Worn me down like a road.
I did everything you told me to do.
But you can't stop thinking about her.

("Worn Me Down," by Rachael Yamagata, 2003)

 

THE DEFINITION OF "WEAR"

PTOA Readers and Students just learned that any on-again, off-again relationship between two surfaces that are sliding or rolling/rotating by each other creates Friction ...

And that Friction eventually causes one of many types of Wear.

 

Even the slickest and smoothest surfaces will eventually be worn down because ...

at the microscopic level ...

the two surfaces are repeatedly adhering to each other and then being torn apart ...  over and over again up to many thousands of times per minute.

Naturally, metal particles are eventually loosened and removed from the surfaces.

Aha! That must mean ...

WEAR is the undesirable removal of metal particles from the surface of a hardware component that is constantly sliding or rolling/rotating by a different surface.

Wear is not associated with the "start up" of a piece of Rotating Equipment but rather the constant motion experienced by an operating piece of Rotating Equipment.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order can sound up-town when paraphrasing the above statement into:

Wear is an outcome of Kinetic Friction but not Static Friction.

And if that statement does not ring a bell then it is time to reread PTOA Segment #177!

The minimization of the Wear that Friction causes is the goal of Lubricants such as waxes, greases, and oils ... a subject that will be  featured in the upcoming PTOA Segment #179.

 

THE TWO FACTORS THAT IMPACT THE MAGNITUDE OF WEAR

Just checking, Fred!

Do you understand that there are many types of Wear but what they all have in common is removing particles of metal from a surface?

Yep!

Good! Here's a new piece of information about Wear ...

The amount of Wear created depends upon two common sense factors:

First, the weight of the hardware component that is in motion sliding or rolling/rotating greatly impacts how much and how fast Wear is generated.

The short-hand jargon word for "the weight of the hardware component" is the Load that must be borne by whatever is supporting it.

So who is surprised to learn that the number of times the two surfaces slide or roll by each other in a minute also impacts how much Friction ... and thus Wear ... is generated?

In  summary, the two factors that impact how much and how fast Wear will show up are:

• The Load (aka Weight) of a sliding or rolling component.
• The rate of sliding or rolling.

 

Aha!

That must mean that Wear can be reduced by fabricating the Load with lighter, less dense materials and/or reducing the rate of sliding/rolling.

Righteeo!

Yet still some Friction would be generated because of the sliding/rolling action!

 

The next line of defense to reduce Wear is to use Lubrication Oils or Greases between the moving components!

 

TYPES OF WEAR

Wear caused by Pitting

Cavitation causes a Wear type called "Pitting."

Pitting is a type of metal erosion that happens when vapor/gas bubbles collapse with great force ... aka 'cavitate' ...on metal surfaces.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already possess a core competent understanding of Cavitation.

In fact, PTOA Readers and Students who have read PTOA Segments #171 and #172  are members of the Cavitation Prevention Club because they all know that Pitting caused by Cavitation will be prevented as long as:

NPSH_a  > NPSH_r

 

Adhesive Wear 

Adhesive Wear is the wear and tear that results when the back and forth sliding movement between two metal surfaces removes metal along the interface of the mating surfaces.

Who's surprised to learn that Adhesive Wear causes adhesions in the metal?

Severe Adhesive Wear causes:

• Scuffing (aka scraping that does not necessarily remove metal).
• Galling (deeper, uneven removal of a metal surface caused by insufficient Lubrication).
• Seizure (metal junctions that are sheared because of sliding motion).

 

The mechanical ways to prevent Adhesive Wear by design are:

• Manufacture the two surfaces with compatible metals that will not interact as much at the microscopic level.
• Apply a surface coating ... such as phosphating ... to reduce the interaction between metals.
• Reduce the speed of the Load.

 

Truth be known, Lubrication is the main method used to reduce Adhesive Wear.

If the Lubricant doesn't seem to be doing its job to reduce Adhesive Wear, the Mechanic or Mechanic Technician will find a suitable, more viscous oil that will separate and cool down the Friction caused by moving parts ...

or perhaps s/he will add "anti-scuff" additives into the Lubrication Oil which is already being used.

 

 

Contact Fatigue and Rolling Contact Fatigue

When the two surfaces are rolling by each other as opposed to sliding by each other, Adhesive Wear is called  "Contact Fatigue."

