Seal Troubleshooting

Troubleshooting mechanical seals at equipment disassembly 3-9

All seals fail for the same reasons:

  • The faces open up and allow dirt or solids to penetrate.
  • One of the seal components has been damaged by either the fluid you are sealing, heat, or a cleaner used to flush the system.

After the failure has occurred you’ll frequently get a chance to analyze the failed components. You’re going to be looking for several things:

  • Evidence of corrosion.
  • Wear patterns on those parts that are in contact.
  • Evidence of rubbing or wear on those components that should not be in contact.
  • Discoloration of any of the seal components, especially the metal parts.
  • Parts that are missing. Springs, set screws and drive lugs are examples.
  • Loose hardware. Either a seal component or a foreign object.
  • Product attaching to a rotating component. Carefully inspect the impeller and rotating part of the seal.

In the following paragraphs we’ll be inspecting the individual components and looking for evidence of the above.

THE CARBON FACE

Chipping on the O.D. of the carbon. Indicating vibration.

  • This can be caused by harmonic vibration, or when the rotating equipment hits a critical speed.
  • Slipstick can occur if you’re pumping a fluid with poor lubricating qualities.
  • Mishandling is a common problem. Look for evidence of drive lug wear to eliminate this as a possibility.
  • Vaporization of the liquid causing the faces to rapidly open and then close as the leaking fluid cools the faces.
  • A discharge recirculation line is aimed at the carbon seal face.
  • The pump is cavitating. Remember there are four + types of cavitation.
  • Water hammer is a another possibility.

Pits in the carbon face. This problem is often associated with poor grades of carbon/ graphite.

  • Exploded carbon. Air trapped in the pores of the carbon expands and expels pieces of the carbon when the seal faces get hot. Prior to being blown out, polished sections will be visible; usually with small cracks visible in the center.
  • If the product solidifies between the faces it will tear out pieces of the carbon at start up. This is a common occurrence with ammonia compressor seals because petroleum oil is mixed with the ammonia and it can “coke” at the elevated temperature.
  • Most petroleum products will “coke” because of the higher face temperature, and pull out small pieces of the carbon as the faces rotate. You’ll see evidence of these small pits if you inspect the carbon face under a magnifying glass.

Chips at the I.D. of the carbon

  • Solids, or a foreign object of some type from outside of the pump, are getting under the gland and are being thrown into the seal faces. This can occur if the seal leaked at some time and the product solidified on the outboard side of the seal. It can also occur if liquid, containing solids, is used in the quench connection of an A.P.I. type gland.
  • If the seal was installed outside of the stuffing box, as is the case with non metallic seals, solid particles in the fluid can be centrifuged into the rotating carbon face.
  • If the stationary face is manufactured from one of the carbon grades, it can be chipped if it comes into contact with the rotating shaft. This is a common problem at pump start up, or if the pump is operating off of its B.E.P.

Phonograph finish on the carbon face.

  • A solid product was blown across the seal face. It rolls from the outside to the inside diameter. This happens in boiler feed water applications.

Chemical attack of the carbon.

  • You’re using the wrong carbon. Something in the product or the flush is attacking the carbon filler. Switch to an unfilled carbon such as Pure grade 658 RC or CTI. grade CNFJ.
  • You’re trying to seal an oxidizing agent. Oxidizers attack all forms of carbon including the unfilled type. The carbon combines with the oxygen to form either carbon monoxide or carbon dioxide.
  • Some forms of de-onized water will pit and corrode carbon faces

Cracked or damaged carbon face.

  • The product is solidifying between the faces. Carbons are strong in compression but weak in tension or shear. This problem is common with intermittent service pumps each time they start up.
  • Excessive vibration can bang the carbon against a metal drive lug.
  • A cryogenic fluid is freezing a lubricant that was put on the face.
  • The elastomer is swelling up under a carbon or hard face.
  • The shaft is hitting the stationary face or the rotating seal face is hitting a stationary object.
  • Mishandling.
  • Poor packaging. The lapped seal faces should be able to survive a 39″ (one meter) drop.r.

A coating is forming on the carbon face:

  • A change in temperature. Many products solidify at temperature extremes.
  • The product is taking a pressure drop across the seal faces and solidifying.
  • Selective leaching is picking up an element from the system and depositing it on the seal face.
  • The stuffing box is running under a vacuum because the impeller was adjusted backwards and the impeller “pump out vanes” are causing the vacuum.
  • The pumping fluid is creating a protective oxide on the piping. This oxide is chipping off and depositing at the faces. In hot water systems we experience this problem with magnetite (Fe3O4) until the system stabilizes.

