FLOWING FLUID PROPERTIES THAT PREDICT FLUID BEHAVIOR … PART 2

I want you!
She's so Heavy
Heavy, Heavy,
Heavy ... HE-A-VY!

("I Want You ... She's So Heavy," by The Beatles, 1969)

THE TWO LIQUID AND GAS PHYSICAL PROPERTIES
WHICH INSTANTLY INDICATE HOW HEAVY THE FLUID IS

PTOA Readers and Students already know how to determine the magnitude of the hydrostatic Pressure on the hull of the sunken Kursk Submarine and how much Force was needed to raise it from the bottom of the ocean.

There are two Fluid physical properties that instantly reveal how heavy or light a Gas Fluid is and four Fluid physical properties that instantly reveal how heavy a Liquid Fluid is.

The two properties that instantly reveal the lightness/heaviness of both Gases and Liquids are:

Density
Specific Gravity

The concepts of Liquid Density and the Relative Density of a Liquid (aka, the Liquid's Specific Gravity) were featured topics in PTOA Segment #145. PTOA Readers and Students used the knowledge to determine the hydrostatic Pressure exerted by any column of Liquid.

Heat Exchange changes the PV Temperature of Fluids; hence Heat Exchange can change several flowing Fluid Properties, including Density and Specific Gravity.

At this juncture PTOA Readers and Students must become core competent understanding how a Fluid's Density and Specific Gravity dynamically change as the feedstocks are processed into intermediate products, and final products.

The Fluid properties of Density and Specific Gravity are greatly impacted when the flowing Fluid experiences changes in the PV Temperature while flowing through Stationary Equipment, Rotating Equipment, and the Piping Network of the processing plant.

Density of a Fluid

Until the Mass Flowmeters featured in PTOA Segment #235 gain more industrial use, PTOA Readers and Students won't use the fluid property Density ... directly in the form of Density ...very frequently. However, knowing a Fluid's Density makes it possible to determine a very useful Fluid physical property, Specific Gravity (aka, Relative Density).

All Brilliant PTOA Readers and Students are already aware that the below definition/mathematical expression for Density applies to Fluids ... and even applies to non-flowing Solids:

The two containers have the same Volume. The one on the left is more dense because it has more mass in it. Density=Mass/Volume.

Density = Mass/Volume.

Common units for Liquid Density in the USA are Lb/gallon or Mlbs/Barrel. In SI Units, Density will be expressed as Kilograms/Cubic Meter (Kg/M³).

In the USA, the common unit expressing the Density of a Gas is Lb/Cubic Foot (lb/ft³) and in SI units gram/Cubic Centimeter (g/cc).

There are two noteworthy values for Density and their derivation goes something like this:

One way to determine the Density of Air! (The Volume of the balloon is not a cubic foot and the Temperature of the room is unknown).

The Weight (lb_f) of a cubic foot of air at STP is 0.0807 lb_f. Because PTOA Readers and Students have read PTOA Segment s #143 and PTOA Segment #144, they know that this information infers that the mass contained in 1 cf of air at STP is equivalent to 0.0807 pound (lb_m). Ergo, the Density of Air at STP is 0.0807 lb_m/ft³.
The Weight (lb_f) of a cubic foot of water at STP is 62.4 lb_f. Brilliant PTOA Readers and Students know they can infer that 1 cf of water at STP contains 62.4 pounds (lb_m) of water. Hence the Density of Water at STP is 62.4 lb_m/ft³. Furthermore, brilliant PTOA Readers and Students know how to use the conversion factors in PTOA Segment #148 and the factor/label method to deduce another commonly used Density for Water of 8.34 lb_m/gal.

Alternatively, Google or other online tools can simplify the conversion calculation. Give thanks to Calculator Soup for this Link to their Density Conversion tool. HERE.

BIG RED NOTE: The Temperature and Pressure used as the Standard Temperature and Standard Pressure vary depending upon what measuring standards are being utilized. The PTOA is written in the USA which is one of the few remaining non-metric-based processing areas. Always check what Temperature and Pressure are defined as the Standard Temperature and Standard Pressure in your local processing environment.

Here's a fun fact:

The greater the Density of the Gas, the easier it is to get the gas to flow! Just stuff that fact in your noggin until you are starting up a Dynamic Compressor with pure hydrogen gas. The concept of Surge was featured in PTOA Segment #224.

Hmmmm.

As Temperature increases (X Axis) the Density of Oxygen Gas decreases (Y axis) because the Volume of the Oxygen expands (the mass is the same). Vice versa when the Temperature decreases. The same behavior is evident in other substances however the rate of expansion/contraction differs between substances due to variance in molecule attraction.

What else does the mathematical equation expressing Fluid Density reveal?

Since brilliant PTOA Readers and Students have read PTOA Segment #143 they know why an amount for Mass is not going to change unless the processing facility is operating at the speed of light.

