Engineering Evaluation Case: Hot Scrubbing Liquid Causes Pump Cavitation and the Packed Bed Quietly Loses Irrigation

A circulation pump can be running while the packed tower receives much less liquid than expected.

One possible cause is cavitation.

This becomes more likely when the scrubber liquid becomes hot and available suction head decreases.

The operating panel may still show:

• motor running;
• valve open.

The packed bed may actually be under-irrigated.

Project Situation

Consider a recirculating scrubber handling hot gas.

Over several hours:

• sump temperature rises;
• pump becomes noisy;
• flow fluctuates;
• outlet contaminant increases.

The operator sees no pump trip.

The tower differential pressure remains relatively normal.

The problem may be on the suction side of the liquid system.

Hot Liquid Has Higher Vapor Pressure

As liquid temperature increases, the margin between suction pressure and vapor pressure decreases.

If available NPSH becomes insufficient, vapor bubbles can form inside the pump.

They collapse farther into the impeller and cause:

• noise;
• vibration;
• unstable flow;
• long-term damage.

Pump Speed Is Not Proof of Liquid Flow

A rotating pump does not prove that the distributor receives the specified circulation.

Actual flow should be measured.

A tower can therefore suffer chemical breakthrough while operators believe irrigation is normal.

Low Sump Level Makes the Problem Worse

Available suction head can also fall when:

• sump level decreases;
• suction piping fouls;
• strainers block.

Hot liquid combined with low level creates a more severe condition.

Distributor Pressure Can Provide Evidence

If pump discharge or distributor pressure falls as sump temperature rises, cavitation or suction restriction becomes more plausible.

Trend together:

• liquid temperature;
• flow;
• pump suction/discharge pressure;
• tower outlet contaminant.

Under-Irrigation Can Damage Process Performance Before Hydraulics Look Abnormal

Lower liquid flow may actually reduce bed pressure drop.

So an operator looking only for a high-pressure-drop alarm can miss the problem.

The tower can be:

• hydraulically easy;
• chemically ineffective.

Pump Selection Should Use the Hottest Credible Liquid

A pump selected from cold-water conditions may not have sufficient suction margin when the real recirculating solution becomes hot.

Actual:

• vapor pressure;
• density;
• viscosity

should be considered.

Engineering Takeaway

The liquid system can fail gradually without a pump trip.

The tower only sees the flow that actually reaches the distributor.