Otherwise stated,

Contact Fatigue is caused by the cyclical contact stresses between rolling/rotating surfaces.

Contact Fatigue stresses cause cracking, pitting, and spalling of one of the surfaces.

What is spalling?

Spalling happens when cracks below the surface cause particles of metal at the surface to break off in fragments.

The below diagram shows how the spalling  begins below the surface of the metal and gradually creates the gash shown on the right most picture.

Contact Fatigue is commonly found in gear teeth, cams, and the rolling element of bearings.

Rolling Contact Fatigue causes small pieces of the supporting surface (like the raceway of a Radial Bearing) to be lifted off in flakes that break up the smooth surface.

The flaking caused by Rolling Contact Fatigue creates regions with a rough and coarse texture.

The mechanical ways to reduce Contact Fatigue by design include:

• Reduce the Load!
• Reduce the frequency of the contact stress by reducing the rate of sliding or rolling.
• Use high quality vacuum melted steels to make the rolling elements.
• Use a less abrasive surface finish.

 

Yet even the most optimally designed rolling elements are going to need Lubrication while in operation.

The Lubricants used for rolling elements like radial bearings would have the following properties:

• Be clean and dry.

• Be sufficiently viscous to create enough pressure between the contacting surfaces.

 

Hey! There's that adjective "viscous" popping up again to describe a desired characteristic of Lubrication Oil !

PTOA Readers and Students learned the definition of the fluid property called "Viscosity" way back in PTOA Segment #162.

Spoiler Alert!

Viscosity is the main fluid property that is used to distinguish one type of Lubricant from another!

 

 

Abrasive Wear

Abrasive Wear is the cutting of furrows into a surface by hard particles.

The nearby picture shows the teeth of a gear which have experienced excessive Abrasive Wear.

 

 

Abrasive Wear occurs one of two ways:

• 'Two body' Abrasive Wear happens when a hard surface rubs on a soft surface and cuts a groove in the soft surface.
• 'Three body' Abrasive Wear happens when a loose abrasive particle rolls between two soft surfaces OR when...
• A hard particle gets embedded in a soft surface ... and perhaps that hard particle came from the suspended film of a poorly filtered Lubricant.

 

Aha!

That must mean that Abrasive Wear can be reduced by:

• Fabricating the two surfaces with harder materials ...or

• Use hard coatings ... especially on the softer surface.
• Get rid of the hard particles (aka "abrasive agents") by improved filtering of the Lubricant or ...
• Use a Lubricant that is free of abrasive particles in the first place and then ...
• Follow clean Lubricating Oil practices ... like filtering and frequent flushing and oil changes.
• Use a more viscous Lubricant.

 

Erosion Wear

Erosion Wear is a lot like Abrasive Wear except in this case the furrows are cut into the metal because hard particles are impacting the surface at a high velocity.

Process Operators should be alert for Erosion Wear any place that a fluid is flowing through a nozzle.

Where would that be exactly?

Nozzles are used wherever a fluid needs to be injected into a certain point of a process stream.

The photo below shows how a quench gas is injected into a process flow line for the purpose of cooling down the process stream gas.

For this application, Process Operators should make certain the integrity of the pipe near an injection point is inspected for the tell tale signs of Erosion Wear whenever the process piping is taken off line for routine maintenance.

The strategies used to decrease Erosion Wear are the same as those listed for Abrasive Wear ... with one important addition:

To minimize Erosion Wear ...

The angle of the impact of the high-velocity injected fluid should be less than 15 degrees.

An injected fluid entering the process line at 45 degrees would certainly cause more Erosion Wear!

 

Corrosive Wear

Corrosive Wear is a type of Wear that is not related to the generation of Friction.

Corrosive Wear happens when a metal surface is chemically attacked by a fluid ... so there's a chemical reaction involved.

 

Here's a Fun Fact:

A rusty nail is one of the very few chemical reactions that automatically happens at Atmospheric Pressure and the typical ambient temperatures found on the Earth's surface.

Seeing rust on a nail is so common that hardly anybody realizes the rust was created by a chemical reaction and is evidence of Corrosive Wear.

At the atomic level, the a chemical reaction between the Iron in the nail and the Oxygen found in air generates the product scientifically known as "Iron Oxide" and commonly known as "rust."