Hence, any change in a Fluid's Density is solely attributable to Volume Expansions (due to an increase in the PV Temperature) and Volume Contractions (due to a decrease in the PV Temperature). For Gases only, the Volume factor of Density is also impacted by changes in the PV Pressure.

Don't fret about the SI units for Density. A glance at the chart reveals that Sapphire is 3.99 more dense than Water. A cubic centimeter of Sapphire would way 3.99 grams and a cubic centimeter of water would weigh 1 gram. Sapphire is thus almost 4 times heavier than water!

The Density expression also reveals that ... given the same Volume of two different liquids ... the denser liquid will weigh more and have the tendency to sink to the bottom. Ditto the occurrence of very light gases mixed with very heavy gases. But ... Beware!

Different Gases can still have the same Density.

Ethane has the same Density as Air but behaves quite differently than Air.

For example, the hydrocarbon gas Ethane (two carbon atoms and 6 hydrogen atoms) has the same Density as air, 0.08 lb_m/ft³. A 1 cubic foot container of each Ethane and Air would each weigh 0.08 lb_f.

Guess what?

Ethane has many different properties than air! Don't confuse the two or KABOOM!

Specific Gravity of a Fluid
(aka "S.G." aka Relative Density)

Specific Gravity of Liquids

Brilliant PTOA Readers and Students ...meaning those who are reading the PTOA Segments in the intended, sequential order ... already know how to calculate the Specific Gravity of a Liquid because they have read PTOA Segment #145 wherein Specific Gravity was introduced as "Relative Density."

From now on Specific Gravity will be used and abbreviated "S.G.".

The Specific Gravity (S.G.) of any Liquid =
(The Density of any Liquid at STP) ÷ (The Density of Water at STP).

and recall from above...

Density of Water at STP is 62.4 lb_m / ft³.

The pressure created and exerted by a liquid depends only on the S.G. and column height of the liquid. Note the lighter the liquid, the smaller the S.G. and the more liquid height is needed to create the same hydrostatic pressure sensed at the bottom of the container.

In PTOA Segment #146, PTOA Readers and Students used S.G. as one of just two inputs needed to determine the hydrostatic Pressure exerted on the bottom of any randomly shaped container, for example human-made tanks or a dammed-up reservoir lake or natural containers like the ocean.

S.G. is also used for dynamically flowing Liquids, not just Liquids in a container.

Given the S.G. of a mystery Liquid, the S.G. data can be used to determine:

Whether or not the Liquid will float above water or sink below it.
The Density of the Liquid.

This Table compares Specific Gravity on the Y Axis to API Gravity on the X axis. A Specific Gravity of 0.80 falls into the category of Light Oil. Light Duty trucks run on diesel which is a Light Oil.

For example, presume the plant laboratory returns data indicating that a mystery Liquid has an S.G. = 0.80.

Brilliant PTOA Readers and Students already know they can infer the mystery liquid is lighter than water and will float on top of it given the chance.

Furthermore, the Density of the mysterious Liquid can be determined simply by rearranging the defining expression for S.G:

0.80 X (The Density of Water at STP) = The Density of Any Liquid at STP

Now filling in what is known to be true:

0.80 X (62.4 lbm/ft³) = 49.9 lbm/ft³

The Board Operator does not get feedback in the form of Labels like "Gasoline" and "Diesel" shown in the nearby graphic. Feedback will be in terms of the Gravity of the Crude Oil Fraction and the corresponding distillation temperature range.

In the petrochemical and fuels refinery, the Control Board Operator uses the S.G. data from the lab to ascertain whether or not the products are "on specification."

For example, Distillation Columns and Fractionating Columns are just two examples of Process Units that rely on S.G. feedback to inform the Control Board Operator how well s/he or they are doing their job.

Specific Gravity of Gases

The Specific Gravity of any Gas =

(The Density of any Gas at STP) ÷ (The Density of Air at STP).

and

Density of Air at STP is 0.0807 lb/ft³.

Air is composed of mostly Nitrogen Gas (78%), approximately 21% Oxygen Gas, and trace amounts of other gases as shown in this graphic.

Note that Gases use Air at STP instead of Water at STP to determine the S.G. of any gas.

Obviously, the S.G. of Air is 1.0.

Given the S.G. of a mystery gas, any PTOA Reader or Student can easily determine:

Whether or not the Gas will diffuse above Air or move more slowly and be more concentrated below Air molecules.
The Density of unknown Gas can be determined by rearranging the above equation and solving for The Density of any Gas at STP.

Thank you to Engineering Toolbox for access to a list of S.G.s for different gases. Note this tables state STP to be 68^oF and 14.7 psia. Access the link HERE.

The S.G. of the gases that make up farts is 0.87 ... lighter than air, these gases diffuse upward quickly.

A gas that has a Specific Gravity less than 1 is lighter than air. In a mixture of gases, given sufficient time, lighter gases will diffuse faster than heavier gases because they can rise more easily above the heavier gas molecules.

