Seal design problems

SEAL DESIGN PROBLEMS S009

  • The wrong carbon or hard face has been selected. The material is not compatible with the fluid you are sealing, and the cleaner or solvent used to clean or flush the system.
  • Original equipment companies supply unbalanced seals. They generate more heat than balanced seals.
    • Unbalanced seal designs require excessive flushing or cooling to remove unwanted heat.
  • Face flatness problems:
    • The face cross section is too narrow causing temperature or pressure distortion problems.
    • The material modulus of elasticity is too low.
    • The wide face is not hard enough.
    • All clamping forces must be “equal and opposite” to prevent face distortion. In many designs they are not.
    • The differential expansion between the seal face and its holder can cause the face to go out of flat.
    • The faces were not lapped at a cryogenic temperature and the seal is being specified for cryogenic service.
    • Bad packaging can cause the lapped faces to be damaged in storage.
  • Poor heat conductivity:
    • Carbon is a poor conductor of heat compared to most hard faces.
    • The dynamic elastomer is located too close to the seal faces. The heat generated at the faces is affecting both the elastomer and the seal face.
    • Most ceramics are not good conductors of heat; silicon carbide is an exception.
    • Plated or coated faces can heat check due to a differential expansion rate between the coating and the base material.
    • A gasket or elastomer sometimes insulates the seal face.
    • Some seal faces are glued in. The glue acts as an insulator preventing the face heat from conducting to the metal holder.
    • The coefficient of friction between the lapped faces varies with face combinations and various sealing products.
    • Carbon/ metal composite faces conduct heat better than solid carbon/ graphite, as long as there is a pressed in interference fit and they are not glued together to hold them in place. Graphite impregnated silicon carbide is one of the newer materials that has good heat conductivity
  • Low-expansion steel face holders are not usually corrosion resistant.
  • If the seal face is too wide the hydraulic force will generate excessive heat.
  • If the carbon seal face is too narrow the spring force can cause excessive heat.
  • Seals mounted in vertical pumps must be vented to remove air trapped in the top of the stuffing box.
    • An outside metal or elastomer bellows seal is almost impossible to vent.
  • Speeds above 5000 F.P.M. (25 m/sec) require a special hydraulic balance ratio and less spring load. A 60/40 balance and a face load of 8 psi. to 15 psi. (0,07 to 0,2 N/mm2) would be normal.
  • Spring loaded elastomers cause varying seal face loads. The actual load depends upon shaft tolerance and installation dimension.
  • Many single spring designs are uni-directional requiring both right handed and left handed seals on a double-ended pump.
  • No vibration damping has been provided to prevent slip-stick vibration problems. This is a major problem with metal bellows seals.
  • The carbon must be dense enough to prevent entrained air pockets from expanding and causing pits in the carbon face. An “unfilled carbon” with four impregnates is the best.

The springs or bellows.

  • Springs in the fluid can clog easily, especially small springs.
  • Stainless steel springs and bellows are sensitive to chloride stress corrosion problems. Choose hastelloy “C” material instead.
  • A single spring can be wound in the wrong direction and loosen in operation. The English seal manufacturer Flexibox has this problem
  • Thin bellows plates and small cross section springs are sensitive to abrasive wear.
  • Rubber bellows experience a catastrophic failure mode when the bellows ruptures.
  • Stressed metal corrodes faster. Springs and metal bellows are subjected to high stress.
  • Too much spring or bellows movement will cause an early fatigue of the metal. Misalignment is a major contributor to this failure.

The dynamic elastomer (the one that moves)

  • Some elastomers do not move to a clean surface as the face wears.
  • Spring-loaded elastomers tend to stick to the shaft or sleeve and are sensitive to the shaft diameter and finish.
  • Elastomers positioned in the seal face are subject to the heat generated between the seal faces.
  • All dynamic elastomers are sensitive to the shaft or sleeve tolerance and finish.
  • Dynamic elastomers flex, roll and slide on polished metal surfaces better than carbon surfaces.

Operating conditions too severe for the design.

  • Elastomers and some seal faces are sensitive to temperature extremes.
  • Excessive pressure can distort seal faces causing them to go out of flat.
  • Excessive pressure can cause elastomer extrusion.
  • High speed can separate the seal faces in rotating seal designs.
  • High speed can cause excessive heat at the seal faces.
  • Excessive shaft movement separates faces also.
  • Hard vacuum can “out gas” an elastomer causing it to leak.

Dual seals

  • Rotating “back to back” designs:
    • Centrifugal force throws solids into the inner faces.
    • The inner seal blows open if barrier fluid pressure is lost.
    • The inner stationary face is seldom positively retained to prevent movement, if the barrier pressure is lost between the faces.
    • When the outboard seal fails, the inboard will fail also due to the pressure drop between the faces.
    • The inner seal has to move into the sealing fluid as the face wears. This is a major problem if the fluid contains solids.
    • Failure to use two way hydraulic balance causes the inner faces to open with a reversal in barrier fluid pressure.

Pump design problems that cause excessive shaft movement

  • An elbow is installed too close to the pump suction inlet.
  • The mass of the foundation is not five times the mass of the pump and its driver.
  • Wrong size pump was specified because of safety factors and, as a result, the pump is operating off the best efficiency point (BEP).
  • The pump was selected oversize in anticipation of a future need.
  • A centerline design should have been selected when the operating temperate exceeded 200°F (100°C).
  • The shaft L3/D4 is too high.

The pump is cavitating due to a design problem.

  • Too high a NPSH is required. You need a double suction pump.
  • The suction specific speed number is too low.
  • You are using too low a specific speed impeller.
  • A reducer has been installed up side down, letting an air pocket into the suction.
  • The impeller to cutwater clearance is too low.
  • There is too much suction resistance due to excessive piping.
  • Too much suction lift for the fluid temperature.
  • An elbow has been installed too close to the pump suction.

Other design problems

  • Most seal designs cannot compensate for thermal shaft growth or impeller adjustment. Cartridge versions are needed for this feature.
  • The pumping fluid is located at the inside diameter of the seal faces.
    • Solids will be thrown into the lapped faces destroying some face materials.
    • Solids will pile up in front of the movable faces, preventing them from compensating for wear.
    • Most seal faces are weak in tension.
  • Hysteresis (delay or lag) problems caused by the seal mass and sliding elastomers.
  • Poor packaging that allows face damage during shipment and storage.
  • Designs that frett (damage or groove) the shaft or sleeve.
  • High speed requires the use of stationary seal designs. Centrifugal force can open rotating designs above 5000 fpm. (25 m/sec.)
  • The seal is positioned too far from the bearing housing.
  • Lack of a self-aligning feature is causing excessive face movement.
  • A tapered stuffing box can cause face damage.
  • No vent has been provided to vent the stuffing box in a vertical application.
  • Hardened shafts and sleeves can cause the seal set screws to slip.
  • A discharge recirculation line is aimed at the lapped faces, causing them to wear and interfering with the seal movement.

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Posted

  • On February 17, 2018