Cavitation by design creates value, avoids destruction
In its simplest form, cavitation can be defined as: the creation of a vapor bubble through a change in energy level in a liquid and then a rapid collapsing of that same bubble with additional changes in energy level. It comes in many forms and is created by a number of conditions.
The condition can be considered good, bad or neutral. Using a simplified neutral example, if you’ve ever stood by a kitchen stove and observed an open pot with water in it getting ready to boil but not quite boiling, you will hear a high pitched sound and observe tiny bubbles emanating from the bottom surface of the pot (the hottest point) and disappearing as the bubbles travel toward the surface. What is happening is the process of cavitation.
In this case, it is neutral in nature as it neither hurts nor helps its surroundings. The vapor bubbles are created in the pot by adding thermal energy to the water at the bottom and the bubbles of course attempt to rise to the surface. As they make their way upward they encounter cooler water (remember we are not talking about water brought to a full boil here) somewhere between the bottom of the pot and the surface. As the bubbles hit the slightly cooler water, at some point the bubbles implode, reverting back to liquid. Seems simple, but it is not.
Enter the theater of your mind and observe the surface of the vapor bubble in the liquid. The reduced temperature hits the vapor bubble surface as it rises in the cooler liquid and immediately begins to collapse. The surface of the bubble gets smaller and smaller. The liquid behind the surface of the bubble moves right along with it, of course. Keep in mind that the vapor is a compressible fluid and the liquid non-compressible.
Taking a three-dimensional view, the left side of the liquid is accelerating going faster and faster toward the center of the bubble, the right side of the liquid is accelerating likewise towards the center but from the opposite direction. The upper and lower liquid walls are racing towards each other as are those paired in the front and the back.
When all three dimensional acceleration masses meet, they are at their fastest speed. When they all meet at once, they make contact and the level of the liquid acceleration reverses to super deceleration like rapidly applying the brakes to your car with both feet, no ABS and no tire skidding, except faster. The deceleration goes off the charts because all of a sudden there is no more compressible vapor, only non-compressible liquid banging into each other like a hammer hitting an anvil only three dimensionally.
The energy that is released from the implosion is in the form of vibration, noise (the high pitched sound) and a slight rise temperature rise of the liquid as the enthalpy of the vapor transforms into sensible heat in the liquid phase. Of course when the pot comes to full boil the temperature of the vapor and the liquid are now the same and the components of cavitation disappear, and after the addition of some olive oil and salt you’re ready to cook your favorite pasta. After all, why would you be staring at a pot of water?
Cavitation that is designed to be controlled and utilized for its good aspects can have very positive effects. Cleaning solid objects of unwanted matter is a major use of cavitation and some of the more popular utilizations are mechanical parts cleaning and plaque cleaning from one’s teeth.
Parts cleaners may use a bath, usually of water, that is subjected to ultrasonic frequencies generated from an electric transducer located at the walls of the tub. The ultrasonic energy, which operates above 20,000 cycles per second, causes the molecular forces that hold water in the liquid state to transition to the vapor state for an extremely short period of time and back to the liquid state above 20,000 times a second on an extremely small bubble scale. When the collapsing bubbles contact cleaned, attached parts less hard than the parent material become dislodged.
Your dentist may use an ultrasonic scaling tool to clean your teeth which uses a stream of water that is subjected to ultrasonic energy at its tip. As the tool is placed against your tooth, the tool rides on a film of water flow where the collapsing microscopic bubbles dislodge plaque. If you have ever experienced this kind of cleaning, you will recall the signature sound which not coincidently is similar to the sound of the near boiling pot of water on your stove.
On the negative side, cavitation can be an unwitting enemy and be very damaging to mechanical equipment in industry in which systems are poorly designed. In the common industrial steam system design where saturated steam is generated, distributed and utilized for its change of phase from vapor to liquid, the condensate returned to the boiler can harbor many potential problems.
Consider a properly operated boiler generating near pure steam. Let’s call it pure vapor. Since we are dealing with saturated steam, the mass flow is constantly on the vapor/liquid cusp and has a high propensity to change into the liquid state with just the slightest removal of heat. When we distribute it we have to deal with two phase flow in the same pipe as the greatest insulation in the world cannot prevent heat loss from the inside of the distribution pipe to its surroundings.
Having both liquid and vapor in the same pipe at the same temperature makes the situation potentially ripe for cavitation if the energy balance is upset under certain conditions. Condensate return, with liquid often at its boiling point and often mixed with flash steam, comprises even more potential cavitation problems.
The difference between good cavitation and bad cavitation is one of design or lack thereof. With well-designed and applied ultrasonic vibrations, we see the application of intense cleaning techniques utilizing cavitation. In a steam cycle many potential cavitation sites exist where the collapsing water vapor bubble causes destruction of metal if the cavitation occurs at the metal surface for an extended period of time. If the destructive potential is not recognized and incorporated in the design, it becomes costly and possibly dangerous.
The collapsing cavitation bubble, imploding at supersonic speeds and exposed to the metal for a length of time may destructively remove some of that metal by erosion and corrosion on a micro scale giving it a weak and spongy state and finally removing the ‘sponge’ on a macro scale.
The most common destructive cavitation in the steam cycle may show its ugly head in piping systems, control valves and pumps. Each of these areas will be looked at, evaluated and potential solutions put forth in Part 2.
Jack Goyette is an Account Executive for the Mid-Atlantic Region of Spirax Sarco. Content provided by Spirax Sarco. Originally published in Steam News Volume 4 Issue 2. Edited by Brittany Merchut, Project Manager, CFE Media, bmerchut(at)cfemedia.com