You oughta know by now
You oughta know ...you oughta know by now

("Words of Love," by J.Phillips of The Mamas and The Papas, 1966)

 

CONNECTING THE PV PRESSURE DOTS

In this PTOA Segment #162, Your Mentor will begin connecting the dots between the PV Pressure fundamentals and how those fundamentals apply to the operation of Centrifugal Pumps.

Centrifugal Pumps are the go-to rotating equipment used to increase the PV Pressure in liquids and condensed vapors.

"Condensed vapors" are gases that are maintained in their liquid state by increased Pressure and lower Temperature.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order will cruise through this essential information that directly applies to the operation of Centrifugal Pumps and other rotating equipment.

If the above paragraph does not describe yourself ...

 STOP RIGHT HERE!

Do yourself a big favor and access the below link that will jump you  back to the beginning of the PTOA PV Pressure Focus Study Area.

Click Here to Jump Back

Your success with respect to learning what Plant Managers expect you to know about Centrifugal Pumps depends upon taking this opportunity to "own" the PV Pressure fundamentals.

 

STUFF YOU OUGHTA KNOW BY NOW

Absolute Pressure Vs. Gauge Pressure

PTOA Readers and Students must understand the difference between Gauge Pressure (psig) and Absolute Pressure (psia)  measurements.

Gauge and Absolute Pressure measurement were featured in PTOA Segment #150.

PTOA Students and Readers learned that:

P_abs = P_gauge + P_atm

Prior to assessing the performance of Centrifugal Pumps, the PV Pressure measurements recorded around the pump must be on the same measurement basis.

The above statement means that PTOA Readers and Students must be vigilantly alert to first recognize when information that describes the pumping situation is given in multiple Pressure-measuring bases and then be ready to convert all information into one consistent basis.

Although the choice between Absolute and Gauge Pressure is arbitrary, most processing industry pump performance assessments are performed on the basis of Gauge Pressure, psig.

With advanced apologies to the PTOA Readers and Students who do not use the English system of measurement, the PTOA PV Pressure Rotating Equipment Focus Study will only use English measuring units to assess the performance of pumps and compressors.

 

The  PV Pressure ↔ Fluid Velocity Swap

PTOA Readers and Students learned liquids are not "compressible" in PTOA Segment #153.

So PTOA Readers and Students know that suddenly decreasing the Volume of a liquid by half WILL NOT result in doubling the PV Pressure of a liquid as it will for a gas.

Centrifugal Pumps add the PV Pressure to liquids and gases by combining centrifugal force with the PV Pressure ↔ Fluid Velocity Swap, a fluid-flow phenomenon that was explored thoroughly in PTOA Segment #159.

PTOA Readers and Students must invest whatever time and effort is needed to understand the conditions that allow a fluid to swap its velocity for a gain in the PV Pressure.

 

FLUID PROPERTIES YOU OUGHTA KNOW BY NOW

Describing Fluids with Fluid Properties

The properties of fluids that are listed below are used to describe the expected behavior of one fluid when compared to a different fluid.

The standard fluid which provides the basis from which all other fluid characteristics and properties are defined is Water for liquids and Air for gases (measured at 1 Atm Pressure and a specified Temperature).

Since all rotating equipment adds the PV Pressure to fluids, it would  behoove PTOA Readers and Students to become "bilingual" with respect to understanding and expertly using the below list of fluid properties.

Density

The term "Density" was defined in PTOA Segment #145.

The definition expression for Density is:

Density =  Mass  ÷    Volume

PTOA Readers and Students learned that any expression of a Mass divided by a Volume is describing the Density of the mass ... be it a mass of solid or liquid or a gas.

Closer scrutiny of the Density definition reveals that the Density of a substance can change in the following ways:

• The Mass of fluid contained in a Volume can increase or decrease.
• The Volume that contains a Mass of fluid can increase or decrease.

For example ...

The visual aid that was used to illustrate Charles' (common sense) Gas Law in PTOA Segment #152 clearly showed how a change in the PV Temperature causes a change in the Volume of a gas.

 

 

Although it wasn't brought up at the time and saved to this very moment ...

The PV Temperature-Volume relationship observed by Charles also applies to liquids; the Volume change for liquids is not as dramatic compared to gases but nevertheless is sufficiently significant for some liquids to be useful.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order learned way back in PTOA Segment #98 that liquid-in-glass Temperature measuring instruments ... aka "common glass Thermometers" ... work because the enclosed liquid mercury expands and contracts according to the heat transferred into or out of it.

