Preventing premature seal failures 11-5
What is the best way to prevent a premature seal failure? Find out what is causing the failure and then take the necessary steps to prevent it. If you understand this simple logic you will be pleased to learn that mechanical seals fail for only two reasons:
- The lapped faces open.
- One of the seal materials becomes damaged.
Please look at the following diagram. It describes a typical original equipment seal (O.E.M.) with five potential leak paths. If the carbon face were pressed into a metal holder there would be an additional leak path between the carbon and the holder.
|1. Teflon ® dynamic seal|
|2. Seal between the faces|
|3. Stationary face seal|
|4. Sleeve to shaft seal|
As you look at this seal remember that there is a continual axial and radial movement of the shaft due to run out, whip, wobble, vibration, operating off the best efficiency point, etc. Regardless of this movement the lapped seal faces must remain in contact.
The seal is normally set screwed to the shaft or sleeve causing the metal case to follow any and all shaft movement. The springs continuously flex because they are pushing the Teflon® wedge to the shaft as well as putting an axial load on the seal faces. This means that there is a constant relative movement between the Teflon wedge and the shaft sleeve. We see evidence of this rubbing when we observe the frett mark on the sleeve caused by the removal of the sleeve’s protective oxide layer. This passivated oxide layer forms on all corrosion resistant metal parts and can easily be rubbed or polished off exposing the base active metal.
Most fluids contain a certain amount of suspended solids. If the lapped faces open, these solids will penetrate between the seal faces, imbed or stick into the softer carbon face, and cause a scoring or damaging to the hard face. This is the most common seal failure that you will experience.
In addition to suspended solids there are other problems with fluids that can cause the premature opening of the lapped faces. The product we are trying to seal could:
- Vaporize between the lapped faces causing them to blow open.
- Solidify in the springs, or between sliding seal components causing the faces to open as the shaft or sleeve moves.
- Become viscous causing the seal faces to be pulled open as the springs are not able to rapidly respond to the shaft movement.
- Crystallize, restricting the movement of the seal components.
- Build a film on sliding components restricting their movement. The film can also build up on the seal faces causing them to open.
- Be a non lubricant. Sealing fluids that have little to no lubricating qualities can cause “slip stick” problems between the lapped faces that can cause the seal faces to bounce open.
- Be a corrosive and attack the dynamic elastomer causing it to swell and lock up the seal.
There are certain operating conditions that can cause the lapped faces to open:
- Vacuum can over come the face load in many unbalanced seal designs. Some mixer designs alternate between a positive pressure and vacuum.
- High stuffing box pressure can distort some lapped faces.
- High shaft speed can prevent the seal faces from staying in contact because of the affects of centrifugal force. This is a consideration any time the shaft is rotating faster than electric motor speeds.
- Cryogenic temperatures can harden any elastomers in the system, and freeze moisture outboard of the seal. This frozen moisture will prevent the seal from moving forward to compensate for face wear
- High temperature can lengthen the shaft and sleeve changing the seal face loading.
- Cavitation is a major cause of seal face separation and damage.
The condition of the sleeve can cause the seal faces to open:
- The sleeve may have been hardened for packing and causing the soft stainless steel set screws to slip. Although not desirable as a set screw material we are forced to use stainless steel and other soft metals for corrosion resistance.
- The sleeve diameter could be oversize causing an excessive squeeze on the wedge or elatomer. Many packing sleeves have this problem.
- The sleeve finish could be too rough.
Maybe something went wrong during the installation process that is causing the seal faces to open:
- The mechanic made the wrong measurement that would insure proper face loading.
- The sleeve moved when the impeller was tightened against the sleeve.
- The initial impeller adjustment was made after the seal was locked on the shaft.
- The impeller is being adjusted to compensate for wear between the impeller and the volute.
- The hard face has been installed backwards, causing the carbon to run on a non-lapped surface.
- The seal face was damaged during the installation process.
- Thermal growth is affecting the face loading.
- The wrong lubricant was put on the elastomer (O-ring) causing it to swell up and lock the seal components together. In some instances a swollen elastomer can force a seal face out of its holder.
- The seal was mounted outside the stuffing box to prevent the small springs from clogging. The seal now has to move into a dirty fluid as the softer seal face wears.
The seal design is causing the face opening problems:
- The springs are in the liquid and they are clogging with solids suspended in the fluid. Sometimes outside springs get painted by maintenance people. This is more common with dual cartridge seals.
- The sliding elastomer moves to a dirty service, causing the lapped faces to separate.
- A discharge recirculation line is aimed at the seal faces or a sliding component restricting its movement. These high velocity solids can erode metal bellows plates causing a drastic failure.
- The Teflon® wedge is getting hung-up on the frett groove.
- Vibration damping can prevent normal vibration from opening some seal designs. One of the advantages of O-rings positioned in the dynamic face is that an O-ring is a natural vibration damper. This is an important consideration in metal bellows seal designs where the elimination of elastomers has made the seal sensitive to vibration.
The rotating shaft is not centered in the stuffing box. A rotating part of the seal is contacting a stationary part of the stuffing box causing the seal faces to open:
- The pump is operating off of its best efficiency point (B.E.P.).
- Pipe strain.
- Misalignment between the pump and its driver.
- The shaft is bent.
- The shaft assembly is not dynamically balanced.
- Thermal growth.
- Pulley driven shafts.
- The stuffing box face gasket is protruding into the stuffing box and rubbing against the seal.
- A fitting is protruding into the stuffing box and rubbing against the seal.
The typical metal bellows seal has five leak paths also:
- Between the carbon face and the metal holder
- At the static O-ring attaching the bellows seal to the shaft.
- Between the lapped faces
- Between the stationary face and the gland
- Between the gland and the stuffing box face