Posts Tagged ‘engineering’

The Depositor’s Pneumatic Actuator

Thursday, June 21st, 2018

    Last time we learned that a fruit jelly depositor in a food manufacturing plant is an example of a positive displacement pump at work.  Today we’ll see how pieces of equipment on the depositor, known as a pneumatic actuators, work.   Pneumatic actuators do not come in contact with the jelly flowing through the depositor.   In other words, no jelly flows through the actuators.   The jelly only flows through the transfer valve and positive displacement pump as we saw last time.  The pump and valve can’t move by themselves.   So, they need some device to set them in motion.   That’s where the pneumatic actuators come into play.   They impart movement to the pump and transfer valve to get the jelly flowing from the hopper and down through the nozzle and onto the pastry.

    A pneumatic actuator is a device that operates using compressed air.   Compressed air, from an external air compressor, enters into a tube in the actuator known as a cylinder.   Inside the cylinder is a piston that can move along the length of the tube.   Attached to the piston is a piston rod which extends to the outside of the cylinder.

    When compressed air is introduced into the cylinder on the left side of the piston, it forces the piston and piston rod to move towards the right side of the cylinder.   But, air must be vented out to atmosphere from the right side of the piston for this movement to occur.   If no venting took place, trapped air to the right of the piston will get squeezed between the piston and the right end of the cylinder.   When the air gets squeezed, it becomes pressurized.  The pressure will impede the movement of the piston.

    Likewise, when compressed air is introduced into the cylinder on the right side of the piston, it forces the piston and piston rod to move towards the left side of the cylinder.

 The Depositor’s Pneumatic Actuator

The Depositor’s Pneumatic Actuator 

 

    So, depending on which end compressed air is admitted to the pneumatic actuator’s cylinder, the piston rod will move to the left or the right.   In engineering terms, the actuator imparts linear motion to machines.   In other words, the piston rod moves back and forth in a straight line.

    Next time, we’ll see how the pneumatic actuator is connected to the depositor’s pump to impart the linear motion that draws jelly from the supply hopper and sends it streaming out of the nozzle onto a passing pastry.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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Positive Displacement Pumps Are Used in Industry

Tuesday, May 29th, 2018

    Last time we learned that the human heart functions as the greatest of all positive displacement pumps, moving a set quantity of blood through it at precise intervals during its operating cycle.   Today we’ll begin our exploration into how positive displacement pumps are used in industry, specifically within a food manufacturing plant.

    At one point in my career I was employed as a design engineer in a food manufacturing plant.   The plant was owned by the leading manufacturer of bakery products in the United States, responsible for supplying restaurants, bakeries, and grocery stores with finished and partially finished pastry goods that they would then resell.   The plant produced vast amounts of puff pastry dough products, all of which were formed and filled with various fillings while zipping along on a production line conveyor belt.   One of the products was a fruit filled pastry in which the belt moved so quickly, depositing fruit fillings into the dough by hand would be impossible, resulting in a frenzied mess similar to what Lucy encountered when she worked in a candy factory.

    Clearly, an automated machine would work better in this and other scenarios.   We’ll see how one known as a depositor functions on a food pastry line next time.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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Reducing Cavitation With A Booster Pump

Monday, May 14th, 2018

    In our last article, we looked at an example problem involving a cavitating centrifugal pump that was drawing water from a storage tank.   The bottom of the storage tank was sitting at the same level as the centrifugal pump’s inlet.   The water level in the tank could not be increased to raise the pump inlet pressure, and thus eliminate the cavitation.   So, the problem was solved by elevating the tank with respect to the pump inlet.   Okay, what if the tank could not be elevated?  How do we stop the centrifugal pump from cavitating?   Well, we can install a booster pump between the tank and the centrifugal pump.

    A booster pump is, as its name implies, a special kind of pump that is used to boost, or raise, water pressure flowing in a pipe.   With regard to our example problem in the preceding article, the cavitating centrifugal pump inlet water is at 108ºF and a pressure of 1.2 pounds per square inch (PSI).

Reducing Cavitation by Raising Tank Elevation--Before

Reducing Cavitation With A Booster Pump — Before

   

    Referring to the thermodynamic properties of water as found in tables appearing in engineering texts, we determine that if we keep water temperature at 108ºF but raise the pressure at the pump inlet from 1.2 PSI to 1.5 PSI we can stop the centrifugal pump from cavitating.   We can install a booster pump to boost the pressure by the required 0.3 PSI and say goodbye to our cavitation problems.

Reducing Cavitation With A Booster Pump -- After

Reducing Cavitation With A Booster Pump — After

   

    This wraps it up for our series on cavitation in pumps.   Next time, we’ll begin learning about some different topics.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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Reducing Cavitation by Raising Tank Elevation

Monday, May 7th, 2018

    Last time we learned that the risk of damaging cavitation bubbles forming at a centrifugal pump’s inlet can be eliminated by simply increasing the water level inside the tank.   Today we’ll do the math that demonstrates how reducing cavitation can be accomplished by raising tank elevation.

