Seals, preventing failure

Preventing premature seal failure 10-5

Here is that three question quiz again. I asked the same questions in a previous paper

Question How do you prevent premature mechanical seal failure?

Answer Find out what is causing the seals to fail and prevent it from happening.

Question How long should a mechanical seal last?

Answer Until the sacrificial carbon seal face wears away.

Question How often do seals wear out?

Answer Less than ten percent of the time.

In paper 10-4 we learned that seals fail for only two reasons:

  • The lapped faces opened.
  • One of the seal components becomes damaged.

In this paper we will discuss a few of the reasons that seal components become damaged.

Damage is easy to see. It will show up as corrosion or some type of physical damage.

Do not make it too complicated. If you inspect a failed seal and see no evidence of component damage, then the seal faces must have opened because seals fail for only two reasons.

Too much shaft radial movement, whip, wobble or run out is a common cause of a rotating seal component hitting a stationary part of the pump or a rotating shaft hiting a stationary seal component. When a rotating component hits a stationary component you will see evidence of rubbing or damage.

Here are some of the reasons that shafts are displaced radially from the center of the suffing box:

  • There are at least four types of cavitation. Cavitation means random and excessive radial and axial movement.
  • Operating off the pump’s best efficiency point (BEP) causes shaft deflection at 60° and 240° as measured from the pump cutwater in the direction of shaft rotation.
  • Non-lubricants can cause “slip stick” vibration problems.
  • Harmonic vibration occurs when the seal is vibrating in harmony with another piece of equipment. Look for this problem with the seals in standby pumps. You will see lug wear and carbon chipping on the outside diameter of the carbon face.
  • Hitting a critical speed. This happens with variable frequency motors and pumps designed with flexible shafts.
  • The pump pedestal does not have enough mass to support the pump and its driver.
  • The pump pedestal is not wide enough
  • Pipe strain.
  • Misalignment between the pump and its driver.
  • The shaft is bent.
  • The sleeve is not concentric with the shaft.
  • A bolted on stuffing box can slip if there is excessive vibration.
  • The rotating assembly is not dynamically balanced.
  • Water hammer.
  • Thermal growth.

High stuffing box temperature is another cause of component damage.

  • The elastomer will take a compression set and harden as the heat increases.
  • You can crack some ceramic seal faces, especially the 85 type.
  • Coated hard faces can heat check as the dissimilar materials expand at two different expansion rates.
  • Carbon can loosens in its metal holder
  • Coke formation between the lapped faces pulls out pieces of carbon. Look for this problem in hot oil applications.
  • Fillers and binders can melt in some carbon compounds.
  • Air trapped beneath the surface of the carbon can expand and blow out pieces of the carbon leaving pits in the seal face.

Cryogenic or low temperature can also cause damage to a component.

  • Elastomers will harden at low temperature.
  • Faces will go out of flat if they were not lapped at cryogenic temperature.
  • Low temperature can freeze moisture outboard and under the seal restricting the seal’s axial movement.
  • A special carbon is needed at cryogenic temperatures because graphite will not release from the carbon/graphite mixture.

Damage can be caused by high stuffing box pressure.

  • It can cause excessive face loading that will raise the temperature between the faces.
  • It can distort the lapped faces.
  • It can extrude the dynamic or static elastomers causing the seal face to hang up, or create a new leak path.

Other causes of seal damage.

  • A discharge recirculation line aimed at a metal bellows can cut the thin metal plates.
  • The wrong choice of seal materials will cause corrosion problems.
  • Hastelloy C springs are needed to prevent chloride stress corrosion problems associated with the 300 series of stainless steel.
  • AM 350 bellows metal is proving to be inferior for long seal life in high temperature petroleum products.
  • All of the seal components must be chemically compatible with what you are sealing as well as any cleaners or solvents in the lines.
  • The wrong lubricant on the o-ring can cause it to swell up and seize the shaft.
  • The fluid is at the inside diameter of the seal face. This happens with non-metallic, outside mounted seals and the inner seal of rotating, back to back, dual seal designs.
  • The internal pressure will cause the materials to go into tensile stress. Most seal faces are strong in compression, but weak in tension.
  • Solids will be thrown into the faces causing abrasive wear. This is a common failure in back to back rotating dual seal designs.
  • Solids pile up in front of the movable face restricting its movement.

A few things seal manufacturers can do to lessen seal component damage, and prevent face opening:

  • Hydraulically balance the seal faces to stop the generation of unwanted heat.
  • Use low friction face combinations.
  • Try to select universal materials. There is no need for the numerous types of carbons we find used in the sealing industry.
  • Design in two-way balance if there is a chance of the pressure reversal we find common in dual seal applications.
  • Design the springs out of the fluid to prevent them from clogging.
  • Keep the fluid at the seal outside diameter to take advantage of centrifugal force centrifuging the solids trapped in the fluid.
  • Design the dynamic elastomer to move to a clean surface.
  • Install environmental controls to prevent the product from changing state to a solid or gas.
  • Utilize stationary or self aligning designs to lessen the affect of pipe strain and pump driver misalignment.
  • Use cartridge mounted seal designs to compensate for thermal growth and impeller adjustment.
  • Use finite element analysis design techniques to lessen the affects of pressure and temperature distortion on the lapped faces.
  • Use suction recirculation as the standard method of providing stuffing box cooling. Be aware of the few instances where discharge recirculation could be a better choice:
    • Durco pump designs that adjust to the back plate.
    • When the entrained solids have a higher specific gravity than the fluid. (they float).
    • Double ended pump designs where the stuffing box is at suction pressure.
    • If the pumping fluid is close to its vapor point. Lowering the stuffing box pressure could cause the fluid to flash in the stuffing box.
  • The seal manufacturer can supply a steam quenching connection in the gland to prevent the formation of ice outboard the seal.