Coking

  • This is a problem with all types of oils, and petroleum products in particular.
  • Coking is caused by the combination of high temperature and time. Contrary to popular belief the presence of air or oxygen is not necessary.

Shiny spots, cracks and raised portions of carbon.

  • The carbon is not dense enough, causing the expanding gases trapped beneath the surface of the carbon to explode through the face.
  • Product is solidifying between the faces and pulling out pieces of the carbon as the seal revolves.

Excessive carbon wear in a short period of time. Evidence of excessive heat is usually present.

  • Heat checking of the hard face. It shows up as a cracking of the hard face. This is a problem with coated or plated hard faces. Cobalt base tungsten carbide can have this problem..
  • The shaft is moving in an axial direction because of thrust. This can cause an over compression and heating of the seal faces
  • The impeller is being adjusted towards the back plate. This is problem with seals installed in Flowserve pumps or any other pump that adjusts the open impeller against the back plate.
  • The inner face of a “back to back” double seal application is not positively locked in position. A snap ring must be installed to prevent the inboard stationary face from moving towards the rotating face when the high pressure barrier fluid pressure is lost or overcome by system pressure.
  • The seal was installed at the wrong dimension. It’s overcompressed
  • A cartridge double seal was installed by pushing on the gland. Friction, between the shaft and the sleeve O-ring is compressing the inner seal.
  • A vertical pump was not vented.
  • Solids have penetrated between the faces.
    • The faces are not flat.
    • The movable face is sluggish.
    • The product is vaporizing between the faces because of either high temperature or low stuffing box pressure .
  • Non lubricants will cause rapid face wear. A non lubricant is any fluid with a film thickness less than one micron at its load and operating temperature..

The carbon has a concave or convex wear pattern

  • High pressure i causing the distortion.
  • The stationary face is not perpendicular to the shaft.
  • Some companies lap a concave pattern as standard. Check with your manufacturer.
  • The shaft is bending because the pump is running off of its B.E.P.

The carbon is not flat.

  • Mishandling.
  • Poor packaging.
  • The hard face has been installed backwards and you’re running on a non-apped surface.
  • The seal was shipped out of flat.
  • The metal/ carbon composite hasn’t been stress relieved and it’s distorting the carbon.
  • When the carbon was lapped the lapping plate was too hot and as a result, not flat.
  • The carbon was lapped at room temperature and the seal is running at cryogenic temperatures.
  • Solids are imbedded in the carbon. The faces have opened.
    • The seal was set screwed to a hard shaft.
    • The elastomer (rubber part) is spring loaded to the shaft causing the faces to open as the shaft moves due to end play, vibration or carbon wear.
    • The shaft/ sleeve is over sized causing an excessive interference between the elastomer and the shaft/ sleeve.
    • The sleeve finish is too rough.
    • The product has changed from a liquid to a solid.
    • Dirt or solids are interfering with the seal movement.
    • Some one put the wrong compression on the faces.
    • Shaft fretting is hanging up the face.
    • The face has been distorted for some reason allowing solid particles to enter.
    • The sliding elastomer has swollen up causing too much interference on the shaft/ sleeve.
    • Poor centering is causing the rotating face to run off the stationary face. Keep in mind the gland bolts are not always concentric with the shaft.
    • If you seal was designed with a single spring, it is wound in the wrong direction. This happens when the seal is put on the wrong end of a double ended pump. These pumps need right and left handed wound sprins.
    • An “out of balance rotating assembly” or bent shaft is causing the rotating face to “run off” of the stationary face.

THE HARD FACE.

Chemical attack.

  • Some ceramics and silicon carbides are attacked by caustic fluids. Check to see if your seal face contains silica. As an example: both reaction bonded silicon carbide and 85% ceramic have this high silica content.

Cracked or broken.

  • The product is solidifying between the faces. Most hard faces have poor tensile or shear strength.
  • Excessive vibration will cause cracking at the drive lug location..
  • A cryogenic fluid is freezing a lubricant that was put on the face.
  • The elastomer is swelling up under an outside seal face. This problem can also occur if the seal design allows a spring to contact the I.D. of the hard face.
  • The shaft is hitting the stationary face or the rotating seal face is hitting a stationary object.
  • Mishandling.
  • Poor packaging.

Heat check (a common problem with coated or plated faces)

  • Caused by a high heat differential across the face. Most hard coating have only one third the expansion rate of the stainless steel base material.

