Engineering Evaluation Case: A Positive-Pressure Packed Scrubber Leaks Corrosive Gas Through Manways and Joints

Many wet scrubbers operate under negative pressure.

Some systems place the blower or fan upstream, causing part or all of the scrubber to operate at positive pressure.

This changes the meaning of a small leak.

Under negative pressure, a leak may draw ambient air inward.

Under positive pressure, untreated or partially treated gas can escape outward.

That makes sealing part of the process-containment design.

Project Situation

Consider a packed acid-gas scrubber with the fan located before the tower.

During operation, internal pressure is slightly above atmosphere.

The system contains:

• corrosive gas;
• liquid distributor;
• packing;
• demister;
• access manways.

Operators begin noticing:

• odor near the manway;
• corrosion around flange areas;
• wet marks at joints.

Tower removal efficiency measured at the stack may still appear acceptable.

The problem is fugitive leakage.

Small Openings Can Become Emission Paths

A packed tower may contain many potential leak points:

• manway gaskets;
• inspection covers;
• nozzle flanges;
• instrument penetrations;
• drain connections.

Under positive pressure, these locations see outward driving force.

The integrity of each joint becomes part of emission control.

Corrosive Condensate Makes Leakage Worse

The escaping gas may be wet or may condense as it cools at the joint.

This can create concentrated corrosion at:

• bolts;
• flange edges;
• external steelwork.

A small gas leak can therefore become a mechanical deterioration problem over time.

Packing Pressure Drop Changes the Internal Pressure Profile

The tower pressure is not identical at every elevation.

Pressure upstream of the packed bed may be higher than near the outlet.

As packing fouls and pressure drop increases, the lower section can operate at even higher positive pressure.

A gasket that was adequate when the tower was clean may begin leaking after hydraulic resistance rises.

Fan Location Should Be Included in the RFQ

A tower supplier may know the gas flow but not whether the fan is:

• upstream;
• downstream.

That information matters.

It helps define whether small joint leakage tends to produce:

• air ingress;
• process-gas egress.

The containment philosophy can therefore influence mechanical detailing.

Manway Sealing Needs Real Operating Conditions

The gasket material must tolerate:

• chemical exposure;
• temperature;
• compression;
• repeated opening.

A gasket that swells, hardens, or loses compression can create leakage.

Overtightening is not always the answer because it can damage:

• FRP flanges;
• plastic components;
• gasket seating.

Drains Can Also Become Gas-Leak Paths

A drain without a proper seal arrangement can discharge gas as well as liquid.

The tower may therefore need:

• liquid seals;
• closed drains;
• another controlled pressure arrangement

depending on the process.

Drainage design and containment are linked.

Positive Pressure Changes Maintenance Risk

Opening a manway or sampling connection while the tower remains pressurized can release process gas.

Operating procedures should include:

• isolation;
• depressurization;
• verification.

This is a plant-safety issue rather than a packing-selection issue, but the tower design should support safe isolation.

Leakage Can Bypass the Stack Measurement

This is an important performance issue.

If part of the untreated gas escapes before reaching the stack sampling point, the measured stack outlet may appear compliant while the plant still has uncontrolled fugitive emissions.

A system balance should therefore consider both:

• stack discharge;
• equipment leakage.

FRP and Plastic Towers Need Suitable Flange Support

Large nonmetallic flanges can deform if:

• bolt loading is uneven;
• pipe loads are excessive.

Poor flange alignment can create recurring leaks.

Connected duct and piping loads should not distort the vessel nozzle.

A Pressure Survey Can Help Troubleshooting

Measure or review pressure at:

• gas inlet;
• below packing;
• above packing;
• outlet.

This reveals where the maximum positive pressure occurs and whether increasing pressure drop is driving new leakage.

Do Not Fix Containment by Reducing Packing Performance

Removing packing to lower pressure drop might reduce internal pressure.

It may also reduce scrubbing efficiency.

The correct response is to determine whether:

• tower pressure is excessive;
• fan operating point changed;
• packing is fouled;
• joints are inadequate.

Engineering Takeaway

A positive-pressure scrubber must both treat the gas and contain it.

Hydraulic pressure drop, fan arrangement, joint sealing, drains, and gasket materials all influence fugitive emissions.