## Archive for February, 2018

### A Centrifugal Pump’s Curved Features are Key to Functionality

Wednesday, February 21st, 2018
 Last time we learned how centrifugal pumps can create a low pressure environment at the pump’s inlet, which can allow water inside the pump to boil at temperatures far lower than normal.   Ultimately, this results in the formation of tiny but destructive cavitation bubbles.   Today we’ll see how a centrifugal pump’s curved features are key to its functionality. A Centrifugal Pump’s Curved Features are Key to Functionality         Even a casual glance at a centrifugal pump will disclose its many curved features.   As the illustration shows, both the housing and internal impeller blades, are curved.   These curves are known as volutes.   The volutes’ dimensions are mathematically generated by engineers to facilitate the precise flow of water from inlet to discharge by way of the pump’s impeller blades.     Next time we’ll see how a centrifugal pump is home to both low and high water pressure, creating a volatile environment in which cavitation bubbles form and collapse. opyright 2018 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________

### Centrifugal Pumps Can Create Vacuums

Thursday, February 8th, 2018
 Last time we performed an engineering experiment that demonstrated how we can lower the boiling point of water inside a lidded pot without applying heat if we use a vacuum pump to lower the pot’s internal pressure.   We discovered that when pressure was lowered to 0.25 pounds per square inch (PSI), the water inside the pot turned to steam at a mere 59ºF, which initiated the cavitation process.   Today we’ll see how centrifugal pumps can also create vacuums to initiate cavitation.   Centrifugal Pumps Can Create Vacuums         As we learned in a past blog, centrifugal pumps contain rotating impellers within a housing called a volute.   This housing has an inlet, known as an eye, where water flows into the pump from a pipe, and an outlet, known as a discharge, where water flows out of the pump.   The centrifugal pump creates a vacuum by mimicking the action of sucking soda through a straw.    The spinning impeller draws water into the housing by creating low pressure at the inlet, and if the pressure gets low enough, we’ll recreate what happened in our vacuum pump and pot experiment.   Water will boil at temps far lower than normal boiling point of 212 ºF.   Just as in our experiment, if pressure is lowered to 0.23 PSI, water present at the pump inlet will boil at 59ºF, causing thousands of tiny steam bubbles to form and the pump to cavitate.     They’re just tiny bubbles, so what harm can they do? We’ll find out next time. opyright 2018 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________