SUBJECT: Evaluating Mechanical Seal designs. 6-2
For every seal application in your plant there are twenty-five seal salesmen ready to supply a seal design at widely varying price levels. Since the designs vary greatly it would seem logical that you should test the most promising proposal to see if the seal performance justifies the difference in price.
Here is where your problem starts. How do you test a mechanical seal? No two pumps are exactly alike; what is a fair test? Too often we test only how the seal performs when all the operating conditions are known and the seal is installed correctly. There is nothing wrong with performance testing, but you really should be considering the following:
1. How will the seal eventually fail? Will there be a massive leak similar to the type we find when rubber or metal bellows seals ruptures, or will the leak be controllable as it is with an A.P.I. (American Petroleum Institute) type gland, or back up seal?
2. Can the seal handle misalignment? Is the dynamic elastomer (rubber part) an o-ring that can pivot easily or is it some other shape that is spring loaded and restricts movement?
3. Can it handle shaft dynamic unbalance? Is the hard face wide enough to prevent the narrow face from "running off" when the shaft experiences the "run out" that accompanies dynamic unbalance?
4. What happens if you have a bearing failure? When the shaft starts its erratic movements will metal seal components contact the pump parts, causing sparking and severe damage?
5. Does the seal have built in environmental controls?
6. Can you make an impeller adjustment without upsetting the seal face loading?
7. Is the seal a non fretting design, or will you need to use a shaft sleeve that will probably increase the shaft L3/D4 number?
8. Is the gland a universal design that will fit most pumps of the same shaft size, or will individual glands have to be purchased?
9. Has the seal manufacturer provided a sensible method of insuring that the narrow seal face is centered in the wider face, or is he depending upon the pump manufacturer to provide this feature?
10. Is the seal an "off the shelf" design that is readily available, or does it carry a part number requiring a special inventory? Will it be stocked by a local distributor?
11. Will any of the seal components (especially the elastomers) be adversely affected by steam or cleaning of the lines with a caustic cleaner?
12. Do any of the seal components have a "shelf life" that can easily be exceed in yours or the distributors stock?
13. Are the seal components mass produced or are they made individually? Individually made components have severe quality control problems, especially with mult-machining operations.
14. Are all the seal components clearly identified by material and grade? You don't need mystery materials in your expensive equipment.
15. Does the seal have a wide range of operating limits? Pressure surges and water hammer are common, and seals do occasionally have to run dry.
16. How much radial and axial travel can the seal accommodate? Sleeve bearing equipment, higher temperature pumps and mixers or agitators need seals with a lot of radial movement capability.
17. Has vibration damping been provided? Poor lubricants cause "slip-stick" problems with the seal faces.
18. Is the seal balanced for both pressure and vacuum and are the springs placed out of the sealing fluid? If not, you are going to need a lot of clean flushing fluid to remove the unwanted solids and heat.
19. Will the seal be used in abrasive service? Thin metal bellows plates can wear severely unless the design rotates the abrasive fluid with the seal.
20. Are the components chosen from universal materials? There are over 100 different grades of carbon/ graphite used in seal designs.
21. How do you dispose of the contaminated parts when the seal is repaired? Can they easily be crushed or are the parts bulky?
22. Is there a spare part kit readily available? What does it cost? Do you have to return the seal for repair? The United States "Right To Know Law" makes returning used equipment to the manufacturer very impractical.
23. If it is a double seal, can it take reversing pressure without opening the lapped seal faces? Pressurized barrier fluids do fail at times.
24. Is installation easy?
25. If you are using a dual seal, does it have a built in pumping ring?
26. Do you have any self aligning feature that will prevent the "cocking" of the rotating face in a stationary, cartridge seal design?
A running test usually proves that most seals work. The above points determine if you are going to be satisfied with the seal life. Most automobiles run fine on a straight road. It's in the mountains and on winding roads that the difference in automobile design shows clearly.
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