Engineering Evaluation Case: A Vacuum Degasser Has Enough Packing but Dissolved Oxygen Remains High Because Air Leaks into the System

Vacuum degassing reduces dissolved gases by lowering the gas-phase partial pressure above the liquid.

The packed bed creates contact area.

The vacuum system creates the driving force.

If ambient air leaks into the vessel, the tower can lose performance even though the packing is:

• clean;
• correctly distributed;
• hydraulically stable.

This is a system-sealing problem.

Project Situation

Consider a packed vacuum degasser used to remove dissolved oxygen from:

• process water;
• boiler feedwater;
• high-purity water.

The tower has:

• liquid distributor;
• packed section;
• vacuum pump;
• demister.

Over time, outlet dissolved oxygen rises.

Inspection finds no major packing fouling.

The vacuum gauge may even appear close to normal.

Air leakage should be considered.

Vacuum Degassing Depends on Low Oxygen Partial Pressure

The purpose of the vacuum is not merely to show a low total pressure.

It is to reduce the gas-phase partial pressure of the component being removed.

If atmospheric air continuously enters the system, it brings:

• nitrogen;
• oxygen.

The vacuum pump may maintain pressure while still processing a much larger non-condensable gas load.

The oxygen concentration in the gas space can therefore be higher than intended.

Total Pressure Alone Can Hide Leakage

Suppose the vacuum pump has sufficient spare capacity.

It compensates for a leak and still maintains the set pressure.

Operators see a normal pressure reading and assume the vacuum system is healthy.

But the pump is now continuously pulling in fresh atmospheric oxygen.

Degassing driving force can deteriorate.

Common Leakage Paths

Potential locations include:

• manway gaskets;
• instrument fittings;
• pump seals;
• flange joints;
• valve stems;
• drain seals.

Small leaks can matter because the entire system is intentionally below atmospheric pressure.

Liquid Seals Can Lose Their Seal

Drain or overflow lines can provide an air path if:

• liquid level drops;
• seal leg is too short;
• siphoning occurs.

A drain that works hydraulically may therefore become a significant vacuum leak.

The complete tower and liquid system should be reviewed.

Vacuum Pump Load Can Reveal the Problem

Indicators can include:

• increased pump power;
• higher ejector steam demand;
• more non-condensable vent flow.

If outlet oxygen rises together with increasing vacuum-system load, air ingress becomes more likely.

Packing Height Cannot Correct Continuous Oxygen Ingress

Adding more packing improves gas-liquid contact.

It does not stop the tower atmosphere from being replenished with oxygen through a leak.

If the process is limited by the gas-side boundary condition, more contact area can provide little benefit.

Distributor Performance Still Matters

After sealing problems are addressed, the tower still requires uniform water distribution.

Poor distribution can independently cause:

• incomplete degassing;
• channeling.

The diagnosis should therefore avoid assuming that every high-oxygen result is caused by leaks.

Water Temperature Influences Degassing

Temperature changes gas solubility.

Cold water may require different operating conditions than warm water.

Therefore, compare leakage symptoms only after accounting for:

• water temperature;
• flow;
• pressure.

Instrument Calibration Matters

A vacuum transmitter can drift.

A dissolved-oxygen analyzer can also produce misleading results.

Before modifying equipment, verify both.

A genuine process problem should be separated from an instrumentation problem.

Leak Testing Should Be Planned

Depending on the system, the plant may use appropriate vacuum-leak detection methods during maintenance.

The objective is to identify:

• air path;
• leakage magnitude.

The packing supplier can flag the issue, while the plant or vacuum-system specialist performs the detailed testing.

Demister Condition Also Affects the Vacuum System

Liquid carryover into the vacuum equipment can:

• reduce pump performance;
• create maintenance issues.

Therefore, the demister remains an important part of vacuum degasser reliability.

Engineering Takeaway

A vacuum degasser needs both contact area and a controlled gas-phase environment.

Air leakage can destroy the intended oxygen-removal driving force without changing the packed bed at all.