Why mechanical seal faces open prematurely

WHY MECHANICAL SEAL FACES OPEN PREMATURELY ST003

Here are the most common reasons that lapped faces open prematurely:

  • The setscrews slipped on a hardened shaft sleeve causing the seal to lose its spring load.
  • The dynamic elastomer (the rubber part that moves) stuck to the shaft or sleeve. The spring load on the wedge, V-rings or U-cup used with most pusher type seals was too high. The Crane #9, Durametallic ROTT and Borg Warner “U” are typical of these designs. A high spring load causes too much interference fit on the shaft.
  • The shaft or sleeve diameter was too large; + 0.000 inches to – 0.002 inches (+ 0.00 mm to – 0,05 mm) is a good tolerance for mechanical seals.
  • The sleeve finish was too rough. You want a finish of 32 rms (0,8 microns) or better, unless you are using rubber bellows designs that call for a finish of no better than 40 RMS.
  • The O-ring groove was out of tolerance. It is causing the O-ring to have too much interference with the shaft preventing the O-ring from flexing and rolling.
  • The elastomer swelled up because it was not compatible with the pumping fluid or a cleaner that was circulated through the lines. In some designs this expansion of the rubber part can force the lapped face out of its holder or break the hard face.
  • Too much axial movement of the shaft:
    • The seal was installed on the shaft and then the semi-open impeller was adjusted to the pump volute for the initial clearance setting, or to compensate for normal impeller wear. This is a very common problem with all open and semi-open impeller designs.
    • Thermal growth of the shaft. Every inch of shaft will grow 0.001 inches per inch of shaft for each 100°F (0,001 mm/mm/50°C) of temperature increase. The growth takes place both axially and radially.
    • Sleeve bearings allow excessive axial movement. Some centrifugal pump designs utilize this type of bearing as a thrust bearing.
  • Too much shaft vibration and no vibration damping. This is a big problem with the metal bellows seals we find in high temperature applications. Vibration damping interferes with the frequency of the vibration. The O-ring found in many seal designs is a natural vibration damper.
    • Harmonic vibration is a major source. The seal is vibrating in harmony with another piece of equipment
    • Cavitation is very common in many applications. Remember that there are four types of cavitation and all of them can cause vibration problems.
    • “Slip stick” vibration can occur between seal faces if the pumping fluid is not a lubricant. Hot water is a good example of a non-lubricating liquid. Most gases and dry solids are obvious non-lubricants.
    • Hitting a critical speed is another cause of vibration. Pumps that have this problem are said to have flexible rather than rigid shafts.
  • Installation errors:
    • Not enough load on the lapped seal faces. Do not be tempted to increase the designed load because the additional heat generated can be a cause of seal face damage.
      • The seal was installed at a wrong initial setting.
      • The sleeve moved when the impeller was tightened. Measurements should be taken after the sleeve to shaft gasket is compressed.
      • The mechanic misread the installation print.
      • Some people need glasses to read a print or scale. They can see the 1 and 2 on the scale, but not the funny little lines in between.
      • The mechanic used the old setscrew marks as a guide and their location was not correct.
      • No print was available at the installation site. The boss has it locked in his filing cabinet so it will not get dirty or lost.
      • The impeller was adjusted after the seal was attached to the shaft. Duriron pumps are the exception because their impeller adjusts towards the back plate causing the seal to over compress.
      • Shaft or sleeve thermal growth. In most seal designs this will unload the seal faces. The seal must be attached to the shaft after all thermal growth has occurred. You are going to need a cartridge seal to do this. Outside mounted, non-metallic seals can be an exception. They will over compress with thermal growth.
  • Rotating type mechanical seals need the stationary face installed square to the shaft to prevent excessive axial movement. Stationary designs need the rotating face to be installed square to the rotating shaft. This is much easier to do as long as the seal face is not set screwed to the shaft. It should be butted up against a square shoulder or some other type of “squareness” must be provided.
  • The wrong lubricant was used on the dynamic o-ring causing it to swell up and lock the seal to the shaft. As the shaft moves, the faces open.
  • Reversing stuffing box pressure can cause most unbalanced seal designs to open.
  • High shaft speed will cause centrifugal force to drive the rotating face square to the shaft, opening the lapped faces. 5000 fpm (25 meters/sec) is just about the limit for rotating seal designs.
  • The shaft is fretted (grooved from the dynamic elastomer) causing the moveable face to hang up as it tries to compensate for wear.
  • The lapped faces are not flat.
    • The faces never were lapped flat
    • The lapped face was installed backwards. You are running on the non-lapped side. Some manufacturers mark the non-lapped side.
    • High stuffing box pressure can distort a lapped face.
    • Thermal distortion can distort a seal face.
    • Seal used in cryogenic service (very cold) must be lapped at cryogenic temperatures.
    • The lapped face was dropped. It did not break, but the worker is afraid to tell his boss.
  • The product changed state and is restricting the movement of the seal.
    • The product can crystallize if you change the temperature in the stuffing box.
    • A temperature change or agitation can cause a product to become viscous and interfere with seal movement.
    • Some liquids can solidify with a change in temperature, pressure or agitation.
    • A change in temperature or pressure can cause a product to build a film on the seal sliding surfaces.
    • The product vaporized between the faces and blew them open. This can happen with an increase in temperature or a decrease in stuffing box pressure.
  • Solids clogged the springs or some other part of the seal, restricting seal movement.
  • Solids outboard the seal can restrict axial movement as the seal moves to compensate for carbon wear.
    • Ice can form when some products vaporize, or cold weather can freeze moisture in the air.
    • Crystals and solids can form outboard because of seal leakage or dirty quench fluid.
    • If you are using a gland quench connection, the quenching fluid must be clean or it will deposit contaminants outboard the seal. Shop water is not a good source of quench water, use condensate or low pressure steam instead.
  • Discharge recirculation lines aimed at the moving seal parts can restrict their movement.
  • The seal face hung up in the fretted groove that we find so common in most original equipment seal applications.

The easiest way to tell that you are having seal face opening problems is to inspect the hard face for evidence of wear. Common sense dictates that carbon cannot wear a hard seal face.

If the faces open they will allow solids to penetrate between the lapped faces and these solids will embed into the softer carbon when the faces close. The contaminated carbon will then act as a grinding surface making wear marks in the harder face.

One of the main contributors to the opening of seal faces and seal damage is the movement of the shaft. Let’s look at the different types of shaft deflection:

Posted

  • On February 18, 2018