The classification of mechanical seals.
Talk to a seal manufacturer's sales representative, send for all of the brochures, and you'll learn that the subject is very confusing. What do we really know about mechanical seals? A few things for sure:
* All legitimate seal companies have the essentially the same materials available to them. There are no secret or "mystery materials". There are, however, companies that purchase standard materials and then change the name to give the impression that they're supplying special materials and proprietary compounds. Shame on them!
* We're not sure of what is happening between the lapped seal faces. Testing has shown that :
o Sometimes there is a film of lubricant
o Sometimes the faces are running on vapor.
o Sometimes the faces run dry.
o Sometimes the faces run on a combination of all three mentioned above.
For all practical purposes seals should not leak visibly. Look at the seal in the water pump of your automobile, or the seal in the air conditioning unit in your car and ask yourself if you think either of them is leaking. Fugitive emission regulations have changed the definition of leakage to less than a few parts per million, depending upon the chemical involved.
* More than 85% of all mechanical seals fail long before they wear out. In other words there's plenty of wearable face left on seals when they're removed because of leakage.
* Seals are affected by pressure
o Extra heat is created at the faces. Heat is almost always destructive.
o The lapped faces will be distorted at some pressure. If they're distorted beyond five light bands ( 58 millionths of an inch or 1,5 microns) of flatness they'll leak.
o The elastomer will be extruded and become damaged at some pressure.
* Seals are affected by heat.
o All of the seal materials have an upper temperature limit. The elastomer (the rubber part) has the lowest. Its limit is determined by the type and grade of material being used. Some grades of carbon and most coated, or plated hard faces have restricted temperature limits.
o Thermal expansion can misalign components, put lapped surfaces "out of flat", and alter the seal face load.
o The pumping fluid can be altered if subjected to high heat. It can crystallize, solidify, vaporize, coke etc. This will restrict the free movement of the seal components.
o Corrosion always increases with heat.
* Stuffing box, or seal environmental controls are necessary a great deal of the time.
o With the exception of split seals, most applications require a large seal chamber with a stuffing box recirculation line connected to the suction side of the pump.
o The temperature of the fluid in the sealing chamber should be kept within certain limits. These limits are determined by the specific gravity. viscosity, concentration, vaporization point, etc. of the fluid.
o The pressure in the sealing chamber, often installed in dual seal application, can be controlled to prevent vaporization of the product or vaporization of the solvent carrying the product.
o If the product presents too big a sealing problem it can often be flushed away with a cool, clean, lubricating liquid.
* Non-clogging features are desirable and necessary in mechanical seal design.
o The spring or springs should be located out of the sealing fluid to prevent clogging and corrosion. Stressed metal corroded at a rate faster than unstressed metal and the springs are under a constant stress.
o The elastomer must move to a clean surface as the seal face wears.
o Centrifugal force should be used to throw solids away from the lapped seal faces.
o The moveable portion of the seal must move to a clean area as the seal faces wear. "Back to back" double seals and outside mounted seal designs should be avoided in dirty service.
* Vibration damping is necessary in all mechanical seals.
o To prevent the faces from vibrating open.
o To prevent damage and wear to the driving mechanism (drive lugs, spring, or bellows) and seal faces.
o To prevent damage (chipping at the outside diameter) to the carbon face.
* Back up seals or dual seals make sense in several applications. These include:
o Costly products.
o Dangerous products. The danger could be an explosive, fire hazard, carcinogen, toxic, bacteria, radiation, etc..
o When there is no spare pump and you can't afford an unexpected shut down caused by a seal failure.
o To keep oxygen or air away from a product.
o To prevent a product pressure drop across the seal faces, you can instll a back-up seal and pressurize beween the dual seals.
o Sealing pollutants
o To prevent freezing of the shaft on the outboard side of a seal.
* Proper seal face loading is more critical than you suspected. It can change with :
o Improper installation.
o Thermal expansion.
o Impeller adjustment. This includes both the initial setting and the adjustments that have to be made to compensate for wear.
o Face wear
o Axial play in the shaft bearings. Especially the sleeve or babbitt type.
If you like the brand of seal you're using ,have the manufacturer repair it at his facility, or purchase the spare parts from him to insure you'll be getting the correct materials and tolerances. It doesn't make sense to do anything else.