Hard coating coming off of the face.

  • The base material not compatible with the sealed product. These coating are very porous, so if the product attacks the base material the coating will come off in sheets.
  • The plating process was not applied correctly.

Analysis of the wear track on the hard face.
Deep grooves&emdash;excessive wear. Solids imbedded in the carbon are causing the problem. The solids were trapped between the faces when the seal faces opened.

  • The seal was set-screwed to a hard shaft.
  • The elastomer is spring loaded to the shaft preventing it from flexing as the shaft vibrates..
  • The shaft/ sleeve is over sized causing the dynamic elastomer or bellows vibration damper to hang up..
  • The shaft/ sleeve finish is too rough
  • The product has solidified in the seal components.
  • Dirt or solids are interfering with seal movement.
  • Not enough spring compression on the faces.
  • Fretting of the shaft/ sleeve is hanging up the face.
  • The face has been distorted by either excessive temperature or pressure.
  • The sliding elastomer has swollen up due to chemical attack of the product or a cleaner that was flushed through the lines. The wrong choice of rubber lubricant, at installation, can also cause the problem
  • Poor centering is causing the rotating face to run off of the stationary face..
  • The single spring was wound in the wrong direction for the application.

The wear track is wider than the carbon.

  • Worn bearings.
  • Bent shaft.
  • Unbalanced impeller.
  • Sleeve not concentric with the shaft.
  • Seal not concentric with the sleeve.
  • In a stationary seal, the stationary carbon is often not centered to the shaft, causing a wiping action.

The wear track is narrower than the carbon.

  • The soft face (carbon) was distorted by pressure.
  • The hard face was over tightened against an uneven surface.
  • The hard face clamping forces are not “equal and opposite”.
  • The face never was flat, or it was damaged during shipment.

Non Concentric pattern. The wear track is not in the center of the hard face.

  • The shaft is bending because the pump is running off of its best efficiency point.
  • Poor bearing fit.
  • Pipe strain.
  • Temperature growth is distorting the stuffing box.
  • The stationary face is not centered to the shaft.
  • Misalignment between the pump[ and its driver.

Uneven face wear. The hard face is distorted:

  • High pressure.
  • Excessive temperature.
  • Over tightening of the stationary face against the stuffing box.
  • The clamping forces are not equal and opposite.
  • The hard face is not wide enough.
  • You are using a two bolt gland and the gland is too thin causing it to distort.
  • You are using a pump seal in a motion seal application.

The product is sticking to the seal face. The product is changing state and becoming a solid. Most products solidify for the following reasons:

  • A change in temperature.
  • A change in pressure.
  • Dilatants will solidify with agitation. As an example: cream becomes butter.
  • Some products solidify when two or more chemicals are mixed together.

The hard face is not flat.

  • Mishandling.
  • Poor packaging.
  • The hard face has been installed backwards and you are running on a non lapped surface.
  • It was shipped out of flat.

THE ELASTOMER.

Compression set. The O-ring has changed shape.

  • High heat is almost always the cause unless you are dealing with Kalrez, Chemraz, or a similar material where a certain amount of compression set is normal.

Shrinking, hardening or cracking.

  • High heat.
  • The shelf life was exceeded. This is a big problem with “Buna N” that has a shelf life of only twelve months.
  • Cryogenics will freeze just about any elastomer.
  • Chemical attack normally causes swelling, but in rare cases can harden an elastomer.
  • Oxidizing liquids can attack the carbon that is used to color most elastomers black.

Torn nibbled, or extruded.

  • Mishandling.
  • Sliding over a rough surface.
  • Forced out of the o-ring groove by high pressure.
  • The liquid has penetrated the elastomer, vaporizing inside and blowing out pieces. This is a problem with ethylene oxide.
  • Halogenated fluids can penetrate the Teflon coating on an elastomer and cause the base material to swell up, splitting the Teflon jacket.

Swelling, changing color, weight or size. Almost always caused by:

  • Chemical attack.
  • Be careful of the lubricant used to install the elastomer.
  • Solvents or cleaners used in the system may not be compatible with the elastomer.
  • Some compounds are sensitive to steam. Most Vitons are a good example of this problem.
  • The elastomer is not compatible with something in the fluid you are sealing.

Torn rubber bellows.

  • The bellows did not vulcanize to the shaft because you used the wrong lubricant.
  • The shelf life was exceeded.
  • The seal faces stuck together and the shaft spun inside the bellows.
  • The pump discharge recirculation line was aimed at the rubber bellows. Solids entrained in the high velocity liquid are abrading the bellows.

