Cavitation Created by Joshua Toebbe NOHS 2015

Cavitation What is cavitation? Lets consider a control valve in a process pipe. When the valve closes, the fluid above the valve creates fluid hammer or fluid shock as it transfers its energy to the valve. But the fluid below the valve also has kinetic energy and continues to flow… Created by Joshua Toebbe NOHS 2015

Cavitation Fluid continues to flow past the closed valve. But, there is nothing behind it to fill in the space. As it moves away from the valve a gap forms behind it. Created by Joshua Toebbe NOHS 2015

Cavitation This gap starts out as a vacuum, since there was nothing behind the fluid to fill the gap. The vacuum pressure causes the adjacent fluid to “boil” off (like in a vacuum chamber). The gap rapidly fills with low pressure fluid vapor. Created by Joshua Toebbe NOHS 2015

Cavitation Even though this gap isn’t a vacuum its still pretty close (kind of like the atmosphere of mars, its so thin that there’s not much difference between it and space). This low pressure area is surrounded by a much higher pressure fluid, creating a pressure gradient. Created by Joshua Toebbe NOHS 2015

Cavitation The pressure gradient causes the cavity or bubble to violently collapse. This process releases a tremendous amount of energy. And creates a high pressure micro-jet of fluid. Created by Joshua Toebbe NOHS

Cavitation If the bubbles collapse in the middle of the fluid nothing really happens. If they collapse against the valve or pipe surface, the jet of water created by the collapse can damage the structure. Created by Joshua Toebbe NOHS

Industrial high pressure jets On a side note, high pressure water jets can do a lot of damage. They are routinely used in industry to make precision cuts through metals and stone. Created by Joshua Toebbe NOHS

Cavitation While the pressure in the jets created by cavitation is not quite that strong, its not that far off. Cavitation pitting can ruin control vales, as well as weaken and break pipes. Created by Joshua Toebbe NOHS

Cavitation Cavitation can occur anytime there is a rapid unchecked change of pressure in a fluid. After the closing of a control valve Against the blade of a propeller At a sudden change in direction or elevation of a pipe Behind orifice plates and flanges. Created by Joshua Toebbe NOHS content/

Cavitation example #1 Cavitation occurs if the pressure in the pipe drops below the fluids. saturated vapor pressure (svp) Using Bernoulli’s equation and the continuity equation the pressure can be calculated, and cavitation determined. Created by Joshua Toebbe NOHS

Cavitation example #1 For water at 20 ˚C the svp is kpa. If the fluid in a pipe with an area of.23 square meters flowing at 6 m/s passes a control valve that begins to close and gets forced into a smaller area of square meters, does cavitation occur? Created by Joshua Toebbe NOHS

Cavitation example #1 first we use the continuity equation to calculate the fluid speed in the valve Created by Joshua Toebbe NOHS 2015

Cavitation example #1 Created by Joshua Toebbe NOHS 2015

Cavitation example #1 Created by Joshua Toebbe NOHS 2015

Cavitation example #1 Created by Joshua Toebbe NOHS 2015

Cavitation example #1 If the pressure down stream rises above the svp the bubble collapse and cavitation occurs. If the pressure down stream does not rise above the svp then cavitation does not occur. Instead, flashing is said to have occurred. Down stream of the valve a pressure gauge registers 213 kpa, which is greater than kpa Cavitation occurs Created by Joshua Toebbe NOHS 2015

Cavitation example #2 To prevent cavitation at on orifice plate (used to help calculate flow rates of fluids) the gap must be large enough to prevent a pressure drop below the svp. If the fluid has an svp of kpa. The upstream pressure is 312 kpa with an area of square meters. The fluid enters the orifice plate at 13 m/s, how big must the opening be to prevent cavitation? Created by Joshua Toebbe NOHS 2015

Cavitation practice #1 If a fluid has an svp of 9.65 kpa. The upstream pressure is 212 kpa with an area of 0.17 square meters. The fluid enters the orifice plate at 7 m/s, how big must the opening be to prevent cavitation? (Will be collected at end of class) Created by Joshua Toebbe NOHS 2015