We'll now look at the various methods of classifying mechanical seals and in the process learn which to specify for our applications. I'll give a brief description of each type and list the most obvious advantages and disadvantages of each. Needless to say the advantage of one is almost always the disadvantage of the other
The rotating seal. The springs/ bellows rotates with the shaft
* Advantage. Lowest cost and reasonably self cleaning, especially in those designs where the springs are located outside of the sealing fluid.
* Disadvantage. Sensitive to misalignment between the shaft and the stuffing box face. This causes the seal to move back and forth twice per shaft revolution. Gaskets and thermal expansion increase the misalignment problem. Most of these designs cannot pass a fugitive emissions test.
The stationary seal The springs/bellows do not rotate with the shaft.
* Advantage. Misalignment and sealing fugitive emissions is not a problem unless the seal is cartridge mounted. Cartridge mounted stationary seals need some type of self aligning feature.
* Disadvantage. Alignment requires that the rotary unit be absolutely square to the shaft and, in a cartridge seal, this is very difficult to accomplish because the cartridge tends to cock the face when the set screws are tightened to the shaft.
o Not your first choice in slurry applications because centrifugal force will not throw the solids away from the moveable components. Slurry is defined as solids in liquid. Their size and quantity are not important.
o You must be careful when introducing cooling to this type of seal because the unit does not rotate, causing an uneven cooling of the lapped face.
o In recent years this seal has only been available in cartridge designs adding to the misalignment problem and increasing the initial cost considerably.
The inside mounted seal. All components are in the pumping fluid.
* Advantage. The elastomer can move to a clean surface as the seal face wears. Centrifugal force throws solids away from the seal components allowing the lapped seal faces to stay in co
* Disadvantage. All the metal components must be corrosion resistant to the pumping fluid.
* If the product solidifies or crystallizes when the pump is stopped, the seal can become inoperable.
The outside mounted seal. None of the metal components are in contact with the pumping fluid. Most designs clamp to the shaft rather than using set screws that damage ceramic or glass coated shafts.
* Advantage. This is the most common solution to non-metallic pump sealing.
* Disadvantage. Centrifugal force throws solids into the lapped seal faces and prevents the sliding components from moving freely.
o Higher stuffing box pressure can cause the retaining clamp to slide on the shaft
The single seal. It has only one set of sealing faces.
* Advantage. Lowest cost and usually a simple installation.
* Disadvantage. The equipment will be shut down with a seal failure. In most cases the resultant excessive leakage cannot be tolerated.
Dual seals. More than one set of faces are installed in a variety of configurations including:
* Back to back. The worst possible choice if used in the rotating seal version. Stationary versions are acceptable because the sealing fluid is located at the outside diameter of the seal faces where we can take advantage of centrifugal force
* Tandem. One seal behind the other requiring a low pressure buffer fluid between the seals. This arrangement cannot be used if a higher pressure barrier fluid is required or desirable.
* Face to face. All of the advantage of tandem sealing without the assembly problems. Usually the two seals share a common stationary face. This can be dangerous if the stationary face fractures because you'll lose both seals
* Concentric. One seal mounted inside the other, sharing a common mating face. These seals require a lot of radial space and are therefore usually limited to mixer applications. If the common face fractures, you'll lose both seals.
In all of these configurations two-way balance should be specified for safe operation.
* Advantage. Back up protection that will almost guarantee no unexpected seal failure.
* Disadvantage. Higher cost, and in some instances, space restrictions.
The unbalanced seal. The seal faces are subjected to full system hydraulic and surge pressures in addition to the spring pressure..
* Advantage. None, other than lower cost
* Disadvantage. Limited application. Usually requires a larger seal inventory because both balanced and unbalanced versions would have to be stocked for the common shaft sizes.
The balanced mechanical seal. The design allows the seal faces to see only a small portion of the system hydraulic pressure.
* Advantage. WCan handle a wde range of operating conditions from vacuum to high suction pressure, as well as unexpected pressure surges in the system.
* Disadvantage. The inside version (the most popular one) requires more radial room because of the need for a balance sleeve. The sleeve also adds to the initial cost. In cartridge seals it would be silly to build an unbalanced version, because the sleeve is always present, but some companies do it any way.
