## Archive for January, 2018

### How Decreasing Pressure Contributes to Cavitation

Sunday, January 28th, 2018
 Last time we learned how the thermodynamic properties of water contribute to the phenomenon of cavitation, and how liquids exist in three states, solid, liquid, or vapor, depending on temperature and surrounding air pressure.   Our example of an open pot of water being heated on a stove demonstrated that once water temperature rose above 212ºF, it changed to steam, which initiated the cavitation process.   Today we’ll see how decreasing pressure contributes to cavitation. How Decreasing Pressure Contributes to Cavitation         Cavitation can occur without a heat source.  In our pot example, we can start the cavitation process by simply decreasing the pressure of the air resting on top of the water, thereby also decreasing the water’s pressure.     Normally atmospheric pressure on Earth exists at around 15 pounds per square inch (PSI).   But if we introduce a vacuum pump to an enclosed space, we can create an internal pressure which is lower than the surrounding atmospheric pressure outside the pot.  In other words, we create a vacuum.   A vacuum is any air pressure lower than atmospheric pressure.   This vacuum environment produces an entirely new set of circumstances under which cavitation can occur.   In fact, creating a vacuum makes it possible to boil water without using any heat!     As we learned in a past blog on the different forms of heat energy, the boiling point of water varies depending on the location of the stovetop, whether it’s in a place of low altitude, like New Orleans, or higher altitude, like Denver.   But if we apply a tight lid to the pot and isolate its internal atmosphere from surrounding atmospheric pressure, you create a closed environment.   This allows us to manipulate the pot’s internal pressure.   When we attach a vacuum pump to remove air, we reduce the air pressure bearing down on the water inside.   With much of the air removed, pressure inside the pot drops below normal atmospheric pressure existing outside the pot, and we discover that at 0.25 PSI water turns to steam at a mere 59ºF and cavitation can begin.   That’s right, you can boil water without using heat.     Next time we’ll apply our knowledge of water pressure and temperature to an industrial setting and see how cavitation occurs inside pumps. opyright 2018 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________

### Thermodynamic Properties of Water and Cavitation

Monday, January 15th, 2018
 Last time we introduced the phenomenon of cavitation, which simply stated is the rapid formation and collapse of vapor bubbles within liquids.   It’s a destructive force that eats away at the metal parts of water pumps, used in power plants and other industrial settings.   To understand how cavitation comes into play, we’ll explore a branch of engineering known as thermodynamics.     Cavitation doesn’t occur in a glass of water resting on a counter, but bring that water to a boil and the cavitation process will begin.   That’s because cavitation is initiated when liquids change form from one physical state to another, in this case from a liquid to a vapor we commonly call steam.   All liquids exist in three states, namely solid, liquid, and vapor, but in our thermodynamic analysis we’ll only consider two, liquid and vapor, because cavitation can’t occur in solids. Thermodynamic Properties of Water and Cavitation         At normal atmospheric pressure of 15 pounds per square inch (PSI) which exists in the average kitchen, water remains in a liquid state between the temperatures of 32ºF and 212ºF.   Above 212ºF water begins to boil, transforming into steam vapor.   The state in which water exists depends on two thermodynamic properties, namely temperature and pressure.   Change one of these variables and it affects the other, and thereby the conditions under which cavitation will occur.     We’ll take an in-depth look at this next time when we revisit the topics of pressurization and vacuums. opyright 2018 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________

### Boiler Feed Pumps Experience Cavitation

Wednesday, January 3rd, 2018
 Shortly after I graduated with my engineering degree I worked as a power plant engineer at an electric utility.   One day I was walking through the plant and heard a loud racket coming from the boiler feel pumps.   These are the massive centrifugal pumps that deliver pressurized water to the boiler.   The water is transformed into steam to drive steam turbines and spin electrical generators, which ultimately results in electrical power.   The noise was so loud, it sounded like rocks were being ground up.   I asked a coworker what was going on, and he replied matter-of-factly, “The pumps are cavitating.” Boiler Feed Pumps Experience Cavitation         So what exactly is cavitation?   We’ll find out next time when we explore the mechanics of this noisy phenomenon as it applies to boiler feed pumps and other centrifugal pumps. opyright 2017 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________