Pump Cavitation in Chilled Water and Condenser Water Systems: The Gravel Sound and What the Suction Gauge Already Knows

• A pump's published net positive suction head (NPSH) required is where it has already lost 3 percent of its head. It is a limit, not a safe operating point.
• Hotter water needs more suction pressure: vapor pressure climbs from about 0.26 psia at 60 °F (16 °C) to about 7.5 psia at 180 °F (82 °C).
• Where the impeller is pitted tells you why. The hidden side of the vanes points to too little suction head; the visible side points to low flow recirculation.
• When one of two parallel pumps trips, the survivor runs out on its curve and can start cavitating during the very event redundancy was built for.

Pump cavitation is the formation and violent collapse of vapor bubbles inside a pump. Where the local pressure at the impeller eye drops below the liquid's vapor pressure, the water flashes into vapor.

The bubbles are carried outward into higher pressure regions of the impeller. There they collapse violently against the vane surfaces, and the repeated collapses pit the metal.

Temperature sets how easily this happens. The vapor pressure of water rises steeply as it warms:

• 60 °F (16 °C): about 0.26 psia.
• 85 °F (29 °C): about 0.60 psia, typical condenser water.
• 180 °F (82 °C): about 7.5 psia, typical heating water.
• 212 °F (100 °C): 14.7 psia, the pressure of the atmosphere at sea level.

Hotter water needs more suction pressure to stay liquid. That is why heating water pumps and condenser water pumps need more care than chilled water pumps, even when they look identical.

Net positive suction head available (NPSHA) is the suction head above vapor pressure that the system delivers to the pump. NPSH required is what the pump needs, and it is set by the pump's design.

The rule is simple: NPSHA must stay above NPSH required, with a margin. NPSHA belongs to the installation. It falls when tank or basin levels drop, when strainers load up, when suction pressure is lost and when water gets warmer.

NPSH required belongs to the pump. It rises as flow increases, which is why a pump pushed out to the right on its curve is the one that starts to cavitate.

Because NPSHA is built from pressure and temperature, most of its warning signs show up on gauges the site already has. That is the theme of this guide: the suction gauge often knows before anyone hears the pump.

A pump cavitates at its rated NPSH required because of how the rating is defined. The Hydraulic Institute (HI) defines NPSH required as NPSH3: the NPSHA at which the pump's head has already dropped 3 percent because of cavitation (Hydraulic Institute).

HI has used this 3 percent convention since 1932. At NPSH3 the pump is already cavitating; the bubbles are simply large enough to show as lost head.

Bubbles start forming well before the head visibly drops, and erosion can occur with NPSHA above NPSH3 (WaterWorld). Treating NPSH3 as a safe minimum is the most common NPSH mistake.

How much margin is enough

HI publishes recommended NPSH margin ratios, NPSHA divided by NPSH3, in ANSI/HI 9.6.1. The ratio varies with suction energy, a measure of how hard the pump's inlet works (Empowering Pumps).

A margin ratio of roughly 1.3 to 1.7 can be needed just to reach full rated head. High suction energy pumps operating at low flow need much more. The practical point: a pump with a ratio near 1.0 is living on its cavitation limit.

Cavitation sounds like gravel or marbles passing through the pump. The noise is random and broadband, and it is not tied to running speed the way a bearing or imbalance noise is.

It is loudest at the suction side of the casing, so acoustic listening right at the pump is one of the best checks on a walk past. The sound usually changes as flow changes, which is a strong clue in itself.

• Ultrasonic crackle: Airborne ultrasound picks up the crackle of collapsing bubbles at the casing.
• Broadband shaking: vibration analysis shows broadband, high frequency energy rather than clean peaks at running speed.
• Feel: the casing and suction piping can feel rough or buzzy to the hand.

Cavitation is not the only gravel sound in a plant room. Entrained air and recirculation can sound similar, which is why the next two sections matter.

Recirculation is cavitation caused by too little flow rather than too little suction head. Below a certain flow, liquid turns back on itself inside the impeller, forming vortices that cavitate.

• Suction recirculation: the flow turns back at the impeller eye.
• Discharge recirculation: the flow turns back at the vane tips.

Suction recirculation commonly begins somewhere below about 60 to 80 percent of best efficiency flow, depending on pump design (Pumps and Systems). It can happen even with plenty of NPSH margin.

The two problems need opposite fixes. Classic cavitation needs more suction head or less flow. Recirculation needs more flow, which is why throttling a recirculating pump makes it worse.

Variable frequency drives (VFDs) raise the risk. A pump slowed far below design flow can enter suction recirculation, which is why minimum speed and minimum flow settings exist.

