I'm fixing a hole where the rain gets in
And stops my mind from wandering
Where it will go.

("Fixing a Hole," by the Beatles, 1967)

 

MOVING ON TO "THE POWER SIDE" OF THE CENTRIFUGAL PUMP

Way back in PTOA Segment #163, PTOA Readers and Students were introduced to ...

• The nearby graphic of a single stage Centrifugal Pump  ... and
• The real world photograph below the graphic which could represent the same single stage Centrifugal Pump installed at a facility.

 

As of this very moment ...

those same brilliant PTOA Readers and Students ...

who have been reading the PTOA Segments in the intended sequential order ...

have a far better understanding of the form and function of the hardware components found within "The Pumping Side" of a Centrifugal Pump compared to many, many currently working Process Operators!

Knowledge is Power!

Now that PTOA Readers and Students possess a fundamental understanding of "The Pumping Side" of a Centrifugal Pump as well the importance of a pump's auxiliary Lubrication Oil System ...

PTOA Readers and Students are well prepared to focus on the form and function of the hardware components that are found on "The Power Side" of a Centrifugal Pumps!

This PTOA Segment #181 describes:

• The form and function of common hardware components found in the Stuffing Box of Centrifugal Pumps.
• The hardware components of Mechanical Seals which have replaced the braided, fiber Packing that prevents leakage from many Centrifugal Pumps!

 

THE FORM AND FUNCTION OF STUFFING BOX COMPONENTS

Let's recap what we know to be true!

• 

  The gold Enclosed Impeller is on the right. The Shaft is attached to the Impeller and extends through the Stuffing Box in the middle and Bearing Box on the left.

  The Impeller cannot rotate on its own.

• The Impeller is attached to a Shaft.

 

• The Shaft is "coupled" to a Driver and

 

• The Driver is the origin of the sliding/rotating movement that spins the Impeller!

 

Heck!

That means there is a hole where the Shaft pierces "The Pumping Side" Casing!

This hole would allow the "pumped up" liquid to leak from "The Pumping Side" of the pump and into the area behind the Impeller!

Righteoo!

 

 

And that area of the Pump adjacent to the pierced pump Casing is called the Stuffing Box.

The term "Stuffing Box" refers to the cavity of space within the interior of the pump Casing where the Shaft pierces "the collar"  ... or whatever physical barrier separates "The Pumping Side" of the pump from "The Power Side" of the pump.

A "Stuffing Box" might be called a "Packing Box" in some literature and training videos.

The hardware components within the Stuffing Box perform the important function of minimizing the leakage of "pumped up" process fluid out of the Stuffing Box; without Packing or a Mechanical Seal in the Stuffing Box, leakage would naturally occur since the rotating shaft pierces the Pump Casing.

 

How To Identify a Stuffing Box

The nearby cutaway graphic of a Centrifugal Pump shows:

• The Casing/Housing of the Stuffing Box labelled "A."
• The Packing Rings labelled "B" which are encircling ...
• The Shaft Sleeve labelled "D" which protects the much more expensive
• Shaft which is labelled "C."

The Stuffing Box Housing in the nearby real life photograph is likewise situated in the same relative position as the cutaway schematic shown above.

 

In the above photo the exterior Casing of both "The Pumping Side" of the pump and the Stuffing Box are both painted blue ... so its kind of hard to distinguish the difference.

That "open area" is where The Shaft is extending from the Stuffing Box Casing and into the Bearing Housing.

 

WHAT'S THE STUFF THAT IS PACKED INTO A STUFFING BOX?

The media used to "stuff" or "pack" the Stuffing Box for the purpose of preventing leakage must have the following properties:

• Be chemically compatible with the "pumped up" process liquid.
• Minimize damage to the Shaft Sleeve ... so the Packing cannot be abrasive and should be somewhat "slick" so that the Packing minimizes the heat created from friction.

 

The Chesterton Packing You Tube

Some folks say "a picture is worth a thousand words."

So that must mean a "You Tube is worth a million words!"

Correctamundo!

Explaining the form and function of Packing in a Stuffing Box is best done via You Tube!

Thanks to the A.W. Chesterton Company for providing the below 8.5 minute video which features replacing Packing in a Stuffing Box.

Guess what?

Process Operators are not expected to replace the Packing in the Stuffing Box of a Centrifugal Pump; that's the job of the Mechanics and Mechanic Techs.

The Chesterton video gets relevant to Process Operators around minute 3 and a half  ... so hang in there!

Access the Chesterton Video HERE ... or access the You Tube directly below.

