Because the world is round it turns me on
Because the world is round

("Because," by The Beatles, 1969)

 

WHY THIS PTOA SEGMENT #176
IS NOT AN INTRODUCTION TO IMPELLERS

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order probably think there is nothing left to learn about Impellers for good reason ... by now any one of them could generate a Power Point entitled "Purpose and Function of Impellers."

 

 

Most recently PTOA Readers and Students learned that:

• The Impeller is on the list of "Pumping Side" essential hardware ... that's the side of a Centrifugal Pump that gets wet because it is in direct contact with the process liquid that is being pumped-up (PTOA Segment #174).
• The number of Impellers on the shaft/rotor of a Centrifugal Pump defines the number of pump stages and the Pump Casing must be fabricated to accommodate all of the Impellers on the shaft.
• Ergo, a Pump Casing must have an Inlet Nozzle and Volute ... and the Casing for a Multistage Centrifugal Pump must have inter-stage Diffusers and Guides located between Impellers ... so that the pumped liquid flows directly into the Eye of each Impeller (PTOA Segment #175).

And the above statements made perfect sense to PTOA Readers and Students because they had already expertly understood the following:

• The Impeller spins/rotates because it is attached to a shaft that is twirled by a Driver/Prime Mover (PTOA Segment #161).
• The spinning Impeller creates a Centrifugal Force which acts upon the liquid ... flinging it radially outward ... all the while increasing the liquid's Velocity (Centrifugal Force was featured in PTOA Segment #173).
• The liquid that is flung radially outward from the Eye of the Impeller has its Velocity converted into the PV Pressure within the Volute or Diffusers (Centrifugal Action was introduced in PTOA Segment #167 and featured in PTOA Segment #173).

 

WAIT THERE'S MORE!

• Because the Centrifugal Action that is created by Impellers and Volutes/Diffusers dynamically adds the PV Pressure into the discharged pumped-up liquid, the Characteristic Curve of a Centrifugal Pump is expressed in feet of  Total Dynamic Head (TDH) and not something that is easier to understand ... like PSI. (PTOA Segment #167).

• A Centrifugal Pump with a given Impeller diameter that is rotating at a specified speed in rpms will develop the same feet of Total Dynamic Head no matter what the liquid being pumped is. (PTOA Segment #167).
• The diameter of the Impeller significantly impacts the real-world performance of the pump and this pump performance can be represented in a Chart of Performance Curves wherein the pump's TDH, Efficiency, and Brake Horsepower are predicted as the pump's Capacity increases; however, there are physical limits with respect to how long or short the Impeller's diameter can be (Impeller Diameter Affinity Laws were featured in PTOA Segment #169).
• Operating the Centrifugal Pump at the optimal condition of Efficiency, TDH and BHP will prevent Cavitation; Cavitation will erode if not destroy the Impeller which will result in pump repairs or even catastrophic pump failure (PTOA Segment #171).
• PTOA Segment #170 demonstrated how the rotational speed of the Impeller in rpms also impacts the real-world function of the pump which can likewise be represented in a Chart of Variable Speed Performance Curves ...

however, there are also limitations with respect to how fast or slow an Impeller can be rotated ...

and those limits on the rotational speed of an Impeller will be explained in this PTOA Segment!

These other Impeller topics will also be featured:

• Common features found on all Impellers.
• The three Impeller styles.
• Impeller Vane/Blade designs and their pros and cons.

 

So keep on reading so that you will learn  ...

Everything Process Operators Need to Know About Impellers!

 

COMMON IMPELLER FEATURES

There are several Impeller styles yet there are common features about all Impellers.

 

First: Every Impeller style will have Vanes which are sometimes called Blades.

 

 

The job of the Impeller Vane/Blade is to push out and increase the radial Velocity of the pumped fluid.

Second: Every kind of Impeller will be designed to "fit snuggly" into the Casing that surrounds it as was previously mentioned in described in PTOA Segment #175.

Why?

Imagine being a drop of liquid in the process stream that is flowing through Suction Line, heading into the Suction Flange and hence sucked into the Suction Nozzle of a  Centrifugal Pump.

You would be drawn into The Eye of the Impeller (the area within the dotted circle in the nearby graphic) and immediately flung harshly outward to the edge of the Impeller where you would finally be able to slow down a bit in the wider volume of the Volute.

But what would keep you ... now a pumped-up drop of liquid ... from slipping behind the Impeller instead of flowing into the Pump's Discharge Nozzle like a good drop of pumped-up liquid would do?

A snug fitting, specially cast Volute for one ...

And the fact that on the back side of the Impeller there is a Collar that fits snugly into the Casing.

The Collar might well be called something else, like "Frame Plate Liner". Unlike their counterparts in the Process Instrumentation manufacturing, Pump Manufacturers don't get hung up on nomenclature.

The way to identify the device performing the function of a Collar would be to notice:

• The Collar is located at the back of the Impeller.
• The Collar has the same interior dimension as the Eye of the Impeller.
• The Collar functionally separates the "wet end" of the Pump from the Stuffing-Packing/Sealing Hardware of the Pump.

