SUBJECT : Common sense and mechanical seals: 7-6
Is success in your business difficult to obtain? Sure it is. But you can succeed in any business if you're offering a product that helps your customer in working towards a sensible solution to his problems.
We would all like instant solutions to our seal and pump problems, but those instant solutions seldom exist. "Voodoo and black magic" seldom work in my industry. Most of the time the manufacturer has to be satisfied with extending the operating life of his customer's equipment and lengthening the time between his overhauls and failures.
In my rotating equipment schools I talk a lot about common sense, that elusive concept that is anything but common. It's like a popular definition of pornography, "You can't define it, but you know it when you see it". One philosopher defined common sense as, "a set of beliefs that appear to be obvious amongst people of a common culture". That doesn't mean that the observation is always correct, it just means that it is generally accepted.
Let's look at some of this "COMMON SENSE" as it applies to mechanical seals:
A mechanical seal should not leak until the sacrificial components are worn away. Leakage prior to this is a premature failure and always correctable. Most people will accept this statement if you define "sacrificial components" and "leak".
Since face wear is not a major cause of seal leakage and at least ninety five percent (85%) of existing single seals could not pass the current fugitive emission standards we have to agree that common sense tells us that 85% of existing seals fail prematurely.
A seal should not be designed to damage or destroy the pump shaft or sleeve. Because it is such a common problem many customers correctly use sleeve damage to justify the replacing of inexpensive packing with a more costly mechanical seal.
The more a seal moves the more likely the lapped seal faces will open, allowing leakage of the product and the introduction of solids between the faces.
It is always desirable to use less flushing water, or to eliminate it completely.
It is always better to use clearly identifiable seal materials.
Superior, universal materials are better than individually selected materials that are chosen to satisfy only one set of operating conditions.
Seals fail for only two reasons:
Since there are only two kinds of damage, corrosion or physical you should be able to see and identify it. If no damage is visible, the lapped seal faces must have opened and you should be able to figure out why that happened.
Seals that are "set screwed" to the shaft or positioned against a shoulder can not be reset when the pump open impeller is adjusted either manually of thermally. If you want to keep your open impeller pump efficient you must use a cartridge design.
If the seal survived for six months and the metal parts are not corroded, it should have lasted for many years. If the speed were too high, the pressure too great, the product too dirty, the vibration or misalignment too severe etc., the failure would not have taken six months to happen. Whatever failed the seal hast to be easy to correct.
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