How to Retrofit a Packed Tower That Repeatedly Dries Out Between Liquid Pump Trips

Packed towers depend on reliable liquid irrigation. In some services, short pump trips or interruptions occur often enough that part of the packed bed repeatedly drains and dries before circulation is restored.

The tower may not suffer catastrophic failure, yet repeated dry-out can create unstable performance, salt deposition, hot spots, material damage, and delayed recovery after each restart.

This is different from ordinary low-flow operation.

The key problem is loss and restoration of wetting.

Understand What Happens During a Pump Trip

When liquid flow stops, residual liquid continues draining through the bed.

Depending on packing type, fluid properties, and tower geometry, some areas remain wet longer than others.

Gas may continue flowing during part or all of the interruption.

This can create dry regions exposed to process gas without the cooling, reaction, or absorption normally provided by liquid.

Check Whether Gas Continues During Loss of Liquid

This is one of the most important operating questions.

If both gas and liquid stop simultaneously, the effect may be limited.

If gas continues at full rate while liquid stops, consequences can include:

• reduced absorption;
• higher packing temperature;
• salt deposition;
• process breakthrough;
• increased material attack.

The retrofit should be based on the actual trip sequence.

Review Packing Material

Some plastic packings rely on liquid flow for temperature control.

A hot gas stream during a liquid interruption may expose the packing to temperatures much higher than normal wet operation.

Metal or ceramic packing may tolerate temperature better, but process performance still collapses without irrigation.

Material selection should therefore consider the dry-gas condition.

Examine Salt or Solids Deposition

When a wetted surface dries, dissolved salts can crystallize.

Repeated pump trips can gradually accumulate deposits even if each event lasts only a short time.

These deposits may concentrate near:

• top packing layers;
• high-temperature regions;
• distributor outlets.

Eventually the tower develops permanent pressure-drop problems.

Improve Distributor Re-Wetting

After liquid flow returns, the packing should be re-wetted uniformly.

A distributor that restarts unevenly can create dry channels that persist long after the pump has recovered.

Check whether all outlets resume flow promptly.

Distributor geometry, operating head, and trapped gas may influence restart behavior.

Consider a Minimum Emergency Flow

Some systems may benefit from maintaining a smaller backup liquid flow during main-pump interruption.

Whether this is practical depends on the plant system and should be addressed by process engineering.

From the tower perspective, the objective is to prevent full bed dry-out when the consequences are serious.

Check Liquid Inventory Above the Bed

A distributor with some retained liquid may continue irrigation briefly after a pump trip.

However, intentionally increasing distributor inventory also increases process holdup.

The appropriate balance depends on trip frequency and required protection time.

Review Operating Interlocks

Equipment retrofit should be coordinated with control logic.

If dry-gas exposure is damaging, gas flow may need to be reduced automatically after loss of liquid circulation.

A more expensive internal may not be the best solution if the process can simply avoid prolonged dry operation.

Inspect for Thermal Damage

Repeated dry periods can cause hidden damage.

Plastic packing should be checked for:

• distortion;
• brittleness;
• softening;
• cracking.

Distributor components near hot gas zones should also be inspected.

Establish Restart Criteria

Operators should know when the tower can return to normal gas load after liquid flow resumes.

Allowing a short pre-wetting period may improve stability.

The necessary time depends on tower size, packing, and distributor behavior.

Track Trip Frequency

A problem occurring once every five years may not justify major modification.

A tower losing liquid circulation several times per month requires a more robust strategy.

Maintenance and control-system data can help quantify the true frequency.

Focus the Retrofit on Resilience

The objective is not to make the tower perform normally without liquid.

That is impossible for most gas-liquid contacting duties.

The objective is to prevent damage, deposition, and poor recovery during unavoidable short interruptions