Damage of the carbon-graphite face

DAMAGE OF THE CARBON-GRAPHITE FACE ST008

Chipping on the outside diameter of the carbon. Indicating vibration of some type.

  • Slipstick can occur if you are 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. Chipping without drive lug wear is a good indication of mishandling.
  • Vaporization of the liquid between the lapped faces causing the faces to rapidly open and then close as the leaking vapor cools the faces.
  • A discharge recirculation line is aimed at the carbon seal face. Each pass of an impeller vane puts a pulse into the recirculation line.
  • The pump is cavitating. Remember there are five types of cavitation.
  • Water hammer is an another possibility.
  • Running at, or passing through a critical speed will cause vibration problems

Pits in the carbon face. This problem is usually 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 ejection polished patches 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 the oil can coke at the elevated face 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 will see evidence of these small pits if you inspect the carbon face under a magnifying glass. This is one of the main reasons that carbon/ graphite seal faces have trouble passing fugitive emission standards in hot petroleum applications.

Chipping at the inside diameter of the carbon

  • Solids or a foreign object of some type from outside of the pump is getting under the gland and is 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 API (American Petroleum Institute) 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 carbon 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 best efficiency point (BEP)
  • Vertical pumps experience this problem when solids fall between the seal and the shaft.

Phonograph finish on the carbon face.

  • A solid product was blown across the seal face. Since one of the faces is rotating the solid particle follows a circular path until it is expelled out of the inside diameter of the lapped faces. This happens frequently in boiler feed water applications.

Chemical attack of the carbon.

  • You are using the wrong grade of carbon/ graphite. Something in the product or the flush is attacking the carbon filler. Switch to an unfilled carbon such as Pure grade 658 RC or C.T.I. grade CNFJ.
  • You are 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.
  • You are trying to seal a halogen like chlorine, fluorine , bromine, astintine or iodine.
  • Some forms of de-ionized 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 or standby 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 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.
  • The fluid, containing solids, is underneath the carbon face. Centrifugal force is throwing solids or abrasive material into the soft carbon. You see this problem in dual, back to back, rotating seals and outside mounted non-metallic seals.

A coating or layer of product is forming on the carbon face:

  • Look for a change in temperature in the stuffing box. Many products solidify at temperature extremes, but some solidify or build a film with a minor change in temperature.
  • The product is taking a pressure drop across the seal faces and solidifying as it passes through the lapped faces.
  • The fluid you are pumping has been demineralized or deionized. Selective leaching is picking up one or more missing elements from the piping system and depositing them on the seal face. This accounts for the copper plating you sometimes see on the carbon face in boiler feed pump applications.
  • The stuffing box is running under a vacuum because the impeller was adjusted backwards and the impeller “pump out vanes” are pumping out the stuffing box.
  • Corrosion resistant materials form a protective oxide coating. This protective oxide is depositing at the faces. In cast iron pipe hot water systems we experience this problem with magnetite (Fe3O4) until the system stabilizes.

Coking. The forming of a hard black layer on the seal face. It is difficult to see on carbon, but you might notice that the carbon nosepiece is getting longer.

  • Coking is a problem with all 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 does not have enough density, causing the expanding gases trapped beneath the surface of the carbon to bubble at the face.

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 is a typical example. These cracks will act as knife blades and shave the carbon face. You often find carbon dust underneath the carbon face.
  • The shaft is moving in an axial direction because of thrust. This can cause an over compression and heating of the seal faces. Pumps equipped with sleeve bearings are a bigger problem
  • The impeller is being adjusted towards the back plate. This is problem with seals installed in Duriron pumps or any other pump that adjusts the open impeller against the back plate.
  • A number of installation problems can cause excessive wear of the carbon face:
    • 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 has too much spring compression
    • 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.
    • You are using an unbalanced seal in a balanced seal application.
  • Solids have penetrated between the lapped faces. Here are some common causes:
    • The faces are not flat or maybe they never were flat
    • The movable face is sluggish. It has trouble following shaft run out.
    • 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 operating load and operating temperature.

The carbon has a concave or convex wear pattern

  • High-pressure distortion.
  • The gland holding the stationary seal carbon face is not perpendicular to the shaft. It is causing an uneven loading on the carbon face.
  • Some companies lap a concave pattern as standard. Check with your manufacturer.
  • The shaft is deflecting because the pump is running off of its best efficiency point (BEP)

The carbon is not flat.

  • Mishandling.
  • Poor packaging. The seal should be packaged to survive a one-meter (39 inches) drop without seal component damage.
  • The seal was shipped out of flat.
  • The seal was rebuilt by a facility that cannot check flatness, or the person responsible for doing it does not know how.
  • The metal/ carbon composite has not been stress relieved and it is distorting the carbon.
  • The carbon was shrunk into a metal holder. It should have been pressed in and sheared to conform to irregularities in the holder diameter.
  • When the carbon was lapped, the lapping plate was too hot and not flat.
  • The carbon was lapped at room temperature and the seal is running at cryogenic temperatures.

Solids are imbedded in the carbon.

  • The seal faces opened, letting the solids penetrate between the lapped faces.
  • Some one used lapping powder to lap the carbon face. The carbon should have been lapped dry, on ceramic stones.

See:

Carbon face physicals by grade gr011

Carbon face manufacturing C010

Carbon/ graphite seal face; where it doesn’t work C011-1

Posted

  • On February 18, 2018