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THE RELATIONSHIP BETWEEN THE PV FLOWRATE AND THE PV TEMPERATURE

The Process Technology and Operator Academy Posted on by

 

Do you get déjà vu? (Ah), hmmDo you get déjà vu, huh?

She thinks it's special
But it's all reused

("Deja Vu," by Olivia Rodrigo et. al., 2021)

 

CONNECTING THE PV FLOWRATE-PV TEMPERATURE DOTS

High Five to PTOA Readers and Students! The hard work that has been devoted to learning Process Technology starts to pay off in this PTOA Segment because there are no new Process Technology concepts introduced.

Instead, this PTOA Segment connects the dots between previously presented content while exploring the PV Flowrate-PV Temperature Relationship.

The Fluid's Phase Determines the PV Flowrate-PV Temperature Relationship 

Brilliant PTOA Readers and Students ... meaning those who are reading the PTOA Segments in the intended, sequential order ...  already learned in PTOA Segments #3 and #237 that the terms "Fluid" and "Flowrate" apply to stuff that flows ... Liquids and Gases. Sometimes Gases are called "Vapors."

PTOA Readers and Students also recently learned in PTOA Segment #240 that the properties of Liquids and Gases differ in significant ways. These diverse fluid properties yield distinct flowing fluid behaviors while the PV Temperature increases or decreases, eventually reaching a phase-changing Temperature.

For example:

Gases have much more space between gas molecules than liquids have between liquid molecules. Ergo, Gases are much less dense than the liquids they were vaporized from.

Furthermore, Gases are compressible, and liquids are not. "Compressible" means that the when the Volume of a gas decreases, the PV Pressure of the Gas will increase, and vice versa.

 

When the Volume of a Gas changes, so does the Gas's Density and hence, Volumetric Flowrate.

The Viscosity of liquids Mercury, Water, SAE 10 Lube Oil and Lube Oil #3 DECREASE the Temperature on the X-Axis is increased. The Viscosity of gases Air, Hydrogen, and Helium INCREASE as the Temperature is increased.

Liquids are not compressible. A change in the PV Pressure does not change a Liquid's Density or Volumetric Flowrate.

Also, an increase in the PV Temperature will increase the Viscosity of a Gas but decrease the Viscosity of a Liquid.  Since Viscosity means "Resistance to Flow" a change in the PV Temperature can significantly change a fluid's PV Flowrate.

For all the reasons above, the relationship between the PV Flowrate and the PV Temperature is governed by whether the flowing fluid is a Gas or a Liquid ... and especially impacted if the flowing fluid is approaching a phase-changing PV Temperature.

Once a Gas is vaporized from its Liquid state, the Gas behaves as a Gas. And once a Liquid is condensed from its gaseous state, that Liquid behaves like a Liquid.

Since the PV Flowrate behavior is predictable when the phase of the flowing fluid is known, this PTOA Segment reviews what makes a Liquid stay in the Liquid state and what makes a Gas stay in the gaseous state.

Spoiler Alert!  The answer is the PV Temperature for Liquids and the PV Temperature and the PV Pressure for Gases!

A prerequisite to understanding the PV Flowrate-PV Temperature Relationship is remembering the PV Temperature fundamentals.

 

Quickie Review of the PTOA PV Temperature Focus Study Area Outline

Brilliant PTOA Readers and Students have already completed the PTOA PV Temperature Focus Study Area (PTOA Segments #1 through #137).  The PTOA PV Temperature Focus Study Area was structured to:

  • Schematic of a Fired Heater. Fluids flow through the Tubes.

    Introduce the concept of the PV Temperature to brand new, novice PTOA Readers and Students.

  • Introduce the industrial processing equipment/systems that is/are used to increase the PV Temperature, lower the PV Temperature, and exchange heat between two fluids for efficient management of the PV Temperature.
  • Introduce and study the 3 Heat Transfer Methods which make it possible to raise, lower, and efficiently manage the PV Temperature.
  • Instruct how Process Operators keep PV Temperature management equipment/systems efficiently operating via illustrating how the 3 Heat Transfer Methods are incorporated into the design of PV Temperature process equipment/systems.

 

HOW THE PV TEMPERATURE OF A FLUID IS INCREASED OR DECREASED
(A Review)

The transfer of heat is what causes the PV Temperature of a substance to increase. Heat always transfers from a hotter area to a warmer area. The greater the difference in Temperature, the more heat is transferred in a unit of time.

PTOA Segment #58 focused upon the difference between "Temperature" and "Heat."

PTOA Readers and Students learned that the PV Temperature simply indicates how much thermal energy in the form of "heat" has been transferred into or out of a substance.

PTOA Readers and Students learned that:

  • The driving force for Heat Transfer is the ΔT between a hot area and a cooler area.
  • The greater the ΔT, the more heat is transferred within a unit of time.
  • Heat always flows from the hottest area to the coolest area.

