FLOWING FLUID PROPERTIES THAT IMPACT FLUID BEHAVIOR … PART 1

Well you could've been anything that you wanted toI can tellThe way you do the things you doI like the way you do the things you do

("The Way You Do The Things You Do," Smokey Robinson and the Miracles, 1964)

SO WHAT MAKES A FLUID A FLUID?

What makes a Fluid a Fluid?

A Fluid is any substance that can flow! And the PV Flowrate applies to anything that can flow!

Brilliant PTOA Readers and Students... meaning those who are reading the PTOA Segments in the intended, sequential order ... read PTOA Segment #3 so they already know that the term "Fluid" means a Liquid or a Gas.

The above schematic illustrates Phase Changes. PTOA Readers and Students already know that a solid turns into a liquid once the solid's molecules are excited by sufficient heat. The liquid will turn into a gas once the liquid's molecules are excited by sufficient heat.

In the absence of the special circumstances explained below, Solids do not flow. However, a Solid that changes phase into a Liquid immediately acquires the characteristics of a Liquid.

PTOA Readers and Students learned way back in PTOA Segment #2 that one important reason to monitor the PV Temperature of a Fluid while it is flowing through the facility's Piping Network is to make sure that the Fluid is in the desired phase.

Otherwise stated,

The PV Temperature determines whether a substance is in the Liquid phase.

The PV Temperature and PV Pressure determine whether or not a substance is in the Gas phase.

And ... as was stated above ... once any substance is in the Liquid or Gas phase, the PV Flowrate applies.

Fluids ... unlike Solids ... do not have definite shapes.

One big difference between a Gas Fluid and a Liquid Fluid is:

Liquids have a definite Volume, but Gases DO NOT HAVE a definite Volume.

Ergo, even though Gases and Liquids are both Fluids, they still have unique properties. The physical properties that predict the behavior of flowing Gases and Liquids are featured in this PTOA Segment and a couple that follow.

Guess what?

Granular Solids like sand can be "fluidized." After being "fluidized" the sand will not have a definite shape and will be able to flow.

This bin of sand is fluidized via sparging of air from the bottom.

The "fluidization" of sand-like catalyst was first mentioned in PTOA Segment #140. Several hydrocarbon processes fluidize granular catalyst particles thereby making it possible for those particles to flow with the feedstock through the Reactor. The spent catalyst is subsequently separated from the Reactor Effluent, regenerated, and then recirculated to the Reactor.

A Multi-Phase Flow of Gas and Liquid flows into the Separator at the Inlet. Gas exits from the top (Gas Out). Liquid exits from the bottom (Liquid Out).

This PTOA Segment compares and contrasts Liquid Fluids and Gaseous Fluids. Liquid Fluids and Gaseous Fluids flow together in Multi-Phase Fluids until the two Fluids are separated in Stationary Equipment called "Separators."

COMPARE AND CONTRAST LIQUID FLUIDS TO GASEOUS FLUIDS

Focus on Shape, Volume, Compressibility.

A dude named Boyle observed that when the Volume of a Gas is decreased, the Pressure of the gas increases. Gases are Compressible and do not have a definite shape or Volume.

Both Liquids and Gases exert the PV Pressure on everything they touch. The molecules of a Gas exert Pressure equally in all directions.

As was stated above, Gases have an indefinite shape ... they just expand to fill up whatever container they are enclosed in. The fact that a Gas will expand to fill up the Volume of any container is directly related to the Compressibility behavior of all Gases.

Brilliant PTOA Readers and Students already know that the fancy word "Compressibility" simply means that the Volume of a Gas will decrease when the Pressure of the Gas is increased, and vice-versa.

The "PT" is transmitting the flowing Pressure of the Gas. The "TT" is transmitting the flowing Temperature of the Gas. The values will be corrected to STP to determine a PV Flowrate in terms of SCF per unit time.

Since both the Temperature and the Pressure of a Gas can change its Volume ... (and thus also change the Volumetric Flowrate featured in PTOA Segment #235) ... determining a generally understood Volume (and the Volumetric Flowrate) of a Gas requires knowing the PV Temperature and the PV Pressure of the flowing Gas.

As was previously stated in PTOA Segment #235, the flowing Gas Flowrates are corrected to 60°F and 14.7 psia (1 Atm). The corrected Gas Flowrate is expressed as Standard Cubic Feet per unit time.

These two plastic bottles are full of water. The bottle on the left has holes vertically poked into its side. The pressure exerted by the water (aka "the hydrostatic Pressure) becomes progressively greater from top level to the bottom of the container. At each distinct level the water exerts the same Pressure on everything the water touches.

The molecules of a Liquid are more attracted to each other; they slip over each other and are not separated by vacant space. Unlike Solids, Liquid molecules move too fast to form an identifiable shape. The loosely attracted Liquid molecules will "take the path of least resistance" while flowing through a pipe or while filling up whatever container they are in. Ergo, Liquids have a definite Volume but not a definite shape.

A Liquid exerts the same PV Pressure in all directions; however, the Pressure exerted by the Liquid depends upon the level that the Liquid molecule is at (see PTOA Segments #146 and #147).

"Liquids are not Compressible. Otherwise stated, the Volume of a Liquid is not impacted by a change in Pressure."

