Failure Analysis John Crane

Performance Analysis of Mechanical Seals KRL 12/02

Performance Analysis Mechanical Seal Life is Affected by: Design      

Seal Pump Support System Bearing Coupling Flush

Installation  Seal  Pump

Operation  Pump  Support System  Flush System

Performance Analysis A Definition of Seal Failure Undesirable passage or leakage of fluid past sealing components causing:  Dilution of a process stream  Poisoning of the environment  An immediate safety hazard

The actual threshold for "failure" criteria is defined by the user or a governmental agency.

Performance Analysis Classification of Seal Failures  Mechanical – 24%  Operation – 40%  Poor Fluid Circuit Design – 19%  Incorrect Seal Selection – 9%  Miscellaneous – 8%

Performance Analysis Diagnostic Path  Failure Mode: Criteria, volume, when  System Status: Changes, modifications, damage  History: Run length, symptoms, trends, system influences

 System and Equipment Checks  Disassembly and Visual Inspection

 Comprehensive Examination

Performance Analysis Notable Indicators on Site  Noise: Continuous or periodic  Visual: Carbon dust, leak volume  Temperature changes: Process, support, environment

 Leakage: Continually, startup, static, during excursions  When: Startup, seasonal,

 Changes: procedure, human, conditions, fluid  Failure frequency: Immediate, variable, diminishing

Stylus

Surfanalyzer

Perfectly Flat Reference Line

Part is High

Part is Low

John Crane Test Lab

Scanning Electron Microscope

Full Contact Pattern

Contact Pattern Mating Ring

Primary Ring

Wide Contact Pattern

Contact Wider Than Width of Primary Ring Face

Drive Notch Wear

Drive Notch Wear

Off Center Wear Track

Mating Ring Damage from Off Center Shaft

Sleeve Damaged by Off Center Shaft

Sleeve Damaged by Excessive Shaft Runnout

Sleeve Damage by Excessive Shaft Runout

Heavy Outside Diameter Contact

Possible Edge Chipping

Rotation Due to Pressure

Heavy to Moderate Contact

Light Contact

Heavy Inside Diameter Contact

Possible Chipping

Rotation Due to High Temperature

Light Contact

Heavy to Moderate Contact

One High Spot on Mating Ring Face Mating ring misaligned, due to a high drive pin, or upset on support surface. Look for wear at drive notches on the primary ring.

Wear Track Through 270° Full contact through approximately 270° as seen on seal ring face. Pattern fades away at low spot. The primary ring may show "wire drawing" in a position over the mating ring low spot.

The seal ring is being distorted mechanically or is not flat

Wire Draw on Carbon Face

Two High Spots on Mating Ring Two large contact spots are noted while the pattern fades away between these areas.

The mating ring is being distorted mechanically. Pump casing split line misalignment is often the cause.

Gland Bolt Distortion Contact spots noted at gland stud locations on mating ring. Mating ring is being distorted mechanically by uneven gland surface.

Most often the gland nuts have been over tightened.

No Contact

Contact at High Spots Near Gland Bolts

No Unusual Wear

Heat Checking of Entire Face Many small radial surface cracks through the entire wear track, normally visible to the eye. Squealing or popping sounds will often be observed during operation.

Caused by inadequate fluid film at the seal interface. This usually results from inadequate vapor pressure margin or flush rate.

Heat Checking and Pullouts

Heat Checking 180° From Flush Port Thermally distressed area on seal face, located opposite of the flush port location. Insufficient flush rate is most likely cause.

Undamaged Surface at Flush Inlet

Heat Checked Area

Patches of Heat Checked Surface Patches of thermally distressed surface on face.

The sealed liquid, probably a light hydrocarbon, is vaporizing at the seal interface.

Good Contact Pattern

Distressed Surface or Hot Spot

Abrasive Wear High wear of seal face through 360° showing a “phonograph pattern” appearance.

Caused by abrasive particles in the process fluid or from seal face breakdown resulting from inadequate flush conditions.

Abrasive Wear on Carbon Face

Abrasive Wear; Dry Run of SiC Face

Abrasive Wear; Dry Run of SiC Face

200 X

100 X

Abrasive Wear on TC Mating Ring

Blistering A seal face has one or more (several) uplifts or “Blisters”. Some may have been pulled out leaving a shallow crater. Normally associated with carbon materials. Caused by high stresses in the interface which exceed the rupture strength of the face material. Viscous fluids, including lube oils, are most often the cause.

Blistering

Bond Failure of Coated Seal Ring A blistered or peeled surface layer on faces which have been coated with a hard material.

Coated parts should be used with caution. Chemical activity and mechanical action can easily disturb the coating.

Fractured Seal Ring One or more cracks extend all the way through the part.

Often caused by mishandling or improper assembly. Can be caused by excessive torque, thermal shock, mechanical (contact) loading, or uneven loading of brittle face materials.

Erosion of Seal Parts Extensive erosion of a component at a location that is exposed to process fluid. An abrasive laden seal flush which is directed at the part(s) in question is a common cause. Trapped abrasives in a seal chamber can damage both pump and seal components.

