FLOWING FLUID PROPERTIES THAT PREDICT FLUID BEHAVIOR … PART 4

Honey stickin' in the hot water ooh its soothing ya

("Honey Water," by Jackii Kennedy, 2021)

THE VISCOSITY BEHAVIOR OF FLUIDS

Proof that Honey is more Viscous than Water:At the same moment in time, two identical marbles are released at the top of a container of Water and a container of Honey. Honey is more viscous than water, thus the ball that flows through Honey is just approaching the half way point in its container when the ball that flows through Water reaches the bottom of its container.

Viscosity is yet another important fluid characteristic that predicts fluid behavior and is also related to the heaviness/lightness of a fluid.

Brilliant PTOA Readers and Students ... meaning those who are reading the PTOA Segments in the intended, sequential order ... already know that "Viscosity" is a fancy word that quantifies a Fluid's "resistance to flowing."

PTOA Readers and Students were introduced to Liquid Viscosity in PTOA Segment #162 which featured the generalized interrelationships of several Liquid properties caused by a change in the PV Temperature.

For example, an increase in PV Temperature will decrease the Viscosity of a Liquid, and vice versa:

Fluid Temperature ↑ =

Fluid Volume ↑ & Volumetric Flowrate ↑
& Density ↓ & Specific Gravity ↓ & Viscosity ↓

and vice versa:

Fluid Temperature ↓ =

Fluid Volume ↓ & Volumetric Flowrate ↓
& Density ↑ & Specific Gravity ↑ & Viscosity ↑

KINEMATIC VISCOSITY (aka "FLOWING FLUID VISCOSITY")
IS MEASURED IN CENTISTOKES and/or MILLIMETERS²/SECOND

The Viscosity of a flowing fluid had a fancy name ... Kinematic Viscosity.

Kinematic Viscosity is measured in Centistokes (cSt) and/or mm²/s.

Happily, the conversion between the two measuring units is easy because:

1 cSt=1 mm²/s.

When two Fluids are compared at the same Temperature, the fluid with the lowest magnitude of cSts will be the less Viscous fluid.

The greater the magnitude of cSts, the more Viscous the fluid is.

Dynamic Viscosity is a close relative to Kinematic Viscosity.

This PTOA Segment includes way too many graphs! Some graphs show a change in a fluid's Dynamic Viscosity as the Temperature increases. Other graphs show a change in a fluid's Kinematic Viscosity as the Temperature increases. Dynamic Viscosity and Kinematic Viscosity are intimately related. Kinematic Viscosity is equal to a fluid's Dynamic Viscosity divided by the fluid's Density.

THE VISCOSITY BEHAVIOR OF LIQUIDS

Heating a liquid will make the liquid thinner. A thinner liquid is less Viscous ... meaning the liquid will flow faster because the liquid has less resistance to flowing.

If the Honey in the nearby picture were heated to 140 °F (60 °C) its Viscosity would match the much lower Viscosity of Water at 68 °F (20 °C).

Compared Viscosity of Various Liquids at 70 deg F.

Because Viscosity changes with Temperature, the Temperature at which a Viscosity measurement is made must be stated.

This photo shows steam tracing. Whatever is flowing through the flanged pipe section must be kept warm.

In the processing facility, highly Viscous liquids flow through pipes that are steam-traced to maintain a flowing Temperature.

Alternatively, a pipe-within-a-larger-diameter-pipe configuration is used to keep very Viscous liquids flowing. The Viscous liquid is enabled to flow through the inner pipe because a hot liquid circulates in the outer pipe, maintaining the flowing Temperature that is required.

Likewise, cooling a liquid will increase the liquid's Viscosity ... meaning the liquid will flow more slowly because the fluid becomes thicker, hence more resistance to flow develops.

Fred has observed that ...

when held at the same Temperature ...

Honey flows more slowly than Water.

Thus, Fred has accurately concluded that Honey is "more Viscous" than Water at the same Temperature.

Fred has also observed that Honey appears visually thicker than water. He accurately predicts that a container of Honey will weigh more than the same sized container of water.

This graph illustrates how the VISCOSITY AND DENSITY OF WATER change as the Temperature on the X-Axis is increased from 2 to 80 °C. Viscosity = Red Line = decreases from 1.70 to 0.35. Density=Black Line = decreases from 1.00 to 0.972.

Thus, Fred has concluded that the Honey is both "more Viscous" and "more dense" than Water.