The stinky combination of Gases that make up farts have a Specific Gravity of 0.87; These lighter-than-air Gases rise above air quickly.

Don't Miss The Point!

Alternatively, two gases with similar S.G.s will not easily diffuse from each other.

Listen Up!

Nitrogen asphyxiation of Oil and Gas industry workers is a Real Thing. Here's how it can happen:

Prior to inspection, the atmosphere in the Reactor must use Nitrogen to purge Hydrocarbon gas. Then Air is used to purge the Nitrogen for safe entry.

The Reactor in the nearby graphic operates in a hydrocarbon atmosphere while on-line. Assume that the hydrocarbon gas atmosphere has an S.G. of 0.27.

Prior to being inspected during a Turnaround, the hydrocarbon atmosphere must be changed into an air atmosphere that human beings can safely breathe.

However, purging the Reactor with air would not be safe because hydrocarbon in the presence of oxygen (air is 21% oxygen) will cause a big boom should a source of ignition be nearby.

Once the Reactor is out of service it will be purged with nitrogen (an inert gas) to displace the hydrocarbon gas atmosphere.

Pure nitrogen gas has a S.G. of 0.9969, very close to the 1.0 S.G. of Air (which is 78% by volume nitrogen), yet much heavier than the 0.27 S.G. of the hydrocarbon gas.

During the hydrocarbon purge with nitrogen, the much heavier nitrogen will concentrate at the bottom of the Reactor. The Reactor must be tested at both inlet and outlet ports to ensure that a non-explosive atmosphere exists throughout the Reactor prior to purging the nitrogen from the Reactor with air.

The Safety Professional is testing the atmosphere before and periodically during a Confined Space Entry.

The S.G.s of Air and Pure Nitrogen are too close for comfort! The nitrogen will not be successfully purged with Air until both the top and bottom of the Reactor confirm the presence of 21% Oxygen.

Too many contract workers preparing tall Reactors and other confined space vessels for inspection have died due to nitrogen asphyxiation. This happens when an air atmosphere is confirmed at the top levels of the Reactor, but nitrogen gas has not been displaced at the bottom. The contract workers black out and are not able to get to higher levels where more oxygen is present.

Returning the inspected Reactor to operating service requires purging the air atmosphere (S.G.=1) with nitrogen (S.G.=0.9969) and then purging the nitrogen with significantly lighter compressed pure hydrogen gas (S.G. = 0.0696).

A successful nitrogen purge of the air can be declared once the top and bottom of the Reactor test to zero oxygen percent.

A successful hydrogen purge of the nitrogen can be declared once the top and bottom of the Reactor test to have potentially explosive atmospheres.

TAKE HOME MESSAGES: Density and Specific Gravity are two physical properties of both Gases and Liquids that quickly reveal how heavy the Gas Fluid or Liquid Fluid is. PTOA Readers and Students are already familiar with Liquid Density and Specific Gravity to determine hydrostatic Pressure. This PTOA Segment illustrated how Density and Specific Gravity predict the behavior of flowing Fluids.

Specific Gravity is a dimensionless physical property of a Fluid. Specific Gravity compares the Density of the flowing Fluid to the Density of Air (for Gases) and Water (for Liquids and Solids) at STP.

STP is defined as 60 °F and 14.7 psia for most USA processing industries.

The two Densities listed below are used as the basis to determine Specific Gravity of a Fluid.

Density of Air at STP is 0.0807 lb_m/ft³.
Density of Water at STP is 62.4 lb_m/ft³

Since compared Densities assume the same Volume has been used in the comparison, the difference in mass explains their different Densities.

Alternatively, since Mass does not change, a change in Density can only be explained by a change in Volume. A change in Volume occurs when a Fluid is exposed to an increase in PV Temperature (the Volume expands so Density decreases) or a decrease in the PV Temperature (the Volume contracts so Density increases). The rate at which a substance will expand or contract due to PV Temperature changes is unique to the substance.

A change in Volume due to changes in the PV Temperature changes the Volumetric Flowrate of the Liquid or Gas substance.

Gases are also impacted by a change in PV Pressure because they are Compressible: as Pressure increases, Volume decreases and vice versa.

The greater the Density of a Gas, the easier it is to get the Gas to flow.

Substances can have the same Density and yet behave quite differently!

This PTOA Segment explained more thoroughly why Fluid Flowrates are corrected to a Standard Temperature and Pressure (STP). In the USA processing industries, STP is 60°F and 14.7 psia. Different countries use different STPs as does the scientific community. Always check the basis of STP when looking up Density and Specific Gravity of a substance.

This PTOA Segment illustrated how dynamic Specific Gravity is practically used in the workplace to guide the Control Board Operator to keep production "on spec," as well as aide in safe entry into a confined space.

PTOA PV FLOWRATE FOCUS STUDY AREA
PV FLOWRATE FUNDAMENTALS FOCUS STUDY

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