Mechanical seal repair
The days of sending mechanical seals back to the manufacturer, or having a local repair facility repair your seals will shortly come to an end. There are several reasons why this will be true:
- Most new seal designs are oriented to “in house repair”.
- Recent and impending legislation like the “Right To Know Law” in the United States will make the paper work involved in sending seals “off site” very prohibitive.
- Contamination and disposal problems will make outside facilities very reluctant to deal with products exposed to the chemicals involved in the process industry.
- The perceived threat to job security will make your people committed to keeping repair work in house.
- You have now, and will in the future have an increasing need to know exactly what materials and tolerances were used during the repair.
Environmental and liability problems will necessitate the documentation of this information. Too many “off site” repair facilities use “unnamed materials” and material grades. Original tolerances are impossible to be duplicated by any one other than the original manufacturer.
What is involved in a normal seal repair? Let’s take a close look at a typical sequence:
1. Inspect and troubleshoot the various components for evidence of corrosion, rubbing, wear or damage. This type of failure analysis should be a normal part of repairing seals. Why put in another seal until you have learned why the present seal failed? The troubleshooting sections of these web pages will help you with this determination.
2. Disassemble the seal and throw away the following components:
- The carbon/graphite face.
- The elastomers and gaskets.
- The spring or springs.
- The set screws
- Any drive lugs or anti-rotation pins.
- Cracked or broken hard faces.
- Cracked or broken bellows.
3. Clean the remaining components such as the sleeve, face holder, adapter, etc.
4. Inspect the cleaned components to be sure they meet the manufacturer’s original dimensions, tolerances, and finish.
5. Some metal-carbon composites have to be stress relieved after assembly to remove manufactured stress that will prevent the carbon from remaining flat during the storage and shipment period. Watch out for seals that are used in cryogenic service. They should be lapped at their cryogenic operating temperature.
If you intend to insert a carbon into a metal holder, you will be better off pushing it in with an arbor press. If you try to expand the holder and shrink it to the carbon you’ll have problems maintaining face flatness unless you stress relieve the assembly.
6. Reassemble the components using only new parts that have been supplied by the manufacturer. Be sure to use original equipment parts because many seal manufacturers design components that have been created from “finite element analysis” or similar techniques. Solid hard faces can be relapped if they are not damaged in any way. Plated or coated faces must be stripped and re-plated before lapping.
7. Check the assembled seal for flatness. A vacuum check should be performed on the movable assembly to insure there is no leak path through or around any of the components. If you elect to check cartridge mounted double seals with an air test, avoid immersing the seal in water. If the seal was later installed in hot oil, or a similar application, the water trapped in gaskets and small crevices would flash to steam generating possibly dangerous pressures.
8. Repackaging varies with the manufacturer. Here are some points to consider:
- The packaged seal should be able to survive a thirty nine inch drop (one meter) without injury to any seal component including the lapped seal faces.
- A protective coating (Silicone is a common one) should be put on the lapped faces.
- A new set of installation instructions and an installation print should be placed in the box with the rebuilt seal. A box is always superior to “bubble packaging.”
- The box should be clearly labeled as to its contents. The use of only a part number leads people to open the box, often causing unnecessary damage to the mechanical seal while it is being handled.
During the rebuilding process you should keep in mind that troubleshooting a rebuilt seal is a frustrating experience. A trained troubleshooter is looking for evidence of rubbing, damage and corrosion. You should be careful to clean up evidence of any of these symptoms prior to repackaging the seal, to avoid confusion the next time the seal failure is analyzed.
In the event you should decide to have another facility repair your seals, have that facility remove the manufacturers name and part number from the seal and replace them with their own. It’s not fair to have the original manufacturer be responsible for unknown seal materials that might have been substituted by the last repair house.
- On February 17, 2018