdoor scrubbers are normally designed around process liquid flowing from the distributor downward.

Heavy rain can introduce liquid from the opposite direction:

through the stack opening and down into the tower.

For a large stack, rainfall can add a meaningful temporary liquid load.

The problem is particularly important during:

• shutdown;
• low gas flow;
• storm conditions.

Project Situation

Consider an outdoor packed wet scrubber with a vertical stack directly above the demister.

The plant is located in an area that experiences intense rainfall.

During a storm or shutdown, operators observe:

• unexpected increase in sump level;
• water dripping from upper sections;
• wet or partially flooded demister;
• dilution of process liquid.

The scrubbing pump may not even be running.

The added water is entering from outside the process.

Rain Can Reach the Demister Directly

Depending on stack geometry, falling rain can enter the open exhaust path.

During normal high gas flow, some water may be prevented from falling deeply.

During shutdown, there may be little upward gas velocity.

Rain can then reach:

• demister;
• upper tower walls;
• distributor;
• packed bed.

The Demister Is Designed for Upward Gas and Downward Drainage

Rainwater entering from above changes the normal flow pattern.

A mesh pad may become heavily wetted from the top.

If drainage capacity is limited, water can accumulate.

On restart, high gas velocity can strip this retained liquid and send a large temporary carryover downstream.

Sump Level Can Rise Unexpectedly

Rainwater eventually drains into the bottom system.

For a large open stack, the cumulative volume during a severe storm may be significant.

If the sump level is already near its normal upper range, the extra water can cause:

• overflow;
• dilution;
• bed submergence in extreme cases.

The plant liquid balance should include credible storm ingress where relevant.

Chemistry Can Be Diluted

In a reactive scrubber, rainwater changes:

• reagent concentration;
• pH;
• salt concentration.

The system may therefore restart with weaker absorbent than expected.

A pH controller may compensate eventually, but the initial operating period can be different.

Outdoor Stack Design Is Part of Scrubber Reliability

Possible architectural features can include:

• stack caps designed for the airflow;
• rain shields;
• drainage arrangements.

The exact design must preserve exhaust performance and meet local requirements.

A rain-control feature should never create excessive backpressure or unsafe recirculation of the exhaust.

Wind-Driven Rain Is Different from Vertical Rain

Rain does not always fall straight down.

Wind can drive water through:

• side openings;
• stack terminations.

Therefore, simple projected stack area may not fully describe the exposure.

Site weather and termination geometry matter.

Drainage Paths Should Avoid Distributor Damage

Large quantities of water falling onto a plastic distributor can create:

• localized impact;
• unexpected loading.

The tower should ideally direct incidental rainwater toward safe drainage rather than allowing it to collect on delicate internals.

Shutdown Is Often the Worst Condition

When the fan is running, upward gas can resist downward water entry.

When both fan and pump are off:

• no upward gas barrier exists;
• no controlled circulation removes water.

This means the environmental shutdown case can control the rain-ingress risk.

Restart Inspection May Be Appropriate After Severe Storms

If significant rain ingress is known or suspected, operators can verify:

• sump level;
• reagent condition;
• demister drainage;
• distributor condition

before full gas flow is restored.

Do Not Misdiagnose Storm Carryover as Demister Failure

If abnormal liquid carryover appears only after heavy rain, replacing the demister may not address the root cause.

The investigation should include:

• stack ingress;
• retained water;
• sump level.

Engineering Takeaway

An outdoor scrubber interacts with weather as well as process gas.

Rainfall can become a temporary liquid feed that enters the tower from the wrong end.