THE METAL CASE OR BODY OF THE SEAL.

Corrosion.

  • General or overall. This is the easiest to see and predict. The metal has a “sponge like” appearance. The corrossion always increases with temperature.
  • Concentrated cell or crevice corrosion. Caused by a difference in concentration of ions, or oxygen in stagnant areas causing an electric current to flow. Common around gaskets, set screws, threads, and small crevices.
  • Pitting corrosion. Found in other than stagnant areas. Extremely localized. Chlorides are a common cause. Can be recognized by pits and holes in the metal.
  • Stress corrosion cracking. Threshold values are not known. A combination of chloride, tensile stress, and heat are necessary. Chloride stress corrosion is a serious problem with the 300 series of stainless steels used in industry. This is the reason you should never use stainless steel springs or stainless metal bellows in mechanical seals.
  • Inter-granular corrosion. Forms at the grain boundaries. Occurs in stainless steel at 800-1600 F. (412-825 C.), unless it has been stress relieved. A common problem with welded pieces. Stabilizers such as columbium are added to the stainless steel to prevent this. Rapid cooling of the welds, the use of 316L and stress relieving after the welding are the common solutions.
  • Galvanic corrosion. Occurs with dissimilar materials in contact with and connected by an electrical current. Common in brine, caustic, and salt water applications.
  • Erosion / Corrosion. An accelerated attack caused by a combination of corrosion and mechanical wear. Vaporization, liquid turbulence, vane passing syndrome, and suction recirculation are special cases often called cavitation. Solids in the liquid and high velocity increase the problem.
  • Selective leaching. Involves the removal of one or more elements from an alloy. Common with demineralized or de ionized water applications.
  • Micro organisms, that will attack the carbon in active stainless steel.

Rubbing–All around the metal body.

  • A gasket or fitting is protruding into the stuffing box and rubbing against the seal.
  • The pump discharge recirculation line is aimed at the seal body.
  • The shaft is bending due to the pump operating off of its best efficiency point.
  • Pipe strain.
  • Misalignment between the pump and its driver.
  • A bolted on stuffing box has slipped.

Partial rubbing — On the metal body.

  • Bent shaft.
  • An unbalanced impeller or rotating assembly.
  • Excessively worn or damaged by corrosion or solids in the product.
  • The product has attached its self to the impeller.
  • The impeller never was balanced.
  • The impeller was trimmed, and not re balanced.
  • The seal is not concentric with the shaft, and is hitting the stuffing box I.D..

Discoloration. Caused by high heat. Stainless steel changes color at various temperatures.

FAHRENHEIT
COLOR OF THE METAL
CENTIGRADE
700 – 800
Straw Yellow
370 – 425
900 – 1000
Brown
480 – 540
1100 – 1200
Blue
600 -650
> 1200
Black
> 650

NOTE: To tell the difference between discoloration caused high heat and product attaching to the metal part, try to erase the color with a common pencil eraser. Discoloration will not erase off.

Product sticking to the metal surfaces.

  • Heat is the main cause.
  • The product pressure has dropped.
  • Air or oxygen is getting into the system.
    • Valves above the water line.
    • Through the stuffing box.
  • The product was not deaerated.
  • The pump suction is not completely submerged.
  • The bypass return is too close to the pump suction.
  • The liquid is vortexing in the suction line.
  • A non o-rring elastomer is being used in the seal, allowing air to enter the stuffing box when you are sealing a vacuum application.
  • The system protective oxide coating is depositing on the sliding metal components.

The following applications cause a vacuum to be present in the pump stuffing box.

  • Pumps that lift liquid.
  • Heater drain pumps.
  • Pumping from an evaporator.
  • Pumping from the hot well of a condenser.
  • Pumps that prime other pumps.
  • The open impeller was adjusted in the wrong direction and the impeller pump out vanes are causing the vacuum.

The Teflon coating is coming off some of the metal parts.

  • Coatings are very porous. They do not provide corrosion resistance. The base material is being attacked by the product.

DRIVE LUGS, PINS, SLOTS, etc.

Broken.

  • Chemical attack.
  • Excessive side load.
  • The seal faces are glued together because the product has solidified.
  • A cryogenic fluid is sticking the faces together.

Wear on one side of the drive lug or slot.

  • Vibration.
  • Slipstick.
  • The stationary is not perpendicular to the shaft.

The drive pins are falling out of the holder.