Cavitation practice #2 For water at 20˚C the svp is kpa. If the fluid in a pipe with an area of.23 square meters flowing at 6 m/s and a pressure of 277 kpa passes a control valve that begins to close and gets forced into a smaller area of 0.13 square meters, does cavitation occur? (Will be collected at end of class) Created by Joshua Toebbe NOHS 2015

Prevention (practice #3) How can cavitation be prevented? Its extremely damaging, causing costly repair. So how can the rapid changes in pressure be prevented? Lets look at a control valve situation. How can cavitation be prevented at a control valve? (This is an answer you want to right down) Close the valve slower Created by Joshua Toebbe NOHS 2015

Prevention (practice #4) What about at a pump impeller? How can cavitation be prevented along a run of pipe entering a pump? hints: Changing _____________ causes cavitation to occur Bernoulli’s principle relates a change in ___________ to a change in ____________ (This is an other answer you want to right down) To prevent a rapid change in pressure, slowly accelerate the fluid in the pump. (the longer the run of pipe before the pump, the more continuous the flow of fluid) Created by Joshua Toebbe NOHS 2015 pressure velocity pressure

Prevention (practice #5) Consider an entire pipe system, and the svp curve of the fluid. How can cavitation be prevented through out an entire pipe system? (This is another answer you want to right down) Lower the fluid temperature Created by Joshua Toebbe NOHS 2015

Biological processes What about in nature? Outside of man made processes does cavitation occur? Lets look at Tuna, yes… the fish… Tuna are fast swimmers. Multiple fish have been pulled from the water with lesions on their tail. Due to their swimming speed and the speed at which their tai must move, these are probably cavitation scars. Created by Joshua Toebbe NOHS 2015

Biological processes Dolphins are also fast swimmers, but their speed is limited not by their power, but by their pain threshold. Dolphins can only swim so fast, because if they go faster, collapsing cavitation bubbles on the tail generate to much pain. Tuna, unlike dolphins do not have this problem, their boney tails have no nerve endings. But the cavitation creates a film around the fin decreasing its effectiveness, and increasing its drag. Created by Joshua Toebbe NOHS 2015

Biological processes There is also much evidence that the mantis shrimp and the pistol shrimp strike so fast that they produce a low pressure region generating cavitation. The collapsing bubbles produce a shock wave that kills, stuns, or opens the shells of their prey. Created by Joshua Toebbe NOHS 2015

Biological processes Some people even believe that cavitation may be crucial in generating enough pressure in magma chambers to restructure carbon into diamonds. (especially kimberlites) Created by Joshua Toebbe NOHS 2015

Application So while cavitation can be devastating and a lot of time, energy and effort goes into preventing it (or repairing its damage); is there any reason we would want to cause it on purpose? (other than for breaking glass, of course) Ultrasonic sound waves create regions of high and low pressure. This pressure oscillation can produce cavitation bubbles in certain fluids (especially within the body). The force generated by the cavitation can be used to break-up kidney stones. (Or a similar process takes place with a non- invasive type of “liposuction”) Created by Joshua Toebbe NOHS 2015

The End Don’t forget to go with the flow… (I know, I know…) (and turn in your practice problems please) Created by Joshua Toebbe NOHS 2015

Sources Engineering Tool Box. (nd). Equation of Continuity. Retrieved January 28, 2015, from continuity-d_180.htmlhttp:// continuity-d_180.html Modtech (Sept. 29, 1999). Fluid Flow and the Continuity Equation. Retrieved January 28, 2015, from Brice, T., Hall, T. (Jan 20, 2014) Vapor Pressure. Retrieved January 28, 2015, from Brahic, C. (2008, March 28). Dolphins swim so fast it hurts. Retrieved January 29, 2015, from dolphins-swim-so-fast-it-hurts.htmlhttp:// dolphins-swim-so-fast-it-hurts.html Patek, S. (2004, Feb.). The shrimp with a kick! Retrieved January 29, 2015, from Created by Joshua Toebbe NOHS 2015