While watching the video, be on the lookout for these features:

• At the very end of the video, Chesterton shows how the pre-lubricated Packing fibers are woven together from separate fiber strands.
• The "ropes" that result after braiding are formed into Packing Rings with a diameter that will fit over the Shaft Sleeve, a covering that protects the  Shaft.
• Note that the Packing Rings are split in half. So two half-circles make one Packing Ring!
• When carefully "stuffed" into the Stuffing Box, the "split" of the two half circles is intentionally radially spaced at 15 degree intervals on the Shaft Sleeve.  This important Packing installation technique helps reduce leakage of the "pumped up" process liquid from the Stuffing Box.
• Note also how the Shaft in the You Tube is protected with a Shaft Sleeve. The Shaft Sleeve bears most of the wear of rubbing from the Packing. A Shaft Sleeve would be much easier and much less costly to replace than the entire Shaft!

 

 

Packing Media Must be Lubricated!

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order expertly understand why the rotating/sliding parts in the above You Tube must be lubricated.

The Shaft Sleeve in the nearby photo shows scoring caused by lack of lubrication.

 

Sometimes, the Centrifugal Pump is designed to allow the "pumped up" process liquid to intentionally leak back into The Stuffing Box for the purpose of continuously lubricating the Packing.

The right side photo below is a cutaway of a pump with Packing that is lubricated by the "pumped-up" process liquid.

But sometimes the chemical properties of the "pumped-up" process liquid are not sufficiently compatible with the Packing to perform the lubricating function.

When that situation exists, an external source of lubricant is delivered into the Stuffing Box and distributed by a Lantern Ring. A Lantern Ring is shown in the above left photo.

Lantern Rings sound like they emit light. They don't!

 

Lantern Rings

As stated above and repeated here ...

When an outside source of lubrication is required to lubricate Packing, a Lantern Ring is installed within the Packing.

The smooth, brass Lantern Ring in the nearby photo is surrounded by the dark, braided fiber Packing Rings.

 

A Lantern Ring is just a metal ring with holes in it that allow the lubricant flow onto the Shaft Sleeve, Shaft, and Packing.

Below is another photo of the same Shaft that is shown above. In this photo it is easier to see how the Shaft is attached to the Impeller. It is also easy to see The Stuffing Box ... the area behind the Impeller that includes all the important hardware that is needed to prevent leakage of the "pumped up" process liquid out of the Stuffing Box.

See how the black, braided fiber Packing Rings and the bronze Lantern Ring encircle the Shaft Sleeve? That Shaft Sleeve protects the much more expensive Shaft from the chafing that would be caused by rotation.

 

Fred is wondering:

How does the lubricant get into the Pump Casing?

 

Lantern Rings are strategically located below an opening in the Casing through which lubrication oil flows into the Stuffing Box.

 

Just like Packing Rings, Lantern Rings rotate with the Shaft. The lubrication oil is distributed to the Shaft Sleeve, Shaft, and Packing via the holes in the spinning Lantern Ring.

Some Centrifugal Pump designs pipe a small stream of "pumped up" liquid from the Pump Discharge Line directly into the lubrication access hole in the Pump Casing.

Now Fred is wondering ...

Where does the spent lubricant/process fluid go in this type of pump design?

Because the PV Pressure of the discharged "pumped up process liquid" (aka lubricant) in the Stuffing Box is greater than the PV Pressure on "The Pumping Side" of the Pump, the lubricant/process fluid will naturally "leak back" into "The Pumping Side" of the pump!

Regardless of whichever pump lubrication design is used, the Outside Process Operator must constantly determine whether or not the leakage rate from the Stuffing Box of a Centrifugal Pump is as expected.

Hey! The below pump is clearly exhibiting too much leakage!

 

MECHANICAL SEALS

The Benefits of Mechanical Seals Over Packing

The Chesterton Video showed the "allowable leakage" from the Stuffing Box of a real, working industrial pump.

Packing will decrease leaking to a controlled minimum ...  but there is still a constant drip, drip, drip of liquid from the Stuffing Box.

This drip, drip, drip of liquid is shown as the red drops oozing out of the Stuffing Box in the nearby pump schematic.

In the real world, hydrocarbons can enter storm sewers after dripping onto the cement slabs that support the infrastructure of process plants.

Just before the turn of the century, the general public "got woke" to the environmental damage caused by the "allowable leakage" of all the industrial pumps in service worldwide.

Then the USA Environmental Protection Agency implemented regulations that advocated for clean water by requiring Mechanical Seals to be installed on new pumps and retrofit onto many pumps in service.

And guess what, PTOA Readers and Students?

This regulation is yet another example of a regulation everybody lives better with!

The industrial waste water that flowed into sewers and storm drains became cleaner and Plant Owners discovered they made much more money over the operating life of a pump that had Mechanical Seals instead of Packing!

That's because when less of the "pumped up" liquid is wasted as leakage over the life time of the pump, more of the money spent on powering the Driver is being spent efficiently!

The benefits of Mechanical Seals over fiber Packing also include:

• Much less maintenance over the life of pump compared with Packing.
• Greatly reduces pump leakage when the pump operates at extreme PV Pressure or PV Temperature.
• Greatly reduces leakage when the pump is operating at high rpm or is in hazardous liquid service.