So the general cause-effect statement between a changing PV Temperature and the Volume of a fluid can be stated in shorthand:

Fluid Temperature ↑ =  Fluid Volume ↑

and vice versa:

Fluid Temperature ↓  =  Fluid Volume ↓

Wow!

That must mean that the change in Temperature which causes a Volume change in a fluid must also change the Volumetric Flowrate of the fluid ...  for example the gallons per minute being pumped through a pump!

Yes Indeedo!

The above cause-effect relationship can be expanded:

Fluid Temperature ↑ = Fluid Volume ↑ & Volumetric Flowrate ↑

and vice versa:

Fluid Temperature ↓  = Fluid Volume ↓ & Volumetric Flowrate ↓

Connecting even more dots ...

The change in PV Temperature that causes a change in Volume and Volumetric Flowrate must also cause a change in the fluid's Density!

Righteeo!

Density =
Mass / Volume

 

• An increase in Volume will decrease the fluid's Density because the Mass of fluid is divided by a greater Volume.
• A decrease in Volume will increase a fluid's Density because the Mass of fluid is divided by a smaller amount of Volume!

Hence ... the cause/effect relationship on a fluid's properties when the PV Temperature changes can be expanded to:

Fluid Temperature ↑ =

Fluid Volume ↑ & Volumetric Flowrate ↑
& Fluid Density ↓

and vice versa:

Fluid Temperature ↓  =

Fluid Volume ↓ & Volumetric Flowrate ↓
& Fluid Density ↑

 

Specific Gravity

The fluid property called Specific Gravity was also defined in PTOA Segment #145.

PTOA Readers and Students learned that Specific Gravity was simply a quickie way to evaluate the relative density of a gas or liquid compared to the density of water (for liquids) and air (for gases) measured at standard conditions.

PTOA Readers and Students who put their thinking caps on can easily figure out why the cause-effect relationship string that results when the PV Temperature changes can be  expanded as shown below to include the impact on Specific Gravity:

 

 

 

Fluid Temperature ↑ =

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

and vice versa:

Fluid Temperature ↓ =

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

In summary, PTOA Readers and Students must understand the impact of the PV Temperature changes on the above fluid properties ... and one more ... Viscosity.

Viscosity

Although it wasn't emphasized at the time and saved for this very moment, PTOA Readers and Students were introduced to the fluid property Viscosity in PTOA Segment #145.

PTOA Readers and Students were asked to consider which liquid would flow faster ... honey or water?

The Viscosity of a liquid characterizes how easily the fluid will flow.

However those who defined Viscosity must have been glass-half-empty folks because they defined it in terms of "a fluid's resistance to flow."

Basically the more goopy looking a fluid, the longer time it will take to pour. This "resistance to flow" means that the Viscosity of a goopy fluid will be greater than the Viscosity of a different fluid that is observed to flow more freely.

Honey takes a longer time to pour than water. Therefore Honey is more viscous than water.

The nearby chart shows that honey and corn syrup are 2000 to 3000 times more viscous than water (measured at 70 °F).

Viscosity is yet another fluid property that is  definitely impacted by Temperature ... think about how much easier honey or syrup flows when it is heated and how much harder it pours when first removed from a refrigerator.

Viscosity is typically measured in centistokes (cSt); The amount of cSt measured for cool honey will decrease as the honey is warmed up.

So the final cause-effect relationship string that shows the impact of Temperature on (liquid) fluid properties is:

Fluid Temperature ↑ =

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

and vice versa:

Fluid Temperature ↓ =

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

 

Fluid Properties and Phenomena Related to a Gas/Liquid Interface

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order have already devoted brainwork  contemplating what happens at the liquid/vapor interface of a liquid that is stored in a container.

The Vapor Pressure that is created by the gas/vapors that linger above the liquid level can have a significant impact on Centrifugal Pump operations.

Ergo, it is worthwhile to review the below liquid/gas interface case studies to completely understand how a Centrifugal Pump works.

 

Case 1: The Total Pressure Exerted By A Gas Blanketed Tank

In PTOA Segment #147, PTOA Readers and Students learned that a gas blanket ...

typically nitrogen or in some cases natural gas ...

is purposely piped into a tank to occupy the vapor space above the liquid level and thereby intentionally prevent the volatile liquid from interacting with the oxygen content in air.

