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”.
- There is only one sacrificial component in most mechanical seals and that is the carbon-graphite face. It’s the only seal component that is designed to wear.
- The definition of leakage is presently undergoing a major change. Until recently it meant “no visible leakage”. The new definition defines it as parts per million leakage with allowable limits diminishing as governments institute new laws and regulations.
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.
- Inexpensive, original equipment manufactured (O.E.M.) mechanical seals often wear or “frett” under the dynamic elastomer.
- Rotating metal bellows designs damage the shaft under the ant-vibration, damping lugs.
- The bearing grease or lip seals frett and damage the shaft close to the bearing location.
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.
- Hysteresis or delay in the ability of the lapped faces to stay together, is the major cause of fugitive emission problems.
- Rotating seals and most cartridge mounted stationary seals have to move excessively to compensate for pump-driver misalignment, pipe strain, operating off of the pump bep, and stuffing box faces that are not square to the shaft. Only self aligning and stationary designs can compensate for this movement and pass fugitive emission standards.
- Unfortunately many stationary seal designs are cartridge mounted to facilitate easier installation. Set screwing the cartridge to the pump shaft destroys the “squareness” that is critical in stationary designs. Some new concepts appear to have solve this squareness problem with a self aligning feature; check with your supplier to insure you are receiving this design.
It is always desirable to use less flushing water, or to eliminate it completely.
- Seals with springs in the sealing fluid are more likely to clog with solids than those designs where the springs are placed outside the fluid. Springs in the sealing fluid are also more sensitive to corrosion because of their high stress.
- Balanced seals generate less heat and therefore require less cooling.
- Suction recirculation makes sense in most applications with the exception of those applications that are being sealed near their vapor point, fluids containing solids with a low specific gravity, double suction, single stage pumps, and most Flowserve (Durco) semi- open impeller designs
It is always better to use clearly identifiable seal materials.
- You cannot troubleshoot “mystery materials”. You must know what material and grade of material is being used in the component to make a sensible decision as to what went wrong.
Superior, universal materials are better than individually selected materials that are chosen to satisfy only one set of operating conditions.
- The number of spare parts can be reduced significantly.
- You lessen the chance of an expensive “mix up” of materials because many seal materials look alike (black O-rings and stainless steel are a couple of examples).
- Flushing fluids and cleaning chemicals may attack the carefully selected low cost materials.
Seals fail for only two reasons:
- The faces open.
- One of the components becomes damaged.
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.