Reducing Cavitation by Raising Tank Elevation--Before

Reducing Cavitation by Raising Tank Elevation–Before

   

    In our example we’ll suppose that we’re having a problem with cavitation bubbles forming at the inlet, where water temperature is 108ºF and water level inside the tank stands at 33 inches.   We are using the formula,

P = γ × h                                                                                    (1)

    Equation (1) was introduced previously to correlate water pressure, P, with the specific weight of water, (0.036 pounds/inch3), and the height, h, of the water surface in the tank.   If h is 33 inches, then we obtain,

P = (0.036 pounds/inch3) ×  (33 inches) = 1.2 pounds/inch2         (2)

    So, the weight of the water in the tank exerts a pressure of 1.2 pounds per square inch (PSI) at the bottom of the tank and the pump inlet when it sits at the same elevation as the tank.

    We know that if we increase the water depth in the tank relative to the pump inlet, we can raise the pressure at the pump inlet in accordance with equation (1).   Raising the pressure will eliminate the cavitation bubbles that can form there.   But, our tank is of fixed volume, and we can’t add more water to raise water depth beyond 33 inches.    However, we can increase the elevation of the tank with respect to the inlet, which will produce the same effect.   We’ll use equation (1) to determine the tank elevation, h, that will provide the needed increase.

    Referring to the thermodynamic properties of water as found in tables appearing in engineering texts, we determine that if we keep water temperature at 108ºF but raise the pressure at the pump inlet from 1.2 PSI to 1.5 PSI, while maintaining current water depth in the tank, cavitation will cease.   In other words, we need to increase P by 0.3 PSI.

Example of Reducing Cavitation by Tank Elevation--After

Example of Reducing Cavitation by Tank Elevation–After

   

    Plugging our known values into equation (1) we solve for h,

0.3 PSI = 0.036 pounds/inch3 × h                                                  (3)

h = 0.3 PSI ÷ 0.036 pounds/inch3                                                  (4)

h = 8.3 inches                                                                              (5)

    Cavitation will cease when we elevate the tank by 8.3 inches with respect to the pump.

    Yet another means of increasing inlet pressure is to install a booster pump.  We’ll talk about that next time.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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One way to Reduce Cavitation by Increasing Water Pressure

Monday, April 16th, 2018

    Ever hear the old saying, “There’s more than one way to cook a goose”?   The statement is meant to encourage creative thinking when problem solving.   This forward thinking can be applied to the problem of destructive cavitation bubbles as well.   Finding ways to reduce cavitation is something engineers are well versed in.   As discussed in our last blog, one way to prevent cavitation is by lowering water temperature at a centrifugal pump’s inlet.    But sometimes that isn’t possible.   Today we’ll discuss another way, reducing cavitation by increasing water pressure.

One way to Reduce Cavitation by Increasing Water Pressure

One way to Reduce Cavitation by Increasing Water Pressure

    If you’ve ever seen a movie featuring divers, you’ll no doubt be aware that the deeper a diver goes, the more water pressure there is bearing down on him from above.   The same goes for a centrifugal pump’s storage tank.   The higher the water level inside the tank, the higher the pressure bearing down on the pump’s inlet, which is located at the bottom of the tank.   This is the area in which cavitation bubbles are likely to form.   The mathematical equation that illustrates this relationship is,

P = γ  × h                                                                   (1)

where, P is water pressure at the bottom of the tank, γ is the Greek symbol gamma, representing the specific weight of water, (0.036 pounds/inch3), and h is the depth of the water inside the tank.

    Let’s see what happens when we increase the water level, h, from 72 inches, shown on the left, to 144 inches, on the right.

P = (0.036 Lb/in3)  × (72 in) = 2.592 PSI                      (2)

When the water level is raised to 144 inches, P becomes,

P = (0.036 Lb/in3)  × (144 in) = 5.184 PSI                     (3)

    We see that by raising the water level in the tank from 72 to 144 inches, pressure at the bottom of the tank where the inlet is located is increased from 2.592 PSI to 5.184 PSI, pounds per square inch.

    Next time we’ll see how simply elevating the tank has an impact on cavitation.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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Manipulating Water Temperature to Control Cavitation

Monday, April 9th, 2018

    As we learned previously, cavitation bubbles form at a centrifugal pump’s inlet when the thermodynamic properties of water, namely temperature and pressure, are right.   Today we’ll see how just manipulating water temperature can control cavitation.

 Manipulating Water Temperature to Control Cavitation

Manipulating Water Temperature to Control Cavitation

   

    Some centrifugal pumps draw water from an external heat source such as a heat exchanger in order to provide heat to buildings, generate power, and perform manufacturing processes.   On some exchangers heat is applied at a fixed rate and can’t be varied.   On others heat can be varied by using a heat exchanger fitted with a temperature control.   This makes it easy to reduce or lower water temperature introduced at the pump’s inlet.   If the temperature is kept low enough relative to the pressure at the inlet, cavitation bubbles won’t form.

    Let’s say water enters the pump’s inlet from a heat exchanger at 59ºF and internal pump pressure is 0.25 pounds per square inch (PSI).   With these parameters in place water boils and cavitation bubbles will form in the pump inlet.   But if the heat exchanger is adjusted so that temperature is lowered by a mere two degrees to 57ºF, cavitation ceases.   This is in accordance with the boiling points of water, listed for various pressures and temperatures, as published in engineering thermodynamic texts.

    If it’s not possible to lower water temperature at the pump inlet, an alternate method to control cavitation is to raise water pressure, which can be accomplished in different ways.   We’ll review those options next time.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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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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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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