An increase in the Atmospheric Pressure or an increase in the gas blanket pressure above the surface of the liquid would not change the Volume of the water enclosed in the nearby plastic bottles. Because why, Fred?

Yep, Liquids are not Compressible!

Because Liquids are not Compressible, only the Temperature in the vicinity of a Liquid PV Flowrate measurement needs to be known to correct the liquid Flowrate to the industry Standard Temperature of 60°F.

MULTI-PHASE FLUIDS, EMULSIONS, AND SLURRIES

The top one half of 1 percent of Process Operators and Control Board Operators are so familiar with fluid properties that they can visualize the various process streams and how they change while flowing through the Piping Network of the processing facility.

Before delving more deeply into the Fluid Properties that impact a Fluid's behavior, PTOA Readers and Students must be aware that the flowing Fluid in the pipes could be any one of the following:

The Inlet to this Separator is on the upper left hand side of the vessel. The purpose of the Separator is to separate Gas and Water from the desired product, Oil. The gas entrained with the Liquid-Liquid Oil-Water mixture is separated from the liquids and exits the Separator at the top of the vessel. The Specific Gravity of Oil is less than the Specific Gravity of Water; Thus the Oil will float on top of water if given sufficient room and time to separate. A weir system and PV Level Control are used to draw off the desired Oil product (Oil Out) and send Water to the Oily Water Sewer (Water Out).

A Liquid Fluid ... for example, only Liquids should be flowing through the Suction Line of a Pump.
A Gas Fluid ... for example, only a Gas should be flowing through the Suction Line of a Compressor or entering the blades of a Gas Turbine.
A Liquid-Gas Fluid is a Multi-Phase Fluid composed of some Gas and some Liquid. For example, the Fluid flowing into the nearby Separator graphic will contain Gas Fluids that flow out from the top of the vessel and Liquid Fluids which are collected and then separated at the bottom of the vessel. The difference in relative heaviness of the two liquids (aka: the difference in their Specific Gravities) results in the heavier water exiting via the line labelled "Water Out" and the lighter Oil exiting via the line labelled "Oil Out."
The oil mixed with water in the Separator graphic is an example of a Liquid-Liquid Fluid that is not "miscible." "Miscibility" refers to how well two fluids will mix together. The molecules of oil do not mix well with the molecules of water. A Liquid-Liquid Fluid that does not mix well is called an Emulsion. The mixture is still a Fluid because the substance flows. The success of the Emulsion not to separate into two liquids depends upon how "miscible" the Liquids are. Specialty chemicals made by chemical companies can be injected to improve the miscibility of the Fluid's components. Specialty chemicals can also be injected to change a Fluid's Viscosity which will make the Fluid easier to flow.
A Liquid-Solid Fluid is a Multi-Phase Fluid called a Slurry. Solid particles are suspended in the Liquid and the Slurry still flows. Manure used for fertilizing is an example of Liquid (Water)-Solid (Manure) Multi-Phase Fluid which can be pumped via Slurry Pumps over agricultural fields.

TAKE HOME MESSAGES: The PV Flowrate pertains to anything that can flow. Otherwise stated the PV Flowrate pertains to Fluids because the capability to flow is exactly what makes a Fluid a Fluid.

There are two types of Fluids: Liquid Fluids and Gas Fluids. Liquid Fluids and Gas Fluids have some common physical properties and some unique physical properties which predict their flowing behavior.

Neither Liquids nor Gases have a rigid molecular structure that yields a defined shape (as does a Solid). Although Liquids have a defined Volume, Gases do not. Gases just expand to occupy the shape of whatever container they are in.

Gases are Compressible and Liquids are not.

Both Gases and Liquids exert Pressure to everything they touch. However, a Gas exerts Pressure equally in all directions. The Pressure exerted by a Liquid depends upon the level at which the Liquid molecules are at.

Both Liquids and Gases are impacted by an increase or decrease in the PV Temperature. To determine a PV Flowrate for a flowing Liquid and Gas, the PV Temperature of the flowing Fluid must be known and thence corrected to the Standard PV Temperature of 60 °F.

Only Gases are impacted by an increase or decrease in the PV Pressure (because Liquids are not Compressible!). To determine a PV Flowrate for a flowing Gas, the PV Pressure of the flowing Gas must be known and thence corrected to the Standard PV Pressure of 14.7 psia (1 Atm).

The upcoming PTOA Segment explains how the PV Temperature and PV Pressure impact the physical properties of flowing Fluids.

The substance flowing through the Piping Network in a processing facility might be:

A Liquid, meaning a single Liquid that can be defined by physical properties.
A Gas, meaning a single gas that can be defined by physical properties.
A Multi-Phase Fluid, meaning a mixture of Gas and Liquid or maybe even a Gas and more than one Liquid.
An Emulsion, meaning two Liquids that do not mix well together; Otherwise stated, the two Liquids are not "miscible."
A Slurry, a Multi-Phase Fluid with solid particles that flow with a Liquid.

Stationary Equipment called "Separators" can separate Gases from Liquids.

PTOA PV FLOWRATE FOCUS STUDY AREA
PV FLOWRATE FUNDAMENTALS FOCUS STUDY

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