Engineering Evaluation Case: A Demister Drain Works at Atmospheric Pressure but Fails When the Scrubber Runs Under Negative Pressure

A mist eliminator must not only capture droplets.

The collected liquid must leave the separator and drain correctly.

In a negative-pressure scrubber, drainage piping can interact with the pressure difference between:

• tower interior;
• atmosphere;
• drain tank.

A drain arrangement that works during an atmospheric workshop test may behave differently after the fan pulls the tower under vacuum.

Project Situation

Consider a vertical packed scrubber with:

• top mesh pad;
• liquid drain connection;
• induced-draft fan downstream.

The demister drains through a small line toward an external collection point.

During water testing with the fan off, drainage appears normal.

During operation under negative pressure, operators observe:

• liquid accumulation in the demister;
• re-entrainment;
• irregular drain flow.

The mesh pad is blamed.

The pressure balance of the drain should be checked.

Negative Pressure Can Pull Air Through the Drain

If the drain is open to atmosphere without a suitable liquid seal or pressure arrangement, the scrubber can pull air inward through the drain line.

This can oppose normal liquid drainage.

The drain effectively becomes an uncontrolled gas inlet.

Collected Liquid Needs Enough Hydrostatic Head

Liquid must overcome the pressure difference between the tower and the drain destination.

If the available liquid head is insufficient, the liquid can:

• remain in the separator;
• back up into the drain tray.

As the accumulated liquid increases, the demister becomes more prone to:

• re-entrainment;
• higher pressure drop.

A Seal Leg Can Solve One Problem and Create Another

A liquid seal or seal pot may be used so the drain can operate while the vessel remains under negative pressure.

However, the seal must be designed for the actual pressure difference.

If too shallow, the fan can pull gas through it.

If poorly arranged, it may:

• siphon;
• retain solids;
• freeze outdoors.

Therefore, the drain should be engineered as part of the pressure system.

Pressure Changes with Operating Flow

Tower vacuum may not be constant.

It can vary with:

• fan speed;
• damper position;
• fouling;
• gas flow.

The drainage system should remain functional over the expected pressure range.

A seal sized only for one normal point may fail during maximum suction.

A Plugged Drain Can Look Like a Separator-Capacity Problem

If liquid cannot leave the demister, even a correctly selected pad can become overloaded.

Possible symptoms include:

• carryover;
• high differential pressure;
• visible liquid downstream.

Replacing the mesh density does not solve a blocked or pressure-locked drain.

Drain Line Diameter Is Not the Only Issue

A larger pipe does not fix an unfavorable pressure balance.

The project should check:

• pressure difference;
• vertical head;
• line route;
• destination pressure;
• solids or crystal risk.

External Drain Tank Pressure Matters

If the drain discharges into another closed vessel, that vessel may itself operate under:

• positive;
• negative pressure.

The pressure at both ends of the drain should therefore be defined.

Horizontal-Flow Separators Can Be Especially Sensitive

A horizontal vane pack may collect liquid into a bottom drain channel.

If drainage fails, liquid can rapidly accumulate in the separator zone.

High gas velocity can then strip the retained liquid back into the outlet stream.

Instrumentation Can Help

Useful checks include:

• demister differential pressure;
• drain sight indication;
• seal-pot level;
• tower pressure.

Correlating carryover with increased vacuum can reveal a drainage-pressure problem.

Winterization Matters Outdoors

A seal leg filled with water or process liquid can freeze.

A drain system designed hydraulically for summer may fail during winter shutdown.

Insulation, heat tracing, or another approved strategy may be required by the plant.

Engineering Takeaway

Demister drainage is governed by both gravity and pressure.

A separator cannot perform reliably if collected liquid has no stable path out of the negative-pressure vessel.