End face mechanical seal
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An end face mechanical seal, also referred to as a mechanical face seal but usually simply as a mechanical seal, is a type of seal utilised in rotating equipment, such as pumps and compressors. When a pump operates, the liquid could leak out of the pump between the rotating shaft and the stationary pump casing. Since the shaft rotates, preventing this leakage can be difficult. Earlier pump models used mechanical packing to seal the shaft. Since WWII, mechanical seals have replaced packing in almost all applications.
An end face mechanical seal uses both rigid and flexible elements that maintain contact at a sealing interface and slide on each other, allowing a rotating element to pass through a sealed case. The elements are both hydraulically and mechanically loaded with a spring or other device to maintain contact. For similar designs using flexible elements, see radial shaft seals and o-rings.
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[edit] Mechanical seal fundamentals
A mechanical seal must contain four functional components: 1) Primary sealing surfaces, 2) Secondary sealing surfaces, 3) a means of actuation and 4) a means of drive.
1) The primary sealing surfaces are the heart of the device. A common combination consists of a hard material, such as silicon carbide or tungsten carbide, embedded in the pump casing and a softer material, such as carbon in the rotating seal assembly. Many other materials can be used depending on the liquid's chemical properties, pressure, and temperature. These two rings are in intimate contact, one ring rotates with the shaft, the other ring is stationary. These two rings are machined using a machining process called lapping in order to obtain the necessary degree of flatness.
2) The secondary sealing surfaces (there may be a number of them) are those other points in the seal that require a fluid barrier but are not rotating relative to one another.
3) In order to keep the two primary sealing surfaces in intimate contact, a means of actuation must be provided. This is commonly provided by a spring. In conjunction with the spring, it may also be provided by the pressure of the sealed fluid.
4) The primary sealing surfaces must be the only parts of the seal that are permitted to rotate relative to one another, they must not rotate relative to the parts of the seal that hold them in place. To maintain this non-rotation a method of drive must be provided.
[edit] Seal Categories
All Mechanical seals must contain the four elements described above but the way those functional elements are arranged may be quite varied. The standards of modern mechanical seals are widely defined by API Standard 682 - Shaft Sealing Systems for Centrifugal and Rotary Pumps.
Mechanical seals are generally classified into two main categories: "Pusher" or "Non-Pusher". These distinctions refer to whether or not the secondary seal to the shaft/sleeve is dynamic or stationary. Pusher seals will employ a dynamic secondary seal (typically an o-ring) which moves axially with the primary seal face. Non-pusher seals will employ a static secondary seal (either an o-ring, high temperature graphite packing, or elastomeric bellows). In this case, the face tracking is independent of the secondary seal which is always static against the shaft/sleeve.
A "cartridge seal" is a prepackaged seal that is common in more complex applications.
Gap seals are generally used in bearings and other constructions highly susceptible to wear, for example, in the form of an O-ring. A clearance seal is used to close or fill (and join) spacing between two parts, e.g. in machine housings, to allow for the vibration of those parts. An example of this type of seal is the so-called floating seal which can be easily replaced. These seals are mostly manufactured from rubber or other flexible but durable synthetic materials.
[edit] Seal Piping Plans
Since the rotating seal will create heat, this heat will need to be carried away from the seal chamber or else the seal will overheat and fail. Typically, a small tube connected to either the suction or the discharge of the pump will help circulate the liquid. Other features such as filters or coolers will be added to this tubing arrangement depending on the properties of the fluid, and its pressure and temperature. Each arrangement has a number associated with it, as defined by American Petroleum Institute "API" specifications 610 and 682.
[edit] Tandem and Double Seals
Since almost all seals utilize the process liquid or gas to lubricate the seal faces, they are designed to leak. Process liquids and gases containing hazardous vapors, dangerous chemicals or flammable petroleum must not be allowed to leak into the atmosphere or onto the ground. In these applications a second "containment" seal is placed after the primary seal along the pump shaft. The space in between these two seals is filled with a neutral liquid or gas called a "buffer" or "barrier" fluid.
In a tandem seal, the seal will leak into the buffer fluid contained in the unpressurized cavity. If the cavity registers a dramatic increase in pressure, operator will know that the primary seal has failed. If the cavity is drained of liquid, then the secondary seal failed. In both instances, maintenance will need to be performed. This arrangement is commonly used when sealing fluids that would create a hazard or change state when contacting open air. These are detailed in API Piping Plan 52
In a double seal, the barrier liquid in the cavity between the two seals is pressurized. Thus if the primary seal fails, the neutral liquid will leak into the pump stream instead of the dangerous pumped fluid escaping into the atmosphere. This application is usually used in gas, unstable, highly toxic, abrasive, corrosive, and viscous fluids. These are detailed in API Piping Plan standards #53a, 53b, 53c; or 54. Plan 74 may also be considered a double seal flush plan, although it is used exclusively when describing a dry gas barrier seal support system. The barrier fluid used in a Plan 74 system is simply a gas, not a liquid. Typically, nitrogen is used as it's inert nature makes it advantageous due to mixing with the process stream being sealed.
Tandem and double seal nomenclature historically characterized seals based on orientation, i.e, tandem seals mounted face to back, double seals mounted back to back or face to face. The distinction between pressurized and unpressurized support systems for tandem and double seals has lent itself to a more descriptive notation of dual pressurized and dual unpressurized mechanical seal. This distinction must be made as traditional 'tandem seals' can also utilize a pressurized barrier fluid.
[edit] Origins
The Mechanical Seal was invented by George Cook and was originally called a "Cook Seal." He also founded the Cook Seal Company. Cook's seal (which actually did not have a means of drive) was first used in refrigeration compressors.
The Cook Seal company was a sideline product for Cook and he sold the company to Muskegon Piston Ring Company where it was renamed as The Rotary Seal Division of Muskegon Piston Ring Co. Muskegon Piston Ring sold the Rotary Seal Division to EG&G Sealol who in turn was largely acquired by John Crane Industries of Morton Grove, IL.
John Crane was founded in 1917 as Crane Packing Company, the company established several facilities throughout the United States, Canada and England. John Crane had been in the Mechanical seal business for many years prior to this acquisition and in fact had long since exceeded Sealol in overall market size.
Today, John Crane's main competitors are Flowserve, Burgmann (aka EagleBurgmann), AESSEAL, and A.W. Chesterton Co.
Smaller, niche, manufacturers include Global Seals Direct, Vulcan, First 4 Seals and Roplan.
In 1990 the world market for Mechanical Seals was estimated at $1 billion.
[edit] See also
- Stuffing box
- O-ring
- Lip Seal
- Labyrinth Seal
- Clearance Seal