This graph illustrates how the VISCOSITY AND DENSITY OF HONEY change as the Temperature on the X-Axis is increased from 20 to 40 °C. Viscosity = Red Line = decreases from 14,000 to 1700. Density=Black Line = decreases from 1.420 to 1.407.

Because Fred understands the relationship between Density and Specific Gravity, Fred accurately predicts that the Specific Gravity of Honey will be greater than the Specific Gravity of Water (again, assuming the Temperature is the same for both the Honey and the Water).

Right, again, Fred! S.G._honey=1.42, S.G_water= 0.998 (@ 20 °C).

The above paragraphs might lead Fred to the understandable yet erroneous conclusion that the Specific Gravity of all liquids and the Viscosity of all liquids are directly correlated.

Otherwise stated, Fred might erroneously conclude that when the Specific Gravity of two liquids is compared, the lightest liquid (the liquid with the lowest S.G.) will also automatically be the less Viscous of the two liquids ... and vice versa.

The two nearby graphs seem to support Fred's erroneous conclusion. The higher up graph shows data for Water and the lower down graph shows data for Honey.

Both graphs illustrate the changes in Viscosity (left side Y-Axis) and Density (right side Y-axis) as the Temperature is increased on the X-Axis.

Note that the scales for the Density of Water (graphed Black Line) and Viscosity of Water (graphed Red Line) are much less magnitude than the scales used for the Density of Honey (also a graphed Black Line) and the Viscosity of Honey (also a graphed Red Line).

Fred's theory that the lightest liquid is also less Viscous is correct when comparing Water and Honey.

However:

The Gravity/Viscosity relationship cannot be assumed to be linear for all liquids.
For example, Water is more dense/heavier than oil but less Viscous.

Brilliant PTOA Readers and Students learned about API Gravity in PTOA Segment #239 so they are aware that the lightest hydrocarbon liquid will have the greatest magnitude of API Gravity.

The below table compares the Kinematic Viscosity and API Gravity for Water and 3 types of Crude Oil.

Remember! The lower the magnitude of Kinematic Viscosity, the faster the fluid will flow.

LIQUID NAME KINEMATIC VISCOSITY AT 20 °C API GRAVITY

Water 1.0 10

Kutubu Crude Oil 2.1 44

West Texas Intermediate Crude 4.9 39

Arabian Light Crude 10.7 34.2

Water, Water, Everywhere! Water is a miraculous liquid with several unique properties that govern its behavior.

The above data table reveals that the API Gravity of Water is heavier than any of the crude oils, yet:

Kutubu Crude flows just over twice as slowly as Water (2.1 compared to 1.0).
West Texas Intermediate Crude flows almost 5 times slower than Water (4.9 compared to 1.0).
Arabian Light Crude flows almost 11 times slower than Water (10.7 compared to 1.0)

However, amongst the 3 crude types, Fred's assumed correlation between the comparative heaviness of a liquid and the Viscosity of the liquid appears to be accurate!

SAE 30 lube oil (RED CIRCLES) is much more viscous than SAE 5W lube oil (BLACK SQUARES). When the SAE 30 is blended with SAE 5W, the resulting SAE 5W-30 lube oil (BLUE TRIANGLES) has a blended Viscosity that is in-between the Viscosity of either pure component.

As the API Gravity decreases, the crude oil is heavier and the Kinematic Viscosity of the crude oil ...its resistance to flow ... increases.

One more thing important thing about liquid Viscosity is that it can be blended downward or upward with a different liquid that has a heavier or lighter Viscosity, respectively. The two liquids must be Miscible, of course!

THE VISCOSITY BEHAVIOR OF GASES

Gases have a completely different Viscosity/Temperature relationship than liquids do!

An increase in Temperature increases the Viscosity of a Gas!

The nearby graph illustrates the compared Viscosity of several common gases.

As the Temperature increases (X-Axis), each gas's Viscosity (Y-Axis) also increases. Thus ... as the Temperature increases on the X-Axis ... the graphed Gas Viscosity lines start at lower magnitudes and rise upwards to greater magnitudes.

As a Gas is heated, the Viscosity of the Gas INCREASES. Totally different behavior than the Viscosity behavior of liquids!

Upon being heated, gas molecules bang into each other more frequently which causes more friction between the molecules, hence more Viscosity (resistance to flow).

Why is the Viscosity Behavior of Gases upon being heated the opposite of the Viscosity Behavior of Liquids upon being heated?

Prior to being heated, there is vacant space already existing between Gas molecules. Unlike a Liquid substance, heating a Gas does not make the Gas substantially thinner.

