I like dragging you around, I like dragging you down
I did it again and then I did it again
You let me do it again and so I did it again

("Dragging You Around," by Greg Larswell featuring Sia, 2012)

 

 UNDERSTANDING "SPECIAL" DYNAMIC PUMPS

The  Relationship Between the "Venturi-Shaped Pipe," "The Venturi  Effect," and The PV Pressure ↔ Fluid Velocity Swap

The prerequisite to understanding the operational theory behind many "Special" Dynamic Pumps is understanding The Venturi Effect.

The Dude History Lesson For Today

Venturi was a dude who … in the late 1790s  … wrote down his observations regarding the interesting behavior of flowing fluids.  Venturi did not understand why the behavior of fluids did what he had observed. Fortunately another dude named Bernoulli mathematically quantified and explained what the deal was.

 

So the lesson here is to write down and publish any observation of nature … no need to explain it …  so that it might become named after you. Because 90 years later that is exactly what a truly brilliant hydraulic engineer named Clemens Herschel did. Herschel designed a flowmeter based upon Venturi's documented observations and named his invention a "Venturi Flowmeter." Nowadays the name Venturi is sustained in process technology and Herschel's name is fading from history (albeit his many hydraulic engineering accomplishments … like the Niagara Falls hydroelectric power plant … live on).

 

 

Venturi observed what the PTOA calls the PV Pressure ↔ Fluid Velocity Swap!

Venturi had observed the flowrate of air and water as they flowed through a pipe which had a  gradually contracting diameter connected to a pipe with a larger diameter. What has become to be known as a Venturi Shape is somewhat like a horizontal hourglass (except this "hourglass" has a wider diameter at the contraction in the middle).

Did Venturi observe the flowing fluid getting bunched up and backed up as it flowed through the restricted diameter of the pipe?

Nope!

Venturi noticed that the velocity of the flowing fluid increased, cramming more particles per second through the restricted diameter of the pipe!

Otherwise stated, the velocity of the fluid simply increased while flowing through the restricted cross sectional area of the pipe.

Venturi measured the PV Pressure of the fluid before the pipe constriction, at the pipe constriction, and after the pipe constriction … where the flowing fluid entered a larger diameter pipe and slowed down to its original velocity.

Venturi observed that the PV Pressure of the flowing fluid decreased as the velocity of the flowing fluid increased.

This relationship between the PV Pressure and the flowing fluid's velocity as the fluid flows through a "Venturi Shape" pipe is poetically referred to as "The Venturi Effect."

The PTOA refers to "The Venturi Effect" as the PV Pressure ↔ Fluid Velocity Swap … a lot less poetic but gets the point across 100% better.

PTOA Readers and Students learned about The PV Pressure ↔ Fluid Velocity Swap way back in PTOA Segment #159. The content was presented way back then because swapping PV Pressure for fluid velocity is the operating principle behind all Dynamic Pumps … like Centrifugal Pumps!

The below photo of an industrial setting features blue painted industrial piping.

Hey, I spy a "Venturi Shape" pipe!

The tubing for a low PV Pressure tap and a high PV Pressure tap can be seen hanging downward from the center (constricted diameter pipe) and to the right of the picture (a pipe with a larger diameter). The difference in the PV Pressure measurements can be converted into flowrate measurement for the fluid that is flowing through the pipe.

 

The point is that anytime a PTOA Reader or Student spies a "Venturi Shape" in the industrial setting, s/he or they should immediately recognize that the PV Pressure ↔ Fluid Velocity Swap is a-happening. In the upcoming PV Flowrate Focus Study, PTOA Readers and Students will learn exactly how Venturi's observations are used to measure the PV Flowrate so don't stress about that now.

 

Venturi Also Observed That A  "Motive Fluid" can Drag A "Suction Fluid"...
Thus Mixing with the "Suction Fluid" While Flowing in the Same Direction 

Venturi made a second observation that is the operating principle behind many types of "Special" Dynamic Pumps.

Venturi observed that …

as air or water gain velocity to get through the pipe constriction ...

these fluids seem to "magically" drag a different fluid that enters the pipe from a connecting pipe!

Of course there are a couple of important stipulations:

The orientation of the Venturi Shaped pipe to the connecting pipe must be VERY specific

 

AND

A fast moving Motive Fluid must be forced to flow through  the Venturi Shaped pipe!

