PV PRESSURE FUNDAMENTALS REVIEW

ANGELICA:So so so— so this is what it feels like to match wits with someone at your level!

("Satisfied," by Lin-Manual Miranda's soundtrack to the musical "Hamilton," 2015)

Way to Go!

PTOA Readers and Students have finished the PTOA'S PV Pressure Introduction!

Next Up ...

The Interrelationship of the PV Pressure with the PV Temperature, the PV Flowrate, and the PV Level

Before all that fun begins let's take a moment to reflect upon all the fundamentals PTOA Readers and Students have learned about the PV Pressure ...

because the fundamentals are the building blocks that support understanding everything else about the PV Pressure!

THE ROLE THE PV PRESSURE PLAYS
IN THE PROCESS INDUSTRIES

PTOA Segment #140 described several industrial applications in which the PV Pressure was needed to convert raw materials into more valuable final products.

PTOA Readers and Students began to realize that Plant Owners invest mucho energy to create and maintain a wide range of Pressures that are needed for processing.

Due to its relationship with the PV Flowrate, PTOA Readers and Students learned that Pressure Differential is a form of the PV Pressure that is likewise crucially used in the process industries.

Next ...

PTOA Segment #141 featured the PV Pressure ISA symbols that represent remote and local automatic instrumentation on PFDs and P&IDs.

All of the above was relatively easy to understand and whetted the whistle to learn what exactly is meant by the word "Pressure."

THE DEFINITION OF "PRESSURE"
AND HOW IT IS IMPACTED BY ITS COMPONENTS:
MASS (m) AND THE ACCELERATION OF GRAVITY (g)

The PV Temperature was a lot easier to understand, wasn't it?

You're either hot or cold or somewhere in between ... and thermometers make it so much easier to gauge exactly where.

The concept of "Pressure" required more brainwork to comprehend ...

precisely because changes in the atmospheric pressure (P_atm) that surrounds us are not encountered on a regular basis ...

so not too many people besides fisherpersons and weatherpersons wake up and wonder what the P_atm will be on any given day.

In PTOA Segment #142, PTOA Readers and Students took their protein pill and rolled up their sleeves to begin learning that a "Pressure" is created anytime a Force is applied over a Surface Area.

Pressure = Force / Area aka P = F / A

The below graphic depicts the magnitude of the Pressure that would be created by forcing a vaccine through a big diameter syringe and a smaller diameter syringe.

We can assume that the amount of vaccine in both syringes is equivalent as is the time interval over which the vaccines are delivered into their respective vaccine needles.

Given those two important assumptions, the Pressure on the needle attached to the large diameter syringe would be LESS than the Pressure on the needle attached to the smaller diameter syringe ...

simply because the surface Area over which the equivalent Force of the vaccines is distributed is greater; thus Force distributed over more Area means less Pressure!

Your Mentor added these paragraphs to illustrate that Pressure can be applied in a sideways direction ... in fact ... Pressure can be applied in any direction!

PTOA Segment #142 focused only on Pressure that is created in the straight downward direction ...

because straight downward directed Pressures are created by "gravitational forces."

That's why PTOA Readers and Students were introduced to Sir Isaac Newton ...

because he was the true genius that figured out how a gravitational Force is created when a Mass (m) is pulled downward and free falls by the acceleration of gravity (g):

Force = (m) * (g)

Sir Isaac also determined that "g" is a constant.

Another way of stating the observation about "g" is:

The velocity of a mass that is free-falling because of gravity increases at a constant rate.

Which just means that it takes the same amount of time for a small Mass and a large Mass to hit the ground when pushed from the same height.

Then PTOA Readers and Students learned that Sir Isaac's decision to name the English measuring unit of Force "the pound of Force (lb_f)" doomed the users of the English measuring system to evermore spend great time and effort distinguishing a lb_f from the English mass unit ... "the pound of Mass (lb_m)."

Ergo ...

the sole purpose of PTOA Segment #143 was to clarify the difference between a lb_f and lb_m ...

which first necessitated alerting PTOA Readers and Students that ...

when the mass is not free-falling but rather at rest on Earth ...

the acceleration of gravity applied to this mass is none other than that annoying entity all of us Earthlings recognize as "Weight."