 

 

 

Heat Transfer occurs by the following processes:

 

How these 3 methods of Heat Transfer are used to heat soup in a saucepan was featured in PTOA Segment #67.  The same 3 methods of Heat Transfer are used to raise the PV Temperature of process fluids while they flow through the tubes of a Fired heater in PTOA Segment #68.

Industrial processing operational units that are purchased and installed to purposely increase the PV Temperature of a fluid via the three Heat Transfer methods are:

 

Exchanging the heat from the Liquids and Gases that have been heated in Fired Heaters and Exothermic Reactors and the Steam produced by Package Boilers occurs in:

 

Industrial processing operational units that are purchased and installed to purposely decrease the PV Temperature of a fluid include:

 

  • Shell and Tube Heat Exchangers (see list of relevant PTOA Segments above).

 

 

 

BOILING POINTS: THE LIQUID-to-GAS PHASE CHANGE TEMPERATURE
(A REVIEW)

"Boiling Point" Defined and How the PV Pressure Changes Them 

When sufficient heat is tranferred into a liquid, the liquid will vaporize into a gas. The Temperature at which the vaporization begins is called the Boiling Point.

At 1 ATM, Water boils at 212 deg F. Industrial Processes typically operate at much higher Pressures than 1 ATM.

Boiling Points vary with the PV Pressure.

PTOA Readers and Students are mostly familiar with Boiling Points determined at 1 ATM, which is Atmospheric Pressure at sea level.

For example, the Boiling Point of Water at 1 ATM (aka 101.3 kPa) is 100 °C which is also 212 °F.

The nearby chart illustrates how the Boiling Point of Water varies as Atmospheric Pressure decreases below 101.3 kPA. The last line indicates that the Boiling Point of Water increases when the Atmospheric Pressure increases above 101.3. kPA.

New York City, New York is a mere 10 feet above sea level. As the nearby chart indicates, the Boiling Point of water in NYC is 100 °C (aka 212 °F).

Summit Mt. Everest and the Atmospheric Pressure will drop to 32 kPA (aka 0.315 ATM). Without as much Atmospheric Pressure pushing against them, an ambient Temperature of 70 °C (158 °F) will cause water molecules to start boiling.

As was mentioned in PTOA Segment #151, lowering the PV Pressure below Atmospheric Pressure for the purpose of lowering the Boiling Point of heavy hydrocarbons is a processing technology used in the Vacuum Towers of fuels refineries. Vacuum Towers/Vacuum Units separate much more valuable intermediate products from the heavy feedstock fed to the tower. Each hydrocarbon product separated from the feedstock will have its own flowing properties. The bottoms product from the Vacuum Tower will be much more viscous than the Vacuum Unit feedstock because the lightest components of the feedstock have been removed via distillation.

 

The Manufactured PV Pressures and PV Temperatures in a Processing Facility
Determine the PV Flowrates of Liquids and Gases. 

More typically, Process Units that upgrade feedstocks into more valuable products operate under much higher PV Pressures than Atmospheric Pressure.  PTOA Readers and Students have already completed the PTOA Process Variable Pressure Focus Study Area (PTOA Segments #138 through #232) and thus already know that Pumps and Compressors are purchased, installed, and maintained specifically to infuse Pressure Energy into Liquids and Gases, respectfully.

The Atmospheric Crude Tower operates around 34 psi (234 kPa). The Feedstock Crude Oil has been heated to 350 deg C (662 deg F) to vaporize light ends out of the crude at the elevated tower Pressure.

Time to connect more dots!

Since the industrial PV Pressures are higher than 1 ATM, the Boiling Point Temperatures required to turn feedstocks into valuable products are significantly greater than the Boiling Point Temperatures observed and recorded at 1 ATM ... Atmospheric Pressure.

Condensation ... the phase change from a Gas into a Liquid state ... more readily occurs when the PV Pressure is greater than 1 ATM.

To attain the Boiling Points needed for processing and to keep Gases in the gaseous state at elevated PV Pressures, high PV Temperatures are manufactured by Fired Heaters, Exothermic Reactions, and heat exchange with steam. These manufactured high Temperatures make Liquids boil and keep Gases in the Gase Phase ... even at elevated PV Pressures.

So how is the Process Operator supposed to know if a Liquid will stay in the Liquid state or is approaching the Boiling Point? How is the Process Operator supposed to know if a Gas will stay in the gaseous state or is approaching its Condensation Temperature? The answers are important because the PV Flowrate between a Liquid and a Gas are significantly different.

The Mass Balance part of the Process Flow Diagram will specify the design PV Pressure, PV Temperature, and composition of each major process stream in the processing unit.

The Process Flow Diagram of a processing unit reveals the PV Flowrate, PV Temperature, PV Pressure, and composition of each major process stream as the feedstock(s) are converted into more valuable product(s).   The nearby Process Flow Diagram was featured PTOA Segment #22. Although actual operating conditions may change from the design operations on the PFD, the information is helpful.