 

Basic Mechanical Seal Hardware Components

The red pump in the nearby cutaway does not have rows and rows of Packing in the Stuffing Box.

This pump minimizes leakage with a Mechanical Seal.

Mechanical Seals primarily prevent leakage around the Shaft of a pump by mating the flat face of a ring-shaped Rotating Seal to the flat face of a ring-shaped Stationary Seal.  There simply isn't sufficient space between the two flat surfaces to leak through!

The ring-shaped Stationary Seal does not rotate with the Shaft and is made out of a non-galling material like carbon.

The ring-shaped Rotating Seal does rotate with the Shaft and is fabricated from a hard material like silicon-carbide where it comes into very nearly direct contact with the Stationary Seal Ring.

A secondary line of defense against leaking is created by an O-Ring that rotates with the Rotating Seal Ring, thus making a tight seal between the Rotating Seal and the Shaft.

 A Spring (or Metal Bellows) provides the tension that keeps the Rotating Seal in place so it can't move up and down the spinning Shaft.

Heck! That must mean that a Spring Holder must be on one side of the Spring, enabling it to push the O-ring (or other type of Elastomer) against the Shaft and Rotating Seal.

 

Lubrication/Seal Oil for Mechanical Seals

In the nearby graphic of a Mechanical Seal, the light blue coloring defines rotating hardware components in the Stuffing Box.

The gray surfaces are stationary hardware components.

Brilliant PTOA Readers and Students ...

who are all knowed-up on Tribology ...

are wondering ...

Why doesn't the interface between the blue and gray hardware components that are touching each other fuse together?

That's because all PTOA Readers and Students who are reading the PTOA Segments in the intentional sequential order expertly understand that the friction created by a rotating surface rolling by a stationary surface would generate mucho heat.

Righteeo!

The interfaces between light blue and gray touching surfaces would most certainly heat up and fuse in a Mechanical Seal.

For this reason, Seal Oil Pots are installed right next to every pump with a Mechanical Seal to insure their internal hardware components are always cooled with Seal/Lubrication Oil.

 

Thanks again to API for providing an animated graphic that illustrates how the Seal Oil circulates through a Seal Pot ... and how the heat transferred into the Seal Oil is exchanged with cooling water so that cool lubricant/Seal Oil is returned to the Mechanical Seal.

Access the animated graphic HERE.

And now all PTOA Readers and Students know the form and function of "the stuff" found in a Centrifugal Pump's Stuffing Box!

Now it's time to learn what the form and function of hardware in The Bearing Box.

 

TAKE HOME MESSAGES: The interior of the Stuffing Box is a cavity where the Shaft pierces the Pump Casing.

The hardware components within the Stuffing Box perform the important function of minimizing the leakage of the pumped-up process fluid; without Packing or a Mechanical Seal in the Stuffing Box, leakage would naturally occur since the rotating shaft pierces the Pump Casing.

Packing is made from braided fibers that:

• Are chemically compatible with the "pumped up" process liquid.
• Minimize damage to the Shaft Sleeve ... so the Packing cannot be abrasive and should be somewhat "slick" so that the friction created by the Packing is minimized.

 

A Shaft Sleeve is a cylinder of metal that protects the Shaft so that the Shaft does not endure the friction caused by Packing. It is much easier to replace a Shaft Sleeve than it would be to replace an entire shaft.

Ropes of Packing are cut into rings made of two semi circles. The rings are "stuffed" around the Shaft Sleeve using a specific technique that minimizes leakage. All packed pumps still leak.

Packing must be lubricated; the heat of created friction must be removed via lubrication. Some pumps use the "pumped up" liquid as a lubricant and some use an external source of lubrication/seal oil that flows into the pump Casing and onto a Lantern Ring.

Lantern Rings are metal rings with holes that distribute lubricant to the Packing, Shaft Sleeve, and Shaft.

Mechanical Seals are replacing Packing in Pumps.

The advantages of Mechanical Seals over Packing include:

• Much less maintenance over life of pump compared with Packing.
• Greatly reduces pump leakage when pump operates at extreme PV Pressure or PV Temperature.
• Greatly reduces leakage when pump is operating at high rpm or is in hazardous liquid service.

 

The basic parts of a Mechanical Seal are:

• Primary leak reduction via a tight fit between a Stationary Seal and a Rotating Seal.
• A secondary line of defense against leaking created by an O-Ring that rotates with the Rotating Seal Ring, thus making a tight seal between the Rotating Seal and the Shaft.
• A Spring (or Bellows) whose tension keeps the Rotating Seal in place.
• A Spring Holder that holds the tension on the non-seal side of the Spring.

 

Heat from friction is generated at the interface of the Stationary Seal with the Rotating Seal and Shaft. Without circulating cool Seal Oil/Lubricant through the Mechanical Seal, this interface would fuse and the pump would fail.

Pumps with Mechanical Seals are easily identifiable by Seal Pots that contain Seal Oil/Lubricant.

©2018 PTOA Segment 0181
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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