The location of the pitting tells you which problem caused it, because each one collapses its bubbles in a different place. When an impeller comes out for repair, photograph it before it is cleaned or scrapped.

• Classic cavitation from too little NPSH, often at high flow: pitting on the low pressure, hidden side of the vanes near the inlet.
• Suction recirculation at low flow: pitting on the pressure, visible side of the vanes at the inlet.
• Discharge recirculation: damage on the pressure side near the vane tips and at the volute cutwater.

This turns a scrap impeller into a diagnosis. If a new impeller goes in without the cause being fixed, it will fail in the same spot.

Record the finding with the work order in the maintenance management system. The next failure can then be compared with the last one.

Closed chilled water loops still cavitate when the loop loses the static pressure that keeps the pump suction above vapor pressure. The water is cold, but a pump can still pull its own suction too low.

• A waterlogged expansion tank or wrong precharge: the tank stops holding the loop's pressure steady.
• Lost fill pressure: a slow leak drains the loop's pressure over weeks.
• The expansion tank on the wrong side of the pump: the pump suction drops below fill pressure.

The expansion tank connection is the loop's point of no pressure change. Whatever the pump does, the pressure at that point stays at fill pressure.

If the pump pumps toward the tank, its suction side is pulled below fill pressure. If it pumps away from the tank, its suction sits at fill pressure. That is why pumping away from the expansion tank is standard design.

Air entrainment can sound similar to cavitation, but gas bubbles cushion rather than implode. Check air separators and vents before blaming NPSH.

Condenser water pumps cavitate because they draw from an open cooling tower basin at atmospheric pressure, with warm water and a suction line exposed to tower debris. Three causes dominate:

• Low basin level: a low tower basin lets a vortex form over the outlet and pull air into the suction.
• A loaded suction strainer: tower debris fills the strainer and adds friction right where the pump can least afford it.
• Warmer condenser water: higher water temperature raises vapor pressure, which eats into NPSHA.

All three get worse at once on hot days. The tower runs harder, the water is warmer and evaporation lowers the basin, just as the chillers need full condenser flow.

Tower and basin checks belong in routine maintenance, covered in our guide to cooling plant maintenance in office buildings. Cooling tower fans and gearboxes are covered in Part 8 of this series.

When one of two parallel pumps trips, the survivor runs out to the right on its curve and can start cavitating. With less system resistance per pump, it moves more flow than it was selected for.

NPSH required rises steeply at high flow. So the remaining pump can start cavitating during the exact event the redundancy was built for.

This is a design check, not a field repair. Ask what flow the surviving pump reaches when its partner trips, and whether NPSHA still clears NPSH required at that flow with margin.

In data centers this matters most. Redundant chilled water and coolant distribution unit (CDU) pumps exist for the trip scenario, so the survivor must be able to carry the load without eroding itself. Our guide to cooling loads nobody owns covers who should own that check.

The suction and discharge gauges show both the cause and the symptom. The suction gauge shows whether the pump has enough pressure to work with, and the discharge gauge shows whether it is delivering steadily.

• Suction reading low or negative: less suction head than design, from a low level, lost fill pressure or a loaded strainer.
• Fluttering discharge needle: unsteady flow from cavitation or air.
• High strainer differential pressure: a loaded strainer adding suction loss.
• Expansion tank and fill pressure gauges: a closed loop losing the pressure that protects its pumps.

Compare each reading with design and with the sibling pump on the same duty. A pump whose suction reads well below its twin has a problem on its own suction side.

Gauges only help if someone reads and records them. Our guide to reading the gauges nobody monitors covers why a reading at the same point every round beats a perfect reading once a year.

Cavitation damages the impeller, the mechanical seal and the bearings, and it steals head and flow. The cost lands in parts, labor and lost capacity.

• Impeller erosion: pitting that grows until the vanes lose shape and the pump loses performance.
• Mechanical seal damage: vibration and vapor at the seal faces break down the film between them, and the seal leaks. A leak brings its own water damage risk.
• Bearing wear: the constant shaking wears the bearings covered in Part 1.
• Lost head and flow: the pump no longer moves the water it was selected for.

In an office building

The cost shows up as noise carried into occupied floors, wasted pump energy and repeat seal and impeller repairs. A pump that keeps needing the same parts is a sign of a recurring failure with a cause on the suction side.

In a data center

The cost is reduced chilled water or CDU flow when the load needs it most. Our guide to liquid cooling inspection covers what else to watch on CDU loops.

Three problems are most often mistaken for cavitation. Telling them apart decides whether the fix is air removal, bearings or valve operation.