Erosion of Rotating Primary Ring

Erosion of Stationary Mating Ring

Leaching Material exhibits a rough matted appearance on surface areas that are exposed to the process fluid.

Caused by chemical attack of one or more phases of a multiphase material.

Leaching of RBSiC Mating Ring

Corrosive Chemical Attack Metallic materials exhibit a cracked, rough, or irregular surfaces. Carbon materials may become soft. Acids, caustics, chlorides, de-ionized water to name a few. Look at weld zones, crevices, friction areas, springs, etc.

Chemical Attack

Chemical Attack

Hydrocarbon Coking A formation of hard, brittle material between seal and shaft sleeve that may impede free movement of the seal. Caused by oxidation or stripping of hot hydrocarbon fluids in the presence of seal face generated heat. Application of a steam quench often cures this problem.

Solid Deposits “Coking”

Hydrocarbon Coking

Hydrocarbon Coking

Dissolved Solids Deposits Grainy, often abrasive substance that may form on either side of the seal; though typically on seal faces or the atmospheric side. Abrasive wear on the seal faces or hang-up are the most common observations. Application of a liquid quench and possibly the use of hard seal ring materials are typical remedies.

Face Grooving Deposits

Caustic Deposits

Suspended Solids Fouling Process borne polymers, solids, or contaminants that imbed in or adhere to areas on either side of a seal. Polymers may form in the presence of seal face or pump generated heat. An exclusionary flush plan or slurry style seal design may be required.

Deposits

Polymer Deposits (Reactor Application)

Iron Oxide Deposits (Pipeline Application)

Solids Fouling of Metal Bellows

Fouling of Seal Face

Cuts or Tears on Secondary Seal Physical damage on O-rings, U-cups, V-rings or soft packing. Normally caused by mishandling during assembly or a defect.

Always inspect parts for visible defects and inclusions before installation. Mounting surfaces should not have sharp edges or burrs that could cause damage.

O-Ring Extrusion A thin lip has formed on the entire ID or OD of the O-ring. Some materials may exhibit a shredded appearance. Normally caused by application of pressure beyond the limits of the design. Elastomer swell or expansion in a confined space will often result in the same damage.

Normal Ring

Lip Shredding

O-Ring Extrusion

Extrusion of Perfluoroelastomer

Extrusion of Perfluoroelastomer

O-ring Blowout (Reverse Pressure)

Worn Surface on O-Ring One surface will be flat from material being abraded or chaffed. The ring may adhere to, or discolor the adjoining surface. Caused by sliding contact at a point where static contact was intended.

Normal Ring

Flat

Worn ID Surface on Dynamic O-Ring

Worn ID Surface on Dynamic O-Ring

Hard, Cracked Elastomer The part is hard and has several cracks, it is easily broken when bent by hand. Damage may be on process side, atmospheric side, or only in areas in contact with a specific part. Temperature too high for material in use. Possible chemical attack if the damage is evident only on its wetted side.

Hard, Cracked Elastomer

Elastomer Compression Set The part is permanently deformed with flat sides on its sealing surfaces. The most common cause of O-ring failure. Caused by designs that over compress the material, resulting in a permanent set. Some elastomers are more susceptible than others to compression set.

Normal Ring

Flat Surfaces

Chemical Attack on Elastomer The part has become either softer or harder than normal. It may have swelled, formed blisters, or portions have been severely deteriorated.

When in doubt, elastomers should be compared to an experience database or subjected to an immersion test. Original Size

Fluoroelastomer Swell in Methanol

Fluoroelastomer Decay in Ammonia

Process Side Attack from HF Acid

Atmospheric Side in HF Acid Service

Blistered and Ruptured Elastomer Many small blisters and ruptures throughout the part caused by explosive decompression. A fluid which is a gas at atmospheric pressure, is being sealed under high pressure and over time is absorbed into the elastomeric material. When the pressure is released too quickly the fluid which is trapped in the elastomer expands rapidly resulting in damage to the part.

Blister

Rupture

Fretting Metal surface which has been in contact with the secondary seal has a rough and worn appearance. Fretting occurs when continuous small movements inherent to a dynamic secondary seal occur while in contact with a metal shaft sleeve or shaft.

Worn or Distorted Drive Components Drive mechanisms are damaged; worn, distorted, or chipped. Caused by excessive loads or relative movement between drive components and the driven part.

Drive Dent Wear

Sheared Drive Pin

Broken or Distorted Coil Springs Springs are distorted, cracked, broken, or corroded. Stress corrosion cracking, general corrosion, fatigue, excessive shaft speed, and over extension or compression of the spring.

Metal Bellows Separation at One End Excessive vibration of the metal bellows.

Torsional fatigue; usually at 1st or 2nd ID weld from either end. Normally caused by inadequate face film (stick-slip)

Metal Bellows Fatigue (Magnified)

Metal Bellows Over Pressurization Applied pressure greater than rating for the metal bellows. Caused by pressure excursions or expansion of contained fluid.

Metal Bellows Over Pressurization

Normal

From OD

From ID

Cracks in Metal Bellows The bellows has cracks in one or more locations at any point on the bellows, normally at or near a weld. May only be detectable in a leak testing device. Stress corrosion cracking is a common cause.