Archive for the ‘Engineering and Science’ Category

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.

Centrifugal Pump’s Curved Features are Key to 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

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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

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

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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

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

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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

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

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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

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

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Do Time Machines Use Flywheels?

Thursday, December 28th, 2017

    The New Year is upon us, which makes me think about time travel and H. G. Wells’ time machine, and that makes me think about flywheels.   We’ve been talking about crankshafts and flywheels in our just-ended blog series, and when I look at this image of Wells’ famous time traveling device, I can’t help but wonder, Do Time Machines Use Flywheels?   Happy New Year everyone!

Do Time Machines Use Flywheels?

Do Time Machines Use Flywheels?

   

   

opyright 2017 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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Reciprocating Engines Maximize Efficiency When They Employ Flywheels

Thursday, December 21st, 2017

   Last time we had a look inside a marvelous piece of engineering machinery known as a crankshaft.   It plays a key role in converting the reciprocating linear motion of a steam driven engine into the rotary motion required to power externally mounted devices that are attached to it.   Today we’ll finish up our blog series on flywheels when we see how using one in conjunction with a crankshaft facilitates a more even transmission of energy.   Reciprocating engines maximize efficiency when they employ flywheels.

   We learned that the energy in the steam supply decreases as the piston moves in its cylinder, which means a concurrent decrease in the engine’s horsepower and its ability to power machinery.   Without an intervening action, the reciprocating steam engine would stall.   Now, let’s see how adding a flywheel to the crankshaft can solve the problem.

Reciprocating Engines Maximize Efficiency When They Employ Flywheels

Reciprocating Engines Maximize Efficiency When They Employ Flywheels

   

   As we’ve learned before, a flywheel stores up kinetic energy while the engine powering it is performing at full horsepower, but if that power should drop off or cease to be produced, the flywheel gives up the kinetic energy stored inside it so as to keep externally mounted machinery operating until that stored energy is exhausted.   This is all made possible because flywheels are designed to have moments of inertia sufficient to positively contribute to its storage of kinetic energy.   This inertia is a numerical representation of the flywheel’s resistance to change in motion.   Please review our past blog on the subject to refresh your memory.

   The overall effect is that while the engine is operating, there’s an even flow of energy between the engine and flywheel and horsepower is supplied to keep machinery mounted to the crankshaft operating.   Any diminishment in the power supplied will be compensated for by the flywheel’s stored kinetic energy.

   Next time we’ll introduce a new topic, a phenomenon known as cavitation.

opyright 2017 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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A Crankshaft in Motion

Friday, December 15th, 2017

    Last time, we learned that a crankshaft is an engineering device which converts the reciprocating linear motion of an engine’s back-and-forth moving piston into the rotary motion that powers externally attached machinery.   Its movement is shown here,

 A Crankshaft in Motion

A Crankshaft in Motion

   

    We’ll see how a crankshaft and piston’s motion benefits by the use of a flywheel next time.

opyright 2017 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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A Look Inside a Reciprocating Steam Engine

Wednesday, December 6th, 2017

    Last time we developed an engineering formula to calculate the horsepower required to accelerate a flywheel by way of a reciprocating steam engine, which contributes to the storage of kinetic energy inside a flywheel.   Today we’ll gain a clearer understanding of how this works when we take a look inside a reciprocating steam engine.

A Look Inside a Reciprocating Steam Engine

A Look Inside a Reciprocating Steam Engine

   

    A reciprocating steam engine performs the work of transforming steam’s heat energy into the mechanical energy needed to move a piston contained within a cylinder.   During a complete operating cycle this piston travels from one end of the cylinder to the other, then back again.   This is made possible because during the first half of the cycle pressurized steam enters one end of the cylinder and expands inside it, forcing the piston to move.

    This process inside the cylinder results in movement of a piston that’s attached to a piston rod, which in turn is connected to a crankshaft via a connecting rod and crank rod.   The crankshaft is a device which converts the reciprocating linear motion of an engine’s piston into rotary motion and in so doing facilitates the powering of any externally mounted rotating machinery attached to it.   So long as there’s ample steam to power the internal piston, over time, energy in the form of horsepower will be available to externally mounted devices.   The energy in the steam decreases as the steam expands behind the moving piston. So, the engine’s horsepower, will decrease as the piston travels to the end of the cylinder.   If the energy in the steam should become depleted, the reciprocating steam engine will stall. The engine will no longer be able to perform work.

    Next time we’ll see how a crankshaft works when we take a look inside it.

opyright 2017 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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Horsepower Required to Accelerate a Flywheel

Friday, November 24th, 2017

    Last time we discussed how torque is created as a flywheel spins.   This torque is a factor of the flywheel’s moment of inertia, which is dependent on how far the masses of the flywheel’s parts are located from its center of rotation.   Today we’ll present a formula to compute how much horsepower is required to accelerate a flywheel.   And here it is,

Horsepower Required to Accelerate a Flywheel

Horsepower Required to Accelerate a Flywheel

where, T is the torque created on the flywheel’s shaft in units of inch-pounds.   The term n is the flywheel shaft’s speed of rotation in revolutions per minute, RPM.   Horsepower, HP, is engineering shorthand for a unit of power equal to 6600 inch-pounds per second, and the number 63,025 is a constant needed to convert torque, T, and the spinning shaft’s rotations per minute, RPM, into horsepower units.

    Torque is present whether the flywheel’s spin accelerates or decelerates.   During acceleration torque is created, which contributes to the production of kinetic energy that’s stored inside the flywheel.  When a flywheel’s spin decelerates, its mass experiences the effects of negative acceleration, and stored kinetic energy is released.

    As we learned awhile back, horsepower is a function of torque in any moving machinery, including engines and flywheels.  An engine must produce horsepower to accelerate a flywheel connected to its shaft.   By the same token, when the engine’s horsepower output diminishes or stops, the flywheel begins to decelerate.   This deceleration causes kinetic energy stored within the flywheel to be released, providing horsepower necessary to keep the engine and flywheel spinning.   That is, until the power output of the engine returns or the stored kinetic energy of the flywheel is ultimately exhausted.

    We’ll see how that works next time when we take a look inside a reciprocating engine.

opyright 2017 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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