Otherwise stated:

Force (in lb_f) = m (in lb_m) * (g)

becomes

Weight (in lb_f) = m (in lb_m) * g

A weight scale was thus revealed to be the device Mankind uses to infer the amount of any Mass!

PTOA Readers and Students finished PTOA Segment #143 expertly understanding why the Mass of any body or thing remains consistent throughout the Universe even though the Weight of that body or thing will change with the local gravitational force ... which is different on the Moon and on Jupiter!

At this juncture in the PTOA PV Pressure Introduction, Fred quite righteously pointed out that the expression Sir Isaac used to define Force does not have consistent units on both sides of the expression!

And Fred was right! Sometimes, he really surprises Your Mentor!

So ...

PTOA #144 was thus dedicated to a one time demonstration that showed how the fudge factor (g_c) magically equates the units of Mass and "g" to the units of Force on the other side of the expression ...

a seemingly insignificant demonstration which nonetheless makes it possible for defiant instructors ...

meaning those like Your Mentor who reject the use of "the slug" as the unit of mass in the English system ...

to shamelessly swap between the English units used for Mass and Force ... yet still demand that our students ALWAYS make certain unit consistency exists on both sides of any equation!

Dang!

By now all PTOA Readers and Students realized that Pressure really does take more cerebral effort to understand than Temperature did.

And that's because Pressure depends on a Force ...

which is dependent upon a Mass and ... in this case ... gravity (plus throw in the confusion between lb_f and lb_m in English measuring units):

P = F/A = (m*g) / A

Thus having completely dissected how the components of Force are applied over a surface Area to create a Pressure ...

PTOA Readers and Students had gained the knowledge needed to comprehend how any contained liquid generates and exerts the PV Pressure on the internal surfaces and bottom of the container.

ANY CONTAINED LIQUID GENERATES PRESSURE

Throughout PTOA Segments #145, #146, and #147 ...

PTOA Readers and Students extrapolated the theoretical concepts they had just learned about Pressure ...

to the real world application of a Pressure that is created and exerted on the interior walls and bottom of any container that holds a liquid.

Eventually, PTOA Readers and Students learned that the long winded phrase ...

"the pressure created by a contained liquid and which is exerted on the interior surfaces and bottom of the container" ...

is simply called "hydrostatic" or "head" Pressure in the process industries ...

and is the basis of hydraulic pressure systems and the renewable energy source of electricity generated via hydropower.

Along the journey, PTOA Readers and Students learned that the magnitude of a "head" or "hydrostatic" Pressure depends only on these two things:

The liquid's specific gravity (aka "relative density") and
The "head" of the liquid ... aka the vertical distance from the surface of the liquid to the level where the Pressure is being measured.

By the time they were done reading these three PTOA Segments, PTOA Readers and Students were:

Experienced pros at determining the hydrostatic/head Pressure created and exerted by any contained liquid at any level in the container and on the bottom surface.
Understood the derivation of "0.433"; the constant used in all simplified English unit head pressure calculations.
Understood why the Pressure Profile created by any contained liquid would indicate the minimum Pressure at the liquid's surface and the maximum pressure at the bottom of the container.
Expertly understood the significance of the concepts as well as how to determine fluid and solid "density" and "specific gravity" (aka "relative density)."
Understood why the Total Pressure exerted by a contained liquid must add the contribution of a gas blanket at any level where Pressure is being measured below the surface of the liquid.
Understood that head Pressure can be used in a series of linked tanks to move a liquid process stream between them ... without using mechanical means like a pump!

FOCUS ON ATMOSPHERIC PRESSURE (P_atm)

In PTOA Segment #149

PTOA Readers and Students extrapolated what they had learned about determining the head Pressure of a liquid to the totally analogous ability to determine the magnitude of Atmospheric Pressure (P_atm) based upon ...

(1) The density of the air and ...

(2) The altitude at which P_atm is being measured.

PTOA Readers and Students learned that the device called an altimeter on an airplane is actually measuring P_atm because the relationship between P_atm and altitude is a one-to-one linear correspondence within the range of altitude that modern passenger aircraft fly in.

In summary:

The higher the altitude, the less the P_atm ... and vice versa.

And since we Earthlings dwell at the bottom of what Your Mentor calls the "Air Head" ...

we Earthlings experience the maximum P_atm on our head and shoulders 24/7 which has been been determined by measurement committee to be 14.7 psia (101.3 kPa) at sea level.

