Editing Cavitation (section)

===Pumps and propellers===
Major places where cavitation occurs are in pumps, on propellers, or at restrictions in a flowing liquid.

As an impeller's (in a pump) or propeller's (as in the case of a ship or submarine) blades move through a fluid, low-pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blade moves, the lower the pressure can become around it. As it reaches [[vapor pressure]], the fluid [[vaporization|vaporizes]] and forms small [[liquid bubble|bubble]]s of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may be audible and may even damage the blades.

Cavitation in pumps may occur in two different forms:

====Suction cavitation====
[[Suction]] cavitation occurs when the pump suction is under a low-pressure/high-vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump, where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition can have large chunks of material removed from its face or very small bits of material removed, causing the impeller to look spongelike. Both cases will cause premature failure of the pump, often due to bearing failure. Suction cavitation is often identified by a sound like gravel or marbles in the pump casing.

Common causes of suction cavitation can include clogged filters, pipe blockage on the suction side, poor piping design, pump running too far right on the pump curve, or conditions not meeting [[Net positive suction head|NPSH]] (net positive suction head) requirements.<ref>{{Cite news|url=https://info.triangle-pump.com/blog/pump-cavitation|title=Common Causes of Cavitation in Pumps|last=Kelton|first=Sam|date=May 16, 2017|publisher=Triangle Pump Components|access-date=2018-07-16|archive-date=2018-07-16 |archive-url=https://web.archive.org/web/20180716224100/https://info.triangle-pump.com/blog/pump-cavitation|url-status=dead}}</ref>

In automotive applications, a clogged filter in a hydraulic system (power steering, power brakes) can cause suction cavitation making a noise that rises and falls in synch with engine RPM.  It is fairly often a high pitched whine, like set of nylon gears not quite meshing correctly.

====Discharge cavitation====
Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller, it must pass through the small clearance between the impeller and the pump housing at extremely high flow velocity. This flow velocity causes a vacuum to develop at the housing wall (similar to what occurs in a [[Venturi effect|venturi]]), which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump housing. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.{{citation needed|date=July 2023|reason=applying the principle of the venturi would require the dynamic pressure to be equal to or greater than the total pressure. Is this the case here?}}

Discharge cavitation in joint fluid is thought to cause the popping sound produced by bone [[Cracking joints|joint cracking]], for example by deliberately cracking one's knuckles.

====Cavitation solutions====
Since all pumps require well-developed inlet flow to meet their potential, a pump may not perform or be as reliable as expected due to a faulty suction piping layout such as a close-coupled elbow on the inlet flange. When poorly developed flow enters the pump impeller, it strikes the vanes and is unable to follow the impeller passage. The liquid then separates from the vanes causing mechanical problems due to cavitation, vibration and performance problems due to turbulence and poor filling of the impeller. This results in premature seal, bearing and impeller failure, high maintenance costs, high power consumption, and less-than-specified head and/or flow.

To have a well-developed flow pattern, pump manufacturer's manuals recommend about (10 diameters?) of straight pipe run upstream of the pump inlet flange. Unfortunately, piping designers and plant personnel must contend with space and equipment layout constraints and usually cannot comply with this recommendation. Instead, it is common to use an elbow close-coupled to the pump suction which creates a poorly developed flow pattern at the pump suction.<ref>{{cite web| last=Golomb| first=Richard| title=A new tailpipe design for GE frame-type gas turbines to substantially lower pressure losses |url=http://cat.inist.fr/?aModele=afficheN&cpsidt=1454644|publisher=American Society of Mechanical Engineers| access-date=2 August 2012}}</ref>

With a double-suction pump tied to a close-coupled elbow, flow distribution to the impeller is poor and causes reliability and performance shortfalls. The elbow divides the flow unevenly with more channeled to the outside of the elbow. Consequently, one side of the double-suction impeller receives more flow at a higher flow velocity and pressure while the starved side receives a highly turbulent and potentially damaging flow. This degrades overall pump performance (delivered head, flow and power consumption) and causes axial imbalance which shortens seal, bearing and impeller life.<ref>[[Pulp & Paper]] (1992), Daishowa Reduces Pump Maintenance by Installing Fluid Rotating Vanes</ref>
To overcome cavitation:
Increase suction pressure if possible.
Decrease liquid temperature if possible.
Throttle back on the discharge valve to decrease flow-rate.
Vent gases off the pump casing.