When these two criteria exist the fast flowing Motive Fluid will cause the second fluid to be "entrained to flow" in the direction of the Motive Fluid.

Otherwise stated:

 

 

The Motive Fluid flowing through a Venturi Shape pipe can drag a "Suction Fluid" with it, causing both fluids to mix and flow in the same direction.

Note that the Suction Fluid is the fluid that needs to be moved. Getting the "Suction Fluid" to move is the reason to purchase and install the "Special" Dynamic Pump in the first place.

Venturi was more eloquent in his description of the interaction of the two fluids. He wrote that the fast moving Motive Fluid  was "impressing its motion" upon the dragged fluid "in lateral communication of motion."

Not bad for not really understanding the physics behind his observations!

Time to "You Tube and Chill" with this Venturi Water Pump Animation created by Trenchless Group.

Note that the Motive Fluid is none other than compressed air. In this pump the dragged fluid … aka Suction Fluid … is water.

The video states that this simple pump can suck water out of a tank at a rate of 85 liters per minute (22 gpm) to a head of 4.5 meters (14.7 feet).

Note that no complicated driver or lubrication oil is needed in this simple pump because there are no moving parts.

Don't forget to give the Trenchless Group You Tube a Thumbs Up!

The "Venturi Effect" illustrated in the Trenchless Group You Tube is the operating theory behind Jet Eductor Pumps and Jet Ejector Pumps, two types of "Special Dynamic Pumps."

 

"SPECIAL" DYNAMIC PUMPS

Behold the familiar "Classification of Pumps … With Examples" diagram which was first introduced in PTOA Segment #206. As has been already emphasized many times, this graphic shows that there are only two main types of pumps, Positive Displacement Pumps (on the right) and Dynamic Pumps (in the middle).

The PTOA focus on Positive Displacement Pumps begins directly after the PTOA focus on "Special" Dynamic Pumps concludes. .

The graphic shows the two lineage lines of Dynamic Pumps, Centrifugal Pumps on the top and "Special" Pumps toward the bottom. "Special" Dynamic Pumps include:

• Ejector Pumps and (Liquid) Eductor Pumps
• Gas Lift Pumps
• Electromagnetic Pumps

 

This PTOA Segment #207 features the operating principle and service for Ejector Pumps and (Liquid) Eductor Pumps. The operating principle and service for Gas Lift Pumps and Electromagnetic Pumps are featured in the next PTOA Segment.

PTOA Readers and Students should note the following about the (Jet) Ejector and (Jet) Eductor "Special" Dynamic Pumps: 

• They don't look like traditional pumps! They look like pumping "systems" designed to move a target fluid with a supplied, fast moving different fluid!
•  Since the Motive Fluid (aka Power Fluid) mixes with the Suction Fluid, some type of separation may be required downstream of the pump discharge and thus becomes an integral part of the "pump system."
• They do not have any moving parts so there is no stress regarding generation of friction heat and thus no need for lubrication, priming, etc.
• Except for whatever type of Rotating Equipment is used to supply the Motive (aka Power) Fluid,  these pumps are ultra reliable and require little maintenance.

 

THE STEAM JET EJECTOR PUMP

In PTOA Segment #151 PTOA Readers and Students learned that Vacuum Pressures … Pressures less than 1 Atmosphere (Atm) or 14. 7 psi … must be created since they will not occur naturally and have important roles to play in process industry.

 

One example given in that PTOA Segment was The Vacuum Distillation Tower found in oil refineries. The relationship between the PV Temperature and the PV Pressure was featured in PTOA Segment  #157. Creating a vacuum PV Pressure makes it possible to boil and thence separate very heavy hydrocarbons at much lower temperatures than their boiling points at 1 Atm or 14.7 psi.

The device … or "Special" Dynamic Pump … that is used to create the Vacuum PV Pressure in the Vacuum Distillation Tower is called a Steam Jet Ejector. Many thanks to Mechanicalengineerigsite.com for allowing use of their Steam Jet Ejector graphic which is shown nearby.  Note the Venturi Shape outlined in orange includes sections which are labelled "Diffuser" and "Mixed Fluid Outlet."

Logically, steam is the Motive Fluid for the Steam Jet Ejector (like Duh!).

The descriptor "Jet" is a hint that the Motive Fluid steam flows first through a Nozzle before entering the Venturi.

The PV Pressure of the steam is very low at the Nozzle exit, thus the velocity of the steam is very high at the Nozzle exit.