The Mass Balance part of a PFD reveals the designed flowing Flowrate, Pressure, Temperature, and composition of each major stream in the process.

The alert Process Operator will consult the Mass Balance part of the Process Flow Diagram to become aware of the designed PV Pressure, PV Temperature, PV Flowrate and the composition of each major process stream within a processing unit.

Given this process information, the Process Operator can operate the process unit to ensure that the Liquids that are intended to be Liquid fluids stay in the Liquid state and the Gases that are intended to stay in the gaseous state do so.

Gas and Liquid Fluid Flowrates that vary from what is expected may indicate something within the process is abnormal and/or the process is not being operated as efficiently as possible.

 

PROCESS EQUIPMENT/SYSTEMS DESIGNED FOR VAPORIZATION (BOILING) AND CONDENSING

A Packaged Boiler only works if Steam is being generated and Steam Vapor is being constantly condensed continuously.

Some processing equipment/systems is/are intentionally installed to accommodate a  phase change.

The Package Boilers cited above constantly vaporized highly treated Boiler Feed Water into Steam while condensing saturated steam back into condensate.

The ISA symbol for a Condenser is also a part of the PTOA logo. This symbol indicates a phase change from a Gas to Liquid could be happening.

The Heat Exchangers cited above remove heat from a hot fluid.  When the ISA symbol on the PFD indicates a Fin Fan or Shell and Tube Heat Exchanger is a "Condenser," this type of Heat Exchanger is not only cooling a Gas process stream but removing sufficient heat from the Gas to condense the Gas into a Liquid.

Evaporation is a Liquid-to-Vapor phase change. Evaporation is like boiling, except the phase change occurs at ambient Pressure and Temperature. The Cooling Towers cited above remove heat from hot water via evaporation into the surrounding ambient air.

Equilibrium between the Liquid and Vaporized Gas.

And some industrial hardware is designed to promote "Equilibrium" between a Liquid and the Vaporized Gas that forms above the liquid level. "Equilibrium" exists when the rate of vaporization and condensation is balanced.

Since maintaining the same Liquid Level is a goal of "Equilibrium," the phase changes that occur during "Equilibrium" will be featured in the upcoming PTOA PV Level Focus Study Area.

 

 

Summarizing the PV Flowrate-PV Temperature Relationship

This Outside Process Operator is monitoring and recording PV Temperatures, PV Pressures, PV Flowrates, and PV Levels.

Process Operators become familiar with the expected phase of every fluid flowing through the pipes within the processing area that s/he or they are responsible for.

Outside Process Operators check PV Temperatures and PV Pressures routinely to verify that each process stream is staying in the desired state of matter. Because ...

Once in the Gas Phase, a Gas Fluid behaves like a Gas. Once in the Liquid Phase, a Liquid Fluid behaves like a Liquid.

The most brilliant Control Board Operators and Outside Operators will constantly assess whether or not the magnitude of Gas and Liquid PV Flowrates are as expected at the flowing PV Temperatures and PV Pressures ... and follow through with action when the Gas and Liquid PV Flowrates are not as expected.

Wow! For not including any new information, this PTOA Segment sure used a lot of previous PTOA content to connect the dots that explain the PV Flowrate-PV Temperature Relationship!

 

TAKE HOME MESSAGES: The PV Flowrate-PV Temperature Relationship depends upon the phase that the flowing Fluid is in.

Gases and Liquids have significantly different flowing behaviors, and those behaviors impact the PV Flowrate. The flowing behavior of a Liquid fluid is governed by the PV Temperature. The flowing behavior of a Gas is governed by the flowing PV Temperature and the flowing PV Pressure.

Process Operators must understand how increases, decreases and exchanges of heat are impacting the PV Temperature of the process streams that s/he or they are responsible for because the PV Temperature determines the phase(s) of the process stream. 

Some industrial equipment is intentionally purchased and installed to increase, decrease, and manage the PV Temperature of process streams. 

Some industrial process equipment is intentionally purchased and installed to promote phase changes. Process Operators must be vigilant regarding operation of this hardware because the success of the phase change directly impacts downstream operations.

The best Process Operators will be able to detect when the PV Flowrates of Gases and Liquids vary from what is expected.  Much more Gas or Liquid flowrate than expected or vice versa can indicate an abnormality in the process or indicate the process is not being operated efficiently.  

The Mass Balance of the Process Flow Diagram will feature the expected PV Flowrate, PV Temperature, PV Pressure, and composition of each major process stream within a processing unit.  Albeit design criteria may be different than actual operating conditions, the PFD is helpful to the Process Operator defining what is a "normal" and "abnormal" PV Flowrate within a processing facility. 

 

©2023 PTOA Segment 0242

PTOA PV FLOWRATE FOCUS STUDY AREA
THE RELATIONSHIP OF THE PV FLOWRATE WITH PV TEMPERATURE, PV PRESSURE, AND PV LEVEL

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