The pressure of the blanket is regulated and typically stated as a gauge pressure (psig).

PTOA Readers and Students must always remember to take the gas blanket pressure into account when determining the Total Pressure exerted on the bottom of the tank.

The Total Pressure exerted on the bottom is determined by adding the gas blanket pressure to the head pressure generated by the liquid.

Total Pressure exerted on Bottom of Tank =

(S.G. * 0.433 lb_f/ft * h (ft)) + Blanket Pressure (psi)

PTOA Readers and Students who do not understand the above paragraphs absolutely must return to the beginning of the PTOA PV Pressure Focus Study Area at this juncture.

 

Case 2: The Total Vapor Pressure exerted on the Liquid Level 

When there is no gas blanket, the vapor space is filled with a combination of vaporized gas particles that have escaped from the liquid mixture's surface.

The Pressure that is generated by the vaporized liquid particles and which is exerted on the liquid level is the "Total Vapor Pressure of the liquid."

 

 

The Total Vapor Pressure of the liquid must likewise be taken into account when determining the total PV Pressure exerted on the bottom of a tank.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order have already been  introduced to the concept of an individual component's vapor pressure:

• PTOA Segment #102  featured intentionally choosing volatile liquids to fill the bulbs of fluid-filled PV Temperature measuring devices.
• The more recent  PTOA Segment #154 featured Gas Laws that are always present and doing their thing while Process Operators are blissfully unaware.

PTOA Readers and Students learned that Dalton's Gas Law of Partial Pressures determines the Total Vapor Pressure exerted on a liquid level by adding up the individual Vapor Pressure contributions from each component in the liquid mixture.

The example liquid mixture that was used to define and explore the Total Vapor Pressure that hovered above a liquid level was a simple binary mixture of two components, Hexane and Heptane.

Not clarified at any previous point and saved to define exactly right now is how the individual component Vapor Pressure is determined.

The fluid property called "Vapor Pressure" is determined by measuring the pressure that a gas/vapor exerts on the liquid surface of a container that is storing just the pure liquid component.

The container is held at a constant reference Temperature while the Vapor Pressure measurement is recorded.

A high Vapor Pressure indicates the tendency for the liquid to easily change into its vapor state.

 

 

A table or graph of compared components and their individual Vapor Pressures quickly reveals which components will most easily vaporize and therefore be present in a higher concentration in the vapor state that hovers above the liquid level of a mixture.

Here are more Vapor Pressure Fun Facts:

• Vapor Pressures are measured in absolute pressure units (kPa and psia and even Torr) and will reference the temperature at which the measurement was made.
• The lower the Boiling Point Temperature of the component, the higher its vapor pressure will be.

 

 

So, Fred wants to know:

Why do we care about the Vapor Pressure of a gas that will form from a liquid when just liquid is supposed to be flowing through a Centrifugal Pump?

 

The question practically answers itself; the main goal of successfully operating a Centrifugal Pump is to make certain the fluid flowing through it remains in the liquid state and thus prevent the formation of bubbles from vaporizing liquid.

The pump impeller in the nearby photo was not so lucky.

 

 

TAKE HOME MESSAGES: PTOA Readers and Students must expertly understand the fundamentals related to the PV Pressure so that they can apply the fundamentals to the operation of Centrifugal Pumps ... and other types of rotating equipment.

Liquids are defined by the following characteristics, all of which are impacted by a change in the PV Temperature.

• Density
• Specific Gravity
• Viscosity, the resistance of a fluid to flow.
• Vapor Pressure
• Volumetric Flowrate

 

A Cause-Effect statement is a string of relationships that uses arrows to easily indicate how a change in a Process Variable ... for example, Temperature ... will impact other properties ... for example, the properties of fluids.

The Vapor Pressure of a liquid component is the pressure that its vaporized particles will exert on the surface of a stored sample of the pure liquid ; the Temperature at which the Vapor Pressure is measured must be recorded because Vapor Pressure changes with Temperature. 

Vapor Pressure is inversely related to Boiling Point Temperature; Liquid components with high Vapor Pressures will have low Boiling Point Temperatures and vice versa.

The Total Vapor Pressure of a mixture is the additive vapor pressure contribution from each component in the mixture.

If the PV Pressure of a pumped fluid decreases below the Total Vapor Pressure of the pumped fluid, the lightest components in the pumped fluid will start vaporizing and ruin the impeller of a Centrifugal Pump. 

©2017 PTOA Segment 0162
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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