Follow the arrows in the below nifty Centrifugal Pump "blow-up" and observe:

• The Collar is called a "FPL Insert" and is Yellow.
• The Impeller in front of the FPL Insert/Collar is Orange.

ATTENTION PLEASE! Note the label "Metal Wet Parts" in the above graphic and focus on the arrows that point to four pieces of hardware.

The label and arrows indicate that  FPL Insert/Collar is "The Final Line of Defense" between the metal parts that are designed to get wet and the Stuffing-Packing Box/Sealing Hardware that is supposed to remain dry.

Hey!

What up, Fred?

 

 

Fred is in the back of the room waving his hand and expounding about how there would still be too much clearance between the Casing and the Impeller to keep the higher Pressure pumped-up liquid from leaking between these two slick metal surfaces.

Correctamundo! Fred, I'm proud of you! Let me explain about ...

Third: Wear Rings are used to further minimize the clearance between the front and back sides of the Impeller and the interior of the Casing.

Wear Rings are accurately named because they bear the brunt and suffer the consequences of wear and tear that would otherwise be borne by the spinning Impeller and stationary Casing.

The main job of the Wear Ring is to protect the much more expensive hard ware (Impeller and Casing) and wear out before the much more expensive equipment does. Wear Rings are much easier and cheaper to replace than the main internal parts of a Centrifugal Pump!

The second job of a Wear Ring is to minimize leakage and thereby direct the flow through the pump as is desired. 

There are two kinds of Centrifugal Pump Wear Rings.

A Casing Wear Ring is a stationary ring that protects the interior surface of the Casing.

An Impeller Wear Ring rotates with the Eye of the Impeller.

Both Wear Rings minimize leakage of the higher Pressure discharged liquid.

The Impeller Wear Ring shown in the above graphic is second from the right, next to the blue Volute.  The Impeller Wear Ring looks like a ring of metal carbon steel.

Fourth: No matter what ... some pumped up liquid will STILL get by the Casing Wear Ring and enter the Collar.

Fortunately, this higher-Pressure liquid will flow back into the lower-Pressure Suction through holes that are drilled into the Impeller.

The nearby photo clearly shows intentionally drilled Impeller holes.

These holes are called Balance Holes because they help equalize the PV Pressure that impacts an operating Impeller.

Fifth: All Impellers have an opening in the dead center that is used to connect the Impeller to the Shaft.

A Single Stage Impeller may be bolted on to the Shaft.

A Multi Stage Pump Shaft may be thread through several Impellers.

 

IMPELLER STYLES

Open Impellers 

The Open Impeller Style has its openly exposed Vanes/Blades centered around the Eye of the Impeller.

The Eye of the Impeller is the centered inlet ... the sucking inlet of the spinning Impeller ... into which the process stream liquid flows.

The Impellers used to propel recreational boats through water and air planes through the fluid called "air" are Open Style Impellers.

Their common name is "propeller" because the axial thrust generated by the Open Style Impeller "propels" the boat and airplane to move forward.

Aha!

Propellers (aka Open Style Impellers) are intentionally used to generate the axial thrust that is desired when pulling water skiers and  making air travel possible!

Spoiler Alert!

Guess what is needed when the Centrifugal Action generated by a rotating Impeller is desired but the axial thrust is not desirable?

Axial Thrust Bearings are always needed to counteract the natural tendency of a rotating Impeller/Propeller to "propel" a shaft forward!

 

 

Semi-Open (aka Partially Open) Impellers 

The Semi-Open (aka Partially Open) Impeller Style has its Vanes/Blades attached to one supporting Plate or Shroud.

The Red and Yellow Impellers in the nearby photo are Semi-Open (aka Partially Open) Impellers.

Note that the Vanes/Blades on the Red and Yellow Semi-Open/Partially Open Impellers are straight and radial from center of the Impeller to the edge of the supporting Plate/Shroud.

Here's another photo of a Semi-Open/Partially Open Impeller.

This Impeller has Backward Curved Vanes ... a Vane/Blade design that will be explained soon.

 

 

 

Enclosed Impellers 

Enclosed Impellers are logically named because their Vanes/Blades are enclosed between a bottom Plate/Shroud and a top Plate/Shroud.

The Enclosed Impeller Style has an opening on the top Plate/Shroud that purposefully directs the Suction-side liquid into the Eye of the Impeller.

The high-Velocity liquid that has been flung radially outward from the Eye of the Impeller exits through the large openings that are visible between the Plates/Shrouds.

 

IMPELLER VANE/BLADE DESIGNS AND ENERGY EFFICIENCY

The straight radial Vanes/Blades shown in the nearby photo that are attached to the Red and Yellow Semi-Open Impellers are much cheaper than the curved Vanes shown below.

 

 

However,

The Backward-Curved Vane design is much more efficient with respect to adding the PV Pressure into liquids than either the Radial Vane design or the Forward Curved Vane design.

The nearby graphic attempts to illustrate how the Power Requirement (P, graphed on the Y-axis) would vary with a Forward Curved Vane, Straight Radial Vane, and a Backward Curved Vane.