• Entrained air: gas bubbles sound similar, but they cushion rather than implode. Check air separators and vents first.
• Bearing failure: a grind or rumble that does not change with flow. Covered in Part 1.
• Water hammer: a sharp bang during valve or pump transients, not a steady crackle. Covered later in the Cooling Plant Performance series.

Vibration faults such as imbalance and misalignment show clean peaks at running speed rather than broadband noise. They are covered in Part 2.

A walk past can read the gauges, hear the sound and see the conditions that cause cavitation. It cannot measure the exact NPSH margin or see the impeller. Each finding carries a label for how much it shows.

• Directly observable: a suction gauge reading low or negative, a fluttering discharge gauge needle, a low basin level or a visible vortex, and a high strainer differential.
• Strong indicator: a steady gravel noise and broadband ultrasound at the pump casing that changes with flow.
• Suspected: the exact NPSH margin and the impeller's condition. These need a hydraulic calculation or a teardown.

That is the limit of any route based check: it catches the symptom and its likely cause, and the calculation or teardown confirms it. A walkthrough record with photos of gauge faces and basin levels makes the trend easy to prove.

This is also why no single sensor is enough. The sound, the gauges, thermal imaging of the seal and bearings, and the vibration spectrum each rule out different suspects. Our predictive maintenance guide covers turning those readings into planned work.

• Listen at the suction side of the casing for crackling.
• Read the suction and discharge gauges and compare them with design and with sibling pumps.
• Read the suction strainer differential pressure where gauges exist.
• Read the expansion tank and fill pressure gauges.
• On open loops, check tower basin level and look for vortexing at the suction.
• Note which pumps are running and which are on standby.

Coming Soon

1. 1. What is the design suction pressure for each pump, and what does the gauge read today?
2. 2. When was each expansion tank's precharge last checked?
3. 3. What happens to the remaining pump's operating point when its partner trips?
4. 4. What is the minimum speed setting on each VFD driven pump?
5. 5. When were the condenser water suction strainers last cleaned?

What does pump cavitation sound like?

Cavitation sounds like gravel or marbles passing through the pump. The noise is random and broadband rather than tied to running speed, and it is loudest at the suction side of the casing.

What is the difference between NPSHA and NPSHR?

NPSHA, net positive suction head available, is the suction head above vapor pressure that the system delivers. NPSHR, net positive suction head required, is what the pump needs, set by its design. NPSHA must stay above NPSHR with a margin.

Can a pump cavitate in a closed chilled water loop?

Yes. A waterlogged expansion tank, a wrong precharge or lost fill pressure can drop the suction below what the pump needs. So can an expansion tank connected on the wrong side of the pump.

What is suction recirculation?

Suction recirculation is flow turning back on itself at the impeller eye when a pump runs at low flow. It commonly begins somewhere below about 60 to 80 percent of best efficiency flow and causes cavitation damage on the visible side of the vanes.

How do you tell cavitation from air in the system?

Both can sound like gravel, but air bubbles cushion rather than implode. Check air separators, vents and tower basin vortexing before blaming low suction head, and compare suction gauge readings with design.

Does cavitation damage the impeller?

Yes. Collapsing vapor bubbles pit the vane surfaces, and the location of the pitting shows the cause. Damage on the hidden side near the inlet points to too little suction head, while damage on the visible side points to recirculation.

What causes cavitation in a condenser water pump?

The usual cause is a low cooling tower basin level that lets a vortex pull in air. A suction strainer loaded with tower debris and warmer condenser water that raises vapor pressure are the other two.

Can a VFD cause pump cavitation?

Yes. A pump slowed far below design flow can enter suction recirculation, which causes cavitation damage. That is why VFD driven pumps have minimum speed and minimum flow settings.

What does the 3% head drop in NPSH mean?

The Hydraulic Institute defines a pump's NPSH required as the suction head at which its head has already dropped 3 percent because of cavitation. At that point the pump is already cavitating, so it is a limit, not a safe operating point.

• Bearing Failure, Stage by Stage: Why Ultrasound Hears It Before Vibration Feels It and Heat Shows It
• Imbalance, Misalignment, Looseness and Soft Foot: Four Faults, Four Vibration Signatures
• Reading the Gauges Nobody Monitors: What Your Facility Already Knows and Never Writes Down
• Ultrasonic Sensing in Data Centers: Hearing Faults Before They Fail

• Hydraulic Institute: NPSH basics and the 3% convention
• WaterWorld: sufficient NPSH margin and pump reliability
• Pumps and Systems: recirculation onset and true operating flow range
• Empowering Pumps: NPSH margin ratio guidance (ANSI/HI 9.6.1)