WHERE OH WHERE DOES THE PV PRESSURE GO?

I'm looking through you,
Where did you go?
I thought I knew you,
What did I know?
You don't look different, but you have changed.
I'm looking through you, you're not the same.

("I'm Looking Through You," by The Beatles, 1967)

WHERE OH WHERE DOES THE PV PRESSURE GO?

Who amongst the brilliant PTOA Readers and Students has noticed that Your Mentor has yet to explain exactly why it is necessary to keep adding the PV Pressure to flowing process streams?

Of course, PTOA Readers and Students know: No ΔP = No Flow!

But why doesn't the flowing liquid just stay at the pump's Discharge Pressure?

Guess what?

Just the act of flowing through the pipe will reduce the PV Pressure of any fluid ... liquid or gas.

Pressure Drop caused by Friction Losses is a BIG DEAL!

Uh-oh!

Fred's getting all panicky and sweaty again.

Don't worry, Fred!

This PTOA Segment #165 explores what Friction Losses and Pressure Drop are and how they impact any piping and pumping system.

FRICTION LOSSES AND PRESSURE DROP
IN ONCE-THROUGH AND CIRCULATING PUMPING SYSTEMS

TheTrans-Alaska Pipeline in Alaska conveys oil from the north of Alaska to the south of Alaska. Twelve Pump Stations (shown as PS) are needed to restore the PV Pressure to the oil so that it may continue flowing.

Consider Alaska's 800 mile long Trans-Alaska Pipeline which delivers crude oil from Prudhoe Bay to Valdez.

Pressure Drop due to Friction Losses eat up the Discharge Pressure delivered by the pumps at Pump Station 1 (PS1) so Pumping Station 2 (PS2) is strategically placed to restore the PV Pressure on the 48-inch diameter pipeline and keep the flow a-going!

The pumps installed at each successive Pumping Station are sized to create a Discharge Pressure that can deliver the oil to the next Pumping Station.

Without Pumping Stations, the PV Flowrate of the oil would cease.

The Trans-Alaska Pipeline is an example of a one-direction, series pumping system.

Some pumping systems are like the Lube Oil System shown below and use the same pump to restore the PV Pressure to the lube oil that is circulating in a closed loop.

PTOA Readers & Students know that ANY circulating system is subject to Heebie-Jeebies build-up as was explained in PTOA Segment #40.

The PV Pressure in the circulating lube oil line decreases from its maximum at the Pump Discharge as it flows through the pipes that deliver the fluid to → Coolers→ Filters →Check Valve → Compressor Train → Pump Suction Line.

Whew! Flowing through all that piping and hardware would consume the flow energy ... aka PV Pressure ... out of any fluid!

That's why IN ANY CIRCULATING PUMP OR COMPRESSOR SYSTEM:

The Suction Pressure is the Lowest Pressure in a Circulating Closed Loop System

and

The Discharge Pressure is the Highest Pressure in a Circulating Closed Loop System

All the PIs on the pipes of the circulating system will indicate progressively less PV Pressure between the maximum observed at Pump Discharge and the minimum observed at the Pump Suction.

Okay! Okay! The above information falls under the category of "Good To Know."

But why does the darn PV Pressure of a flowing fluid decrease in the first place and where or where does that PV Pressure go?

POOF ... PV PRESSURE CHANGES INTO FRICTION!

The PV Pressure of a flowing fluid is lost due to the Friction created by rubbing on the interior pipe wall.

Hey! You've already been sort of introduced to this concept!

The Velocity Profile of Laminar flow that was featured in PTOA Segment #159 is caused by Friction between the flowing fluid and the interior surface of the pipe.

Schematic of cone nose shape formed by Friction in Laminar Flow region.

PTOA Readers and Students know that the Velocity Profile shows how the fluid Velocity gradually increases "in radial layers" from the slowest flow velocity observed at the pipe's interior surface to the fastest flow velocity observed at the center of the pipe.

So PTOA Readers and Students already know that the Velocity Profile of Laminar flow can be identified by a cone shape.

Real life cool photo of cone formed in liquid flowing through this pipe.

