Swimming pool, swimming pool
Swimming pool, swimming pool
Indigo, deep blue, deep blue
Oh, baby I know where you've been
I know where you've been, in the deep end

("The Deep End," by Lykke Li, 2018)

 

PRESSURE IS CREATED BY CONTAINED LIQUIDS

See the dude.

See the dude do a cannonball into the pool.

See the dude reach the bottom of the pool.

OUCH!

All PTOA Readers and Students feel their ears hurt while swimming in the deep end of the pool.

Why does that happen?

Pressure is created by liquids that are held in any kind of container.

The swimming pool is a container of water.

When swimming at the bottom of the deep end, the pressure created by the depth of the water pushes in on the ear drums and makes them hurt.

This PTOA Segment #145 continues the focus on pressures that are created by liquids that are stored in any kind of container.

At the processing plant, there are many, many tanks and towers and vessels that are storing or in the process of accumulating all kinds of liquids.

Why is it important for Process Operators to know how much pressure is created by a liquid held in a container?

Here's one good reason:

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order learned in PTOA Segment #140 that pressure differentials must exist before a fluid will flow from an area of high pressure to an area of lower pressure.

The pressure that is created by liquids that are stored in the tank, tower, or vessel contributes to the pressure differential that is needed to make flow happen!

 

HOW MUCH PRESSURE IS CREATED BY CONTAINED LIQUIDS?

The amount of pressure that a contained liquid creates depends upon just two things:

• The density of the liquid.
• The depth of the liquid ...  which is also the height of the liquid in the container if you were on the very bottom of the container looking up.

Okay ... there's one more important thing that creates pressure and that's the pull of gravity on the mass of liquid in the container.

The calculations that are used to determine pressures created by contained liquids use the same magic that was explained toward the end of  PTOA Segment #144 to account for gravity.

Stay tuned!

In the next PTOA Segment #146 PTOA Readers and Students will see how easy it is to figure out the pressure created by any liquid in any kind of container ...  not just water as is shown in the below graphic:

This PTOA Segment #144 focuses on determining the density and specific gravity of a liquid that is held in a container.

 

THE DEFINITION OF "DENSITY"

"The time has come," the Walrus said, " ...to learn about "density."

Density is simply the amount of stuff (aka mass) that will fit into a container (aka volume).

Ergo ...

The units of density are mass divided by volume or:

Density = Mass / Volume.

Whenever your ears hear someone express a certain amount of mass in a container of any volume ...

well ... that's an expression of the density of the fluid or solid that is in the container!

For example ...

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order just learned in PTOA Segment #144 that :

"62.4 lb of water will fit in 1 cubic ft (ft³) container."

My, my ...

That's a density!

 The density of Water is  = 62.4 lbm/ft³ 

which in SI units is 1 kg/L  ... isn't that easy to remember!

Here's are some other SI density expressions that your ears should start recognizing:

• grams/ml = grams per milliliter
• kg/M³ = kilograms per cubic meter.

Here's another common expression for density in English units that your ears should recognize if you plan to be working in the USA processing industries:

• lb/gallon = pounds (of mass) per gallon of liquid.

 

IDENTIFYING LIQUIDS THAT ARE MORE DENSE THAN WATER

Some liquids are "more dense" than water ...

and all PTOA Readers and Students know that statement just means that they have more stuff/matter/mass packed into the same volume compared to an equivalent volume of water.

Do you think water or honey is more dense?

Doesn't honey just seem goopier than water ... harder to get to flow?

Doesn't your gut just tell you that one cubic foot of honey would weigh more than one cubic foot of water?

So ... between water and honey ... which is more dense and would have the greater density?

Yep!

Honey will always have more mass than water no matter what the volume of the container is:

More grams per ml!

More lbs per gallon or ft³!

More kg per Liter or M³!

Honey is just dang heavier than water!

In fact a cubic foot of honey would contain 91.7 lbs.

Otherwise stated:

"The density of honey is 91.7 lbs  per ft³."

(aka "1.47 kg per liter")

You know what that means?

In a mixture of water and honey, the honey would be so heavy it would collect on the bottom and the water level would be on top!

IDENTIFYING LIQUIDS THAT ARE LESS DENSE THAN WATER

What would happen if the water were poured out of the one cubic foot container and it was then filled up with gasoline?

First you need to know that a one cubic foot container would hold 46.8 pounds of gasoline.

Otherwise stated:

"The density of gasoline is 46.8 lbs/ft³."

(aka "0.75 kg per liter" in SI units)

Wow, gasoline is a lot lighter than water!

You know what that means?

In a mixture of gasoline and water ... the gasoline will float on top!

 

DEFINITION OF RELATIVE DENSITY

It's a lot easier to classify liquids according to how they relatively compare with water.

The Relative Density of a Liquid is simply calculated:

The Density of the Liquid 

÷

The Density of Water

So that means that the Relative Density of Water is:

Density of Water / Density of Water =

62.4 lbs Water/1 ft³  ÷ 62.4 lbs Water/1 ft³ =

Relative Density of  Water = 1.0

No surprise there!

The density of water divided by itself is equal to ONE!

