One of the most common fault symptoms in liquid ring vacuum pumps is a crackling or gravel-like rattling sound from the pump, accompanied by increased vibration. These symptoms are, more often than not, a sign of cavitation. This article covers how cavitation physically forms inside a liquid ring pump, the conditions under which it's more likely to occur, and which design and operating practices are effective at preventing it.
Table Of Contents
- What Is Cavitation, and How Does It Form in a Liquid Ring Pump?
- Why Do Vapor Bubbles Collapse So Violently?
- What Are the Symptoms of Cavitation?
- What Damage Does Cavitation Cause to a Pump?
- Under Which Operating Conditions Is Cavitation More Common?
- How Does a Two-Stage Design Reduce Cavitation Risk?
- What Is an Anti-Cavitation Line, and How Does It Work?
- What Operating Practices Help Prevent Cavitation?
- Frequently Asked Questions
What Is Cavitation, and How Does It Form in a Liquid Ring Pump?
Cavitation is the sudden formation of vapor-filled cavities (bubbles) within a liquid, occurring when local pressure in that liquid drops below its vapor pressure at the prevailing temperature. The vapor pressure concept covered in our article "Why Does Service Water Temperature Reduce the Vacuum Level of a Liquid Ring Pump?" also underlies cavitation; but what's involved here isn't the service liquid's general evaporation, rather a local, sudden pressure drop within the liquid itself.
In a liquid ring pump, as the rotor turns, the volume between the liquid ring and the rotor blades changes continuously; in regions where that volume expands fastest, local pressure can momentarily drop to very low levels. If that local pressure falls below the service liquid's vapor pressure, sudden vapor bubbles form within the liquid.
Why Do Vapor Bubbles Collapse So Violently?
The bubbles that form travel along with the liquid ring toward the pump's compression zone. As local pressure rises again in that zone, the vapor inside the bubble suddenly condenses back into liquid; this condensation causes the bubble to collapse almost instantaneously (an implosion).
During this collapse, the liquid surrounding the bubble rushes inward at very high speed, generating an extremely high, but extremely small and short-lived, pressure spike (a micro-jet) at the center of the collapsing bubble. When that spike strikes a nearby metal surface — a rotor blade or the casing — it causes microscopic-scale erosion of the material.
What Are the Symptoms of Cavitation?
The most characteristic symptom of cavitation is an irregular, crackling sound from the pump, often described as sounding like the pump is moving gravel or sand. This sound is typically accompanied by increased vibration; in severe cases, that vibration can even be felt in nearby equipment or piping.
Alongside noise and vibration, prolonged cavitation can also bring slight fluctuations in motor current and a noticeable drop in pump performance over time. This cluster of symptoms is one of the most common causes behind field complaints about noise and vibration (see our article "Why Does an Industrial Vacuum Pump Run Noisily and With Vibration?").
What Damage Does Cavitation Cause to a Pump?
While a single bubble collapsing has an extremely small effect, a liquid ring pump can generate and collapse thousands of bubbles simultaneously and continuously. Over time, these repeated micro-impacts create small pits — a phenomenon called "pitting" — and a rough erosion pattern on rotor blade and casing surfaces.
As this erosion progresses, the tight tolerances between the rotor and casing can be disturbed, sealing can degrade, and pump efficiency can drop. Severe cavitation left unaddressed for a long time can reach a point where the rotor or casing requires early replacement.
Under Which Operating Conditions Is Cavitation More Common?
Cavitation risk rises when a pump operates outside its own design range — particularly at a suction pressure much lower than expected (that is, a deeper vacuum than intended). A sudden loss of load in the process (a valve closing abruptly, for example) can let the system "run away" past the pump's design point toward an excessively deep vacuum, noticeably raising cavitation risk.
Service liquid temperature and flow rate are also factors affecting cavitation risk; as covered in our article "Why Does Service Water Temperature Reduce the Vacuum Level of a Liquid Ring Pump?", keeping the service liquid within the correct temperature and flow range helps the pump operate stably, close to its design point.
How Does a Two-Stage Design Reduce Cavitation Risk?
As covered in our article "Should You Choose a Single-Stage or Two-Stage Liquid Ring Vacuum Pump?", a two-stage design splits the compression load between two rotors. This split reduces the pressure differential each stage has to handle; a smaller pressure differential, in turn, reduces the likelihood of local pressure reaching the service liquid's vapor pressure.
Cavitation is generally less of an issue in two-stage designs. This makes the two-stage series worth considering for applications expected to frequently run at low suction pressure, or that have a variable process profile.
What Is an Anti-Cavitation Line, and How Does It Work?
In the general design of liquid ring vacuum pumps, an "anti-cavitation line" or similar protection mechanism — one that admits a controlled amount of gas into the region where cavitation risk is highest, raising local pressure and preventing vapor bubble formation — is commonly used. This type of line works on a principle similar to gas ballast: adding extra gas to keep local pressure above vapor pressure.
To find out whether a specific Gücüm Pompa model includes this kind of protection line, and to get model-specific technical details, contacting Gücüm Pompa's technical team is recommended.
What Operating Practices Help Prevent Cavitation?
Reducing cavitation risk starts with correctly matching the pump to the process's actual suction pressure and capacity needs; a pump that's oversized or undersized for its application is more prone to operating outside its design point. Sudden valve closures or load losses in the process should also be made as gradual and controlled as possible.
Keeping the service liquid within the correct temperature and flow range, checking rotor and casing surfaces for wear during regular maintenance, and acting early when unusual noise or vibration is noticed in the field all help catch and address cavitation before it causes lasting damage to the pump.
Frequently Asked Questions
How Do You Recognize the Sound of Cavitation?
Cavitation is typically noticed as an irregular, crackling sound, often described as sounding like the pump is moving gravel. This sound has a more chaotic character than a steady, regular mechanical noise.
Should a Pump Be Stopped Immediately If Cavitation Is Noticed?
Mild, short-lived cavitation doesn't always require an immediate shutdown, but continuous or severe cavitation warrants checking operating conditions (suction pressure, service liquid) and consulting Gücüm Pompa's technical team if needed.
Can Cavitation Damage Be Repaired?
Minor wear caught early can sometimes remain within acceptable limits, but advanced cavitation damage generally requires replacing the rotor or casing components. An exact assessment requires inspecting the pump and consulting Gücüm Pompa's technical team.
What's the Cavitation Risk Like in Monoblock Pumps?
As covered in our article "In Which Cases Does a Monoblock Vacuum Pump Offer an Advantage Over Coupled Models?", the monoblock series operates at relatively small capacity and a limited vacuum range (up to 33 mbar). Cavitation risk is primarily tied to operating a pump outside its own design point, so using the monoblock series correctly within its own capacity and vacuum range matters just as much.