  • Corrosion.
  • Improper fit.
  • Bad part.
  • Excessive vibration.

THE SPRINGS.

Broken or cracked.

  • The stationary face is not perpendicular to the shaft causing excessive spring flexing in the metal “plastic range”. The spring material has “work hardened” and fatigued.
  • Chloride stress corrosion problems with 300 series stainless steel.

Corroded.

  • Stressed material corrodes much faster than unstressed material. The springs are always under severe stress.

Clogged.

  • Be sure to distinguish between “cause and effect”. If the springs are located outside the liquid, it happened after the failure.
  • If the product solidifies or crystallizes it can clog springs exposed to the pumped fluid.
  • Dirt or solids in the fluid can clog exposed springs.

Twisted.

  • Almost always an assembly problem. The lugs were not engaged in the slots. This is a problem with many seal designs. Check to see if your seals can come apart easily, or if the drive lugs can change position when the seal is not compressed.

The drive lugs or slots are worn on both sides.

  • Excessive vibration.
  • The single spring, rubber bellows seal, was not vulcanized to the shaft.
  • The stationary is not perpendicular to the shaft, causing excessive spring movement.

Broken Metal Bellows.

  • Fatigue caused by over flexing in the plastic range of the metal
    • Harmonic vibration.
    • Slipstick.
  • The discharge recirculation line is aimed at the thin bellows plates.
  • Excessive wear from solids in the stuffing box.
  • Faces sticking together as the product solidifies.
  • Chloride stress corrosion with 300 series stainless steel.

Because these seals do not have a dynamic elastomer to provide vibration damping some other means must be provided or vibration will always be a problem.

THE SLEEVE, OR SHAFT.

Grooves or pits at the seal dynamic elastomer location.

  • Fretting.
  • Concentrated cell corrosion.
  • The rubber bellows did not vulcanize to the shaft/ sleeve.
  • The set screws slipped on a hardened shaft or were not tightened properly. The seal faces stuck together causing the shaft to rotate inside the static elastomer.
  • Salt water applications are particularly troublesome when a static elastomer or clamp is attached to the shaft. Pitting caused by the chlorides and the low ph of salt water are the main problems.

Rubbing at the wear ring location.

  • The pump is running off of its best efficiency point.
  • The shaft is bending.
  • Bad bearings.
  • Excessive temperature.
  • Sleeve is not concentric with the shaft, or the seal with the sleeve.
  • Bent shaft.
  • Unbalanced impeller or rotating assembly.
  • Pipe strain.
  • Misalignment between the pump and its driver
  • High temperature applications require a “center line: pump design.

Corrosion. See above description under metal corrosion

.THE SET SCREWS.

  • Stripped from over tightening.
  • Corroded. Check to see if you are using hardened set screws. This type is normally supplied with most cartridge seals and can corrode easily.
  • Rounded Allen Head. Alan wrenches wear rapidly. They are an expendable tool.
  • Loose.
    • Sleeve too hard. They are not biting in.
    • Sleeve too soft. They are vibrating loose.

THE GLAND.

Rubbing at the I.D.

  • Partial rubbing.
    • The gland has slipped.
    • Improper installation. It was not centered to the shaft.
    • The shaft is bending.
    • Pipe strain.
  • Rubbing all around.
    • The shaft is not concentric with the sleeve.
    • The seal is not concentric with the sleeve.
    • Bad bearings.
    • Bent shaft.
    • Unbalanced impeller or rotating assembly.
    • Solids attached to the shaft, or caught between the shaft, and the gland.
    • Cavitation.

Corrosion.

  • If there is evidence of rubbing the corrosion will be accelerated.

Passages clogged or not connected properly.

  • A.P.I Gland.
    • Hooked up wrong.
    • Flushing connection clogged.
    • Quench connection clogged.

BUSHINGS

Rubbing at the I. D.

  • Partial rubbing.
    • The A.P.I. gland has slipped.
    • Improper installation. It was not centered to the shaft.
    • The shaft is bending.
    • The gland bolt holes are often not concentric with the shaft/ sleeve.
    • Misalignment between the pump and its driver.
    • Excessive pipe strain.
  • Rubbing all around.
    • The shaft is not concentric with the sleeve.
    • The seal is not concentric with the sleeve.
    • Bad bearings.
    • Bent shaft.
    • Unbalanced impeller.
    • Cavitation

Erosion.

  • Dirt and solids are present in the discharge or suction recirculating fluid.

Posted

  • On February 09, 2018