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order were introduced to chemical reactions in PTOA Segment #27 and the Periodic Table of Earth's Elements in PTOA Segment #60 so they know that Oxygen is represented as "O" and Iron is represented as "Fe" in the below chemical reaction diagram.

So who is surprised to learn that removing the corrosive products  ... like rubbing the rust off of a surface ... helps reduce the Corrosive Wear between two surfaces?

The non-chemical methods to reduce Corrosion Wear include:

• Use corrosion-resistant metals.
• Reduce the temperature in the vicinity of the reaction because that will decrease the rate of converting the reactants into the corrosion products.
• Remove the corrosion-causing agent  ... in the example of a rusty nail that would mean getting rid of the oxygen in the air (hard) or ... as appeared first on this list ... use a nail made from a corrosion-resistant metal (easier).

 

The chemical remedy to reduce Corrosion Wear would be to add "Corrosion Inhibitor" into a circulating fluid ... like Lubrication Oil!

As PTOA Readers and Students will soon learn, some Lubrication Oil performs the duo-function of Seal Oil ... which means the oil provides enough PV Pressure to keep corrosive agents (like Oxygen) away from the sliding and rolling hardware components of Rotating Equipment.

 

WHY PROCESS OPERATORS MUST BE AWARE OF WEAR 

Process Operators are not typically expected to be experts in the causes of Wear.

Any large processing facility that is serious about staying online to convert raw materials into products will have Mechanical Engineers, Mechanics, and Mechanic Technicians that are expected to maintain the integrity and reliability of the plant hardware.

However, Process Operators should be sufficiently "Wear Aware" to distinguish between divots in metal caused by spalling versus surface corrosion.

Furthermore, Process Operators must never forget that they have a vested interest in understanding where oh where what type of Wear may materialize because s/he will be the first to be impacted in the event that excessive Wear causes a pressurized line to fail.

Likewise, it is the Outside Process Operator ... not the Mechanical Technician ... who will be the first to respond to a Rotating Equipment failure.

PTOA Readers and Students now understand how Friction causes many types of Wear and the important role Lubrication Oil systems play extending the run life of Rotating Equipment.

PTOA Readers and Students are now "primed" to learn about Lubrication!

 

TAKE HOME MESSAGES:  Wear is the undesirable removal of metal particles from the surface of a hardware component that is constantly sliding or rolling/rotating by a different  metal surface.

Wear occurs while Rotating Equipment is in operation, not during startup. Otherwise stated, Kinetic Friction causes Wear, not Static Friction.

Types of Wear include:

• Pitting Wear, a type of Erosion Wear caused by Cavitation.
• Adhesive Wear, caused by sliding metal parts repeatedly making contact and ripping apart. Scuffing, Galling, and Seizure are all examples of Adhesive Wear.
• Contact Fatigue, a subset of Adhesive Wear caused by rolling metal parts and typically found in gear teeth, cams, and the rolling element of bearings. Contact fatigue causes cracking, pitting, and spalling of metal surfaces. 
• Rolling Contact Fatigue, caused by rolling metal parts impacting nearby metal surfaces. For example Rolling Contact Fatigue causes the metal flaking that creates rough patches on bearing raceways.
• Abrasive Wear, caused by a hard surface etching furrows into a softer surface or an independent abrasive particle rubbing against two moving surfaces.
• Erosion Wear, caused by high velocity fluids impacting surfaces and which can be reduced by decreasing the angle of impact to below 15 degrees.
• Corrosive Wear, not caused by motion but rather by chemical reaction.

 

Wear can be reduced by:

• Designing sliding and rotating hardware components to be less dense and therefore lighter.
• Decreasing the rate of motion between moving hardware components.
• Applying hardening or less reactive coating to metal surfaces. 
• Remove abrasive agents via filtering and flushing of lube oil.
• Use non-corrosive metals in sliding/rotating hardware.
• Reduce corrosion causing chemical reactions by reducing the temperature.

 

Lubrication will always be needed to minimize the Wear between two metal surfaces when one or both of them are sliding or rolling/rotating.

This PTOA Segment emphasized the importance role Lubrication systems play with respect to minimizing Wear caused by Friction.

 

©2017 PTOA Segment 0178
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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