The cone shape develops because of the rubbing of the outermost liquid on the interior of the pipe wall ...

and this rubbing causes some of the PV Pressure of the flowing fluid to just ...

POOF!

Change into Friction Losses that lower the PV Pressure in the process flow line ... thereby creating a noticeable Pressure Drop.

No kidding! ... The PTOA Department of Redundancy Department repeats:

Even with no restriction in the process fluid line, the mere interaction of a flowing fluid with the interior walls of a pipe creates a noticeable Pressure Drop due to Friction Losses.

Just flowing through a pipe causes Pressure Drop due to Friction.

The nearby schematic shows fluid flowing from left to right in a blue pipe. The decreasing PV Pressure is depicted as a decreasing red level in a gauge ... kind of like a barometer!

Here's another redundant way to look at it:

If a set of PIs were stabbed into a length of pipe, the PV Pressure indicated on the PIs would be less and less as the fluid flowed through the pipe.

For example:

The 60 psi PV Pressure in the top left of the nearby pipe drawing decreases to 57 psi after the fluid has flowed through a 50 foot length of Pipe ...

Heck yeah that's a noticeable Pressure Drop of 3 psi!

Otherwise stated ...

Friction Losses have eaten up 3 PSI of the PV Pressure as the process fluid flowed through just 50 feet of pipe!

The Pressure Drop due to Friction Losses is even more severe in a pipe with a smaller diameter!

The first 50 feet of pipe has a pipe diameter of 3/4 inch.

The second 50 feet of pipe has a reduced pipe diameter of 1/2 inch.

This decrease in pipe diameter from 3/4 inch to 1/2 inch greatly increases the Friction Losses that create Pressure Drop!

The 57 psi PV Pressure at the mid point of the top pipe reduces to 42 psi after the pipe diameter is reduced!

Wow! Decreasing the diameter of the pipe by a third creates 5 times the Pressure Drop due to Friction Losses (15 psi versus 3 psi).

OMG!

The Total Pressure Drop due to Friction Losses over just 100 total feet of pipe in the above pipe drawing is 18 psi!

That means (18/60 * 100 =) 30% of the initial PV Pressure was just chomped up by Friction and therefore did not help transport the fluid to its intended destination.

Wow! The above observation also means ...

Thirty percent of the utilities spent on generating the spinning or pushing action of a pump to create the PV Pressure did not even go into doing useful work ... like delivering the fluid where it is needed!

Thirty percent of the utilities spent on generating the PV Pressure of 60 psi is just wasted as Friction Losses!

SOURCES OF FRICTION LOSS ... AND THUS PRESSURE DROP

PTOA Readers and Students just learned that there is a direct link between wasted utilities expense and Friction Losses.

Understanding the sources of Friction Losses is the first step to designing pumping systems that minimize Pressure Drop.

Friction Loss and Pressure Drop Caused by Flow Through a Pipe

Pipe Flow Friction Losses that result in Pressure Drop are impacted by:

What is the AVERAGE VELOCITY when the Velocity Profile changes? The AVERAGE VELOCITY is the average of the flows that appear at the vertical line drawn.

Pipe Length ... the longer the pipe, the greater total Pressure Drop.
The Average Velocity of the Flowing Fluid ... squared! The greater the Average Velocity the much greater the total Pressure Drop!

The Pipe Diameter ... the smaller the Pipe Diameter, the greater the Pressure Drop ... a topic that was explored earlier in this PTOA Segment #165.

The Pipe Interior Smoothness ... the more smooth the pipe, the less Pressure Drop. The below schematic shows how much PV Pressure a flowing fluid retains when the pipe is fabricated from different materials. The graphic shows that smooth PVC pipe retains 50 more psi than the same length of pipe made of cast iron.

Cast Iron Pipe has the most Friction Losses, PVC Pipe has the least Friction Losses.

Now PTOA Readers and Students understand why Your Mentor made the below statement that appeared in PTOA Segment #163:

In the real world, the Suction Pipe that delivers lube oil from the Reservoir to the Lube Oil Circulating Pump will be the shortest, straightest run of pipe possible.