The X-axis is the increase in Capacity of flow through the Pump ... in this case labelled "Q."

The graphic reveals that ...

 

At any value of pump Capacity ...

The Backward Curved Vane consumes significantly less power than either the straight Radial or Forward Curved Vanes.

Ergo, the Backward Curved Vane shape is the most efficient Vane design to use when adding the PV Pressure to a liquid.

But guess what?

The Forward Curved Vane design is used when adding the PV Pressure to a gas ... which takes place in the Rotating Equipment known as "Compressors."

 

YOU'RE SO VANE*

The below cool graphic shows a set of Semi-Open Impellers with an increasing amount of Vanes/Blades from 1 to 7.

The number of Vanes/Blades on an Impeller will be optimized for the wide range of pump services.

Here are some generalizations with respect to optimal number of Vanes/Blades:

• A Single Stage Pump and the first Impeller of a Multistage Pump might have between 3 and 5 Vanes/Blades.
• The Impellers for Multistage Pump stages, pipeline pumps, and other large, high Efficiency pumps tend to have 7 Vanes/Blades.
• An Impeller may have as few as 1 to 3 Vanes/Blades when the process service involves solids.

Who amongst the brilliant PTOA Readers and Students has figured out that the Vanes/Blades are not just welded on to the Plate/Shroud with no thought at all?

The subject is way beyond the scope of the PTOA, but nerdy Mechanical Engineer types spend beaucoup time determining the optimal Vane/Blade angles to incorporate into the design of an Impeller.

Don't stress out, Fred!

You do not need to understand Vane angles!

But there is one good reason to be aware that Vane positioning on an Impeller is not random ...

Read on!

 

*("You're So Vain," by Carly Simon, 1972)

 

LIMITS ON VARYING THE RPM SPEED OF IMPELLERS

The Affinity Laws that can be used to predict "stepping up" and "stepping down" Centrifugal Pump Performance via varying the rpm speed of the Driver/Prime Mover were featured in PTOA Segment #170.

PTOA Readers and Students have just been made aware that Impellers are designed with Vane/Blade angles that optimize several criteria, including:

• The process service of the Pump.
• The designed speed of rotation (rpms) provided by the Driver/Prime Mover.

Ergo ...

If the rpm speed is varied too radically, the Affinity Laws featured in PTOA Segment #170 will not be accurate.

However ...

In most cases the Affinity Laws for speed variance will be sufficiently accurate to predict the TDH, Efficiency, and BHP for a reasonable initial "step up" and "step down" from the base case rpm scenario.

And that tidbit of information concludes ...

EVERYTHING PROCESS OPERATORS NEED TO KNOW ABOUT IMPELLERS!

 

 

TAKE HOME MESSAGES: PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already know a lot about Impellers that will not be re-summarized here!

New information about Impellers included:

All Impellers:

• Have Vanes/Blades that push out and radially accelerate the Velocity of the fluid.
• Impellers are sandwiched between the Pump Suction and Collar.
• Impellers have rotating Impeller Wear Rings and stationary Casing Wear Rings that are expected to wear out before the much more expensive Impellers and Casings wear out. Wear Rings are much easier to replace than Impellers and Casings. Wear Rings also limit the amount internal pump leakage.
• All industrial Impellers will have Balance Holes to equalize Pressure across the Impeller and which also allow the higher Pressure liquid that seeps into the Collar to flow back into the Eye of the Impeller.
• All Impellers have an opening in their dead center which is used to connect the Driver/Prime Mover  to the Impeller ...  so they can rotate at the same speed in rpms.

The Collar of a pump may be called something else; functionally it is the physical barrier between the wet components of the pump and the Stuffing-Packing Box/Sealing Hardware. The Collar component will be located on the back of an Impeller and will have the same internal dimensions as the Eye of the Impeller. 

The 3 styles of Impeller are:

• Open - the Vanes/Blades are centered around the Eye of the Impeller.
• Semi Open - the Vanes/Blades are supported by a single Plate/Shroud ... and centered around the Eye of the Impeller.
• Enclosed - the Vanes/Blades are enclosed in a top and a bottom Plate/Shroud ... and centered around the Eye of the Impeller.

Examples of Open Impellers are outboard motor Impellers and air plane Impellers ... both of which are called "Propellers."  Fans are also Open Impellers.

The Vanes/Blades on the Impeller can have the following design:

• Radial
• Backward Curving
• Forward Curving

Backward Curving Vanes/Blades are the most energy Efficient for liquids and are thus used in industrial Centrifugal Pumps. 

High efficiency pumps used in the process industries will have Enclosed Impellers with 7 Vanes/Blades per Impeller although the first stage may just have 3 to 5.

Forward Curving Vanes/Blades are used to add the PV Pressure to gases/vapors in Compressors.

Vane/Blade placement on an Impeller is not random; the geometry takes into account the process service of the Impeller and the design speed of the Driver/Prime Mover.

Because the design of the Impeller is so specialized, the Affinity Laws used to predict Centrifugal Pump performance via "stepping up" or "stepping down" rpms are only valid for a reasonable initial "step up" and "step down."

©2017 PTOA Segment 0176
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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