What IS worth focusing on is that Relative Density is a dimensionless number!

All the units cancel out!

And yet this dimensionless number instantly gives us an idea of how heavy or light the substance in question is.

The Relative Density of Honey is:

Density of Honey / Density of Water =

91.7 lbs Honey/1 ft³  ÷ 62.4 lbs Water/1 ft³ =

Relative Density of Honey = 1.47

The Relative Density of Honey is a dimensionless number that is greater than "1."

A Relative Density that is greater than "1" instantly delivers the message that the liquid is heavier than water!

And...

The more that the number exceeds "1" ... the more heavy the liquid is compared to water.

Liquids that are heavier than water will sink to the bottom of a container that has both water and the heavy liquid.

On the other hand...

The Relative Density of Gasoline is:

46.8 lbs Gasoline/1 ft³  ÷ 62.4 lbs Water/1 ft³ =

Relative Density of Gasoline = 0.75

The Relative Density of Gasoline is a dimensionless number that is less than "1."

A Relative Density that is less than "1" instantly delivers the message that the liquid is lighter than water!

And...

The further the number is below "1" the more light the liquid is compared to water.

Liquids that are lighter than others will stratify in layers above heavier layers of liquids when they are held in the same container.

 

VIDEO DEMONSTRATION ABOUT RELATIVE DENSITIES

The link listed below includes a GREAT video by stevespanglerscience.com that demonstrates how liquids will stratify according to their relative densities.

Once you access the link, cursor downward until you find the video with the title:

"7 Layer Density - Cool Science Experiment"

Thank you Steve Spangler for helping PTOA Readers and Students visually learn about density and relative densities of liquids.

Did your ears pick up a density expressed as a mass of grams in a volume of milliliters?

The demonstration may be colorful and kid pleasing ... but the exact same thing happens in the process industries because intermediate and final products are blends of liquids with varying  densities.

Believe me! A LOT OF MONEY is spent on mixers and circulation pumps in the process industries!

Here's another real-life situation of fluids separating out due to varying densities:

Have you ever worried about filling your car up with "bad gasoline" ... gasoline that has been contaminated with water?

When that happens, the gasoline stored in the below-ground storage tank has been depleted to the point that some of the low-lying water is getting pumped into your car's tank.

In the USA, there are special filters that are supposed to swell and shut off flow to your car when water is sensed in the fuel delivery system.

 

SPECIFIC GRAVITY = RELATIVE DENSITY

The grown up name for "Relative Density" is "Specific Gravity."

The term Relative Density that was just introduced in this PTOA Segment #145 will hence no longer be used in the PTOA.

Any liquid that has a specific gravity of 1 is water.

A liquid that has a specific gravity less than 1 is lighter than water and will float on top of it.

A liquid that has a specific gravity greater than 1 is heavier than water and will separate out below the water level.

FYI: Water is used as the standard to calculate the specific gravity of both liquids AND solids. The density of Air is used to determine the specific gravity of gases.

 

HOW DENSE IS FRED? WILL FRED SINK OR FLOAT?

Fred is a lanky dude.

When he jumps out of the page, his 3-D Avatar has a height of 6 feet 3 inches (aka 6.25 feet or 1.91 meters).

His length ... measured straight across at the hips  .... 1.5 feet long  (0.46 meters).

When Fred stands sideways, his width is 1 foot (0.305 meter).

All smart PTOA Readers and Students know that Fred's mass can be inferred from his weight ...

And he weighs 175 pounds (79.4 kilograms).

When Your Mentor pushes Fred into the pool ... will he sink or float?

DIY! Figure it out all by yourself!

 

1. You know Fred's Mass.
2. Determine Fred's Volume.
3. Determine Fred's Density.
4. Determine Fred's Specific Gravity.

The answer will appear at the end of the PTOA Introduction to PV Pressure Focus Study!

 

TAKE HOME MESSAGES: Liquids held in containers create pressures.

The amount of pressure created by the contained liquid depends upon:

• The density of the liquid.
• The height of the liquid in the container.

Density is the amount of mass a liquid has divided by the volume that it is contained in:

Density = Mass / Volume

Common English units of density are:

• lbs/gallon
• lbs/ft³

Common SI units of density are:

• g/ml
• Kg/L
• Kg/M³

The density of Water is:

• 8.33 lbs/gallon
• 62.4 lbm/ft³
• 1 kg/L
• 1000 kg/M³

Relative Density is

The density of a liquid or solid

÷

The density of Water.

Relative Density is a dimensionless number that instantly relates how much heavier or lighter a liquid or solid is compared to water.

The scientific name for Relative Density is Specific Gravity and that's what the PTOA will use from now on.

Water has a Specific Gravity of 1.

Liquids that are more dense and heavier than water have a Specific Gravity greater than 1; the more the specific gravity exceeds 1, the heavier the liquid is and the lower it will sink below a less dense fluid.

Liquids that are lighter than water have a specific gravity that is less than 1. The  further below '1', the lighter the fluid is and the higher it will float above a more dense liquid.

©2016 PTOA Segment 0145
PTOA Process Variable Pressure Focus Study Area
PTOA Introduction to PV Pressure Focus Study

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