If a pump sounds like it is passing gravel, it is not passing gravel. It is destroying itself, and it will keep going until someone changes the suction side.
Cavitation is the most misdiagnosed failure in pumping, mostly because the noise comes from the pump while the cause sits upstream of it.
What Is Pump Cavitation?
Pump cavitation is the forming and collapsing of vapor bubbles inside a pump when pressure at the impeller eye falls below the liquid’s vapor pressure. The water boils at ordinary temperature, then the bubbles implode against the vanes, taking a microscopic piece of metal with each collapse.
Key Takeaways
- Cavitation is a suction-side problem, not a pump fault.
- It sounds like gravel, not like a hum or a whine.
- Damage is cumulative and permanent, measured in hours.
- Never throttle a suction line to control flow.
- Available suction head must exceed what the pump requires.
The Five Usual Causes
| Cause | Why it starves the pump | Fix |
|---|---|---|
| Clogged strainer | Restriction before the pump | Clean or resize it |
| Suction lift too high | Less pressure at the eye | Move the pump lower |
| Undersized suction pipe | Friction eats the margin | Go one size larger |
| Throttled suction valve | Deliberate restriction | Open fully, throttle discharge |
| Hot liquid | Vapor pressure rises | Flood the suction inlet |
Notice that four of the five are decisions someone made, not parts that failed. The margin they are eating into is net positive suction head, explained in this reference on NPSH.
How to Recognize It
- Rattling or crackling rather than a steady hum.
- Vibration you can feel through the base.
- Discharge pressure swinging instead of holding steady.
- Pitted impeller vanes that look sandblasted from the front.
- Flow falling off with no change to the system.
The pitting is the giveaway once the pump is open. Grit wear rounds the vane edges evenly, while cavitation eats holes into the low pressure face, and telling those apart is covered in our pump impeller guide.
Why It Wrecks More Than the Impeller
The vibration that comes with cavitation travels straight into the bearings and the seal faces. Faces designed to run parallel within a few microns cannot tolerate being shaken, so they chip and start to weep.
That is why a cavitating pump often presents as a leak first, as described in our pump mechanical seal guide, and the wider parts picture is in the water pump parts guide.
Designing It Out
Cavitation is cheapest to solve before a pump is bought. Keep the suction line short, straight and at least one size above the pump inlet, and put the pump as close to the water level as the site allows.
Sizing matters just as much, because a pump running far to the right of its curve demands more suction head than one near its best efficiency point. That relationship is set out in our guide to reading a pump curve, and a clean, correctly sized foot valve protects the margin you designed in.
Frequently Asked Questions
What is cavitation and why is it bad?
Cavitation is the forming and collapsing of vapor bubbles inside a pump when pressure at the impeller eye drops below the liquid’s vapor pressure. It is bad because each bubble implodes against the vanes and takes a microscopic piece of metal with it. Thousands collapse every second, so the damage accumulates quickly and is permanent once done.
What are the warning signs of cavitation?
The warning signs of cavitation are a rattling or crackling noise like gravel passing through the casing, vibration you can feel through the base, and a discharge gauge that will not settle. Flow falls off with no change to the system, and opened pumps show pitting on the impeller vanes.
What is the most common cause of pump cavitation?
The most common cause of pump cavitation is a restricted suction side, usually a clogged strainer or foot valve. A suction lift that is too high, undersized suction pipe, a partly closed suction valve or hot liquid do the same thing. Almost every cause sits upstream of the pump, not in it.
How to stop a pump from cavitating?
Stop a pump cavitating by giving it more suction pressure than it actually needs. Shorten and upsize the suction pipe, clean the strainer and the foot valve, move the pump closer to the water level and open the suction valve fully. Never throttle the suction side to control flow; throttle the discharge instead.
What are the two types of cavitation?
The two types are suction cavitation and discharge cavitation. Suction cavitation happens when the pump is starved of liquid and bubbles form at the impeller eye, which is the common case. Discharge cavitation happens when a pump runs against a nearly closed valve and liquid recirculates violently inside the casing.
Suction-Side Help at Ken’s Distributing
Ken’s Distributing sizes suction lines, strainers and pumps together across the Franklin Electric range and the Monarch water pump range.
Tell us the lift, the pipe run and the liquid temperature, and we will check the suction margin before recommending a pump rather than after the impeller is pitted.