Upon being heated, Gas Molecules bang into each other and ricochet off of each other with increased frequency as the Temperature rises. This frenetic movement between gas molecules increases the friction between the gas molecules which increases the gas's resistance to flow ... aka increases the gas's Viscosity.

Therefore ...

Heating a Gas increases the Gas's Viscosity and cooling a gas will decrease the Gas's Viscosity.

Brilliant Fred correctly surmises that a heavy gas compared to a light gas will weigh more and be denser.

However, Fred erroneously concludes that a heavier, more dense gas molecule would have a harder time moving than a lighter gas.

That just don't sound right to the ear, do it?

Still, it is true:

The greater the Gas Density...

the easier it is to get the Gas to flow!

Thanks to Engineering Toolbox, the nearby graph illustrates the flowing Viscosity relationship of 4 Liquids and 3 gases as the Temperature increases from 0 to 100 °C on the X-Axis.

The Liquids are easy to identify because their Kinematic Viscosity DECREASES as the Temperature increases. Their graphed lines start higher up on the Y-Axis and slope downward.

The Yellow Balloon is filled with Helium which is lighter than Air. The Blue Balloon if filled with Air and cannot float above Air.

Toward the top half of the graph, the Viscosities of Air (Maroon Line), Hydrogen (Light Blue Line), and Helium (Black Line) INCREASE as the Temperature on the X-Axis increases.

Hydrogen and Helium are much lighter than Air; a balloon filled with Helium floats above Air.

Note that the graph indicates that the Kinematic Viscosities (@ 0°C) of much less dense Hydrogen and Helium are approximately 100,000. However, the Kinematic Viscosity of much heavier Air is 8 times less than the Kinematic Viscosity of the light gases at approximately 12,000 (@ 0°C).

Thanks again to The Engineering Toolbox for illustrating that a more dense gas will still flow faster than a less dense gas ... because that conclusion just don't sound right to the ear!

Every PTOA Reader and Student knows that the Density of a gas can be altered ... because gases are compressible! Since compressing a gas decreases the Volume of the gas, the action of compressing a gas makes the gas more dense.

Many common gases are colorless. To illustrate the faster flow of a Compressed Gas over a non-compressed Gas, consider the steam vapor exiting the Pressure Safety Valve into ambient air which is at 1 ATM Pressure. The water in the vapor makes it possible to observe how much faster the compressed vapor gas flows than the surrounding air.

Ergo, no PTOA Reader or Student is surprised to learn that a compressed gas will flow more easily than a gas that is less compressed. Otherwise stated, more highly pressurized gases flow more easily than less pressurized gases.

In the nearby photo, the pressurized vapor exiting the PSV outlet moves much faster than the surrounding ambient air which is not pressurized at all at 1 ATM Pressure.

Wow! The Viscosity Behavior of Gases is so different than the Viscosity Behavior of Liquids!

Heating a Gas increases its molecular friction and thus increases the Gas's Viscosity.
Cooling a Gas decreases molecular friction and thus decreases the Gas's Viscosity.
Higher Density Gases are less Viscous and thus flow more easily than lower Density Gases.
Compressed Gases are denser than less-compressed gases and thus flow easier than less pressurized Gases.

MISCIBILITY BEHAVIOR OF FLUIDS

The descriptors "Miscible" and "Miscibility" describe how well two fluids will mix up and form a single fluid.

Likewise, the descriptors "Immiscible" and "Immiscibility" describe how two substances will not mix into a single fluid at all but would rather remain separate substances.

The Miscibility Behavior of Liquids and Gases are significantly different:

Not all Liquids are Miscible with other Liquids.

All Gases are Miscible with other Gases.

For example:

Mixing a glass of orange juice with a glass of carbonated water will make a big glass of carbonated orange juice because the two Liquids are Miscible.

"Immiscible Liquids" do not mix at all.

When a light Oil is poured into a container of Water, the Oil and Water molecules separate from each other. A layer of Oil floats on top of the Water.

Oil and Water are two Liquids that will not mix together at all. Otherwise stated, they have a high degree of Immiscibility.

The Immiscibility of Oil and Water is a useful property that is leveraged to remove the lighter hydrocarbon that separates from and floats on top of the heavier Water.

For example, the successful operation of Oil/Water Separators depends upon the Immiscibility of the Oil and Water.

This 3-Phase Separator separates Gas from an Immiscible Oil-Water Mixture. Oil (Brown in the graphic) is lighter than Water and thus floats on top of Water. The Oil is allowed to spill over a weir and is drawn off as Oil Out.