PTOA Segment #149 showed how the P_atm is measured with a barometer.

Who amongst the PTOA Readers and Students recognized how the head of liquid mercury that fluctuates with P_atm could be converted into the English units of P_atm ... 14.7 psia? Proving once again that ANY head of liquid ...big or super small ...creates a Pressure!

P_atm was also equivalent to:

760 mm Hg = 760 Torr = 1 atm = 29.92 inches Hg

Albeit Process Operators and Control Board Operators do not think about P_atm on a day to day basis, PTOA Readers and Students learned one helpful use of barometric pressure variances is the ability to predict weather changes.

And ... Spoiler Alert!

The head pressure created by both gases and liquids will return as a featured PTOA subject due to the critical role that head Pressure plays in the design and operation of the Rotating Equipment that is used specifically to build Pressure up in fluids.

THE THREE TYPES OF PRESSURE AND THEIR APPLICATION TO THE PROCESS INDUSTRIES

In PTOA Segments #150 and #151, PTOA Readers and Students used their expert understanding of Atmospheric Pressure (P_atm)as the basis to separate the three classifications of the PV Pressure.

PTOA Segment #150 focused on delineating Gauge (psig) from Absolute (psia) Pressure.

PTOA Readers and Students learned:

A PI can be assumed to be reading a Gauge Pressure unless the units of measurement are otherwise labelled.
Absolute Pressure PIs will always indicate that they are reading in Absolute Pressure units of measurement and are typically used to accurately indicate what would be a very low Gauge Pressure ... like 2 to 3 psig.

PTOA Segment #151 focused on Vacuum Pressures.

PTOA Readers and Students learned that Vacuum Pressures can be expressed in a variety of Vacuum Pressure measuring units; the choice will match the process service.

PTOA Readers and Students learned that both Gauge and Vacuum Pressures require significant effort and expense to be generated.

For example:

Pumps and compressors are purchased and installed to create Gauge Pressures that exceed P_atm.
Steam jet ejectors and suck pumps create Vacuum Pressures that are lower than P_atm.

PREDICTABLE GAS BEHAVIOURS TO EXPECT IN THE PROCESS INDUSTRIES

PTOA Segments #152, #153, and #154 focused on the predictable behavior of gases and incidentally provided a great segue into the upcoming PTOA Focus Study series:

The Interrelationship of the PV Pressure with the PV Temperature, the PV Flowrate, and the PV Level

PTOA Readers and Students learned that they had a knack for predicting how a gas's Pressure, Temperature, and Volume will interact under certain conditions.

PTOA Readers and Students learned that the application of Gay Lussac's and Boyle's Gas Laws predict the two ways that the PV Pressure of a contained gas can increase:

The gas's Temperature is increasing via heat transfer from some source or...
The density of the contained gas is increasing either by a decrease in Volume or ... more than likely ... more Mass flow entering the container somehow.

PTOA Readers and Students learned that Dalton's Gas Law of Partial Pressure and the Ideal Gas Law are used in the design of Separating Systems and other gas-handling equipment and are also frequently used by Control Board Operators ... even though they don't realize it!

PTOA Readers and Students also learned that Boyle's Law showed that gases are compressible but liquids are not.

In fact, Boyle's Law predicted that decreasing a gas's Volume by half will double the gas's Pressure.

PTOA Readers and Students learned that "Compressible" means that the Pressure of a gas can be increased by squeezing the gas mass into a smaller Volume ... a characteristic that liquids and solids do not have because of the close attraction of their molecules.

Hmmm?

Just how the heck can the Pressure of a liquid be increased if its not compressible?

Stay tuned!!!

TAKE HOME MESSAGES: The PV Pressure takes more brainpower to understand than the PV Temperature.

PTOA Readers and Students must understand all of the PV Pressure fundamentals reviewed in this PTOA Segment #155.

PTOA Readers and Students who do not yet feel they understand the content in this review segment should return to the beginning of The PTOA PV Pressure Introduction Focus Study. Go ahead! Nobody's watching!

Link to PTOA Segment #138 entitled "Where Do We Go From Here (#4 ... Part 1).

Otherwise, proceed to the PV Pressure DIY Answers and then move on to:

The Interrelationship of the PV Pressure with the PV Temperature, the PV Flowrate, and the PV Level

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