Cooling Tower Pump Cavitation: Why It Gets Worse in Summer

The cooling tower pump sounded fine in May.

Then temperatures climbed.

Cooling demand increased, pump flow went up and suddenly there was a rattling or crackling sound coming from the equipment.

One possible cause is cooling tower pump cavitation.

Cavitation occurs when pressure at the pump inlet drops low enough for some of the liquid to form vapour bubbles. As those bubbles move into a higher-pressure region of the pump, they collapse. Over time, this can affect performance and damage internal components. Hydraulic Institute guidance identifies adequate NPSH margin as an important factor in limiting cavitation and maintaining pump reliability.

What Causes Cooling Tower Pump Cavitation?

The pump needs the right conditions at its inlet.

During peak summer operation, greater flow can increase losses through suction piping, fittings, strainers and valves.

Conditions that may contribute include:

• Low cooling tower basin level

• Restricted suction strainers

• Partially closed valves

• Air entering the suction piping

• Excessive suction-side pressure loss

• Pump operation outside its preferred range

• Higher flow demand than the system can properly supply

The hotter and busier the building becomes, the more obvious an existing suction problem may become.

What Does Cavitation Sound Like?

Operators often describe cavitation as a rattling, crackling or gravel-like sound.

But sound alone is not enough to diagnose the problem.

Loose components, bearing wear, air in the system and other mechanical problems can create similar symptoms.

That is why technicians should look at the complete operating picture.

Check pressures, flow, basin level, pump speed and valve positions rather than diagnosing by sound alone.

Watch for Reduced Pump Performance

Noise may be the first thing someone notices, but performance matters too.

A pump experiencing cavitation may not produce the flow or head expected for its operating conditions.

You might see:

• Reduced cooling-water flow

• Unstable pressure

• Increased vibration

• Changing pump sound

• Difficulty meeting cooling demand

Hydraulic Institute notes that operating outside a pump's preferred operating region can contribute to recirculation, vibration and localized cavitation, reducing reliability.

Check the Basin Level

A cooling tower needs to maintain sufficient water level for the pump suction arrangement.

If basin level drops, conditions at the pump inlet can deteriorate.

Check the makeup-water system, level controls and actual operating level before assuming the pump itself has developed a mechanical defect.

A water-level problem can easily become a pump problem if it is allowed to continue.

Inspect the Suction Side

When cooling tower pump cavitation appears, look upstream.

Check:

• Suction strainers

• Isolation valves

• Pipe restrictions

• Air leaks

• Inlet geometry

• Changes made to piping

• Water level

Pump piping can have a significant effect on pump performance. Hydraulic Institute's ANSI/HI 9.6.6 standard specifically addresses how upstream and downstream piping conditions affect rotodynamic pumps.

Don't Just Slow the Pump Down and Walk Away

Reducing speed may change the symptom.

That does not necessarily identify the cause.

If the system once operated properly at the required flow and now cavitates under the same conditions, determine what changed.

Has the strainer become restricted?

Has basin level changed?

Was piping modified?

Is the pump now being asked to provide more flow?

The goal is not simply to make the noise disappear.

The goal is to understand why the pump no longer has the inlet conditions it needs.

Protect Cooling Performance Before Peak Demand

Summer is when cooling tower pumps are often needed most.

A developing suction problem that is easy to ignore during mild weather can become much more significant during sustained cooling demand.

If you suspect cooling tower pump cavitation, investigate it before reduced performance becomes equipment damage or an HVAC interruption.

Academy Pump & Motor provides commercial pump troubleshooting, service and repair throughout Calgary and surrounding areas.

Call Academy Pump & Motor at 403-437-7888 or visit academypump.ca.