Just recently in PTOA Segment #237, PTOA Readers and Students learned how strategically placed weirs are used to isolate and draw off the desired hydrocarbon Oil from the non-desired Water layer that lies beneath.

The Miscibility of Gases is an entirely different story!

Gases mix so well together that a dude named Dalton gets credit for figuring out the Overall Pressure of a Gas mixture can be determined by simply adding up the contributing pressures of each pure gas in the mixture (Dalton's Law of Partial Pressures was featured in PTOA Segment #154).

As illustrated in the nearby schematic, when Gas A is mixed with Gas B and Gas C in a single container, the overall Pressure will be the sum of the partial pressures of each pure gas.

THE DIFFUSION BEHAVIOR OF GASES

"Diffusion" is a characteristic of just Gases and does not pertain to Liquids.

Given sufficient time, a container of pure Hydrogen Gas will diffuse completely and evenly with a container of pure Oxygen. (Just watch out for the explosion).

The Diffusivity rate of a Gas will determine how fast a gas will fill up a container that it has been released into.

The rate of Gas Diffusion depends on the Gas's Density.

Light Gases with an S.G. less than 1 (e.g., the gases that make up farts) will rise quickly above air.

Heavy Gases with an S.G. greater than one will diffuse more slowly.

Which PTOA Readers and Students had a science teacher way back when who sliced up an onion and asked students to raise their hand when they smelled the onion vapor? The Diffusivity rate of Onion Gas was the principle that the teacher was trying to demonstrate.

High Five to PTOA Readers and Students for completing the PTOA PV Flowrate Fundamentals Focus Study!

Next up?

A PTOA Focus Study which features the interrelationship of the PV Flowrate with the PV Temperature, PV Pressure, and PV Level!

TAKE HOME MESSAGES: The Viscosity of a Fluid quantifies how much resistance to flow the Fluid has at a specified Temperature. A change in Temperature will change the Viscosity of the Fluid.

Like Specific Gravity, API Gravity, and Baume Gravity ... the physical property of Viscosity is linked to the relative heaviness/lightness of the fluid.

Kinematic Viscosity is the Viscosity of flowing fluids and is the most pertinent mode of Viscosity measurement with respect to the PV Flowrate. The units to measure Kinematic Viscosity are centistokes (cSt) or mm²/sec; 1 cSt=1 mm²/s.

When comparing two fluids, the fluid with the greater Kinematic Viscosity is more Viscous, ergo has more resistance to flow at a given Temperature.

Kinematic Viscosity is directly related to Dynamic Viscosity; dividing the Dynamic Viscosity by the fluid's Density yields the fluid's Kinematic Viscosity.

The Viscosity Behavior of Liquids and Gases with respect to a change in Temperature are completely different! An increase in the PV Temperature will DECREASE a Liquid's Viscosity but INCREASE a Gas's Viscosity. Vice versa ... ...a decrease in the PV Temperature will INCREASE a Liquid's Viscosity but DECREASE a Gas's Viscosity.

In processing facilities, heat tracing of pipes and pipe-within-pipe (concentric piping) is used to keep Viscous Liquids at a flowing temperature.

The Viscosity/Density Relationships of compared Liquids cannot be assumed to directly correlate; For example, Water is heavier than hydrocarbons yet less Viscous.

The Viscosity of a Liquid can be altered by mixing with a miscible Liquid that has a greater or lower Viscosity.

The Viscosity/Density Relationships of compared Gases don't sound right to the ear. The greater the Gas Density, the easier it is to get the gas to flow. Otherwise stated, the greater Density Gas will flow faster than the less dense Gas.

Compressing a Gas increases its Density. As was just stated, the greater the Gas Density, the easier it is to get the gas to flow. Otherwise stated, a more compressed gas flows faster than the same gas when it is less compressed.

Immiscible Liquids do not mix at all. Miscible Liquids mix well. The Immiscibility of two Liquids can be used to mechanically separate the Liquids.

All Gases are Miscible.

The rate of Diffusion (aka the Diffusivity) of a Gas reveals how fast the Gas will expand to fill up a container. The Diffusivity rate of gases depends upon the Gas's Density. Lighter gases rise above air and move more quickly to fill up a container. Heavier gases diffuse more slowly.

PTOA PV FLOWRATE FOCUS STUDY AREA
PV FLOWRATE FUNDAMENTALS FOCUS STUDY

You need to login or register to bookmark/favorite this content.