The elastomer type of seal , utilizing an o-ring, wedge, chevrons or a U-cup, with the o-ring type having the most advantages.
* Advantage. The elastomer acts as a natural vibration damper to prevent face chipping and separation. Only the o-ring, version can be used for either vacuum or pressure. The o-ring, configuration has the widest selection of materials available and is the most precision elastomer you can purchase.
* Disadvantage. All elastomers have temperature limits. Some modern elastomers have an upper limit of about 700 degrees Fahrenheit (370 C.)
The metal bellows seal. All elastomers have been eliminated from the design.
* Advantage. Wider range of temperature sealing. Excellent in Cryogenic sealing and most hot fluids with the exception of petroleum products. These petroleum products must be cooled to prevent "coking"
* Disadvantage. Higher cost than comparable elastomer sealing. Problems with vibration unless dampers have been installed. All of the low expansion metals used in these designs are not considered corrosion resistant. 316 Stainless steel is not acceptable because of Chloride Stress problems. In abrasive, slurry applications the thin plates are sensitive to wear and eventual fracture.
The cartridge seal. All components of the seal are mounted on a sleeve that can be secured to the shaft from out side the seal chamber.
* Advantage. An easy method of seal installation and a necessary feature for impeller adjustment. These designs allow you to change the seal with out emptying a side entering mixer application. Seal centering is provided for in most designs. There is an advantage to specifying A.P.I. type glands to take advantage the environmental controls and safety these glands provide.
* Disadvantage. Larger space requirement and higher initial cost. When used with stationary seals, you lose the advantage of total misalignment compensation unless the seal has some type of "built in" self-aligning feature..
The non-cartridge seal. The seal attaches directly to the pump shaft or sleeve, or in some cases, against a shaft shoulder.
* Advantage. None at all, except for lower initial cost and the fact that it takes less radial room than most cartridge versions.
* Disadvantage. Subject to all the errors that can be made at an installation. Longer installation time and the seal is unable to compensate for temperature growth or impeller adjustment.
The split seal. In a true split seal all the seal components are split in half so that the seal can be installed without taking the equipment apart. Rubber components are not allowed to be glued together as this would cause a "hard spot", interfering with the free movement of the dynamic elastomer.
* Advantage. The advantages are obvious. No one wants to disassemble any piece of rotating equipment unless it's absolutely necessary.
* Disadvantage. Limited seal materials available and some designs cannot be used in applications that cycle between pressure and vacuum because pressure assists in holding the components together.
The solid seal. The equipment must be disassembled to install the seal.
* Advantage. Can be used in alternating pressure/vacuum applications and, for the time being has a wider range of materials available.
* Disadvantage. Insulation must be removed. Several trades could be involved, the pump must be realigned and the list goes on and on...
Motion seals. They have larger internal clearances along with different spring arrangements and wider hard faces to compensate for excessive radial and axial motion. A radial movement capability of plus or minus 1/8" (3 mm) would be typical.
* Advantage. Ideal for mixers, agitators, sleeve bearing equipment, or any type of rotating equipment requiring excessive axial or radial movement.
* Disadvantage. Larger radial space required. 3/4 inch (20 mm) is typical.
Pump Seals. Manufactured for 3/8 inch (10 mm) packing space. Some designs will go into 5/16" (8 mm).
* Advantage. They fit into existing pump stuffing box space, but there is little to no room for proper operation unless you install either a larger stuffing box or back plate with the larger diameter stuffing box cast in.
* Disadvantage. Can handle only limited axial and radial movement. In slurry applications they clog easily.
Original equipment seal designs. The type you get if you don't specify a specific brand and model number.
* Advantage. Generally the lowest cost seal.
* Disadvantage. No interchangeability, requiring you maintain a much larger inventory. In most case these seals will frett and damage expensive shafts and sleeves. In the majority of cases you'll not be able to identify the grade of carbon, silicon carbide, elastomer etc., and they're always the non-balanced type.
Off the shelf, universal seal designs. Designed to fit into the thinnest, shortest space and still meet the necessary operating conditions. Most are non-fretting designs with universal materials installed as standard.
* Advantage. Lowest cost inventory and no more shaft/ sleeve damage.
* Disadvantage. Since the majority of these designs incorporate slotted glands, they require a centering method to prevent shaft/ sleeve contact.
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