O.E.M. SEAL PROBLEMS O003
The next time you purchase a pump, mixer or some other type of rotating equipment and request that it be supplied with mechanical seals, you are going to be very disappointed in the performance of those seals unless you specify exactly which brand, model and materials you want.
If you fail to do this you will be supplied with an unbalanced mechanical seal, manufactured from unnamed grades of materials, and you will find that the seal will leak long before the sacrificial seal faces are worn out. In other words the seal will have plenty of wearable carbon face left when it starts to leak.
In addition to supplying a poor performing seal, the manufacturer will attempt to put the seal into a piece of equipment that was designed for soft packing. The result will be both poor seal and bearing performance as well as a premature failure of both of them.
What is wrong with these original equipment manufacturer seals? The following paragraphs will describe a few of the problems. The seal that came in your pump was probably supplied with:
- An unidentified grade of carbon-graphite with binders and fillers that could be attacked by the product or cleaners used to flush the lines. In most cases the seal will not have enough density for higher temperature applications
- An unidentified grade of elastomer that has a temperature limit, chemical compatibility problems, possible shelf life and sensitivity to steam cleaning or some lubricants that might be used during the installation process.
- Stainless steel springs that are subject to chloride stress corrosion failure.
- Multiple springs located in the sealing medium that can easily clog when solids are present.
- A spring-loaded dynamic elastomer, Teflon® wedge, or V-rings that are not free to flex and roll on the shaft. These designs will damage the expensive shaft (this is called fretting) so the manufacturer will be forced to reduce the shaft diameter and supply a sacrificial sleeve that will weaken the shaft.
- No method of compensating for axial growth or impeller adjustment. This is a major problem with seals that position against a shoulder on the shaft. You will need a cartridge seal to solve this problem.
- Seal designs that are sensitive to the diameter, tolerance and surface finish of the shaft or sleeve.
- A discharge recirculation line that will fill the narrow stuffing box with solids and abrasives that can interfere with the free movement of the seal or even worse, a filter in this line that will clog up and cause overheating in the stuffing box.
- Rubber bellows designs that are very sensitive to the installation lubricant, shelf life and heat. These designs will experience massive failure as the bellows ruptures (and it will)
- Piloted glands with drilled bolt holes that require a large spare parts inventory and prevent you from using the same gland on most pumps of the same shaft size
- Single spring designs that are wound in one direction and sensitive to the direction of shaft rotation.
- Please look at the following diagram:
- Spring loaded elastomers that cause fretting damage in two places on the shaft or sleeve.
- A design in which the spring-load on the seal faces increases and decreases as the shaft moves axially.
- The inner rotating face moves into the solids as the carbon wears causing “face hang up” and premature failure along with product dilution.
- Faces that are kept closed by barrier fluid pressure and will open if there is a surge of pressure in the system or the barrier fluid pressure is lost.
- Dirt and solid particles are centrifuged into the lapped seal faces of the inner seal.
- If the outside seal fails or wears out (and it is supposed to) the inner seal will blow open when the barrier fluid pressure is lost making the design unsafe with dangerous or expensive products.
- No way of venting air from the seal faces when the seal is mounted in vertical applications.
What can you do about these problems? The answer is obvious; do not use original equipment seals. Do you remember how you handled the packing that came in pumps? You threw that stuff away and used the type that worked in your applications. Tell your supplier that you want balanced O-ring seals or balanced metal bellows seals made from proper materials and the material grades must be identified because you cannot run your facility on “mystery materials”.
To insure longer mechanical seal life, do the following:
- Specify balanced O-ring seals.
- Require that all seal materials are identified by compound and grade.
- Use split or cartridge seals for ease of assembly.
- Connect a suction recirculation line between the front of the stuffing box and the suction side of the pump or any other low pressure point in the system. Please refer to suction recirculation for the details of this arrangement.
CAUTION: A suction recirculating line will lower stuffing box pressure so do not do this if you are pumping close to the vapor point of the liquid. In this case you would connect the line between the stuffing box and the discharge side of the pump to raise stuffing box pressure. A throttle bushing installed in the bottom of the stuffing box will also aid in raising the pressure in the stuffing box.