The steam enters the Venturi Shape piping, dragging (or use the fancy word: "entraining") the Suction Fluid with it and the "Mixed Fluid" exits the Steam Jet Ejector.

The below Steam Jet Ejector Works You Tube from GDNash.com illustrates how the supplied steam (Motive Fluid) creates a vacuum while flowing through the Nozzle and Venturi of a Steam Jet Ejector, dragging the Suction Fluid with it.

Be certain to give the You Tube a Thumbs Up!

The Suction Fluid in the  the Vacuum Distillation Tower is composed of the vapors from the lightest hydrocarbons that have been distilled from the heavy oil that was fed to the tower.

The strength of the vacuum can be adjusted with the flowrate of the steam; greater steam velocity would pull a stronger vacuum and vice versa.

In other words:

Besides getting the Suction Fluid to flow as desired,  Jet Ejectors also establish and maintain the PV Pressure of the pumping system.

Three vertically mounted Venturi Shape pipes can be seen in the nearby photo of a Vacuum DIstillation Tower. These Steam Jet Ejectors are maintaing the vacuum presure on the Vacuum Distillation Tower seen in the photo.

The fluid mechanics and physics behind creating the vacuum PV Pressure is beyond the scope of the PTOA to explain. However, multiple stages of Steam Jet Ejectors can achieve a vacuum pressure of 10 - 40 mm Hg (aka .19 - .77 psi or 0.013 - 0.0526 ATM), greatly reducing the boiling point of heavy hydrocarbons.

 

 

JET EDUCTOR PUMPS

Like Jet Ejectors, Jet Eductors feature a Nozzle through which the Motive Fluid exits at high velocity prior to flowing into the Venturi Shape structure.

One of the main distinguishing features about Jet Eductors is that the Motive Fluid drags 4 times its volume of surrounding Suction Fluid.

For example, the nearby graphic illustrates that a total of 4 gallons of liquid (aka the Suction Fluid) can be dragged into the Venturi Shape pipe by 1 gallon of jet-streamed Motive Fluid.

Unlike Jet Ejectors, Jet Eductors DO NOT establish and maintain the PV Pressure of the "pumping system."

(Liquid) Eductor Pumps are for the service application of mixing and/or blending the contents of a large tank as well as emptying tanks and draining sumps.

You Tube and Chill again with this promotional video from Mead-Obrien regarding Eductors Used for Tank Mixing. And be certain to give the Mead-Obrien You Tube a Thumbs Up!

 

The next PTOA Segment wraps up the PTOA content on the Dynamic Pump Family Tree while focusing on  Hydraulic Jet Pumps, Gas Lift "Pumps," and Electromagnetic Pumps.

 

TAKE HOME MESSAGES:  The noun "Venturi" and "Venturi Shape" describe a pipe with a decreasing diameter (contraction) followed by an enlargement of the same pipe to a larger diameter (expansion). PTOA Readers and Students who become future Process Operators can identify the Venturi shape throughout an industrial processing plant and will understand what is going on inside the structure. 

As he observed water and air flowing through what was decades later called a Venturi Shape pipe, Venturi had no idea the PV Pressure ↔ Fluid Velocity Swap he documented would become the operating principle behind all Dynamic Pumps as well as a method to measure the flowrate of liquids and gases. .

Venturi also did not realize his observations of a Motive Fluid dragging (aka entraining) a Suction Fluid would be the operating principle behind Jet Ejectors, and Jet Eductors … two types of Special Dynamic Pumps.

Jet Ejectors and Jet Eductors do not look like traditional Centrifugal Pumps or Positive Displacement Pumps and are thus aptly classified as "Special Pumps" because they still can move fluids as all pumps do.

The descriptor "Jet" means that the Motive Fluid first flows through a Nozzle prior to flowing into the Venturi part of the Jet Ejector or Jet Eductor.

While moving a Suction Fluid, Jet Ejectors maintain a system's vacuum PV Pressure.   

The Motive Fluid (aka Power Fluid) of (Liquid) Jet Eductors can move 4 times more by volume of Suction Liquid; Typically the Suction Liquid is surrounding liquid. Eductors are ideal for fast mixing, blending, emptying tanks, and draining sumps.

Except for the Rotating Equipment that supplies the Motive Fluid (aka Power Fluid) there are no moving parts in Jet Ejectors or Jet Eductors so there is no need for lubrication or priming.

©2020 PTOA Segment 0207
PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT PUMPS

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