Fluid Properties Impact Friction Losses, Too!

The nitty-gritty details that describe math modelling and quantifying Pressure Drop due to Friction Losses are well beyond the scope of the PTOA and are not fundamental information that Process Operators must know to do their job well.

Viscosity is "Resistance to Flow." The High Viscosity Honey had more "resistance to flow" than Water. Honey will cause more Pressure Drop while flowing than Water will.

However, the factors that influence the Pressure Drop of a flowing liquid are easy to understand and reason out.

Who amongst the PTOA Readers and Students would be surprised to learn that the fluid properties that were featured in PTOA Segment #162 contribute to how much Pressure Loss is observed?

The Fluid's Properties of Density, Specific Gravity and Viscosity logically impact Friction Losses and Pressure Drop.

Imagine water and then honey flowing through a pipe. Honey is much more dense and viscous than water and of course has a Specific Gravity greater than 1.0.

No PTOA Reader or Student would be surprised to know that:

The greater the fluid Density and Specific Gravity, the greater the Friction Losses and Pressure Drop.
The greater the fluid Viscosity (which is defined as "resistance to flow"), the greater the Friction Losses and Pressure Drop.

Pipe Hardware Fittings and Components
Are Additional Sources of Friction Losses and Pressure Drop

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order just learned in PTOA Segment #164 that typical hardware components are found in the Suction and Discharge Lines of the Typical Pump Installation Set Up.

Who amongst the PTOA Readers and Students would be surprised to learn that each hardware fitting ...

like a Gate Valve or Check Valve ...

causes yet more Pressure Drop due to Friction Losses?

The flow through an open gate valve still has to flow around the lifted plug.

Heck! That makes sense!

Because if merely flowing through a pipe causes a Pressure Drop so would flowing through the twists and turns of a Valve because the fluid makes contact with more surfaces than just the interior pipe wall!

The nearby Check Valve animation and graphic together show why Check Valves cause significant Pressure Drop.

The normal flow through the nearby Check Valve animation is from right to left; the Pump Discharge Line would be to the right of the animated diagram.

In the event the ΔP changes and thus changes the flow direction from left to right, the Check Valve closes ... which is desired because the Check Valve was installed to protect the pump internals.

The normal flowrate through this check valve is from left to right and must always flow past the Check Valve flapper ... which causes A LOT of Pressure Loss.

However, the normal flow route must always flow around the flapper of the Check Valve.

This constant contact with the flapper converts the PV Pressure into Friction Losses 24/7!

Friction Losses that cause Pressure Drop in hardware fittings and components explain why Your Mentor made the below statement in PTOA Segment #164:

PTOA Readers and Students will learn that each Valve is installed for a specific purpose ... no Valve is just randomly stuck in there for grins.

Fred! Please confirm that you understand all the TAKE HOME MESSAGES below.

And Fred ...

Have you realized that the Friction Losses that cause a Pressure Drop support the statement: "No ΔP, No Flowrate!"

Good! That means it's time to introduce everybody to the Three Headed Pump Monster!

TAKE HOME MESSAGES: Contact between a flowing liquid and the interior surface of the pipe that the fluid is flowing in creates Friction Losses that result in Pressure Drop.

That means the mere necessity of having a fluid flow through a pipe results in some of the PV Pressure in the line being changed into useless Friction ... with the result being a decrease in the PV Pressure.

Friction Losses are unavoidable yet wasteful because they result in continually wasting the Utilities Expense item in the Plant's Daily Operating Expenses.

The amount of Pressure Drop observed in the flowing fluid depends upon pipe and fluid properties and are listed below:

The Fluid's Average Velocity
Pipe Length, Diameter, and Smoothness
Fluid Properties: Density/Specific Gravity and Viscosity.

Pressure Drop due to Friction Losses is also caused by every hardware component in the piping, like Gate Valves and Check Valves, etc.

In a circulating pump or compressor system, the Suction Pressure is the lowest PV Pressure in the system and the Discharge Pressure is the highest PV Pressure in the system.

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